Category Rocketing into the Future

“Ah, but a man’s reach should exceed his grasp, or what’s a heaven for?” –

Robert Browning

On 4 October 2004 the (unofficial) airplane altitude record of 107.8 km (353,700 feet) established by the X-15 in 1963 was finally broken. Not by a new, large-budget government rocket aircraft but by the privately developed SpaceShipOne rocket plane which pilot Brian Binnie flew to an altitude of 112.0 km (367,400 feet). The project was entirely funded by a private sponsor and the vehicle was developed and flown by a small commercial aircraft company.

This revolutionary development in rocket planes and spaceflight had its roots in the X prize, a $10 million reward announced in 1996 for the first private enterprise to develop and launch a suborbital vehicle. The competition’s rules dictated it had to be capable of carrying three people to the ‘edge of space’, and this, in accordance with International Aeronautical Federation regulations, was defined as an altitude of 100 km (62 miles). An X prize vehicle would give its passengers a thrilling ride, enabling them to view the curvature of the Earth and enjoy several minutes of weightlessness in the same way as experienced by X-15 pilots. To prove its reusability, the X prize organization required the same vehicle to make a second flight within two weeks of the first launch with at most 10% of its dry weight being replaced. In addition to the pilot, it had to be capable of carrying two passengers, but the flights required to win the prize could be made by the pilot only.

The purpose of the prize (in 2004 renamed the ‘Ansari X Prize’ following a multi­million dollar donation from entrepreneurs Anousheh Ansari and Amir Ansari) was to encourage the development of suborbital space tourism and thus kick-start a non­governmental human spaceflight industry. It was modeled after the aviation prizes of the early twentieth century which tremendously boosted aviation, such as the Orteig Prize for crossing the Atlantic that was won by Charles Lindbergh and the Schneider Trophy that encouraged the development of extremely fast seaplanes (the heritage of which was evident in several fighter planes of the Second World War, most notably the Spitfire). Twenty-six teams from around the world declared their participation in the competition, with some intending to employ relatively simple rockets (one even a modern derivative of the A4/V2 design) launched from the ground or slung beneath stratospheric balloons, others choosing rocket powered spaceplanes, and others rather exotic concepts such as pulse-jet driven flying saucers. One group even imagined a do-it-yourself suborbital rocket plane which you would be able to assemble in your own garage and launch from the nearest airfield.

The big surprise, however, was Scaled Composites, the Californian company of famous aircraft designer Burt Rutan, which initially shied away from publicity but in April 2003 revealed a project that was far ahead of its competitors. Not only did the company have a good plan but also real hardware: a fully operational twin-engined turbojet high-altitude carrier plane called the White Knight, a mobile mission control center, a mobile propulsion test facility, and a prototype of the Space – ShipOne air-launched, three-seat rocket plane. The company was by then already known for its innovative small aircraft designs, among them the Voyager aircraft that in 1986 flew around the Earth in just over 9 days without refueling or landing (73% of its weight at take-off consisted of fuel, leading to design constraints somewhat similar to those faced by spaceplane designers).

SpaceShipOne is primarily built using composite materials, a signature of Scaled Composites’ designs, as indicated by the name of the company. The fuselage is bullet shaped, similar in appearance to the X-l. Its stubby wings have a slightly swept-back

SpaceShipOne in a glide flight [Scaled Composites, LLC].

leading edge, a straight trailing edge, and a vertical fin bearing a single horizontal stabilizer at each wingtip. The total length is 8.5 meters (28 feet), the wingspan is 8.2 meters (27 feet), and the total take-off weight is 2,900 kg (6,380 pounds). Its flight profile resembles that of the X-15 by involving an air-drop, boosted ascent, ballistic trajectory into space, re-entry and glide back to the ground. But it is intrinsically a much simpler aircraft, designed not for cutting-edge research flights but purely as a precursor for commercial tourism flights, and it benefits from an additional 40 years’ of developments in aerodynamics, materials and avionics (as well as the considerable experience of the X-15 program). Although its maximum speed is Mach 3 rather than the X-15’s Mach 6.7 and the mission does not call for extreme speed, it does call for extreme altitude. And whereas the X-15 could not survive a steep descent into the atmosphere and so had to fly a 40 degree ascent and descent trajectory over a horizontal distance of some 500 km (300 miles), SpaceShipOne flies up and down almost vertically so that its entire flight occurs within 40 km (25 miles) of its base. This greatly simplifies its operations by not requiring a large network of ground stations, chase planes and emergency landing sites.

SpaceShipOne is propelled by a single, revolutionary rocket motor which is a mix of a solid rocket booster and a liquid propellant motor. This SpaceDev SD010 hybrid motor uses a solid rubber-like HTPB (hydroxyl-terminated polybutadiene) grain as fuel, but in combination with liquid nitrous oxide (also known as laughing gas). The main benefit over a solid propellant booster is that this hybrid engine can be throttled and shut down at any moment by varying the amount of liquid oxidizer that enters the combustion chamber. Without the liquid oxidizer, it is totally safe from explosion during transport and handling. Furthermore these propellants have a higher specific impulse. The hybrid is also simpler than a liquid propellant rocket engine by having only one valve and redundant igniters. In contrast to the complex

XLR99 engine of the X-15 the SD010 uses only non-toxic, easy-to-handle propellants and it has never failed to start. It has a maximum thrust of 75,000 Newton, a specific impulse of 250 seconds, and a maximum total burn duration of 87 seconds.

Prior to re-entering the atmosphere the plane’s two tail booms and the rear half of the wings fold upward on a hinge that runs the length of the wing. This ‘feathered’ position gives the aircraft a high-drag that allows a safe, stable “carefree, hands-off’ penetration of the atmosphere which greatly reduces aerodynamic and aerothermal loads. For this innovative solution Rutan was inspired by a badminton shuttlecock, which always orients itself correctly with the direction of flight. The cockpit has a spacecraft-like environmental control system and features many windows to provide a good view for the pilot and passengers (although no passengers were carried). The aircraft has three flight control systems: a direct manual control for subsonic speeds, an electric control system for supersonic speeds (where muscle power alone is unable to handle the aerodynamic forces), and a reaction control system for high altitudes. The thrusters emit non-toxic cold gas (there is no combustion involved). State-of- the-art instrumentation provides the pilot with the precise guidance information he needs to manually fly SpaceShipOne during the critical boost and re-entry phases. Flight test data is sent to a mission control center during each flight, where it is recorded for careful post-flight analyses.

The only SpaceShipOne aircraft was registered as N328KF, with N the prefix for US-registered aircraft and 328KF chosen by Scaled Composites to stand for 328 К (for kilo, meaning thousand) feet, corresponding to the 100 km altitude goal (registry number N100KM was already taken).

The White Knight plane, SpaceShipOne’s carrier, is itself an innovative aircraft. It too is made mostly out of composite materials. It has two afterburning turbojets, thin wings that have a total span of 25 meters (82 feet) and two tail booms. Most of the cockpit, instrumentation and other internal equipment are identical to those installed on SpaceShipOne, enabling it to flight-qualify much of the equipment intended for SpaceShipOne, thereby sharing the development costs for the two aircraft. The White Knight could be used as a trainer aircraft for SpaceShipOne pilots. The high thrust from its turbojets with afterburners in combination with the low weight, as well as it enormous speed brakes for rapid deceleration meant that rocket plane pilot trainees could use the White Knight to rehearse SpaceShipOne’s boost flight, approach and landing very realistically.

On 21 June 2004 the White Knight took the diminutive SpaceShipOne with 62- year-old pilot Mike Melvill to an altitude of 14 km (46,000 feet). The spaceplane was dropped into a gliding flight, then fired its rocket motor for 76 seconds. Shortly after ignition of the rocket motor, wind shear suddenly made the aircraft roll 90 degrees to the left. Melvill attempted to correct it and unexpectedly rolled 90 degrees to the right. He then managed to level the plane again and proceed with the steep but still somewhat unstable powered boost to a maximum speed of Mach 2.9. During the rocket bum Melville reported a loud bang that was later reahzed to have been caused by the overheating and subsequent crumpling of a new aerodynamic fairing that had been fitted around the rocket nozzle. Fortunately the fairing’s collapse did not affect the flight. After burn-out of the engine the plane continued unpowered to an altitude in excess of 100 km (62 miles). This coasting phase and the following free-fall back to Earth lasted about 3.5 minutes, during which time Melvill opened a bag of M&Ms and watched them float weightlessly around the cockpit. At the highest point of the trajectory the vehicle’s speed was almost zero. Then it began to fall, accelerating to a maximum speed of Mach 2.9 (the same as its maximum speed going up, as potential energy converted back to kinetic energy). During the fall, the two tail booms and rear parts of the wings were put in a vertical position to achieve the high-drag configuration that facilitated a safe, stable penetration of the atmosphere. The thickening air then decelerated the vehicle, and subjected Melvill to a tolerable 5 G deceleration. The re-entry air temperatures remained less than 600 degrees Celsius (1,100 degrees Fahrenheit) owing to the large area of the underside of the aircraft and the relatively modest velocity. There was no need for heat shields or tiles because the hot re-entry phase was brief and the air at high altitude too tenuous to transfer a lot of heat; the skin of the aircraft remained much cooler than the surrounding air (the X-2 flew its ‘heat barrier’ research flights at similar speeds but at much lower altitudes, while the X-15 and orbital vehicles returning from space endure much higher temperatures as a result of their faster entry speeds). In fact, SpaceShipOne’s structure hardly contains any metal parts. At 17 km (57,000 feet) the wings and tail were repositioned and the aircraft reverted to a conventional glider for its descent to the runway in the Mojave Desert in California.

“It was a mind blowing experience, it really was; absolutely an awesome thing,” Melvill said after landing. With this flight he became the first private civilian to fly an aircraft into space, as well as the first person to leave the atmosphere in a non­government sponsored vehicle. (All rocket aircraft except the early, pre-war rocket – boosted gliders were developed under government contracts for military or research purposes.) Measured by the number of world newspapers that carried the story above the fold, the flight was the second largest news event of the year, being topped only by the capture of Saddam Hussein in Iraq.

Work on Scaled Composites’ suborbital spaceplane concept began right after the X Prize announcement in 1996 and the full development program was initiated in April 2001, hidden from the public and the competitors by the inhospitable Mojave Desert. To finance the project the company got a $30 million grant from Paul Allen, Microsoft cofounder and third-wealthiest person in America. Since the X Prize was $10 milhon, Allen could not expect to get a return on his investment any time soon but he was in it for the sense of adventure rather than for the money. The overall plan was to mature the concept, then sell improved vehicles to a space tourism company. “Spaceflight is not only for governments to do,” Allen said. “Clearly, there’s an enormous pent-up hunger to fly into space and not just dream about it.”

SpaceShipOne made its first captive flight on 20 May 2003 and shortly thereafter Rutan announced the project to the public. After a second captive flight there were seven successful glide drop tests before pilot Brian Binnie made the first powered flight on 17 December of the same year (deliberately marking the 100th anniversary of the first ever powered aircraft flight by the Wright brothers). A short burn of the rocket motor pushed the aircraft to Mach 1.2 and an altitude of 21 km (68,000 feet). The left main gear collapsed due to a roll oscillation upon landing but the damage was minor and Binnie was uninjured. After another glide test flight there was a series of progressively faster and higher flights, culminating in the one in June 2004 that put Mike Melvill into space. During the test program SpaceShipOne also became the first privately funded aircraft to exceed Mach 2. All of the flights took place from the Mojave Airport Civilian Flight Test Center, the runway close to Scaled Composites’ premises. The four pilots that flew SpaceShipOne came from a variety of aerospace backgrounds: Mike Melvill was a test pilot, Brian Binnie a former Navy pilot, and both Doug Shane and Peter Siebold were company engineers. They all trained to fly SpaceShipOne using a flight simulator (like the X-15 pilots) as well as by flying the White Knight and other aircraft produced by Scaled Composites.

After Melvill’s space flight, everything was deemed ready to try for the X Prize by making two such flights within a fortnight. On 29 September 2004 Melvill shot up to an altitude of 103 km (338,000 feet), which was slightly less than planned due to a serious roll instability during the rocket-boost phase, but was still above the 100 km requirement. It was quickly followed on 4 October (specifically chosen to mark the 47th anniversary of the launch of Sputnik) by Brian Binnie’s fully successful flight to the record altitude of 112.014 km (367,500 feet) that won the X Prize for Scaled Composites and also made SpaceShipOne the first privately funded aircraft to exceed Mach 3: when the motor cut off at over 61 km altitude (200,000 feet) the maximum speed was Mach 3.09, an equivalent velocity of 3,490 km per hour (2,170 miles per hour). Melvill and Binnie, the two pilots who flew above the 100 km (330,000 feet) mark were issued the first commercial ‘astronaut wings’ by the US Federal Aviation Administration.

No further flights were made, as the prize had been won and the concept and the technology proven. For commercial space tourism flights, Rutan wanted to develop a larger rocket plane that could seat more passengers and incorporate more

redundant systems and aerodynamic stability for increased safety. In addition, he did not wish to risk damaging the unique and now historic SpaceShipOne. Since 2005 the small rocket plane has hung on display in the main atrium of the National Air and Space Museum in Washington D. C., between the Wright Flyer, the Spirit of St. Louis and the Bell X-l, and near the first X-15. As a tribute to SpaceShipOne’s achievement, in 2006 a small piece of its carbon fiber material was cut off and launched on the New Horizons probe heading for Pluto. An attached inscription reads: “To commemorate its historic role in the advancement of spaceflight, this piece of SpaceShipOne is being flown on another historic spacecraft: New Horizons. New Horizons is Earth’s first mission to Pluto, the farthest known planet in our solar system. SpaceShipOne was Earth’s first privately funded manned spacecraft. SpaceShipOne flew from the United States of America in 2004.”

A fiberglass replica of SpaceShipOne created using the same molds used to make the original can be found in the AirVenture Museum in Oshkosh. Another full-scale replica is on display in the William Thomas Terminal at Meadows Field Airport in Bakersfield, while a third is in the Mojave Spaceport’s Legacy Park, and a fourth is hanging above the stairs in the main entrance of Building 43 of Google’s Googleplex campus (Google cofounder Larry Page was a trustee on the X Prize board) and a card taped to the nozzle implores, “Attention Googlers: Please do NOT launch. Thanks.” SpaceShipOne also became a popular model rocket, with Estes Industries currently offering several SpaceShipOne models that you can launch from your own back yard repeatedly by replacing the little solid propellant rocket motor.

Rutan’s company has now teamed up with the Virgin Group, famous for its airline and its entertainment and communications companies, as well as its charismatic and adventurous head, Sir Richard Branson. Under the name ‘The Spaceship Company’, the Virgin Group and Scaled Composites have set up a joint venture to develop the SpaceShipTwo and White Knight Two aircraft which will be operated by a company called Virgin Galactic. At the time of writing, the ‘spaceline’ plans to operate a fleet of five SpaceShipTwo vehicles starting no earlier than 2012. They have been taking bookings at $200,000 per passenger for the early flights, and by late 2011 had over 450 paid customers. It is expected that ticket prices will drop significantly as flight operations mature, increasing the size of the space tourist market.

SpaceShipTwo, based on the same principle, concept and shape as SpaceShipOne is roughly twice the size in order to house two pilots and six passengers. It will be propelled by a larger hybrid rocket motor named ‘RocketMotorTwo’ delivering over

230,0 Newton of thrust. Development of the new rocket plane was delayed when in 2007 an explosion occurred during an oxidizer flow test that was being conducted at the Mojave Air & Space Port. Three staff were killed and another three severely injured; rocket engines are still potentially dangerous devices that have to be handled with great care. White Knight Two is an innovative twin-hull aircraft that carries the SpaceShipTwo rocket plane between its fuselages. It is also designed to operate as a zero-G parabolic-flight aircraft for SpaceShipTwo passenger training or micro­gravity science flights, and as a high-altitude research plane. It could potentially launch other rockets than SpaceShipTwo, such as small sounding rockets with instruments for scientific research.

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SpaceShipTwo technical diagram [Virgin Galactic].

Unlike previous rocket plan projects, environmental impact is now an important issue in aviation. With respect to carbon dioxide (C02) emissions the hybrid engine is not exactly ‘green’ but according to Virgin Galactic, “C02 emissions per passenger on a spaceflight will be equivalent to approximately 60% of a per-passenger return commercial London/New York flight.” This is about 500 kg of carbon dioxide per passenger per flight. So even if SpaceShipTwo flights eventually number 1,000 per year the resulting carbon dioxide emissions would be in the order of one-thousandth of what a major airline typically expels into the atmosphere during a year. Virgin Galactic nevertheless accepts that the environmental impact of their operations could have serious implications for the image and success of their business, and the larger Virgin empire is committed to being as environmentally friendly as is practical. The company therefore plans to run its spaceport(s) with as much renewable energy as possible, which may even make them a net energy producer and potentially “carbon negative” by preventing more emissions of carbon dioxide than its vehicles produce. White Knight Two’s jet engines will initially burn kerosene but are also capable of running on butanol, a biofuel that can be made from algae.

The first SpaceShipTwo, christened VSS (Virgin Space Ship) ‘Enterprise’ (after the legendary Star Trek starship) made its first glide flight on 10 October 2010, being launched by the first White Knight Two aircraft VMS (Virgin Mother Ship) ‘Eve’, and it performed its first ‘feathered’ flight on 4 May 2011. To date, a total of 16 glide flights have been made, and round 100 test flights are expected before the first
passengers will be carried. The first commercial flight is expected no earlier than 2012. The company will initially operate from Spaceport America, a brand new $210 million airport for suborbital vehicles located in New Mexico. There are also plans for a sister spaceport in northern Sweden. Singapore and the United Arab Emirates have both also shown interest in establishing suborbital flight facilities.

A SpaceShipTwo flight will be an incredible adventure offering the possibility, albeit brief, to experience what astronauts (and X-15 pilots) feel and see, without the heavy workload. You will be dropped from the carrier aircraft at an altitude of 15 km (50,000 feet) and then go supersonic within 8 seconds. After 70 seconds of powered flight, during which you attain a maximum speed of just over Mach 3 (equivalent to about 3,500 km per hour, or 2,100 miles per hour) the rocket plane will coast to a peak altitude of 110 km (360,000 feet). The virtually drag-free parabolic trajectory will last for 3.5 minutes, during which you will be able to float about in the relatively spacious cabin and admire the view of Earth below and the curvature of the horizon through the large windows.

Other companies are also working on suborbital rocket planes for space tourism, with microgravity science and high-altitude experiments (as on the X-15) forming a secondary market. XCOR Aerospace, which is based on the same Mojave airfield as Scaled Composites, is developing its ‘Lynx’ rocket plane (superseding its earlier and similar ‘Xerus’ design). Unlike SpaceShipTwo this double-delta-winged vehicle will take off from a runway on its own power and hence will not require a carrier aircraft. This simplifies the development and operations (one rather than two planes) but it means the rocket aircraft has to carry all the propellant for the entire flight itself. The Lynx Mark-I prototype aircraft is considerably smaller than SpaceShipTwo and will only be able to reach an altitude of 60 km (200,000 feet) carrying a pilot and a single paying passenger. A more advanced Mark-II production version is to be able to reach the milestone of 100 km (330,000 feet). The passenger will have to remain strapped in his seat, as the cockpit is too small for weightless acrobatics. On the other hand, the initial ticket price announced by the company is about half that of a SpaceShipTwo flight. XCOR appears to be well advanced in the general development of liquid propellant rocket engines, and has reported that its 13,000 Newton XR-5K18 liquid oxygen and kerosene rocket engine (four of which will be needed to power the Lynx) is almost ready for flight. But propulsion is only one part of a rocket plane, and although the company has done extensive wind tunnel testing using a scale model of the Lynx, its announcement that it expects to start the test flight campaign of its Mark-I prototype in 2012 appears rather optimistic.

