Category Rocketing into the Future

As a Dutch kid growing up in the early 1980s I devoured the ‘Euro 5’ science fiction series by Bert Benson. They were typical boy’s adventures: in each book a secret team of European policemen had some 200 pages to track down a menagerie of rampaging robots, mutant criminals and murderous scientists hell-bent on terrorizing Earth and the solar system. The bad guys were usually seeking world domination, which they inevitably intended to obtain via some overly complicated but fascinating scheme. As required by the genre, the good guys always managed to arrest the interplanetary villains before they could bring their devious schemes to fruition. Just in the nick of time of course. It was ideal literature for a certain somewhat nerdy would-be aerospace engineer. The books were all written in Dutch, and much later I found out that writer Bert Benson’s real name was Adrianus Petrus Maria de Beer, which sounds about as futuristic in Dutch as is does in English. In spite of this, the stories breathed a kind of cosmopolitan atmosphere, with a diverse team of agents from various European countries (the leader was Dutch, naturally) flying to exotic countries, forgotten islands and hostile moons and planets. Their means of transportation was the Euro 5, a wedge­shaped rocket spaceplane with a set of large wings at the back and smaller ‘canard’ wings in front, four rotating ray-guns, and a small boat-shaped plane for short reconnaissance trips (which I now know looks a lot like NASA’s M2-F3 ‘lifting body’ experimental rocket plane of the early 1970s). In the final pages of each book, it was usually this marvelous machine that saved the day, if not the entire universe. For me this gigantic blue vehicle was really the centerpiece of the stories, rather than the colorful team of international heroes. I guess kids in other countries at the time were reading similar adventure books with rocket planes in a starring role.

To me, part of the appeal of the Euro 5 books was that the idea of a spacefaring rocket plane did not seem to be very far-fetched. After all, when I was reading them the Space Shuttle had just entered service and promised to make all those boringly tube-shaped, expendable rockets obsolete. In 1976, NASA predicted that it would be launching around seventy-five Space Shuttles per year; more than six per month. Because most of the system’s hardware could be reused, launch prices would be lower than for the old-fashioned single-use rockets. The low cost and flexibility of the Shuttle would even make it economically feasible to repair satellites or bring

them back to Earth for major refurbishments. Companies would be able to use the Shuttle to build microgravity factories in orbit, and scientists would be able to fly all manner of experiments and instruments. The Shuttle was going to be everything for everybody, a kind of real-life, American Euro 5. And that would be just the first step in reusable launch vehicle development. Many space experts, members of the public, and for sure a certain twelve-year-old, expected that real rocket planes would soon follow, taking off from normal airports, operating more or less like normal aircraft and flying ordinary people (and even Dutch teenagers) into space.

Like many childhood fantasies, that did not come true. The Space Shuttle proved to be a very complex, extremely expensive and dangerous launch vehicle. Instead of taking off like a plane, a Shuttle lift off is a major event with an enormous checklist of things that can easily postpone the launch if not working perfectly. If you headed to Kennedy Space Center to watch a Shuttle lift off, you could count yourself very lucky if it got off on time. You ran the risk of spending many days at nearby Cocoa Beach, watching a continuous parade of launch delay messages on NASA TV and eating lunches that would likely ground you for being overweight if you were an astronaut. Instead of witnessing two launches within a week, as had been predicted by NASA’s optimistic forecasts of the early 1980s, you were more likely to return home for work and other commitments without having seen even one (ruining yet another childhood dream).

The huge numbers of flights per annum did not materialize mainly because the Shuttle system took much more time than anticipated to prepare for each flight, and because NASA had been extremely optimistic in the number of satellite launches and other missions the Shuttle would be required to perform. Although the Space Shuttle performed some spectacular missions, like repairing the Hubble Space Telescope in orbit, retrieving malfunctioning satellites and putting a large space station up module by module, it did not grow into the cheap, regularly flying ‘space truck’ envisioned in the early 1980s. In fact, during its thirty-year operational lifetime it was the most costly way to launch anything or anyone into space, even without taking into account the costs of new safety measures introduced in response to the loss of Challenger in 1986 and Columbia in 2003.

In his 1986 State of the Union address, only five years after the Shuttle’s debut, President Ronald Reagan already called for a successor, “.. .a new Orient Express that could, by the end of the next decade, take off