XCOR modified an existing canard configuration (i. e. tailless) ‘Long EZ’ sports aircraft to demonstrate its rocket engine capabilities by installing two 1,800 Newton restartable, pressure-fed, regeneratively cooled rocket engines which burn isopropyl alcohol and liquid oxygen. This ‘EZ-Rocket’, which is a modest-performance rocket plane in its own right, has made a total of 26 flights including a number of air show demonstrations. In December 2005 the EZ-Rocket set the world record for ‘Distance without Landing’ for a ground-launched rocket powered aircraft with a flight from Mojave to California City, a distance of 16 km (9.94 miles). “That was the shortest long-distance record flight ever!” pilot Dick Rutan exclaimed. XCOR also built and

flew the ‘X-Racer’, a sleek rocket aircraft based on the airframe of the ‘Velocity SE’ canard sports plane. This was a prototype for aircraft to compete in rocket plane races organized by the Rocket Racing League, an organization that seeks to promote rocket aircraft development by flying competitions. The X-racer is equipped with an XR-4K14 restartable, pump-fed rocket engine that burns liquid oxygen and kerosene with a thrust of 6,600 Newton. It made its first flight on 25 October 2007. The test program has now been completed after a total of 40 flights and demonstrations. The X-Racer holds claim to several (unofficial) records including the most flights made in a single day by a manned rocket powered aircraft, and the fastest turn-around for a manned rocket powered vehicle.

Armadillo Aerospace, the small aerospace company of computer game developer John Carmack, who made his fortune by developing popular games such as Doom and Quake, has also made a rocket engine for the Rocket Racing League. It equipped the Rocket Racing League’s current Mark-II and Mark-Ill Rocket Racers, which are also based on the Velocity airframe (in this case the Velocity XL FG version) with a home-grown rocket engine that is fed with liquid oxygen and ethanol and develops a maximum thrust of 11,000 Newton. Seven successful test flights were made by the Mark-II aircraft during August 2008 and both machines are currently used for flight demonstrations. The Rocket Racing League hopes to generate sufficient interest for a number of teams to build or purchase similar rocket aircraft in order to participate in rocket propelled air races. In the meantime, you can download a video game that puts you in the cockpit of a Rocket Racer.

In March 2002 the Space Adventures company that organizes ‘flight participant’ missions to the International Space Station, unveiled a mockup of the ‘Cosmopolis ХХГ (C-21) lifting body-type suborbital rocket plane at Zhukovskiy Air Base near Moscow. This was to be developed by the Russian Myasishchev Design Bureau, be launched from the design bureau’s existing M-55X ‘Geofizika’ high altitude aircraft, and be able to carry a pilot and two passengers into space at $98,000 per ticket with the first flight in 2004. The carrier aircraft with the C-21 attached would first slowly climb to an altitude of 17 km (56,000 feet) and then gather speed in order to make a vertical climb to 20 km (66,000 feet) to release the C-21. The C-21 would then ignite its expendable sohd propellant rocket motor. When this motor burned out it would separate and fall away, leaving the C-21 to follow a ballistic arc to a peak altitude of 100 km (330,000 feet). The rocket plane would glide back to the airport and make a parachute-assisted touchdown. But Space Adventures has abandoned its plans to use the C-21 and instead contracted Armadillo Aerospace to develop a vertical launched, vertically landing suborbital rocket capsule to implement its planned suborbital flight services.

The giant European space company EADS Astrium announced in 2007 that it was to develop a suborbital rocket plane for space tourism. This single-stage, straight-winged plane would take a pilot and four passengers to the edge of space and offer a great view through many large windows and a roomy cabin for weightless antics. It would take off from a normal airport and climb to an altitude of 12 km (39,000 feet) with jet engines, then ignite a Romeo liquid oxygen-methane rocket

engine to reach 60 km (200,000 feet) in just 80 seconds with enough velocity to continue unpowered to its 100 km (330,000 feet) apogee. As the plane fell back the pilot would use small thrusters to control its attitude for re-entry into the atmosphere prior to restarting the jet engines to return to the airport. Jet engines use 10 to 20 times less propellant than rocket motors of the same thrust over the same time and are much more efficient for the first and final phases of a flight (SpaceShipTwo’s carrier aircraft uses jet engines for the same reason) but when they are not providing thrust at high altitudes they are dead weight. SpaceShipTwo effectively leaves them behind once it separates from its carrier. Jet engines are also handy in case of a failure of the rocket engine, as well as for ferry flights between airfields. Astrium expected to require around 1 billion euro to develop their system (much more than SpaceShipTwo is estimated to cost), flights to begin in 2012, and tickets to cost up to

200,0 euro. “The development of a new vehicle able to operate in altitudes between aircraft (20 km) and below satellites (200 km) could well be a precursor for rapid transport point-to-point vehicles, or quick access to space,” the company said.

Famous designer Marc Newson was to take care of the aesthetics of the design, and the images in the brochure published by Astrium sure are beautiful. As Astrium builds the Ariane 5 launcher and its mother company EADS develops and produces the famous Airbus airliners as well as the Eurofighter military jet, the company seems ideally suited to pursuing a suborbital rocket plane project: it has all the necessary knowledge, experts and facilities in-house, and could incorporate a lot of existing EADS aircraft and spacecraft equipment such as cockpit instrumentation, undercarriage and control thrusters.

After their 2007 announcement, however, Astrium remained awfully quiet about their rocket plane, making it appear to have been merely a publicity stunt rather than a real project. But early in 2011 the company announced that it was indeed working on the concept and that after having placed work on hold for several years due to the global economic downturn it was planning to spend a further 10 million euro on it in 2011; a considerable sum but not much in comparison with the 1 billion euro that it had predicted for full development. “We continue to mature the concept, maintaining the minimum team in order that when we find the relevant partnership we are ready and have progressed sufficiently,” Astrium CEO Franfois Auque told reporters in January 2011. Once it has secured the required financial and industrial partners, the company expects to be able to put the rocket plane into service within five years.

In 2004 another big European aeronautics company, Dassault Aviation in France, announced its own suborbital rocket plane design called VSH. This was based on an earlier design for an automated air-launched reusable hypersonic vehicle known as YEHRA (‘Vehicule Hypersonique Reutilisable Aeroporte’) but was intended to be manned and therefore VSH stood for ‘YEHRA Suborbital Habite’. The delta-winged rocket plane would be carried into the air by a commercial aircraft, be released at an altitude of 7.6 km (25,000 feet) and a speed of Mach 0.7, and ignite a liquid oxygen-kerosene rocket engine to climb to the milestone altitude of 100 km (330,000 feet). Design work is progressing in the context of the K-1000 project that Dassault is self-financing with several industrial partners in Switzerland.

Bristol Spaceplanes, mentioned earlier for its Spacecab and Spacebus projects, is working on a rocket plane called ‘Ascender’. This is a delta-winged aircraft with two jet engines and a single rocket motor similar in concept to that of EADS Astrium but only able to seat a pilot and a single passenger. Ascender’s rocket engine, a prototype of which is to fly on a sounding rocket, will use hydrogen peroxide and kerosene as propellants. As such it resembles the Spectre rocket engines developed in the 1950s to power the SR.53 and SR.177. Ascender is also to pave the way for the company’s orbital spaceplane concepts (discussed above). However progress is slow because the company is waiting for a serious investor so that it can afford to appoint a full-time team of engineers.

Virgin Galactic would seem to be the most advanced company in terms of making and flying suborbital rocket planes, but if the space tourism market really takes off there ought to be room for several aircraft manufacturers and operators to compete. This would hopefully lower ticket prices further, resulting in ever more people being able to afford a flight to the edge of space.

The next step foreseen by Burt Rutan is an orbital rocket plane for space tourism, but that poses a tremendous challenge because although the 100 km (330,000 feet) altitude reached by SpaceShipTwo will be sufficiently above the atmosphere to circle the Earth a couple of times, the speed of the vehicle falls far short of that required to

enter orbit. To achieve orbit at that height, a vehicle must have a horizontal speed of

7.8 km per second (4.8 miles per second); i. e. 28,000 km per hour (17,500 miles per hour). SpaceShipTwo reaches Mach 3 at engine burn-out in a steep climb but at the top of its parabolic arc its speed is virtually zero (as all its energy has been converted into altitude). Compared to SpaceShipTwo’s maximum speed of 0.9 km per second (0.6 miles per second) an orbital rocket plane needs to go over 8 times faster; and as kinetic energy increases with the square of the speed that means a propulsion system capable of delivering almost 70 times as much energy! This is why satelhte launchers and orbital spaceplane concepts are so much larger than suborbital rocket planes such as SpaceShipTwo and Lynx; even though they all reach space, in terms of energy and thus propellant volume the difference is huge. Weight constraints are also much more demanding for an orbital spaceplane. Whereas a suborbital rocket plane’s dry weight can be approximately 40% of the vehicle’s overall weight including propellant, the energy needed to go into orbit demands that a plane’s empty weight be no more than 10% of its take-off weight (for both types of vehicle these percentages diminish if multiple stages and/or airbreathing propulsion are employed but the large difference remains). This also has consequences for safety: where normal aircraft structures are usually designed to be able to withstand 1.5 times the highest load expected to occur during the plane’s lifetime (and even 2 times for the undercarriage) this margin will be extremely difficult to meet for reusable orbital spaceplanes. Even for expendable launchers, which are less constrained regarding empty weight, this factor is typically only 1.2, except for crewed launchers where it is 1.4 according to NASA standards.

In short, the difficulty in achieving orbit is not so much to get up to high altitude, it is rather to attain the necessary high velocity with a structure weight that provides a reasonable amount of rehability and safety. Factoring in the much more extreme re-entry temperatures that will require heat shields, and that ‘feathering’ cannot be used for hypersonic re-entry, clearly indicates that an orbital SpaceShipThree will not be merely an upgrade of SpaceShipTwo but a completely new, much larger, and more complicated spaceplane that will be vastly more expensive to develop and operate.

The use of Rocket Assisted Take-Off (RATO) boosters became very common shortly after the Second World War because the early jet engines delivered relatively low thrust when the aircraft was moving slowly. Heavy jet aircraft in particular needed a bit of help to get going. But soon the take-off thrust of jet engines had increased to the point that RATO was only required for heavy cargo aircraft using short runways or airfields in hot places and at high elevations (they are still in use today, but only in very limited circumstances).

A different idea for mixing rocket and jet propulsion that was investigated was to use a powerful rocket booster to shoot an airplane straight into the sky, dispensing with the need for a runway. With this ultimate stretch of the RATO concept, fighter aircraft could be launched anywhere and anytime, even from a truck trailer!

For Cold War military planners the vulnerability of airfields and their concentration of aircraft was always a major issue, especially for the early, underpowered jets that could not take off from rough fields and required especially long runways. This concern led to the need for Vertical Take-Off and Landing (VTOL) aircraft: jet fighters that (as with helicopters) would not need much more than a clearing in a forest to operate from. Most of the VTOL fighter concepts proved to be impractical but the developments ultimately led to aircraft like the swivel-nozzle Harrier, which can take off and land virtually anywhere whilst operating as a conventional jet fighter when in the air.

During the 1950s, blasting jet fighters into the air using rockets was a simpler way of liberating aircraft from their dependence on runways for take-off (although the returning aircraft would normally still need a prepared airfield for landing). The idea had already been pioneered by the British ship-launched, rocket-catapulted Hurricat fighter of the Second World War, but the new jet fighters were much heavier than the old Hurricane propeller fighter and required much larger boosters to get airborne: so large, in fact, that they were impossible to fit into the airframe and (like RATO units) had to be attached externally and jettisoned immediately after use. This had several benefits though. One was that the heavy rocket equipment did not need to be taken with the aircraft during its entire mission. Another was that not much modification to existing aircraft would be needed to accommodate the external rocket boosters. Also of benefit was that because the rockets would be jettisoned soon after take-off, they would not need to be especially light and efficient: relatively simple solid propellant boosters similar to those used in surface-to-air missiles would suffice. This concept became known as Zero-Length Launch (ZEL).

In the early 1950s the US Air Force began a program called ‘Zero Length Launch, Mat Landing’ (ZELMAL) in which a Republic F-84G Thunderjet fighter was to be shot into the air using a large solid propellant rocket booster. (The F-84 was selected because it was sufficiently light that it could be launched by already available rocket boosters.) To solve the problem of the need for a landing strip upon return, the idea was for the fighter to be equipped with a hook to snag an arresting cable suspended close to the ground in order to come to a quick stop, rather like on an aircraft carrier, except that instead of rolling to a halt the aircraft (without lowering its undercarriage) would smack down onto an inflatable mattress measuring 25 x 245 meters (80 x 800 feet) and 1 meter (3 feet) thick. An additional perceived benefit of this technique was that ZELMAL aircraft would not need an undercarriage and so would be lighter than comparable conventional fighter aircraft.

The Glenn L. Martin Company was selected to manage the development of the system, with the Goodyear Tire & Rubber Company making the air-filled mat. Tests started at Edwards Air Force Base, California, on 15 December 1953 with a pilotless F-84G being launched from a trailer normally used to fire Matador cruise missiles (it seems that the same type of rocket booster was employed). As planned, the aircraft was lobbed into the air and then crashed onto the hard desert floor. The next test less than a month later, on 5 January 1954, was equally promising. It was manned by test pilot Robert Turner and the G-levels that he experienced during the launch were no worse than during a conventional catapult launch from an aircraft carrier although he accidentally jerked one hand and throttled the engine back, almost stalling it. Turner made another flight on 28 January. Both flights went surprisingly well, and surviving movie footage (which is on the Internet) shows a very smooth operation with a fluent acceleration and a clean separation of the booster. This indicated that rocket-boosted take-offs were a feasible operational military possibility. In both tests Turner landed conventionally. Landing on the mat would prove much more problematic. The first time the rubber mat was inflated after being transported on a couple of trailer trucks, it was found to leak so badly that parts of it had to be sent back to the manufacturer. The first mat landing on 2 June 1954 became a fiasco when the aircraft’s arresting hook tore the mat wide open and caused a very hard landing. The plane was damaged beyond repair and Turner was rendered inactive for months due to back injuries. Two more mat landings were conducted but the sudden impact on the mat remained too hazardous. Test pilot George Rodney suffered a neck injury from his mat landing: “We tied ourselves into the seat real well, so we wouldn’t pitch forward into the control column and the instrument panel, but unfortunately your head, it goes into a big arc and comes down on your chest.” After 28 rocket launches ZELMAL was terminated.

The sudden lift-off of a ZEL launch must have been rather strange for the pilots: they were sitting in the cockpit of a familiar aircraft but rather than seeing a runway in front of them they were looking up into the sky at a steep angle. And instead of the reassuring, slowly growing push of the jet engine there was a sudden explosion of power hurtling them into the air. It is a bit like sitting in your own car but with an additional dragster racing car’s engine in the boot.

Despite the termination of ZELMAL the idea of launching a fighter with a rocket booster was still believed valid. The Air Force initiated a new program in 1957 that dispensed with the mat landing and so was simply named ZEL. It was to involve the launch of a nuclear-armed strike aircraft from a truck trailer which, since it could be hidden anywhere, would be hard for the enemy to destroy during a first strike. After dropping his bomb, the pilot of the undercarriage-less aircraft would simply bail out over friendly territory. To be able to carry a heavy atomic bomb into Soviet territory the aircraft would have to be much larger than the F-84. The selected F-100 Super Sabre was about twice as heavy as the F-84, so a new rocket booster was developed by Rocketdyne. This solid propellant rocket could deliver a thrust of almost 578,000 Newton for 4 seconds and accelerate the F-100 at about four G. It was affixed under the aircraft’s rear fuselage, at a slight angle so that its thrust was aimed through the center of gravity and would thus not cause any rotation. At burnout, the plane would be flying at 450 km per hour (280 miles per hour) at an altitude of 120 meters (400 feet). Preliminary tests were started with a so-called ‘iron bird’, a structure of steel and concrete that simulated the weight and the mass distribution of the F-100. These tests showed that if the booster were not precisely aligned with respect to the center of gravity of the entire contraption it could perform some very impressive backward summersaults. But this was soon fixed, and the earlier problem of the pilots’ hand on the throttle moving backwards due to the acceleration was remedied by introducing a fold-out handle which the pilot could slip his hand into.

The first manned launch of an F-100 at Edwards Air Force Base on 26 March 1958 went perfectly. Test pilot A1 Blackburn said he found the flight “better than any ride you can find at Disneyland”. On his second launch, however, the rocket did not separate, even when he tried to shake it free using wild maneuvers. He had to use his ejection seat and let the plane crash in the desert because it was impossible to land it with the big booster attached. The investigation showed the attachment bolts had not sheared off as they were intended to. Thereafter explosive charges were provided that could blow the boosters off on command. Another 14 successful flights were made by October. A sign on the trailer claimed it to be the ‘World’s Shortest Runway’. The tests were not kept secret: footage of one of these launches was used in the Steve Canyon television series. There was even a public demonstration of the system, with the pilot showing off by performing a slow roll immediately after booster separation. The technical feasibility of the ZEL concept was proven, but its operational role was not so clear.

First of all there were the practical as well as safety and security issues relating to driving a fighter with a nuclear weapon on a truck through dense forests, over narrow roads and through tunnels. Critics said it would be better to launch ZEL aircraft from fixed, protected positions. To test this idea, a few launches were performed out of a hardened shelter at Holloman Air Force Base in New Mexico, the last of which took place on 26 August 1959. But launching from fixed positions denied the flexibility and elusiveness of the mobile system. A more serious threat to the project was that the idea of sending nuclear-armed fighter aircraft into the Soviet

Union was rapidly being made obsolete by the increasing reliability and accuracy of unmanned ballistic missiles. Although 148 F-lOOs were modified to enable ZEL launches, the program went nowhere.

The concept of a ZEL nuclear-armed strike fighter was picked up once more, this time by the German Luftwaffe in 1963. Working with Lockheed, they organized rocket-launch experiments using an F-104G Starfighter at Edwards Air Force Base. The Rocketdyne booster could push the Starfighter to a speed of 500 km per hour (310 miles per hour) in just 8 seconds. Lockheed test pilot Ed Brown, who performed the flights, was very impressed: “All I did was push the rocket booster button and sit back. The plane was on its own for the first few seconds and then I took over. I was surprised at the smoothness, even smoother than a steam catapult launch from an aircraft carrier.” The successful experiments were followed up by further tests at the German Air Force base at Lechfield. But this project was also canceled for the same reasons as the F-100 ZEL and because the tests using the expendable rocket boosters were quite expensive. In addition, the much more practical YTOL Hawker Siddeley GR. 1 Harrier was by then under development. This made its first flight in December 1967. It could not only take off vertically without rocket booster assistance, it could also land vertically. A German F-104G equipped with a rocket booster and a dummy nuclear missile is on display at the Luftwaffe museum in Berlin-Gatow in Germany.

In France there was a proposal for a ZEL version of the mixed-power Durandal in which the aircraft would be launched from a mobile trailer using a cluster of sohd propellant rockets attached at an angle on the tail, but this was not developed into a real system.

Soon after the US started to experiment with rocket launched F-84Gs, the Soviets initiated a very similar project using their MiG 19 (which, as described earlier, was also converted into a high-altitude mixed-power interceptor around this same time). Rather than launching nuclear-armed strike aircraft, the Soviets intended their rocket-launched interceptor to play a role similar to that they had envisaged for their earher rocket propelled aircraft, namely a fast-reaction point-defense interceptor. Launching these from truck trailers with large rocket boosters would make it possible to station them at remote locations or in battle areas where there were no suitable runways. The MiG design bureau prepared a proposal for a trailer-launch system for the MiG-19. This was given the go-ahead in 1955. MiG came up with a modified version of the MiG-19S, designated the SM-30, which had a reinforced structure to handle the high rocket thrust, the ventral fin was replaced by two new fins straddling the rocket booster, and a special headrest to protect the pilot from whiplash at the onset of the sudden acceleration.

Like the US Air Force and the Luftwaffe, the Soviets used a large, jettisonable

solid propellant rocket booster which was mounted on the rear fuselage and pointed slightly downwards. The PRD-22 booster had sufficient thrust to shoot the 8,000 kg (17,000 pound) plane into the sky. The SM-30 could be transported on a large trailer-truck combination but had to be placed on another type of trailer for the launch. The plane was connected to this trailer by bolts that would shear and release the aircraft upon ignition of the booster. When launched from soft surfaces, such as from within a forest, soldiers would be required to dig a trench behind the booster to prevent the powerful exhaust from creating a fountain of loose dirt that would be visible to the enemy from far away. The first test launch was performed using a remote-controlled unmanned airplane in the autumn of 1956. The launch went smoothly (just like the Americans had experienced) but the trailer was wrecked, thus proving that it needed to be fitted with a blast shield for protection. On 13 April 1957 test pilot Georgi M. Shiyanov made the first manned flight, which was a big success. He had trained with a special launch simulator catapult (even successfully enduring an excessive 18 G acceleration during one test when a technician made an error arming the catapult). Several more SM-30 launches were performed, all of which were successful (film of the launches is on the Internet). However, landing the heavy fighter on rough and small landing strips like those envisioned to be available near forward battle areas was not so easy; getting the plane to stop before it ran out of runway was very tricky using the standard MiG-19 drag parachute and brakes. An arresting-cable system (Uke on an aircraft carrier) was therefore tried out.

In the end, however, the SM-30 project was terminated because of the difficulty of driving the large, heavy aircraft across the countryside and because an airplane which does not require a runway for take-off but does require one for landing is not all that useful. Just as the F-100 and F-104G ZEL were rendered obsolete by the introduction of long-range nuclear missiles, the SM-30 turned out to be less effective for forward air defense than the new mobile battlefield surface-to-air missiles.

“Man is the best computer we can put aboard a spacecraft… and the only one

that can be mass produced with unskilled labor.” – Wernher von Braun

Will a future hypersonic plane have pilots on board or be fully automatic? Nowadays aerodynamics and rocket propulsion are fairly well understood and can be accurately modeled and simulated. As a result, the areas where direct pilot intervention may be needed due to unexpected behavior of an aircraft/spaceplane are rapidly decreasing. This is especially true for vehicles that have a fixed, pre­determined trajectory such as missiles and launch vehicles. So-called Unmanned Aerial Vehicles have become important operational military assets, and these aircraft are steered from the ground or fly their missions completely autonomously as aerial robots. It is therefore likely that future space planes will be flown by a computer under human supervision from the ground rather than directly by a human pilot, particularly as hypersonic vehicles tend to be aerodynamically unstable and therefore require sophisticated avionics for efficient and safe control. For instance, Skylon is to fly automatically; any astronauts to be transported into orbit will be housed inside its payload bay.

Especially on a satellite launch vehicle with relatively little margin for errors and malfunctions, operating without pilots results in a simpler and thus cheaper design; a crew requires a comfortable cabin with regulated pressure and temperature, requires to have an escape capability if the spaceplane is less reliable than a regular aircraft (which rocket vehicles invariably are) and requires higher safety margins to be built into the design. Not having any people on board potentially makes the vehicle less expensive and saves weight and space that can be used for more payload. Moreover, catastrophic failures have less grave consequences; compare the dramatic aftermaths of the losses of the Space Shuttles Challenger and Columbia with those of the many but almost forgotten failures of unmanned expendable launchers.

However, on many occasions having an exceptionally skilled pilot on board saved the X-15 and earlier rocket aircraft. So any fully automatic flight control system on a versatile hypersonic aircraft intended for various types of missions must be smart and capable of reacting very rapidly to unexpected situations and emergencies. That may be difficult, as programming a computer for unforeseen events is near to impossible whilst the human brain excels at improvisation (although developments in so-called neural networks may result in self-learning computers that can quickly react to new situations). And what if a spaceplane carries astronauts onboard? Even if they are not flying the vehicle themselves, the aircraft will still need to incorporate the additional equipment and various reliability enhancing redundancies that an unmanned vehicle can do without. Would it be acceptable for them to ride into space in a fully robotic hypersonic launcher? Or would a pilot with a manual override capability be required, as for astronauts launched on current expendable rockets like the Soyuz and even the Space Shuttle, if only for psychological reasons? The impact on the design would be limited if one of the transported crew could fly the vehicle in an emergency, in order that no additional seat need be assigned to a pilot.

Talking of people on board spaceplanes and rocket planes in general, what about vehicle safety? The early rocket propelled aircraft like the Me 163 were extremely hazardous. Four pilots died and two were severely injured during the X-l, X-2 and X-15 programs and there were also many less serious accidents. Of the 16 individual airframes involved, 10 were completely or largely destroyed in accidents: not a very good safety record given that the X-planes only made a total of some 415 flights, a total that can be readily accumulated by a single airliner in 6 months of operations. Does this mean that rocket planes are inherently dangerous and hence ought never to be used for suborbital space tourism and/or mass transportation into orbit? Surely we have learned much about high-speed, high-altitude flight since those days, and rocket propulsion has also greatly matured. Suborbital flight in particular, benefits not only from the experience gained from the experimental rocket planes but also from high-performance jet aircraft in general.

Furthermore, whilst the high losses among pilots flying rocket planes may appear high today, they were not particularly exceptional compared to the accident rate in experimental aviation and the general testing of prototype aircraft. In the late 1940s and the 1950s test pilot loss rates in the US were in the order of one per week. And crashes of military jets in operational service occurred frequently. Nowadays crashes and aircraft explosions are very rare, even for new types, so there is no real reason to expect suborbital rocket aircraft like SpaceShipTwo to suffer from anything like the loss rates of early jets. However, a suborbital launch is certainly more hazardous than a regular airline flight, and orbital spaceflight even more so. In part this is due to the extreme speeds, altitudes and temperatures involved, in combination with the need to keep the vehicle as light as possible, and in part due to the still experimental nature of human spaceflight. At the time of writing, the number of crewed space missions is less than 290, well below the number of planes in the air on a typical day. There have been even fewer suborbital rocket plane flights into FAI-certified space. Indeed, only two X – 15 flights and the recent three missions of SpaceShipOne ascended above the milestone altitude of 100 km (62 miles), and another eleven X-15 flights exceeded 80 km (50 miles). In today’s world of health and safety regulations, the relatively low trustworthiness of rocket vehicles is certainly a business risk. People have come to expect that even radically new aircraft will not kill anyone, and that suborbital space tourists riding rocket planes should not feel that they are putting their lives on the line. On the other hand, perhaps it is the risk that provides the sense of adventure.

“You’ve never been lost until you’ve been lost at Mach 3.” – Paul F. Crickmore

(test pilot)

Until about the start of the Second World War the strange phenomena which develop at transonic speeds were academic, since the propeller aircraft of the day did not fly that fast. But starting in 1937 mysterious accidents began to occur at high speeds. An experimental early version of what was to become Germany’s most potent fighter of the war, the Messerschmitt Bf 109, disintegrated as its pilot lost control in a fast dive. Pretty soon other new, high performance military airplanes were running into similar difficulties. For these fast, propeller-driven fighters the airflow over the wings could achieve Mach 1 in a dive, making air compressibility a real rather than a theoretical issue.

In the US, the aeronautics community was rudely awakened to the realities of this unknown flight regime in November 1941 when Lockheed test pilot Ralph Virden was unable to pull his new P-38 fighter out of a high-speed dive and crashed (due to the problem of ‘Mach tuck’ described in a previous chapter). It became clear that any propeller fighter pilot who inadvertently pushed his fast plane into a steep dive was risking his life. Aggravating this problem was that bombers were flying ever higher, which meant that in order to reach their prey interceptors had to venture into the thin, cold air of the stratosphere in which the speed of sound was lower, and thus issues of air compressibihty occurred at slower flight speeds than they did when flying nearer the ground.

A really thorough understanding of high-speed aerodynamics was initially not necessary, because measures to prevent control problems focused on limiting the dive speed and temporarily disrupting the airflow to prevent shock waves from forming on the wings and controls. Due to the limitations of propellers and piston engines, it was accepted that conventional aircraft would never be able to fly faster than sound.

Then jet and rocket aircraft appeared. These were quickly realized to require the potential to fly at Mach 1 or even faster for extended durations, so a real understanding of transonic and supersonic aerodynamics rapidly became a ‘hot’ issue that promised real military advantages. The name ‘sound barrier’ had been coined by a journalist in 1935 when the British aerodynamicist W. F. Hilton explained to him the high-speed experiments he was conducting. In the course of the conversation Hilton showed the newsman a plot of airfoil drag, explaining: “See how the resistance of a wing shoots up like a barrier against higher speed as we approach the speed of sound.” The next morning, it was incorrectly referred to in the newspaper as “the sound barrier”. The name caught on because the issues which conventional high-speed aircraft invariably encountered on reaching transonic speeds gave the impression that the magic Mach number was indeed a barrier that would need to be overcome.

It not only represented a physical barrier, but also a psychological one: there were many skeptics who said supersonic flight was impossible because aerodynamic drag increased exponentially until a veritable wall of air emerged. They pointed to the loss of the second prototype of the de Havilland DH 108 Swallow on 27 September 1946. This high-speed jet disintegrated while diving at Mach 0.9, killing pilot Geoffrey de Havilland Jr., and crashing into the Thames Estuary. Shock stall had pitched the nose downwards, and the resulting extreme aerodynamic loads on the aircraft cracked the main spar and rapidly folded the wings backwards. Others, however, noted that rifle bullets could fly at supersonic speeds, so the sound barrier was not an impenetrable wall. Indeed, during the war it had been realized that streamlined bullet shapes were ideal for supersonic speeds. This is how rockets such as the German А4/ V2 got their familiar shape. The V2 achieved Mach 4 as it fell from the sky towards London. In fact, because it fell faster than the speed of sound there was no audible warning of the imminent danger until the impact reduced whole blocks of houses to rubble; only afterwards did the sound arrive. But aircraft need wings, rudders, ailerons and other devices to develop lift and facilitate control, making their aerodynamics much more complicated than those of bullets and rockets. The traditional tool for gathering aerodynamic data and developing new aircraft and wing shapes was the wind tunnel. However, the technology available at that time did not permit accurate and reliable measurement of airflow conditions at transonic speeds: the aircraft models placed in the wind tunnels would generate shock waves in the high-speed air flowing around them, and these in turn would reverberate and reflect across the test section of the tunnel. As a result, there was a lot of interference and the measurements of the model did not correlate to the real world in which aircraft flew in the open air rather than in an enclosed tunnel. Also, you can scale an aircraft but you cannot scale the air, so air flowing around a small-scale model does not necessarily behave in the same way as air flowing around a real airplane.

The least understood area was from about Mach 0.75 to 1.25, the transonic regime where the airflow would be unstable and evolve quickly, and for which no accurate aerodynamic drag measurements and theoretical models were available. It was called the ‘transonic gap’; the aerodynamicists nightmare equivalent to the ‘sound barrier’ so dreaded by pilots. The aerodynamic drag is especially high in this range of speeds, peaking at just below Mach 1. However, it actually diminishes considerably at higher supersonic speeds (which is why modern aircraft either fly well below or well above Mach 1, spending as little time and fuel as possible at transonic speeds). Drag occurs at transonic speeds for two reasons: firstly as a result of the build-up of shock waves where the airflow reaches Mach 1 (typically over the wings), and also because the air behind the shock waves often separates from the wing and creates a high-drag wake. At even higher speeds the shock waves move to the trailing edge of the wing and the drag-inducing air-separation diminishes and finally vanishes, leaving only the shock waves.

All major military powers realized that if their aircraft were to remain state-of- the-art and competitive, then transonic aerodynamics was an area that really needed to be explored and mastered. Specialized and heavily instrumented research aircraft would be needed, speeding through the real atmosphere rather than a wind tunnel. In effect these airplanes were to be flying laboratories. In the US, work was started on the Bell X-l, the first of the famous X-plane series and the first aircraft to break the dreaded sound barrier. The Russians initiated their transonic research using the captured German DFS 346, but soon moved on to designing their own aircraft.

The UK started development of the Miles M.52 research aircraft in 1943. It was to be powered by an advanced turbojet (because the British had considerable experience on such engines, and little on rockets). The jet’s fuel economy meant it would be able to take off using its own power. The M.52 might have become the first plane ever to exceed Mach 1 if the secret project hadn’t been canceled by the new government in early 1946, weeks before completion of the first prototype for subsonic testing. Apart from dramatic government budget cutbacks, one reason for the cancellation was that, based on captured German research, it was feared that the M.52’s razor sharp but straight wings were unfit for high-speeds and that swept-back wings were a must for supersonic flight. The Miles engineers had thought about a delta wing for the M.52 during the war, but discarded it as being too experimental for their short-term project. However, the Bell X-l did not have swept wings either, and both aircraft used all-moving horizontal stabilizers to preclude shock-stall problems (interestingly, the Bell engineers got the idea for the special tailplane from the M.52 team during a visit to Miles Aircraft in 1944). Had the British continued their ambitious project, they could have beaten the US in breaking the sound barrier: in 1970 a review by jet engine manufacturer Rolls Royce concluded that the M.52 would probably have been able to fly at supersonic speeds in level flight.

A 30%-scale radio-controlled model of the original M.52 design powered by an Armstrong Siddeley Beta rocket engine and launched from a de Havilland Mosquito did reach a speed of Mach 1.38 on 10 October 1948. This was quite an achievement, but by then the manned X-l had stolen the show with its record-braking Mach 1- plus flight over the desert of California about a year earlier. In spite of the UK’s prowess in aeronautical design and records, there never would be a British counterpart to the X-plane series of the Unites States.

The development of specialized rocket aircraft purely to reach extreme speeds and altitudes went in parallel with that of the rocket/mixed-power operational interceptor. However, where the interceptors needed only to go as high as the maximum altitude that enemy bombers could achieve, there were no limits for the experimental aircraft: they were meant to provide information on entirely new areas of aerodynamics and aircraft design, and their designers and pilots kept on coaxing ever more impressive performance from them. Rocket engines proved to be very appropriate for propelling research aircraft up to extreme speeds and altitudes, since

endurance was not of great importance. Rocket engines were light and relatively simple compared to jet engines of similar thrust, and because they did not need air intakes this made it much easier to design airframes suitable for supersonic flight. From the 1940s through to the late 1960s the rocket propelled X-planes achieved velocities and altitudes unrivaled by contemporary jet aircraft, with some of their pilots gaining ‘astronaut wings’. The rocket powered interceptor turned out to be a dead end but the early rocket research aircraft led to the Space Shuttle and the current designs for future spaceplanes. New experimental rocket planes are still being developed, although when they fly they are often unmanned.

If flying mixed-propulsion interceptor prototypes was a risky business, then the pilots of early experimental research rocket aircraft had a truly dangerous job. These aircraft were, by definition, going beyond the known boundaries of velocity, altitude and aerodynamics; what pilots refer to as “pushing the envelope”. Such planes had to incorporate new, often hardly tested, technology such as experimental wing designs, powerful rocket engines and innovative control systems. Unsurprisingly, whilst being tested several of these experimental aircraft crashed, blew up, or were ripped apart by aerodynamic forces. There were no accurate computer simulations and knowledge databases to warn of design errors, incorrect assumptions and unexpected situations that are nowadays resolved long before a new airplane makes its first test flight. In fact, the research aircraft of the 1940s, 1950s and 1960s were providing the data required to set up such models, and they had to obtain it the hard way. Modern aerodynamic design tools still depend on the experience gained in those years.

In addition, the means of escaping from a plane heading for disaster were much more limited than for today’s test pilots, who have sophisticated avionics on board to tell them what is happening to their aircraft, and reliable ejection seats which permit a bail-out at any speed and altitude. In a recent interview for NOVA Online, Chuck Yeager, the first man to break the sound barrier in the X-l, summarized the test pilot philosophy of time as follows: “Duty above all else. See, if you have no control over the outcome of something, forget it. I learned that in combat, you know… you know somebody’s going to get killed, you just hope it isn’t you. But you’ve got a mission to fly and you fly. And the same way with the X-l. When I was assigned to the X-l and was flying it I gave no thought to the outcome of whether the airplane would blow up or something would happen to me. It wasn’t my job to think about that. It was my job to do the flying.” The urgency of Cold War developments, as well as an acceptance of loss of life ingrained into pilots and aircraft developers during the Second World War, meant high risks were taken and many test pilots perished as their new aircraft succumbed to some overlooked detail in the design.

“Nothing ever built arose to touch the skies unless some man dreamed that it should, some man believed that it could, and some man willed that it must.” – Charles Kettering

The ‘golden age’ of the rocket plane, whether it is defined in terms of the number of aircraft, speed of progress or number of flights, kicked off with the He-176 in 1939, essentially at the same time as the jet age, and arguably ended with the final flight of the X-15 in 1968. Successful rocket aircraft projects of that period were based upon three vital ingredients: a good aircraft design, a good rocket engine, and a great pilot. If any part of this fundamental triangle was lacking, the outcome was often disaster: aircraft pitched over due to Mach tuck, engines blew up, and pilots overshot landing fields and crashed their expensive aircraft. The extreme speeds that rocket aircraft achieved and the new aerodynamic territories they ventured into meant things could go wrong very fast and very unexpectedly. Pilots who let their powerful, sleek planes get ahead of them often did not make it back. And extensive flight experience did not mean that pilots were safe from making mistakes. For each new, experimental rocket aircraft every pilot was essentially inexperienced. The same applied to the designers, but at least they rarely lost their fives due to a fault in their aircraft or engine.

However, even while they were in the limelight, airplanes with rocket propulsion were rapidly rendered obsolete by improved turbojet engines. The Me 163B was the only pure rocket fighter that ever entered military service, while the only operational mixed-power fighters were the Mirage IIIC, – E, and – S, and for most of the time even these flew without their optional rocket packs. Altogether the rocket propelled fighter plane does not have a very impressive track record when taking into account all the development effort on experimental aircraft and prototypes.

Rocket planes were soon realized only to be really useful as research aircraft to fly at extremely high speeds and altitudes. The X-15 set incredible records for aircraft speed and altitude but the data it collected at the extremes of its flight envelope was so far beyond what was required for operationally useful manned aircraft that there was no need to make a successor to push the boundaries even further. Orbital rocket planes, the logical next step, proved to be too complex, too costly, and ultimately not really needed. Rocket aircraft development therefore stopped at the end of its

infancy and at the peak of its success, and so never matured into really operationally useful series-produced planes. Instead, new military planes relied on advanced jet engines and spacecraft kept using vertical take-off launchers that were usually expendable or at best included a reusable shuttle that was able to glide back from orbit.

By the mid-1980s it seemed that a second golden age was about to begin, with a number of ambitious spaceplanes and hypersonic airliners such as the Sanger-II and HOTOL following up on the experimental rocket planes of the 1950s and 1960s. But these new vehicles would not be pure rocket planes, as they were to rely on airbreathing propulsion for the first part of their flight. Indeed, NASP was initially expected to do without rocket motors. In that respect, they were more similar to the various mixed-power interceptors of the 1960s.

But the revival proved to be a false start. While routine hypersonic flight into orbit appeared to many people to be imminent, the unforgiving numbers in the engineers’ weight budgets and the managers’ cost estimates said otherwise. In part the optimism appears to have been inspired by the ease of imagining a spaceplane flying into orbit as a natural extension of high-speed and high-altitude aviation: an X-15, just flying a bit faster. Looked at Uke this, the intrinsic difficulties seemed smaller than they really are. Spaceplanes are inherently large because of the enormous volumes of propellant required. It is possible to make a small, relatively low cost aircraft, but not a small, cheap orbital spaceplane (indeed, people build simple aircraft in their garage but it is very unlikely that one day your neighbor will roll a hypersonic satellite launcher out of his shed).

Dr. Richard HalUon, a former Chief Historian of the US Air Force recently said of the apparent lack of progress in hypersonic flight (and thus the spaceplanes discussed in this book): “The hope of hypersonics thus became inextricably caught up in what might be termed a hypersonic hype. This led, over time, to a cycle of fits and starts that has largely worked to discredit the potential of the field and taint it with an image of waste and futility. Typically, a program has begun with great fanfare and promise, increased in complexity, and when realistic performance, schedule, and cost estimates are derived, its appeal quickly fades.”

In addition to the canceled X-20 Air Force project, NASA has a long history of abandoned hypersonic projects, including the X-30, X-33, X-34 and X-38. The space agency seems essentially to have given up on spaceplanes, shuttle-type space gliders, and indeed reusable launchers in general for the near future. The Russians had their single Buran flight but never progressed beyond paper studies for real spaceplane concepts. At present, neither NASA nor the Russian Space Agency, nor indeed any other space agency, is willing to risk burning its hands on another shuttle, let alone a spaceplane project.

A modest resumption of interest in the rocket plane was kicked off by the success of SpaceShipOne. Hopefully other suborbital rocket propelled aircraft will soon fly. However, it seems that brief suborbital flights represent the last niche in aeronautics for the pure rocket powered plane to play a useful role: any future hypersonic aircraft or orbital spaceplane will primarily rely on advanced forms of jet propulsion, perhaps in combination with rocket power if really necessary. In fact, even the early

X-planes like the X-l, X-2 and X-15, as well as the D-558-2 Skyrocket, were launched from large turbojet aircraft that can be regarded as airbreathing first stages.

The development of hypersonic launch vehicles will be expensive but by using a one-step-at-a-time approach, also known as ‘crawl-walk-run’, it may be technically feasible as well as affordable with or without government funding. The logic is clear: start with a suborbital aircraft such as SpaceShipTwo, advance to a suborbital hopper that can launch payloads into low orbit at the apogee of its ballistic flight into space, and finally make an orbital spaceplane. Each of these steps could be a commercially viable project in its own right, earning the money needed to fund the next step on the road to a fully reusable launch vehicle. In this regard, NASA’s Commercial Orbital Transportation Services program (which encourages private companies to introduce crew and cargo transportation vehicles to service the International Space Station) is of interest since one of the participants, SpaceDev, is developing the aforementioned Dream Chaser mini-shuttle.

At the same time, the US military’s desire for a long-range hypersonic missile (or even an attack aircraft) able to reach any place on Earth in no-time and fly too fast to be shot down is generating a lot of spaceplane technology. Perhaps in the foreseeable future the quest for the first operational hypersonic aircraft able to routinely fly into orbit and back will finally be concluded. Meanwhile the old quip in the US military remains valid that “hypersonics is the future of airpower and always will be”.

A proper airplane should have pilots on board, but the more that time passes the lower the chance that future spaceplanes will be directly piloted by anyone present. For launch vehicles flying only cargo it seems certain no crew will be required, and pilots may not be needed even for transporting astronauts. The Sanger-II and NASP spaceplanes would have had cockpits and flight crews but HOTOL was specifically intended to fly without, as indeed is its Skylon successor. The technology of orbital spaceplanes will be just as exciting as that of the X-15, but without pilots the concept loses a lot of its glamour and sense of adventure.

So what are the chances of there being a fully reusable, crewed rocket plane (with or without airbreathing engines) like the Euro 5 discussed in the Preface of this book blasting through the air anytime soon? Unfortunately, I think it looks like it will take quite while. The only operational rocket aircraft in the near future will be suborbital. When orbital spaceplanes eventually come around (hopefully) it is likely they will be fully automatic vehicles rather than resembling a hypersonic fighter aircraft piloted by gallant astronauts.

Still, the required technology and the possibilities that are on offer are extremely exciting: if spaceplanes really can dramatically reduce the cost of putting things into orbit then they will at long last open up space for large-scale commerce, production, moonbases, space solar energy satellites, space hotels and other marvelous ideas that are currently wishful artistic impressions and science fiction.

I just cannot accept that the expensive, wasteful expendable rocket as we know it today is the best that we can do and therefore represents the final answer in access to space. Spaceplanes truly represent the last great aeronautical frontier.

The US Army Air Force and NACA (National Advisory Committee for Aeronautics; forerunner to NASA) in 1944 initiated a program called X-l (originally it was XS-1 for ‘Experimental Sonic One’ but the ‘S’ was dropped early on). Its purpose was the development and use of a rocket research aircraft specifically in order to investigate the mysterious transonic region of speed, determine whether there was such a thing as the sound barrier and, if there were not, pass beyond Mach 1. Initially NACA had expected to use an advanced turbojet-powered aircraft which would take off under its own power (just like the British M.52) and, in a very scientific way over a series of flights, study transonic phenomena at different subsonic speeds just short of Mach 1 (because the initial design was not expected to be capable of exceeding the speed of sound). But the Army Air Force was in a hurry to find out whether the sound barrier was a myth, and they pushed for a simpler design based on existing technology that would soon be able to reach and hopefully even surpass Mach 1. Based on previous experience with the Northrop XP-79, as well as early information about the Me 163 Komet, they were confident that a rocket propelled and air-launched, but otherwise fairly conventional aircraft would suffice. As the military was paying for the project, their views prevailed.

The Bell Aircraft Company was awarded a contract for three prototype aircraft in March 1945, just before the war in Europe ended. Consequently, when the X-l was designed the important German wartime discoveries about transonic flight were not yet available. As a result, the X-l had conventional wings rather than the swept-back wings of the revolutionary German type. But the wings were relatively thin, with a maximum thickness of only 10% of the chord (the width of the wing at any point). In comparison, the wing of the Me 163 varied in thickness between 14% at the root and 8% at the tip; for the DFS 346 the maximum thickness was 12%. But because their wings where swept the effective thickness with respect to the air flow was actually less (as explained in the description of the Me 163). Conventional straight wings for subsonic propeller fighter aircraft were generally thicker, with a typical ratio of 15%. The wings of the X-l were made especially strong to be able to handle the powerful shock waves that were expected in spite of their narrow width.

To compensate for the huge amount of aerodynamic drag, a powerful engine was needed. But at that time the US was not as advanced as the UK and Germany in turbojet technology and it did not yet have a jet engine that could provide sufficient thrust to push an aircraft beyond Mach 1. Also, problems were foreseen in ensuring a proper airflow into a jet engine during transonic flight. So as not to delay the project, the X-l designers opted to install a relatively simple, home-grown liquid propellant rocket engine. A liquid propellant rocket engine would also be much smaller than the giant jet engine of the M.52 and would not need air intakes, making integration with the aircraft (both in design and construction) less complicated, which would in turn enable the development to progress faster and with fewer surprises. For instance, not requiring an enormous air duct to pass right through the length of the fuselage meant the wings could be connected by a single spar, resulting a simple, sturdy design with a relatively low weight.

An important requirement was that the propellants be relatively safe and easy to handle, as well as available in large quantities. This excluded the nasty and difficult – to-produce hydrogen peroxide used in Germany, as well as the dangerous nitric acid favored by Russian rocket plane designers. The engine selected for the X-l was the

Reaction Motors XLR1 l-RM-3, which burned a fuel that consisted of a mixture of five parts ethyl alcohol to one part water, in combination with liquid oxygen. These propellants were non-toxic, did not spontaneously ignite on coming into contact, and gave reasonable performance. Moreover, unlike (for instance) gasoline, the alcohol-water fuel mixture could be used to cool the engine: the water improved the cooling capabilities for only a modest decrease in specific impulse. This early version of the XLR11 did not have turbopumps but relied on pressure from a nitrogen tank to drive the propellants into the combustion chambers. It was a pure American design and not based on any German technology, since that was not available when the engine was developed. The four combustion chambers of the XLR11 each produced a thrust of 6,700 Newton, and the engine could be throttled simply by varying the number of chambers ignited at any time. At full power the engine would consume the onboard supply of propellants in less than 3 minutes but this was expected to be sufficient for a short leap beyond Mach 1 if the aircraft were dropped from a carrier plane at high altitude (in contrast, the M.52 would have been able to fly under power for about 20 minutes). After its powered run, the X-l would glide back for landing.

The airframe was constructed from high-strength aluminum, with propellant tanks welded from steel (the patch of frost you can see on many of the rocket X- planes is caused by water vapor in the air freezing on the fuselage at the location of the frigid liquid oxygen tank). For the shape of the fuselage, the designers decided to model it after a 0.50 caliber gun bullet; a piece of hardware which was known to be able to fly faster than Mach 1 and whose shape was based on extensive earlier research on the aerodynamics of munitions. The X-l was basically a bullet with wings. It looks very stubby to us today, and also in comparison to the previously described German DFS 346 that was otherwise very similar in purpose and concept. In order to adhere to the bullet shape there was an unconventional cockpit with its window streamlined flush with the fuselage. Bailing out would have been terribly difficult, because the pilot did not have an ejection seat (a novel technology at that time) or an escape capsule (like the DFS 346 or M.52); he would have had to exit through a small hatch on the starboard side of the nose. It would have been quite a feat in a rapidly tumbling, disintegrating airplane that might be on fire. And even if the pilot were to make it through the hatch, he would have almost certainly struck either the sharp wing or the tail. Health and Safety did not really exist in those days.

In addition to these rather blunt aerodynamic design solutions, the X-l employed one sophisticated idea: an all-moveable horizontal tail plane (inspired by that of the British M.52 concept) set high on the vertical tail fin to avoid the turbulence from the wings. It was known that the elevon controls on conventional stabilizers generated strong shock waves at high speeds, making the airplane impossible to control in the all-important pitch direction and ultimately producing the infamous ‘Mach tuck’ that caused it to nose over into a terminal dive. But if the entire stabilizer is moved, not just a part of it, no shock wave forms on its surface and there is no elevon to become blocked; in other words, it allowed control of an aircraft at transonic and supersonic speeds. This was such a revolutionary discovery that the US hid it from

the Soviets for as long as possible. During the Korean War the all-moving tail gave the US F-86 Sabre jet fighter a real advantage over the agile Soviet MiG-15, whose conventional tail had elevons which made it difficult to control at speeds approaching Mach 1. The all-moving horizontal stabilizer promptly became a standard feature on all supersonic aircraft, including the Russian successors to the MiG 15.

The X-l had good flight characteristics at transonic as well as lower speeds, both under rocket power and while gliding. Pilots found it a delight to fly, very agile with the handling characteristics of a fighter. It had a length of 9.5 meters (31 feet) and a wingspan of 7.0 meters (23 feet). Fully loaded with propellant it weighed 6,690 kg (14,750 pounds). Any propellant left after a powered flight was jettisoned in order to avoid landing with the hazardous liquids on board, and its dry weight was 3,107 kg (6,850 pounds)

Although originally designed for a conventional ground take-off, the X-l was air- launched from a high-altitude B-29 Superfortress bomber to maximize the use of its own propellant to accelerate to supersonic speed in the higher atmosphere, where both the aerodynamic drag and the speed of sound were significantly lower. At sea level a plane must exceed 1,225 km per hour (761 miles per hour) to surpass the speed of sound but at an altitude of 12 km (39,000 feet) Mach 1 is ‘only’ 1,062 km per hour (660 miles per hour). This meant the transonic and phenomena which the researchers were interested in would occur at slower, easier to attain speeds.

The X-l flight tests were to be undertaken at Edwards Air Force Base, at that time named Muroc Army Airfield, the famous test flight airfield out in the Mojave Desert of California. The base is next to Rogers Dry Lake, a large expanse of flat, hard salt that offers a natural runway. The desert also offers excellent year-round weather, as well as a vast, virtually uninhabited area with plenty of free airspace. All this made the base perfect for testing new high-speed and potentially dangerous rocket aircraft, especially if they were to remain secret.

By today’s aviation standards the X-l was a very risky aircraft. Apart from the rather dubious means of escape for the pilot, it also had no backup electrical system. During one flight, test pilot Chuck Yeager found himself in a powerless X-l due to a corroded battery just after being dropped from the carrier aircraft. He could neither ignite the engine nor open the propellant dump valves, since both required electrical power. Luckily, engineer Jack Ridley and Yeager had installed a manual system to get rid of the dangerous fluids just before that very flight, so he could still empty the tanks before landing; the X-l had not been designed to land safely with the weight of a full propellant load.

The original X-l aircraft, the X-l-1, made its first unpowered glide flight on 25 January 1946 over Florida’s Pinecastle Army Airfield, flown by Bell Aircraft chief test pilot Jack Woolams. The first powered flight was on 9 December 1946 at Muroc using the second X-l aircraft, with Bell test pilot Chalmers ‘Slick’ Goodlin (‘Slick’ being a flattering moniker in those days) at the controls. He also piloted the X-l-1 on its first powered flight on 11 April 1947. Two months later the Air Force, unhappy with Bell’s cautious and thus slow “pushing” of the flight envelope in terms of speed and altitude, terminated the flight test contract and took over. Captain Chuck

Yeager, a veteran P-51 Mustang pilot of the Second World War, was selected to attempt to exceed the speed of sound in the X-l-1. After being assigned to the program, which was understood by all involved to be extremely dangerous, he was told by program head Colonel Boyd: “You know, we’ve got a problem. I wanted a pilot who had no dependents.” Yeager responded that he was married and had a Uttle boy, but that this would only make him more careful. This was judged sufficient explanation.

In October 1947, after several glide and powered flights, both pilot and aircraft are deemed ready to officially break the sound barrier. On the 14th, teams of technicians and engineers awaken early in order to prepare the small, bright orange X-l for flight and install it in the bomb bay of its B-29 carrier. Then the four-engined bomber takes off and chmbs to an altitude of 6 km (20,000 feet). At 10:26 a. m., the X-l-1, which Yeager has christened ‘Glamorous Glennis’ after his wife, is dropped at a horizontal speed of 400 km per hour (250 miles per hour). Yeager Ughts the four XLR11 rocket chambers one by one, rapidly climbing as he does so, and then he levels out at about 13.7 km (45,000 feet). Trailing an exhaust jet with shock diamonds (caused by shock waves in the supersonic gas flow) from the four rocket nozzles, the X-l approaches Mach 0.85. Entering the poorly understood transonic regime, Yeager momentarily shuts down two of the four rocket chambers, holding the plane at about Mach 0.95 to carefully test the controls. As on previous flights there is buffeting and shaking due to the invisible shock waves forming on the top surface of the wings, but apart from that the plane responds well to his steering inputs. It is time. At an altitude of 12 km (40,000 feet) he levels off, reignites the third rocket chamber and watches the needle move smoothly up the Mach meter.

Suddenly the buffeting disappears and the needle jumps off the scale (which only went up to Mach 1; apparently not everyone was so confident in the X-l’s supersonic capability). Yeager lets the X-l accelerate further, and for 20 seconds flies faster than Mach 1. At supersonic speed, a strong bow shock wave forms in the air ahead of the needle-like nose, but the flow over the wings has smoothed out and he discovers that the plane behaves rather well. Not only is the X-l able to survive surpassing the dreaded sound barrier, it is functional and controllable beyond Mach 1. Satisfied, Yeager shuts down the engine and glides back to land on the dry lake at Muroc.

The recorded peak flight speed was Mach 1.06 at an altitude of 13 km (43,000 feet), corresponding to an actual airspeed of about 1,130 km per hour (700 miles per hour). On his return to base, Yeager reported that the whole experience had been “a piece of cake”. It may be that he broke the sound barrier on the previous flight when the recorded top speed was Mach 0.997, as inaccuracies in the measurements might have masked a speed slightly over Mach 1. However, no sonic boom was heard on that occasion, whereas it was on the day the sound barrier was officially broken. The loud explosion-like noise scared several people on the ground into believing that the X-l had blown up; no one had ever heard a sonic boom before.

This first-ever officially recorded Mach 1-plus flight made Yeager a national hero and the quintessential test pilot of the new jet age. His 1985 autobiography, Yeager, was a multi-million-copy best seller, and he plays a prominent role in Tom Wolfe’s famous book The Right Stuff, as well as the eponymous movie (in which he has a cameo as the old fellow near the bar in Pancho’s Happy Bottom Riding Club). The introduction to the movie perfectly describes the X-l program: “There was a demon that lived in the air. They said whoever challenged him would die. Their controls would freeze up, their planes would buffet wildly, and they would disintegrate. The demon lived at Mach 1 on the meter, 750 miles an hour, where the air could no longer move out of the way. He lived behind a barrier through which they said no man could ever pass. They called it the sound barrier. Then they built a small plane, the X-l, to try and break the sound barrier.” If you desire a flavor of the rough world of the early jet and rocket plane test pilots and the first seven US astronauts, Wolfe’s book and the movie are indispensable. Some of the tales may seem fictional, inserted to spice up the story, but most of it is true. Bell test pilot ‘Slick’ Goodlin demanding a $150,000 bonus for attempting to break the sound barrier, then being replaced by Air Force Captain Yeager willing to do the job on his government salary of just over $200 a month is true. So is the famous incident in which Yeager breaks two ribs in a riding accident, says nothing to his superiors to avoid being replaced for the historic Mach 1 flight, and then gets his close friend and X-l engineer Captain Jack Ridley to furnish him a piece of a broom handle so that he can pull the lever to close the X-l’s door using his other hand; unfortunately, the historic piece of wood has been lost to history.

Breaking the sound barrier would have been a great publicity coup for the US Air Force, which had recently gained its independence from the Army, but the flight was kept secret in the interests of national security. Then in December the trade magazine Aviation Week (often referred to as ‘Aviation Leak1) unofficially broke the news. The Air Force did not confirm the story until March 1948, by which time Yeager and his colleagues were routinely flying the X-l up to Mach 1.45. The National Aeronautics Association voted that its 1947 Collier Trophy be shared by the main participants in the program: Larry Bell for Bell Aircraft, Captain Yeager for piloting the flights, and John Stack of NACA for scientific contributions. They received the 37-year-old prize from President Harry S. Truman at the White House. Yeager kept the prestigious trophy in his garage and used it for storing nuts and bolts.

The original X-l-1 ‘Glamorous Glennis’ became one of the most famous planes ever. Not only was it the first to fly faster than the speed of sound, it also attained the maximum speed of the entire X-l program: Mach 1.45. Furthermore, it was the only X-l to make a ground take-off (also with Yeager at the controls). On 8 August 1949, on the program’s 123rd flight, Air Force Major Frank K. ‘Pete’ Everest Jr., flew the X-l-1 to the new altitude record of 21,916 km (71,902 feet). Like all X-l records, it was unofficial, as according to FAI rules an aircraft must take off and land under its own power in order to be able to claim an official record (in 1961 this even prompted the Soviets to hide the fact that the world’s first spacefarer, Yuri Gagarin, had landed by parachute separately from his capsule). On the next flight, on 25 August, also with Everest on board, the X-l-1 suffered a cracked canopy and the cockpit lost pressure at an altitude of approximately 21 km (65,000 feet). Fortunately Everest was wearing a pressure suit that quickly inflated to prevent his blood from boiling in the thin air, making him the first pilot to have his life saved by such a suit. The X-l-1 was retired in May 1950 after a total of 82 flights (both gliding and powered) with ten different pilots. It was given a well-earned place in the Smithsonian Air and Space Museum alongside the Wright Flyer and Lindbergh’s Spirit of St. Louis, and it has recently been joined by a distant relative in the form of SpaceShipOne. Upon presenting the X-l to the museum, Air Force Chief of Staff General Hoyt Vandenberg said that the program “marked the end of the first great period of the air age, and the beginning of the second. In a few moments the subsonic period became history and the supersonic period was born.” The XLR11 engine that was used during Yeager’s historic flight is on display separately at the same museum. When I first saw both the aircraft and the engine I was surprised at how crude they appear by today’s standards, dramatically showing the fairly basic technology that was available to the X-l team in tackling the challenge. The Air Force Flight Test Center Museum at Edwards Air Force Base has an X-l replica.

Bell built three aircraft for the program: X-l-1 (serial number 46-062), X-l-2 (46­063) and X-l-3 (46-064). X-l-1 and X-l-3 were flown by the Air Force while X-l-2 was used by NACA, which had by then established a permanent presence at Edwards (initially NACA Muroc Flight Test Unit, it was renamed NACA High­Speed Flight Research Station in 1949 and then NACA High-Speed Flight Station in 1954. After the formation of NASA it became NASA Flight Research Center in 1959 and finally NASA Dryden Flight Research Center in 1976). In their original configuration, the three X-ls made a total of 157 flights between 1946 and 1951, of which 132 were under rocket power. They were flown by 18 different pilots but Yeager, with a total of 34 flights, was the most experienced X-l pilot of the program.

The X-l-2 was essentially identical to X-l-1, and made its first powered flight on 9 December 1946 with Bell test pilot Goodlin at the controls. By October 1951 it had

completed 74 gliding and powered flights, flown with nine different pilots. Then it was rebuilt as the X-1E, one of the second generation of X-l planes.

The X-l-3 differed by having the turbopump-driven XLR11-RM-5 engine (in the XLR11-RM-3 of its predecessors high-pressure nitrogen fed the propellant into the combustion chambers). By using turbopumps, the pressures in the propellant supply lines could be kept relatively low, and metal fatigue problems diminished (concerns of which had resulted in the grounding of the X-l-2 after its 54th powered flight). The lower pressure also resulted in a considerable mass saving on the nitrogen tanks. On the other hand, the high level of complexity of the new turbopump system delayed production. When the aircraft was delivered to Muroc in April 1951 it was three years behind schedule. It gained the nickname ‘Queenie’ for being a Hangar Queen (an airplane that requires extraordinary preparation and maintenance time in the hanger). The X-l-3 made only one glide flight, and that was on 20 July 1951 with Bell test pilot Joe Cannon at the controls. Sadly, the aircraft was lost on 9 November whilst being de-fueled following a captive flight test mated to its B-50 carrier bomber (an improved form of the B-29). As Cannon pressurized the liquid oxygen tank a dull thud was heard, followed by a hissing sound as white vapor escaped from the X-l-3’s center section. Then a violent explosion engulfed the rocket plane and its carrier aircraft in yellow flames and black smoke. Both the X-l-3 and the B-50 were totally destroyed. Cannon managed to get out of the X-l-3, but spent nearly a year in hospital recovering from severe bums on his legs, arms and body. The X-l-3 was the first (but not the last) rocket X-plane to be lost due to a violent, mysterious explosion.

To follow up on the success of the original X-l aircraft, Bell received a contract to build a second generation of X-l aircraft with the potential to fly at speeds exceeding Mach 2. These aircraft, the X-l A to X-1E, were powered by the turbopump XLR11- RM-5 engine that was also incorporated in the X-l-3. It had the same 27,000 Newton maximum thrust of the XLR11-RM-3 and was throttled by varying the number of active combustion chambers. The X-l A resembled the X-l, but had a bubble canopy and a stretched fuselage to carry more propellant for a longer powered flight. It was delivered to Edwards on 7 January 1953. The first ghde flight was made by Bell pilot Jean ‘Skip’ Ziegler, who went on to make five powered flights in it. Afterwards, the aircraft was handed over to the Air Force.

In parallel with the Air Force’s X-1A flights, NACA initiated its own high-speed research with the Douglas D-558-2 Skyrocket (more on this later). On 20 November 1953 Scott Crossfield achieved Mach 2.005 in this aircraft, beating the Air Force to the ‘magic number’ of Mach 2. The Air Force promptly initiates ‘Operation NACA Weep’ in which a series of ever-faster flights culminate on 12 December 1953 with Yeager boosting the X-l A to a new air speed record of Mach 2.44 at an altitude of

22.8 km (74,700 feet). Moreover, Yeager achieves this speed in level flight, whereas Crossfield had required to push his Skyrocket into a shallow dive in order to surpass Mach 2. However, Yeager’s elation is short lived, because soon after setting the new speed record his aircraft starts to yaw, and when he tries to compensate this causes it to suddenly pitch up violently. The aircraft enters an inverted flat spin from which Yeager is unable to recover. Bailing out is not possible at the high speed with which the aircraft is tumbling from the sky because it is not equipped with an ejection seat. Accelerations of up to 8 G throw him so violently around inside the cockpit that his helmet breaks the canopy. Only when the aircraft enters the denser atmosphere, at an altitude of 7.6 km (25,000 feet), is he able to restore control. He has literally fallen 15 km (50,000 feet). Unperturbed, Yeager glides back to Edwards and lands safely. Aerodynamidsts had predicted that such ‘inertia coupling’ might occur when flying at high speeds but the X-1A was the first to experience it. This is a very dangerous phenomenon in which the inertia of the aircraft fuselage overpowers the stabilizing aerodynamic forces on the wings and tail. Aircraft that have low roll inertia relative to their pitch and yaw inertia are especially susceptible to it. In practice, this means that planes having stubby wings and long fuselages, and in which the mass is spread over the length of the plane rather than being concentrated near its center of gravity, will probably have problems at high speeds. With its long, relatively slender fuselage, the heavy rocket engine in the tail, and its Mach 2 + flight speeds, the X-l A matched this profile. Pilots had up to then felt that with experience and a basic flight control system, any situation in the air could be handled. But at the extreme altitudes and speeds that the new research aircraft could attain, inertia coupling would require the development of much more sophisticated flight control systems.

An attempt to surpass Yeager’s record speed with the X-1A would be extremely dangerous and was never tried. However, flying the X-l A to higher altitudes was still possible. On 26 August 1954 USAF test pilot Major Arthur Murray set a new record of 27.56 km (90,440 feet). In September the aircraft was transferred to NACA High­Speed Flight Station, which returned it to Bell for the installation of an ejection seat; all of the Air Force’s high-speed and high-altitude flights had been done without the pilot having a quick and secure means of escape!

On 20 July 1955 NACA test pilot Joseph Walker made a familiarization flight in the modified aircraft. Then, on 8 August, as he is sitting in the cockpit preparing for another drop, there is an explosion in the engine compartment of the X-1A. Flames erupt from the propellant tanks and leave a trail in the B-29’s slipstream. In addition, the X-lA’s landing gear has been blown down into the extended position, making it impossible to land the carrier aircraft without the X-1A touching the runway first and likely breaking apart. Walker manages to get out of the rocket plane into the relative safety of the bomb bay, grabs a portable oxygen tank to breathe, and then returns to dump the rocket plane’s propellant in an effort to save both aircraft. But it is too late, and the B-29 jettisons its burning load. As the X-1A falls it suddenly pitches up and almost hits its carrier, then spirals down and smashes into the desert floor, exploding on impact. Walker and the B-29 crew return to base uninjured. The X-1A had performed a total of 29 flights (including aborts) by four pilots.

The second aircraft of the new series, the X-1B, was similar in configuration to the X-1A except for having slightly different wings (for its last three flights its wings were slightly lengthened). The Air Force used the X-1B for high-speed research from

October 1954 to January 1955, whereupon it was turned over to NACA, whose pilots (Neil Armstrong amongst them) flew it to gather data on aerodynamic heating, a new field of study that became ever more important as aircraft speeds increased.

Aerodynamic heating occurs when the speed of the airflow approaches zero, most particularly in the strong shock waves at the leading edges of the wings and the nose of a supersonic aircraft, where much of the kinetic (movement) energy of the air is converted into heat that can transfer into the aircraft. At extreme speeds the heat can damage the structure of a plane, and even if the temperatures remain relatively low the cycles of heating and cooling that a plane goes through during each flight can still weaken its structure in the long term. Moreover, the aerodynamic heat can make life very uncomfortable for the pilot (and passengers) if no adequate cockpit or flight suit cooling system is installed. For instance, when the Concorde supersonic airliner was cruising at Mach 2.2 its nose reached 120 degrees Celsius (250 degrees Fahrenheit). When the Space Shuttle entered the atmosphere at Mach 25 on returning from orbit its nose reached a searing 1,650 degrees Celsius (3,000 degrees Fahrenheit). Special structural materials (such as the titanium alloy used on the SR – 71 capable of flying at Mach 3) and thermal protection materials (Uke on the Space Shuttle) were required to survive the heat at extreme flight speeds.

To be able to make detailed measurements of the temperatures on different areas on the X-1B, NACA installed 300 thermocouple heat sensors over its surface. During this test campaign the aircraft was also equipped with a prototype reaction control system comprising a series of small hydrogen peroxide rocket thrusters mounted on a wingtip, the aft fuselage, and the tail to provide better control at high altitudes where there is Uttle air for the aerodynamic control surfaces to work with. On the X-1B this system was purely experimental, as the maximum altitude was typically kept to about 18 km (60,000 feet) at which it could still rely on its standard aircraft control system; in fact, the X-1B reached its highest ever altitude of 19.8 km (65,000 feet) three years prior to the installation of the reaction control system. Subsequently a similar system was installed on the X-15, which could fly so high that it was essentially in a vacuum and unable to rely on rudders, ailerons and elevons alone. For the Mercury, Gemini and Apollo spacecraft of the 1960s, thrusters were the only means of controlling the attitude of the vehicle. The X-1B played a pioneering role in the development of such systems.

Moreover, midway through its flight test program the X-1B was equipped with an XLR11-RM-9 engine which had a novel low-tension electric spark igniter instead of the high-tension type of the earlier XLRlls. NACA flew the aircraft until January 1958, when it was decided to ground it owing to cracks in the propellant tanks. It had completed a total of 27 flights by eight Air Force and two NACA test pilots, all of which had been intended to be powered but some had ended up as glide flights due to problems with the rocket engine. In January 1959 the X-1B was given to the National Museum of the US Air Force at Wright-Patterson Air Force Base in Ohio, where it is still on display.

The X-1C was intended to test onboard weapons and munitions at high transonic and supersonic flight speeds, but while it was still under development operational jet fighters such as the F-86 Sabre and the F-100 Super Sabre were already shooting cannon and firing missiles while flying at such speeds, so the X-1C was canceled in the mockup stage.

The X-1D was to take over from the X-1B in testing aerodynamic heating. It had a slightly increased propellant capacity, a new turbopump which enabled the tanks and propellant feed lines to work at a lower pressure, and somewhat improved avionics (i. e. the onboard electrical and electronic equipment). On 24 July 1951 Bell test pilot Jean ‘Skip’ Ziegler made what would turn out to be the only successful flight of the X-1D. On being dropped by its B-50 carrier the aircraft made a 9 minute unpowered glide which ended with a very ungraceful landing due to the failure of the nose gear. The repaired aircraft was turned over to the Air Force, which assigned Lieutenant Colonel ‘Pete’ Everest as the primary pilot. On 22 August the X-1D took to the sky for its first powered flight, partly contained within the bomb bay of its B-50 carrier. But the mission had to be aborted owing to a loss of nitrogen pressure needed to feed the propellants into the turbopump of the rocket engine. Because it would be dangerous to land the B-50 with a fully loaded X-1D, Everest attempted to jettison the propellant. Unfortunately this triggered an explosion and a fire, and once again an X-l had to be jettisoned. Luckily no one was hurt. The explosions of the X-l-3 and the X-1D were finally traced to the use of leather gaskets in the oxygen propellant supply plumbing (which had likely also caused the loss of the X-l A). The leather had been impregnated with tricresyl phosphate (TCP), which firstly becomes unstable in the presence of pure oxygen and can then explode if subjected to a mechanical shock. It was one of the hard lessons learned during the X-l program.

After the loss of the X-l-3 and the X-1D (the crash of the X-1A would not occur until several years later) it was decided to upgrade the X-l-2 and redesignate it as the X-1E to continue the high-speed flight test campaign. It was christened ‘Little Joe’ in honor of its primary Air Force test pilot, Joe Walker. The most visible modifications included a protruding canopy, a rocket assisted ejection seat, and thinner wings with knife-sharp leading edges and a thickness ratio of 4% (better suited to supersonic flight). The surface of the plane was covered with hundreds of tiny sensors to register structural strain, temperatures and airflow pressures. The X-1E made its first glide flight on 15 December 1955 with Walker at the controls. He went on to make a total of 21 flights, attaining a maximum speed of Mach 2.21. NACA research pilot John McKay took Walker’s place in September 1958 and made five more flights, with a maximum attained speed of Mach 2.24. It was permanently grounded in November 1958 owing to structural cracks in the fuel tank wall, and now guards the entrance of NASA Dryden Flight Research Center.

The X-l program thus opened the door to supersonic flight, and its experimental results facilitated a new generation of military jets that could fly faster than the speed of sound. The various X-ls truly adhered to the Edwards Air Force Base motto of ‘Ad Inexplorata’ (Into the Unknown).

In friendly competition with the Air Force’s X-l program, the US Navy, working with NACA, initiated tests using its mixed-power Douglas D-558-2 Skyrocket. The Navy/NACA D-558 program pursued a more conservative approach to the problems of high-speed flight than did the USAF/NACA X-l. In contrast to the decision by the Air Force to go straight to supersonic rocket propelled planes, the Navy started with the transonic D-558-1 Skystreak jet-powered research aircraft. This was more in line with the careful scientific approach which NACA advocated. The D-558-1 had only just been able to surpass Mach 1 in a dive. By using rocket power in addition to a jet engine the D-558-2 was to explore the transonic and supersonic flight regimes and investigate the characteristics of swept-wings at speeds up to Mach 2. The Navy was also particularly interested in the strange phenomenon that made high-speed, swept-wing aircraft of that time pitch their nose upwards at low speeds during take-off and landing, as well as in tight turns. The original plan was to modify the fuselage of the D-558-1 to accommodate a combination of a rocket and a jet engine, but that soon proved impractical. The D-558-2 became a completely new design that had its wings swept at 35 degrees (its predecessor had straight wings) and its horizontal stabilizers at 40 degrees. The wings and the tail section would be fabricated from aluminum, but the fuselage would be primarily magnesium. For take-off, climbing and landing the Skyrocket would be powered by a Westinghouse J34-40 turbojet engine drawing its air through two side intakes on the forward fuselage and producing a thrust of 13,000 Newton. To attain high speeds, a four-chamber rocket engine with a total sea-level thrust of 27,000 Newton would be fitted. The Navy called this the LR8-RM-6 but it was basically the same XLR11 engine as used in the Bell X-l. The design called for a flush canopy similar to that of the X-l in order to obtain a sleek fuselage, but this would have so limited the pilot’s visibility that it was decided to use a normal raised cockpit with angled windows. The resulting increased profile area at the front of the aircraft had to be balanced by a slight increase in the height of the vertical stabilizer. Somewhat reminiscent of the German DFS 346 rocket aircraft, the pilot was housed inside a pressurized nose section that (as on the D-558-1) could be jettisoned in an emergency. The capsule would be decelerated by a small drag chute, and when it had achieved a suitable altitude and speed the pilot would bail out to land under his own parachute.

On 27 January 1947 the Navy issued a contract change order to formally drop the production of the planned final three D-558-1 jet aircraft and substitute instead three of the new D-558-2 Skyrockets.

The Douglas company invited its pilots to submit bids to fly the new rocket plane during the test program. However, at that time Yeager had not yet made his historic Mach 1 flight in the X-l and trying to break the sound barrier was still seen by most test pilots as a quick and easy way to “buy the farm” (i. e. die). Rather than ignore the offer, which would have been bad for their reputations, the pilots conspired to

submit exceptionally high bids that would surely not be accepted by the company. However, John F. Martin was away delivering an airplane for Douglas and unaware of the plot. He submitted a reasonable bid and was promptly accepted as the Skyrocket’s project pilot. On 4 February 1948 Martin took off from Muroc Army Airfield in the first aircraft (Bureau No. 37973; NACA 143) for the Skyrocket’s maiden flight. At that time this aircraft employed a jet engine and was configured only to take off from the ground. It was tested in this configuration by the company until 1951 then handed over to NACA, which kept it in storage until 1954 and then modified it by removing the jet engine, installing an LR8-RM-6 rocket engine, and configuring the aircraft for air-launch from the bomb bay of a P2B (the naval version of the B-29). However, it was subsequently only used for one mission: an air-drop familiarization flight on 17 September 1956 by NACA pilot John McKay. In total NACA 143 made 123 flights, mostly in order to validate wind-tunnel predictions of the Skyrocket’s performance. One interesting discovery was that the airplane actually experienced less drag above Mach 0.85 than the wind tunnels data indicated, thus highlighting the discrepancies between wind tunnel results and actual flight measurements that still existed at that time.

Skyrocket Bureau No. 37974 (NACA 144) had a much more interesting career. It also started out with a jet engine only, in which configuration NACA pilots Robert A. Champine and John H. Griffith flew it 21 times for subsonic airspeed calibrations and to investigate longitudinal and lateral stability and control. They encountered the expected pitch-up problems, which were often severe and occurred very suddenly. In 1950 Douglas replaced the turbojet with an LR8-RM-6 and modified the airframe to be carried by a P2B (B-29) bomber. The release at an altitude of about 9 km (30,000 feet) and the increased thrust compared to the turbojet enabled company pilot Bill Bridgeman to fly this aircraft up to a speed of Mach 1.88 on 7 August 1951, and on 15 August reach a maximum altitude of 24.2 km (79,494 feet) and set an unofficial world altitude record. Bridgeman flew the aircraft a total of seven times.

During his supersonic flights he encountered a violent rolling motion due to lateral instability which was curiously weaker on his Mach 1.88 flight than on a Mach 1.85 flight that he made in June.

It was then turned over to NACA, which started its own series of research flights in September 1951 with legendary pilot Scott Crossfield. Over the next several years

Crossfield flew NACA 144 at total of 20 times, gathering data on longitudinal and lateral stability and control, aerodynamic loads and buffeting characteristics at speeds up to Mach 1.88. On 21 August 1953 Marine Lieutenant Colonel Marion Carl, flying for the Navy, set a new unofficial altitude record of 25.37 km (83,235 feet). NACA technicians then extended the rocket engine nozzle in order to prevent its exhaust gas from affecting the rudders at supersonic speeds and high altitudes (where the exhaust expands into an enormous plume). As explained later in this chapter, such additions also improve the efficiency of an engine at high altitudes; in the case of the D-558-2 increasing the thrust by 6.5% at 21 km (70,000 feet) altitude.

Meanwhile, people in the project where lobbying for the go-ahead from NACA to attempt to cross the Mach 2 boundary. They knew the Air Force was planning to try to fly faster than twice the speed of sound using the X-1A in celebration of the 50th anniversary of the first flight by the Wright brothers. The Navy and Scott Crossfield, who was a Naval officer prior to joining NACA as a civilian test pilot, were eager to claim this record. NACA preferred to focus on a steady scientific approach and leave record setting to others, but Crossfield convinced NACA director Hugh L. Dryden to consent to a Mach 2 flight attempt with the NACA 144 Skyrocket. Some years later Crossfield admitted, “It was something I wanted to do; particularly if I could needle Yeager about it.”

The NACA project team knew their aircraft would need to be pushed to the very limit of its capabilities. The extra thrust from the new nozzle extension would help, but more was required. Extremely frigid liquid oxygen was put into the oxidizer tank 8 hours before the flight to cold-soak the aircraft, because this would reduce fuel and oxidizer evaporation due to the aircraft’s own heat during the flight and thereby leave more propellant in the tanks for several more seconds of powered acceleration. To limit drag as much as possible they cleaned and thoroughly waxed the fuselage, even taping over every little seam in the aircraft’s surface. The heavy stainless steel propellant jettison tubes were replaced with aluminum ones. In addition, these were positioned into the rocket exhaust stream so that they would bum off once the engine was ignited and were no longer required, further reducing the aircraft’s weight and drag. Project engineer Herman O. Ankenbruck devised a flight plan to make the best use of the Skyrocket’s thrust and altitude capabilities. It was decided that Crossfield would fly to an altitude of approximately 22 km (72,000 feet) and then push over into a slight dive to gain a little help from gravity. Despite having the flu and a head cold, Crossfield made aviation history on 20 November 1953 by becoming the first man to fly faster than twice the speed of sound; although barely: his maximum speed was Mach 2.005, or 2,078 km per hour (1,291 miles per hour). But this record stood for a mere 3 weeks, when the X-1A flew considerably faster. No attempts were made to push the D-558-2 to higher speeds; it had reached the limits of its design and there was no way that it could hope to reclaim the speed record from the X-1A.

More flights were made by NACA 144 with NACA pilots Scott Crossfield, Joe Walker and John McKay gathering data on pressure distribution, stmctural loads and stmctural heating. It flew a total of 103 missions, including the program’s finale on 20 December 1956 when McKay took it up for data on dynamic stability and sound-pressure levels at transonic and supersonic speeds.

The third Skyrocket (Bureau No. 37975; NACA 145) could also be air-launched and was equipped with both an LR8-RM-6 rocket engine and a Westinghouse J34- 40 jet engine which had its exhaust pipe exiting the belly of the plane. Taking off under its own power on 24 June 1949 this aircraft became the first Skyrocket to exceed the speed of sound, thereby proving that the design was well suited to supersonic flight; pilot Eugene F. May noted that upon passing Mach 1, “the flight got glassy smooth, placid, quite the smoothest flying I had ever known”. By November 1950 NACA 145 had completed 21 flights by company pilots May and William Bridgeman, and then it was turned over to NACA. In September the following year pilots Scott Crossfield and Walter Jones began flying it to investigate the notorious pitch-up phenomenon. For this, the aircraft was flown with a variety of configurations involving extendable wing slats (long, narrow auxiliary airfoils), wing fences (long but low vertical fins that run over the wing) and leading edge chord (width) extensions. They found that whilst fences significantly aided in the recovery from sudden pitch-ups, leading edge chord extensions did not. This disproved wind tunnel tests which had indicated the contrary, and clearly demonstrated the need for full-scale tests on real aircraft. Wing slats, when in the fully open position, eliminated the pitch-up problem except in the speed range of Mach 0.8 to 0.85. The data obtained from these tests was extremely valuable when developing supersonic fighter aircraft. In June 1954 Crossfield began using NACA 145 to investigate the aircraft’s transonic behavior with external stores such as bombs and drop tanks (the bombs were empty dummies, as only their shape and position were relevant). Pilots McKay and Stanley Butchart completed NACA’s investigations on this, with McKay flying the last of NACA 145’s 87 missions on 28 August 1956.

Together the three Skyrockets flew a total of 313 missions, both taking off from the ground on jet power as well as being air-launched from a carrier. They gathered invaluable data on the stability and control of swept-wing aircraft at transonic and supersonic speeds. The data enabled a better correlation between wind tunnel results and flights by real aircraft in the open sky, making wind tunnel tests more useful in the design of high speed aircraft. Especially benefiting from the D-558’s research, as well from the X-l program, were the so-called ‘Century Series’ supersonic fighters: the F-100 Super Sabre, F-101 Voodoo, F-102 Delta Dagger, F-104 Starfighter, F – 105 Thunderchief and F-106 Delta Dart. The various makers of these magnificent aircraft all exploited the flight research done at Edwards, giving the US military an important edge over their Soviet counterparts.

NACA 143, the first Skyrocket, is on display at the Planes of Fame Museum in Chino, California. NACA 144, the first aircraft to fly at Mach 2, is hanging from the ceiling of the National Air and Space Museum in Washington D. C. NACA 145 can be found outside on the campus of Antelope Valley College in Lancaster, California, not far from Edwards.

In late 1944, as the design of the X-l was getting underway, it became clear to the US Army Air Force that supersonic aircraft would greatly benefit from swept wings like those pioneered in Germany. Bell thus responded to the Air Force request for a successor to the X-l with their Model 37D, which was essentially an X-l that had its wings swept back at 40 degrees. However, aerodynamic and structural analyses soon demonstrated that such an upgrade of the X-l design was not very practical, and the proposal was rejected. In September 1945, just after the Second World War ended, Bell came back with an entirely new and much bolder concept which they called the Model 52. Even although the X-l had yet to fly, the Bell engineers told the Air Force that their new aircraft would be able to get close to Mach 3 at altitudes above 30 km (100,000 feet). The Air Force was sold on the concept and named it the XS-2 (later shortened to the X-2). This revolutionary airplane had wings that were swept back at 40 degrees (as before) but now they were mounted to the fuselage with 3 degrees of dihedral and had a 10% thickness ratio (as explained earlier, swept wings can have a greater relative thickness than a straight wing for a given critical Mach number). The wings had a bi-convex profile (a double-wedged cross section which resembled an elongated diamond) that was expected to be particularly suitable for supersonic flight as already indicated by wind tunnel experiments performed in Italy in 1940 (also the canceled British Miles M.52 would have been equipped with bi-convex wings). Like on the X-l, the horizontal tailplane was all-moveable but an innovation was that the stabilizers had the same sweep as the wings.

Where the X-l series was to surpass the sound barrier, the X-2 was envisioned to best the ‘heat barrier’. The temperatures on its exterior were expected to reach about 240 degrees Celsius (460 degrees Fahrenheit) due to severe aerodynamic heating. To survive this, the wings and tail surfaces were made using heat resistant stainless steel and the fuselage was a high strength copper-nickel alloy called К-Monel. In order to maintain a comfortable temperature in the cockpit, a cooling system weighing 225 kg (496 pounds) was installed which, under normal conditions, was sufficient to keep a room containing 300 people nice and cool.

The X-2 would be air-dropped from a B-50 bomber and land without propellant on the dry lake near Muroc, so its landing gear comprised a deployable center-line skid, a small skid under each wing, and a short nose wheel which hardly protruded beyond the fuselage. (Its peculiar attitude on the ground gave the impression that the front carriage had collapsed.) It looked very much like a manned rocket, with a

rather small cockpit capsule right at the front, housed inside a sharp pointy nose. Just as on the D-558-2 Skyrocket, in an emergency the X-2’s entire pressurized nose assembly would be jettisoned and soon stabilized by a small parachute. The pilot would then have to manually open the canopy at a safe altitude and speed, and bail out. Although NACA was concerned about this system, the Air Force considered it an adequate means of escape at extreme flight speeds and altitudes and approved the design. It is another example of the more careful but slower NACA approach versus the bolder Air Force seeking faster progress in order to stay ahead in aviation (with respect to the Soviets certainly, and probably also in friendly competition with the Navy and NACA).

To propel the X-2 to Mach 3, it was equipped with an advanced Curtiss-Wright XLR25-CW-3 pump-fed dual-chamber rocket engine that ran on water-alcohol and Uquid oxygen and produced a total thrust of 66,700 Newton at sea level; about two- and-a-half times that of the XLR11 used by the X-l. The upper combustion chamber could produce a maximum of 22,200 Newton and the larger, lower chamber twice that. They could be run together or separately, and each could be throttled between 50 and 100% of its full thrust level (whereas the XLR11 could only be adjusted by varying the number of chambers ignited). With full propellant tanks the X-2 weighed 11,299 kg (24,910 pounds), and its landing weight with empty tanks was 5,613 kg (12,375 pounds); both of these weights where almost twice the corresponding figures for the X-l.

The Air Force ordered two X-2 Starbuster research aircraft (airframes 46-674 and 46-675) from Bell Aircraft for the initial flight test program. NACA would initially provide advice and support, and install data-gathering instrumentation, then later use the aircraft for its own test flight campaign.

The X-2 represented a major advance in technology over the X-l. In particular, the development of the XLR25 rocket engine delayed the program by several years and many issues concerning the structure of the aircraft and its flight control system had to be overcome. The planned revolutionary fly-by-wire system where the pilot’s control inputs would be interpreted by a computer and then translated into electrical signals to operate motors of the control surfaces was abandoned in 1952 because its technology was too immature. It was replaced by a conventional and much heavier hydraulic power-boosted system. This unfortunately also meant that the operation of the aircraft was completely up to the pilot, without any intervention from a computer to ensure that no maneuvers were made which would be dangerous at certain speeds and altitudes.

The Air Force purchased a Goodyear Electronic Digital Analyzer (GEDA) analog computer which NACA turned into a rudimentary X-2 flight simulator, the first ever computer simulator to be used in aviation. This machine, which could simultaneously handle the various complex interdependent mathematical equations that described the motions of the X-2, helped pilots to familiarize themselves with the aircraft and its expected handling characteristics. It also allowed detailed preparation and checking of flight plans before assignment to the real aircraft. In due course the measurements made during the actual flights helped to improve the simulator.

Consistent (although probably not intentionally) with the X-l speed indicator only going up to Mach 1, the X-2 cockpit had a meter limited to Mach 3 and an altimeter that only went to 100,000 feet (30.5 km), even though the plane was intended to (and did indeed) fly considerably faster and higher than that! In

addition, the cockpit had a standard gyro system to indicate the plane’s attitude, which the pilots found to be so inaccurate as to be unusable.

Owing to the development problems it was early 1952 before Bell concluded the captive flight tests with the X-2 remaining mated to the B-50. The first glide flight on 27 June 1952 took place at Muroc (which by then was Edwards Air Force Base) with Bell test pilot Jean ‘Skip’ Ziegler at the controls. The plane used on the occasion was the second X-2 (46-675) because it had been decided to leave the first aircraft at the company so that it could be equipped with an XLR25 engine as soon as one became available. Unfortunately, at the end of its first glide flight the plane was damaged by a rough landing that collapsed its nose gear. While this repair was underway, a wider central skid was installed to make landing easier. When testing resumed in October 1952, both glide flights resulted in satisfactory landings.

With the glide tests finished, the plane was returned to Bell for modifications. As the first rocket engine delivered had not yet been installed in the first (untested) X-2, it was decided to put it in the already flown one. More captive flight tests were then performed to verify the proper operation of the new propulsion system (without any ignition) at high altitude. Sadly, Ziegler, a veteran of many flights in the X-l series, died on 12 May 1953 when this X-2 suddenly exploded during a captive flight over Lake Ontario while he was checking the aircraft’s liquid oxygen system. B-50 crew member Frank Wolko also died, but the bomber managed to jettison the burning X – 2 into the lake and land safely. The X-2 was never recovered and the B-50 had been damaged beyond repair. It was later found that the explosion was likely caused by the same inflammable leather gasket problem that caused the loss of the X-l-3 and X-1D, and possibly also the X-1A.

Once the remaining X-2 airframe 46-674 had been equipped with an XLR25 engine, the testing of this aircraft began with a series of glide flights. No problems were foreseen, since the glide landings with the second X-2 had been satisfactory after the wider skid was installed. The flight team was therefore surprised when 46- 474’s first flight ended in a very unstable landing in which the aircraft skidded sideways over the salt lakebed. After repairs, the next flight ended similarly. It appeared that the high position of the aircraft’s center of gravity on the ground due to the tall landing skid booms made it wobble upon touching down. The skid’s height was decreased, changing the plane’s 7-degree nose-down angle to 3 degrees. This did the trick. The aircraft made perfect landings from then on. Now the X-2 was finally ready for its powered maiden flight. The first attempt took place on 25 October 1955 but because of a nitrogen leak pilot ‘Pete’ Everest had to complete the mission as a glide flight. The second attempt was aborted while still attached to the carrier aircraft and ended in another captive flight. On 18 November everything finally worked. As planned, only the smaller of the two thrust chamber was ignited. The maximum speed attained was Mach 0.95. However, a small fire had broken out in the tail of the aircraft. Although this did not look very severe in the post-landing inspection it nevertheless meant several months of repair. Following several more aborted attempts, Everest completed a second powered flight on 24 March 1956, this time using only the larger thrust chamber. If anything, these early flights showed the X-2 to be a complex aircraft that was difficult to fly and to maintain. Due to these problems the development and flight test program was already three years behind schedule.

When both combustion chambers were used on 25 April they enabled the X-2 to fly supersonically for the first time: it reached a speed of Mach 1.40 and a maximum altitude of 15 km (50,000 feet). Everest performed three powered flights in May that pushed the X-2’s speed to Mach 2.53, making him the ‘Fastest Man Alive’. Another pilot, Air Force Captain Iven C. Kincheloe, made a supersonic flight on 25 May, but a malfunction obliged him to shut the engine down early.

In a rocket, the role of the nozzle is to correctly expand the hot exhaust from the high pressure inside the combustion chamber to a considerably lower pressure but a much higher speed. For maximum efficiency (i. e. specific impulse) the expelled gas should reach the same pressure as the ambient atmosphere at the end of the nozzle. Over-expansion (in which the exhaust reaches a pressure lower than that of the air) causes a loss of thrust; as indeed does under-expansion. The higher the altitude the lower the ambient air pressure, which means that at high altitudes the exhaust can be expanded further through a longer nozzle, enabling the same engine to deliver more thrust (at the cost of the maximum thrust at lower altitudes, where the exhaust will be over-expanded). In June 1956 the X-2 received an engine nozzle extension to give it more thrust at high altitudes where there is low aerodynamic drag, thus enabling it to fly faster. Everest made a supersonic checkout of the upgraded X-2 on 12 July 1956, and on the 23rd made his final flight in the aircraft to gather data on

aerodynamic heating. During this mission he reached a speed of Mach 2.87 at an altitude of 21 km (68,000 feet). Kincheloe then took over as project pilot and made a series of flights in an attempt to reach the aircraft’s greatest possible altitude. To achieve this, the X-2 had to make a powered ascent at an angle of 45 degrees. This was difficult to judge using the cockpit instrumentation owing to the inaccurate gyro system, so engineers simply drew a line on the windscreen with a red grease pencil: if Kincheloe kept this line parallel to the horizon while looking out to the side, he would be climbing at the required angle. After two aborted attempts he achieved the very respectable altitude of 26,750 km (87,750 feet) on 3 August 1956. On 7 September he shattered his own record by reaching a spectacular 38,466 km (126,200 feet) flying at Mach 1.7, which also marked the first time anyone had exceeded

100,0 feet altitude (corresponding to 30.5 km, but 100,000 is obviously more impressive as a ‘magic number’). Since at this altitude 99.6% of the atmosphere is below the aircraft, Kincheloe was named the ‘First of the Spacemen’. He later said that at the highest point, “Up sun the sky was blue-black in color and the sun appeared to be a very white spot. The sky conditions down sun, were even darker in color. This dark condition existed through the horizon where a dark gray band appeared very abruptly. This gray band lessened in intensity until eventually its appearance resembled that of a typical haze condition. Extremely clear visual observation of the ground within a 60 (degree) arc directly beneath the aircraft was noted.” As expected of a military test pilot, this report was factual and devoid of any emotional response. On three occasions Kincheloe tried to go higher, but each attempt ended in an abort. His altitude record (unofficial due to the use of a carrier plane) stood until the X-15 rocket plane surpassed it in August 1960.

The X-2 was scheduled to be transferred to NACA in mid-September, which was eager to start a series of missions to investigate aerodynamic heating and study the handling characteristics of the aircraft at extreme altitudes and speeds. However, the Air Force was keen to reach Mach 3, which was the next ‘magic number’ in aviation,

and managed to get an extension and check out another of its pilots, Captain Milburn ‘Mel’ G. Apt. While Apt practiced missions on the GEDA simulator, representatives from the Air Force, NACA and Bell agreed on a flight plan. It was clear the mission would involve a lot of risk, as understanding of the dynamics of a Mach 3 airplane was fairly sketchy in the 1950s. In fact, the limited aerodynamic data gathered from wind tunnel experiments for the X-2 was only valid up to Mach 2.4; what happened beyond that could at that time only be discovered by practical “cut-and-try”.

On 27 September 1956 all was ready to attempt the record flight. Thanks to the grease pencil line on his cockpit window, Apt flew an almost perfect profile of speed and altitude as a predefined function of time and became the first person to fly faster than thrice the speed of sound. The maximum speed attained was an incredible Mach 3.196; equivalent to 3,369 km per hour (2,094 miles per hour). Sadly, the excitement was very short lived. As he turned back towards Edwards, Apt for some reason made too sharp a turn and lost control due to inertia coupling; the problem first suffered by Yeager in the X-1A in 1953 and which may well have been avoided if the intended fly-by-wire flight control system had been implemented in the X-2. After a series of violent combinations of roll, pitch and yaw the aircraft entered a relatively smooth subsonic inverted spin, but Apt could not get it under control. During his attempts he never unlocked the rudder, which had been manually secured prior to accelerating to supersonic speeds in order to avoid dangerous shock waves forming over the vertical stabilizer. We will never know whether unlocking the rudder would have helped to escape from the spin. Realizing that he would not be able to gain control of the plane, Apt separated his escape capsule. Unfortunately he did not manage to get out of the capsule before it slammed into the desert floor (the problem that NACA had warned of when the system was accepted by the Air Force). Ironically, the X-2, now without its cockpit, stabilized itself and continued to descend in a series of undulating glides followed by stalls, before hitting the ground and coming apart.

The most spectacular achievement of the X-2 was therefore also its last, and Apt’s death cast a shadow over the program. It was a highly experimental and dangerous machine, a fact that was downplayed at the time in order to ensure continuing public support. However, the X-2 program had accomplished much of what it had set out to do: identifying the peculiarities of high-altitude flight and speeds exceeding Mach 2. Unfortunately, some of the lessons were learned the hard way. It was now clear that the safe operation of aircraft at very high speeds would require more sophisticated control systems, in particular incorporating so-called ‘stability augmentation’ since at Mach 3 things happen very quickly and a pilot receives little warning before inertia coupling causes loss of control. In fact, X-2 pilots found that above Mach 2.5 the safest thing to do was not to do anything at all, as any small steering correction could give rise to dangerous instabilities. One simple measure implemented during the X-2 flights was the already mentioned mechanical locking of the rudder at supersonic speeds. Everest even had a metal grab bar installed at the top of the instrument panel, on which he would place both of his hands at extreme speeds in order to force himself not to move the stick (a very difficult task for a pilot used to always being in active control of his plane). The extremely successful (and

much better known) X-15 rocket plane program benefited greatly from both the good and the bad experiences of the X-2.

The dangerous nature of their X research aircraft was pretty much downplayed by both the Air Force and Bell Aircraft. The documentary movie Flight into the Future released by the Department of Defense in 1956 duly explained how important and challenging the research work at Edwards was, but it failed to say anything about the risks and accidents, of which there had already been many. It showed pilot Everest kissing his wife goodbye in the morning and going to work just as if he were going to spend his time at a desk. No mention was made of the considerable risks that he was undertaking on a regular basis, and that his wife was probably wondering whether he would survive to have dinner with her that evening. Many test pilots at Edwards died paving the way for the future of aviation, flying various experimental and prototype rocket planes and jet aircraft. The movie included a routine test firing of the rocket engine of the X-2 with personnel standing literally alongside the nozzle, which was a risky thing to do because rocket engine’s were still not all that reliable (as an engine explosion during a ground test of the X-15 would later emphasize).

Not much of the X-2 has survived. The one that was dropped in Lake Ontario was never recovered. The one that crash-landed by itself near Edwards was salvaged, and some thought was given to reassembling the aircraft to continue the test program but this was rejected and the remains were buried (apparently nobody remembers where on the vast base). Souvenir hunters occasionally find bits and pieces at the crash site. A replica of the X-2 was constructed for the 1989 television series Quantum Leap, and it is currently being restored for display at the Planes of Fame Museum in Chino, California.

The X-2 also made it onto the big screen, first in 1956 in the movie Toward the Unknown (apparently a translation of the Latin motto of Edwards Air Force Base). It is a story about a daring test pilot trying to redeem himself after having succumbed to torture while a prisoner of war, and also win back the love of a girl. Other than using actual X-2 footage, the story has little to do with the real flight program. In 2000 the entertaining movie Space Cowboys featured a plane which appears to be a (computer generated) two-seat version of the X-2. In the prologue one of the pilots manages to rip a wing off the aircraft during a flight in 1958, after which both occupants (played by Clint Eastwood and Tommy Lee Jones) employ ejection seats to save themselves. So much for historical accuracy!

The illustrations show the maximum velocity and the maximum altitude that aircraft have achieved over the years, and as such they encapsulate much of the story told in this book.

Since 1939 the (unofficial) maximum velocity records have all been set by rocket propelled aircraft, with the trend being steeply exponential then concluding with the X-15 in 1967. Around the same time that the X-15 program ended, the maximum velocities attained by turbojet and ramjet aircraft also reached their limits. It will be possible to fly faster using airbreathing propulsion but it will require scramjets (work on experimental versions of which continues to this day). It is also interesting to note that velocities that were initially achieved by mixed-propulsion interceptors using jet

as well as rocket engines were soon surpassed by jet-power-only aircraft (rendering mixed propulsion obsolete by about the end of the 1950s).

The (unofficial) maximum altitude records have been exclusively the province of rocket aircraft since 1948, with the exponential trend once again culminating with the X-15. Turbojet/ramjet aircraft cannot fly at altitudes above 30 km (100,000 feet) for extended times and are only able to surpass this during short zoom climbs. Sustained airbreathing flight at higher altitudes will require scramjets.

Given the exponential growth of the maximum velocity and altitude achieved by aircraft over time, it is understandable that many people expected these trend Unes to continue into the 1970s and beyond with aircraft reaching orbital altitudes as well as orbital velocities within a decade or two. Of course the Space Shuttle actually did so in 1981 but it was a vertical take-off, rocket-launched space ghder rather than a true rocket plane. Real spaceplanes possessing rocket engines, sophisticated airbreathing engines or combinations of the two, have yet to progress beyond the drawing board.

SpaceShipOne managed to exceed the highest altitude achieved by the X-15 but got nowhere near that aircraft’s record velocity; it travels about as fast as the fastest airbreathing aircraft. But SpaceShipOne was the first aircraft in four decades to reach the edge of space.

Like the Americans the British and the French, the Russians also understood that the Germans had made great advances in the development of jet and rocket technology during the Second World War. And in spite of the fascist origin of that knowledge the Soviets were not too proud to use it. At the end of the war they had captured the unfinished prototype and wind tunnel models of the German DFS 346, the advanced experimental research plane with swept wings, a pressurized cockpit, and the HWK 109-509C rocket engine. The cigar-shaped fuselage with sleekly embedded rivets was optimized for high speeds and the T-tail had all-moving horizontal stabilizers placed high on the vertical fin to prevent shock stall and disturbances by the wings. To minimize the plane’s frontal cross section the pilot was prone on his stomach and viewed through a Plexiglas nose. The Germans had designed the DFS 346 to be air-launched from a bomber so that the maximum of 2 minutes at full-thrust would suffice to break the sound barrier at high altitude. The plane was to land on a retractable skid, saving considerable weight in comparison to a conventional undercarriage using wheels. For measuring the speed of the aircraft a long spike with a pitot tube projected ahead from the nose. Now standard equipment on any aircraft, this tube measured the relative air pressure, which is a function of the velocity of an aircraft through the air. Poking this pitot tube out in front of the plane ensured that its measurements were not affected by airflow disturbances closer to the fuselage. At least as important as capturing hardware was the recruitment of many of the German engineers who had developed this revolutionary plane, by offering them privileges such as additional food rations as well as the opportunity to continue their research (apparently Stalin had finally understood that positive motivation resulted in more progress than brute force when it came to developing complex technology).

The Soviets planned to use the DFS 346 in order to gain a head start in the Cold War competition for speed and altitude, and therefore converted the German Siebel Flugzeugwerke company, which during the war had been tasked with developing the DFS 346, into the OKB-2 design bureau under the direction of the German engineer Hans Rossing. Soon the factory and its staff were moved from the original location in Germany to Russia, where the team continued their work on the DFS 346. Aleksandr Bereznyak, one of the original designers of Russia’s wartime BI rocket interceptor, was assigned to assist (and no doubt keep an eye on) Rossing. In order to disguise the German origin of the design, the project was renamed ‘Samolyot 346’ (Aircraft 346), and the Russian form of the German engine was designated ZhRD – 109-510.

Wind tunnel tests showed that at high angles of attack and low speeds the angle of the leading edge of the 346’s wing forced some air to flow sideways out towards the

wingtips instead of parallel to the fuselage. At the wing tips the airflow could end up flowing almost completely span-wise, sharply reducing the lift and resulting in a stall on the outer part of the wing and a loss of control of the aircraft. The solution was to add two so-called wing fences, low vertical ridges running from the leading edge to the back of the wing. This solution was later incorporated in most Soviet swept wing fighters of the 1950s and 1960s.

In 1947 the first prototype was completed. Since it had no engine installed it was designated 346P (for ‘Planer’, meaning glider). This version was meant to test flight stability, practice landings, and also try out the mating to and release from the carrier aircraft. It lacked a pressurized cockpit, propellant tanks and other propulsion-related equipment. In 1948 four test flights were carried out with the 346P being dropped from under the right wing of a confiscated American B-29 bomber that had suffered damage during a raid over Japan and then gone on to make an emergency landing in Soviet territory. Interestingly, during these tests the 346P was piloted by Wolfgang Ziese, who had previously been a test pilot for the Siebel company in Germany. In Russia he had prepared for the flights using a modified DFS ‘Kranich’ (Crane) glider that had been fitted with a prone-pilot cockpit and could be towed into the air behind a Petlyakov Pe-2 bomber.

Flying the 346 into unknown areas of aerodynamics, virtually encased in the tiny aircraft in an uncomfortable prone position and having to rely upon its complicated escape system, must have taken a lot of courage. Especially since at that time some aerodynamicists predicted that at Mach 1 an aircraft would slam into a virtual wall of air and inevitably be ripped apart by violent shock waves. The successful breaking of the sound barrier in the US by the Bell X-l leaked by Aviation Week in December 1947 did tell the Soviets that faster-than-sound flight was possible, but exactly what kind of phenomena they would encounter in the 346 was still unknown; naturally, the Americans kept the X-l flight data secret.

On one flight, Ziese forgot to check that the ailerons were in their neutral position before his aircraft was released by the B-29 carrier, so the 346P immediately flipped inverted. Only after losing almost 2,000 meters (6,600 feet) of altitude did he manage

to regain control of the plane. On the whole however, the 346P drops, gliding flights and landings went very well, and it was decided to proceed with the construction of a powered prototype. This 346-1 was completed in May 1949, and had a launch weight of 3,145 kg (6,935 pounds).

On 30 September 1948 the B-29 drops Ziese in the 346-1 equipped with a dummy engine from an altitude of 9.7 km (32,000 feet). He experiences some difficulties in controlling the aircraft and is obliged to land at an excessive speed (the fact that the aircraft does not have flaps for additional lift at low speeds exacerbates the problem). After the first hard touchdown the plane bounces several meters into the air, flies a further 700 or 800 meters (2,300 or 2,600 feet) across the ground, then touches down again. At that moment the ski is pushed back into the fuselage and the plane slides along the runway on its belly prior to coming to a standstill. It is slightly damaged, and the pilot is knocked unconscious but only lightly injured when his head hits the front of the cabin (apparently his seat and safety belt system were not up to the rough landing). Investigators conclude that Ziese had not fully released the skid during his approach, probably because he was fully occupied keeping the aircraft under control.

After repairs and improvements, the plane is redesignated 346-2 and glide flight testing resumes in October 1950 with Russian pilot P. Kazmin. The plane still proves tricky to fly, and on the first flight the skid once again fails to lock when lowered for landing. However, this time the landing takes place on a snow covered field and the belly-sliding does not cause any significant damage. On its second flight the 346-2 is towed by a Tu-2 bomber to an altitude of 2 km (6,600 feet) and released for a free gliding flight. This time Kazmin lands short of the runway. The aircraft is damaged and more repairs are needed. Meanwhile Ziese has recovered from his injuries and, starting on 10 May 1951, resumes flying the engineless 346-2, and starting on 6 June also the newly constructed but still unpowered 346-3 which has thinner wings better suited to transonic flight speeds. During the 346-3 flight tests the confiscated B-29 is replaced by a Soviet copy designated the Tupolev Tu-4 (reputedly copied so literally that rivets missing from the original were omitted).

Finally Ziese and the aircraft are judged to be ready for a powered flight, and on 15 August 1951 the 346-3 is driven through the air on rocket power for the first time. For around 90 seconds Ziese is the ruler of the sky. But the flight is no treat because the plane still has a tendency to roll. And due to a malfunctioning heating regulator the temperature in the cockpit rises to 40 degrees Celsius (104 degrees Fahrenheit), all but making the pilot faint. During this mission, as well as the following flight on 2 September, only the weaker cruise chamber of the engine is used in order to hold the speed below Mach 0.9 because tests in the T-106, the Soviet’s first supersonic wind tunnel have led the designers to fear that the aircraft’s control surfaces will freeze up at transonic speeds. And their fears are soon proven well-founded. On 14 September Ziese is dropped for the third low-thrust flight, ignites the smaller thrust chamber and accelerates into a climb. Shortly thereafter things go wrong at an altitude of just over 12 km (39,000 feet). Ziese reports to the ground that the aircraft is not responding to his control inputs, is rolling uncontrollably and rapidly losing altitude. Evidently the rocket thrust has pushed the plane into the transonic ‘no-go’ zone, resulting in locked control surfaces. On falling to a lower altitude Ziese manages to regain some control and ends up in a dive from which he pulls up at about 7 km (23,000 feet). When the airplane starts to roll wildly once again, Ziese realizes that he is running out of time and altitude. The controllers on the ground tell him to bail out. For the first time he triggers the explosive bolts to separate the cockpit section from the rest of the plane. The system works perfectly. The stabilizing parachute puts the cockpit into a smooth descent, enabling him to scramble out and land safely under his own parachute. The aircraft is obviously lost, along with all the flight measurements recorded and stored inside (there was no real time telemetry link with the ground, as is standard for test flights today). Nevertheless the limited data available enables investigators to figure out what probably happened. It is concluded that when it shot up into thinner air the aircraft entered the transonic flight regime and experienced shock stall at its tailplane and wings, freezing up its controls. Once the plane started to fall it accelerated out of the transonic area and exceeded Mach 1, at which moment the shock waves at the tail moved further to the rear, releasing the elevators. And when Ziese pulled out of the dive the aircraft slowed down and again entered the transonic regime, freezing up its controls once more.

It was clear that the 346 was not well suited to transonic speeds, and the aircraft shape’s aerodynamic speed limit had been achieved even without igniting the rocket engine’s more powerful main combustion chamber. The 346 project was abandoned. In any case, not much valuable data was expected to be gained from further flights because by the late 1940s Soviet jet aircraft were already flying faster than Mach 1. One ‘glass half full’ project report stated that within the speed limits imposed by the obsolete aerodynamic design all the 346-3 hardware had functioned well, including the rocket engine, the skid landing gear, and finally the escape capsule. The German engineers involved in the 346 project were repatriated to East Germany in 1953 (this was apparently standard procedure once Russian engineers felt that they had learned everything they could from their German colleagues.)

In parallel with OKB-2 and its 346 project, OKB-256 under Pavel Vladimirovich Tsybin was working on a transonic rocket plane called the Tsybin LL (with the LL standing for ‘Letayushchaya Laboratoriya’, which means Flying Laboratory). Even though this aircraft was kept very simple in terms of construction and propulsion, it was meant to approach Mach 1 and if possible surpass it. After models were tested in the TsAGI wind tunnels, two prototypes were constructed. They were made almost entirely of wood, with ailerons and flaps operated by a pneumatic system powered by compressed air (the forces on the control surfaces were expected to be very high at transonic speed, and so require more than pilot muscle power to operate). The rocket engine in the tail was a straightforward solid propellant booster called the PRD – 1500, and it could provide an average of 15,000 Newton for a duration of 10 seconds. The first prototype, LL-1, had conventional straight wings and an ejectable dolly take-off undercarriage similar to that of the Me 163. From mid-1947 pilots M. Ivanov, Amet-Khan Sultan, S. Anokhine and N. Rybko together completed a total of 30 flights with this prototype. After being towed by a Tu-2 bomber to an altitude of 5 to 7 km (16 to 23,000 feet) the pilot pushed it into a steep dive of 45 to 60 degrees in order to gain as much speed as possible prior to leveling off and igniting the rocket

motor. Then a very short, horizontal high-speed powered flight was followed by a gliding return to land on a retractable skid.

During the winter of 1947-1948 the second prototype was equipped with forward – swept metal wings, the benefits of which the Russians had learned of from German wartime research, and which had also initially been planned to be incorporated in the previously described Lavochkin 162. Water tanks were installed in the fuselage so as to be able to adjust the center of gravity of the aircraft. This was designated the LL – 3. It made over 100 flights and achieved a maximum speed of 1,200 km per hour (750 miles per hour), corresponding to Mach 0.97, without any significant problems. After the LL-3 tests, the LL-1 was turned into the LL-2 by retrofitting it with swept wings, but it never flew because by then swept-winged jet fighter prototypes had already undergone extensive testing.

A more ambitious project was the Bisnovat 5 developed by aircraft manufacturer Matus Ruvimovich Bisnovat. This was intended to continue where the Samolyot 346 project had ended, providing data on transonic and low-supersonic flight speeds up

to 1,200 km per hour (750 miles per hour) at an altitude of 12 km (39,000 feet), which was Mach 1.1. Bisnovat had prior experience of rocket planes because he had been responsible for the production of the Kostikov 302 prototypes by OKB-55 during the war and later had been involved in a number of missile projects. Similar to the DFS 346-based Samolyot 346, the Bisnovat 5 was an all-metal monoplane that had wings swept back at 45 degrees and augmented by fences, and a pressurized cockpit. It was also to be dropped from a carrier aircraft, in this case from under the right wing of a Petlyakov Pe-8, and then land using a simple ski undercarriage. The main ski under the fuselage was set at an angle to enable the aircraft to land with its nose slightly up to ensure sufficient low-speed lift for a soft impact. UnUke the uncomfortable prone-pilot position and complicated escape capsule of the 346, the pilot had a conventional ejection seat and sat upright, although slightly reclined in order to reduce the plane’s cross section. A single Dushkin-Glushko RD-2M-3VF dual-chamber rocket engine was installed in the tail and fed nitric acid and kerosene propellants by a turbopump powered by hydrogen peroxide. This engine was similar to those of the Florov 4303, Kostikov 302P, the Polikarpov Malyutka and the MiG 1-207 but the combined thrust chambers provided a maximum thrust of 16,500 instead of 15,000 Newton at sea level.

Models were tested in the TsAGI T-104 wind tunnel at up to Mach 1.45 and then one-third-scale models that were powered by small liquid propellant rocket engines were launched from carrier aircraft. After these tests had validated the aerodynamics of the new design, the first flight prototype was constructed and prepared for gliding flights. The first flight of this ‘5-1’ aircraft on 14 July 1948 almost ended in disaster when it hit the Pe-8 carrier shortly after being released. But test pilot A. K. Pakhomov managed to keep the 5-1 under control and made an emergency landing in a rough field. This incident severely damaged the prototype but it was repaired,

and the pylon under the wing of the Pe-8 was revised to carry the Bisnovat 5 with its nose pointing slightly downward to reduce the risk of the aircraft flying up and hitting the carrier after the drop. The next glide flight showed that the aircraft had poor roll and yaw stability. This problem had not yet been resolved when the third flight was made on 5 September 1948 and caused the plane to land tilted to one side, hit the ground with a wingtip and topple over. The plane was almost broken in two and beyond repair, but Pakhomov was okay.

The ‘5-2’ prototype was modified based on the lessons learned during the gliding tests with the 5-1. The vertical tail was swept back further aft to improve directional stability, and the simple metal wingtip bows were replaced by shock-absorbing skids better suited to dampening the impact of touchdown. The test campaign was resumed on 26 January 1949 with pilot Georgi Shiyanov taking the 5-2 on its first glide flight. Again the mission ended in a hard landing with severe damage to the aircraft, this time because the pilot had difficulty in finding the proper approach to the rather short runway and therefore came down beyond it. The 5-2 was repaired and further improvements made. The main ski, which had previously been set at an angle in the vertical direction for improved lift prior to landing, was now put horizontal and thus parallel to the fuselage to improve the pilot’s view of the runway. This meant that the small ski on the tail could be removed and replaced with a ventral keel fin to further improve flight stability. No major problems occurred during the next flight but roll and yaw stability were still insufficient. This led the engineers to install downwards angled fins at the wingtips like those of the Florov 4302. The next six ghde flights showed that the stabihty had improved, and that the plane was controllable at least up to the highest speed of Mach 0.77 that was attained in a dive.

But before the powered flight test campaign could commence, the authorities had shifted their interest to further developing supersonic jet aircraft. On 26 December 1948 test pilot I. E. Fedorov had opened the throttle on his swept-wing Lavochkin La-176 (derived from the La-168 jet fighter), pushed the plane into a shallow dive and attained Mach 1.0, marking the Soviet Union’s entry into the world of supersonic flight (just over a year after Chuck Yeager made his historic flight in the X-l). Hence the authorities did not see much use for a Mach 1 rocket aircraft.

There was never a Russian equivalent to the American Douglas D-558-2 and Bell X-l series of experimental rocket planes, and no subsequent evolution into a vehicle Uke the X-15. The Samolyot 346 flew until September 1951 but never managed to exceed Mach 1 and (as noted above) this project was also terminated after the loss of the 346-3 aircraft.

Nevertheless Soviets engineers continued to develop many supersonic aircraft that were as good as anything in the West, and during the Cold War proved themselves to be masters of aerodynamic theory and design. It is however clear that Soviet spies in the US aviation industry and NACA provided data that was of great assistance to the Russian designers, and at least partly made up for their lack of a supersonic research aircraft program.

Von Braun became fully occupied with the development of the A4/V2 missile at the Army area of the Peenemunde center, but retained his interest in rocket planes. As with the A4, he again tried to sell impractical designs to those who didn’t need them: in July 1939 he proposed to develop a rocket powered interceptor for the RLM. The first design had a cigar-shaped fuselage and straight, tapered wings. His trademark propellant combination of alcohol and liquid oxygen was stored in tanks behind the cockpit. The rocket engine was installed in the tail, and just as with his A3 and A4

rockets he placed four rudder-like jet vanes behind the nozzle to divert the exhaust jet and steer the vehicle by thrust vector control. Tilting the two opposing horizontal vanes up or down in the same direction would make the plane pitch, while the two vertical vanes would control the yaw. Rolling could be achieved by tilting opposite vanes in different directions. The pilot was to be seated in a pressurized cockpit that would be able to maintain a comfortable air pressure at high altitudes, and he would be protected from enemy bullets by armor plating. The vehicle would be armed with either two or four cannon mounted in the wing roots.

Whilst the plane was to land normally, von Braun designed his interceptor to take off vertically. That way the rocket plane could be launched straight up to the target and reach it in minimal time. A simple undercarriage would only require to be able to handle the empty weight of the vehicle at the end of the flight. The airplane was basically a rocket with wings. Yon Braun envisaged large numbers of his planes would be stored vertically in a hangar/launch facility, hanging on the tips of their wings on two rails. When the air raid alarm rang, pilots would quickly board their interceptors via a removable bridge, then the plane and pilot would be rolled out of the building and launched straight off the rails. For the first minute or so, the plane would be remotely controlled from the ground and steered to the target by the help of radar (as was done with conventional air defense fighters). Then the pilot was to take manual control, switch off the main engine and start a smaller rocket motor that would enable the plane to engage enemy aircraft at sufficient speed while using its remaining propellant at a much lower rate. Spewing out rocket planes like a giant candy machine, a strategically placed launch facility would thus be able to quickly swarm enemy bomber formations with heavily armed interceptors. After

completing his attack, the pilot was to glide his plane back to land on a grass field using a built-in skid.

However, the RLM considered von Braun’s concept too impractical owing to the need for liquid oxygen, which was difficult to produce and store, and the specialized launch facilities that had to be constructed and maintained. Such facilities could also be easily identified by the enemy and destroyed by precision bombing. Several years later this fear was shown to be well founded when the elaborate bunkers constructed at the French coast to launch VI and V2 missiles against England were destroyed by bombers, often before they became operational. The reluctance to use liquid oxygen in an operational military rocket system was also valid, as shown near the end of the war by the difficulties experienced in providing the mobile V2 launch systems with this propellant because of the bombing of the production facilities and transporta­tion networks. Another reason that the RLM did not buy von Braun’s proposal was that Germany expected to quickly win the upcoming war using its existing conventional weapons; in 1939 the prospect of large enemy bomber formations venturing far into Germany was not considered to be realistic. Unlike the other objections, however, this particular evaluation would soon be proven incorrect.

Von Braun reacted to the objections by producing a second version of his Vertical Take-Off (VTO) interceptor design. He switched to Visol and SV-Stoff as the rocket propellants because these are easier to store for lengthy periods and are hypergolic, meaning that they automatically ignite upon contact and thus do not need a separate ignition system, as does a hydrogen/oxygen rocket motor. SV-Stoff was mostly nitric acid, which is a very nasty substance; not something a pilot should feel comfortable sitting close to, and especially not in a combat aircraft whose tanks are quite Ukely to be punctured by enemy bullets, but, as we shall see, this was not a major concern in German rocket powered fighter design. Otherwise the new VTO plane was similar to its predecessor, with the vertical tail being a bit smaller and the wings now dihedral for improved flight stability.

Von Braun tackled the RLM’s objections to the need for large ground facilities by proposing to launch his updated design from a mobile system based on a truck which hauled a trailer. These would first be used to transport the plane to wherever it would be needed. Once at the launch location, the truck and trailer would each be outfitted with a sort of tower structure and placed one wingspan apart from each other. A crane would hoist the rocket plane vertically between the two, and rest each of its wingtips on one of the support towers. A small flame deflector would be positioned beneath the rocket nozzle to avoid it burning up the ground or damaging the nearby equipment. In spite of the updated design, von Braun’s VTO interceptor project was rejected by the RLM in 1941 because at that time the war was progressing well for Germany, with its forces continuously on the attack. Expecting the offensive war to finish soon, they saw no need for an interceptor which, because of its very limited range, was only suitable for local defense against intruding enemy planes that were in any case never expected to reach Germany in large numbers.

Undaunted, Von Braun retained his interest in rocket planes, and near the end of the war did launch two A4 rockets fitted with large swept-back wings. The military rationale was to develop a ‘boost-glide’ missile capable of reaching London when

Original drawing showing the launch configuration of von Braun’s updated rocket interceptor.

launched from inland, because at that time Germany was rapidly losing the coastal territory from which it had been launching its A4/Y2 rockets. On 8 January 1945 a winged A4b left launch complex P7 at Peenemtinde but failed in flight. The second attempt on the 24th was more successful: it reached an altitude of 80 km (260,000 feet) and then briefly performed a supersonic glide using its two swept-back wings until one of them broke off. The increasingly chaotic situation in Germany near the end of the war prevented any further flight tests.

The A4b launches were part of a plan to develop an A9/A10 two-stage rocket to attack the United States. This intercontinental ballistic missile was to have a winged, piloted upper stage (resembling the later X-15) to undertake an extended glide phase and accurate aiming. Once the A10 booster was jettisoned, the pilot/astronaut would steer the A9 to its target with the aid of radio positioning guidance from a network
of U-boats along the flight path across the Atlantic. Once confirmed to be on course, the pilot was to use his ejection seat and land by parachute near an awaiting submarine if he was lucky.

Furthermore, Von Braun was planning the A6, which was basically a winged A4 with a pressurized cockpit instead of a warhead, plus landing gear and an auxiliary ramjet engine for continuing flight at extreme speed and altitude after the propellant for the main rocket engine was consumed. It would be launched vertically but land horizontally after gliding down to an airfield. To get funding for developing the A6, which von Braun saw as precursor to a real spaceplane, he offered it to the German military as a photographic reconnaissance aircraft. With an expected top speed of 2,900 km per hour (1,800 miles per hour) and a maximum altitude of 95 km (310,000 feet) he reckoned it would be impossible to intercept. But the Army did not see any urgent need for such an advanced, complicated and expensive machine, and it was rejected.

Von Braun’s original concept for a vertically launched interceptor was also kept alive by Erich Bachem, at that time technical manager of the Fieseler aircraft plant. He proposed two designs for a Fieseler VTO rocket aircraft named the Fi 166-1 high – altitude fighter. It initially involved a modified Messerschmitt Bf 109 from which the propeller and piston engine would be removed and replaced by an aerodynamic nose cover. It was to be launched with its aft belly affixed to a rocket stage with the same

250,0 Newton engine as von Braun’s large A4 rocket, then under development at Peenemiinde. Some sources say the engine of the smaller A5 rocket was to be used, but its 15,000 Newton thrust would not have been capable of lifting the engineless, empty Bf 109 of about 1,500 kg (3,300 pounds) together with a loaded rocket stage. At about 12 km (39,400 ft) the spent rocket would be discarded and parachute back down to be recovered and reused, while the engineless plane would attack enemy bombers during a gliding descent. A modification of this initial concept replaced the Bf 109 with a new, Bachem-designed aircraft which had two Jumo 004 jet engines installed beneath its wings to give the plane an extended flight capability. The RLM deemed the idea impractical. Undeterred, Bachem drafted a plan for a Fi 166-11. He deleted the rocket stage and designed the new two-seat aircraft (which looked very similar to Von Braun’s VTO interceptor but was considerably larger) for a vertical take-off under its own rocket power. As before, the RLM was not convinced of the feasibility and the necessity for such a weapon. When the situation changed later in the war, Bachem revived the idea and developed the much smaller BP-349 ‘Natter’ (discussed later in this chapter). It is also interesting that the concept of launching a plane vertically on top of a large liquid propellant rocket stage would much later be revisited many times for launching winged vehicles into orbit, and is of course the basic concept behind the Space Shuttle.

Whilst conducting the He 112 rocket plane tests at Neuhardenberg, Heinkel and the RLM decide to continue the development of a rocket plane interceptor. A secret

department at the Heinkel factory at Rostock-Marienehe is established to pursue this work. Whereas the rocket propelled He 112s were modifications of an existing type of airplane, the new machine is developed from the start as a true rocket plane. It is called the Heinkel He 176. Until recently it was unclear what the original prototype, the He 176 Yl, looked like, and many books and websites include drawings of the proposed operational successor rather than the actual flying prototype (the fact that this improved version was also designated He 176 obviously caused the confusion). A recently discovered picture of the He 176 VI indicates a configuration optimized for high speed flights: a tiny plane with a bullet-shaped fuselage and extremely thin, razor sharp wings in order to minimize aerodynamic drag. The cockpit is completely enclosed within the fuselage, with a flush upper glazing that can be removed for the pilot to gain entry to the plane. The picture shows two retractable main wheels and a fixed nose wheel that was fitted only for the initial taxi tests; for flights the plane was to land using the two main wheels and its tail.

The He 176 Yl has a Walter HWK RI-203 engine that uses the decomposition of hydrogen peroxide, as with the Walter engine for the He 112, but it is more powerful because the propellant is pumped into the engine rather than being pushed in (with a lesser force) by compressed air. Its maximum thrust is about 6,000 Newton, double that of the engine of the He 112. A second version of the design, the He 176 V2, will use an even more powerful von Braun engine to achieve the objective of a speed of

1,0 km per hour (620 miles per hour). To break speed records, the high thrust of the engine will be combined with a very lightweight fuselage and wing structure. In order to minimize the size of the cockpit, it is tailored closely around Erich Warsitz, the designated test pilot. It is so cramped that he can’t even bend his elbows, and the controls that are to be operated by a particular hand have to be put on the opposite sides of the cockpit! To increase the stability of the plane, the wings have a positive dihedral. The design is quite a step in technology. The propellant tanks for the 82% hydrogen peroxide, for instance, are integrated into the thin elliptical wings and thus require to be welded by using a new process. In order to be able to handle the high accelerations, and also to minimize the frontal area of the cockpit and thus air drag, the pilot adopts an unconventional reclined position. There is no canopy bubble, so the entire nose section is made of Plexiglas for the requisite visibility. At the high speeds the He 176 is to fly, even the smallest movement of the aerodynamic control surfaces will have a big effect (because the generated lift forces are a function of the square of the velocity of the air flow) so these surfaces are kept small. However, at take-off and landing the pilot will have to make large movements with the stick and rudder pedals to produce some steering effect from the small rudder, elevators and ailerons. The sensitivity of these controls had therefore to be adjusted by the pilot to achieve sufficient control at all speeds. With a wingspan of 5 meters (16 feet) and a fuselage length of 5.5 meters (18 feet), the He 176 is very small: it would fit inside a modest living room. Looking at the picture of the tiny plane, you have to admire the bravery of Warsitz for volunteering to fly something so experimental which had such a dangerous engine in such a small package.

If anything were to go wrong in flight, even baihng out was going to be a novel experience. Jumping out of the cockpit in the traditional manner was expected to be extremely difficult at high speeds, if not impossible because the force of the air drag would be strong enough to rip the pilot’s head off. Therefore the whole cockpit and nose formed a separate section that could be ejected from the rest of the plane by compressed air. A braking parachute would then slow it down sufficiently to enable the pilot to get out and land using his own parachute. Wooden mockups of the nose section with a dummy pilot inside (with weight distribution and body measurements reflecting those of Warsitz) were dropped from an He 111 bomber, and established that Warsitz would probably survive a parachute landing inside the cockpit if he did not manage to get out, and even without serious injury if he were lucky enough to set down on soft soil.

The first tests are performed by placing the actual prototype inside the huge wind tunnel of the Gottingen Test Institute. Once complete, the prototype is moved to the Luftwaffe area of Peenemiinde which offers more secrecy than the Heinkel factory. Taxi trials in which the He 176 prototype is towed at speeds up to 155 km per hour (96 miles per hour) behind a 7.6 liter Mercedes car prove to be pretty useless, as the velocity is too low for the small rudder to become effective. Taxi runs are therefore continued on the plane’s own rocket thrust, but all too often the wings hit the ground on the uneven grass airfield. Metal bumpers are therefore installed on the wingtips to prevent them from being damaged; something that can also be seen on the picture of the nose-wheeled He 176 VI. The tests show that the rudder only starts to be useful near the He 176’s take-off speed, making it necessary to steer using the wheel brakes for most of the take-off run. As this costs a lot of energy and makes it difficult for the plane to get up to the desired speed, a rudder is installed in the engine’s nozzle (this solution had also been implemented in the He 112 fitted with the Walter engine, for the same reason).

The first short rocket propelled hops into the air are made in March 1939 with very limited amounts of propellant in the aircraft for safety reasons. Over a hundred of these test runs are performed, with the plane getting no higher than 20 meters (70 feet) over a distance of about 100 meters (330 feet). Modifications are made to the

plane. New concrete runways are built. In May 1939 a demo-hop is made for RLM officials including Ernst Udet, head of technical development for the Luftwaffe, and Erhard Milch, head of the RLM. The demonstration does not have the effect the He 176 team expects. Quite the opposite: Udet deems the plane to be too unstable, too small and too dangerous, and he grounds it! Nevertheless Warsitz talks the visitors into allowing the team to conduct the test flights. On 20 June the team prepares the He 176 for its real maiden flight. To prevent anyone from blocking the attempt, and to limit the repercussions of a failure, no officials are invited or notified – not even Heinkel; Warsitz assumes full responsibility for the historic flight. After take-off he quickly achieves a speed of 750 km per hour (470 miles per hour), then he makes a steep ascent and continues to fly a circuit at 800 km per hour (500 miles per hour): faster than any previous plane. After the 1 minute’s worth of propellant is consumed, he glides back and makes a safe landing. Apart from the expected sensitivity to the controls, the He 176 proves to be a fine flying machine. News about the successful flight quickly gets out and the next day Warsitz performs a demonstration flight for Heinkel, Udet and miscellaneous other officials. On 3 July even Adolf Hitler and Hermann Goring, chief of the Luftwaffe, watch in amazement as it flies at a special air show of new Luftwaffe planes at Roggentin airfield. Coming in to land, Warsitz shuts off the engine too soon and almost flies into a brick wall; a last-second restart of the engine makes the plane suddenly rise some 50 meters and hop over the wall prior to landing safely. Most of the spectators think this spectacular maneuver is a part of the demonstration. At the same show, an impressive demonstration is made of the Walter take-off assist rocket pod with a pair of He 111 bombers, one with two 4,900 Newton thrust Walter RI-200 rockets and the other without. After starting at the same moment, by the time the standard He 111 leaves the ground the assisted one is already boosted to 200 meters (660 feet) altitude by the powerful Walter engines! The Walter rocket pods, which are dropped after burn out, deploy parachutes and are recovered for reuse, soon become standard equipment in the Luftwaffe’s bomber squadrons in the form of the RI-202.

The more powerful He 176 V2 fitted with a von Braun engine is never built; on 12 September 1939 Hitler issues an order to halt all development work on weapons that cannot be made operational within one year, which is the time he expected Germany would to need to successfully conclude the recently started war. The He 176 V2 was to have had a rocket thrust exceeding the weight of the plane so that it could lift off vertically. It might even have been able to attain the magic number of 1,000 km per hour (620 miles per hour). Hitler’s order also ended the test flights of the He 178, the world’s first jet plane that was also flown by Warsitz. The He 176 VI was put into a sealed container and sent to the Aviation Museum in BerUn to be displayed after the war but it was destroyed by an air raid in 1943. Sadly, no pictures or movies of the historic He 176 flights are available; according to Warsitz the Soviets obtained all the documentation when they captured Peenemunde at the end of the war and they kept everything secret.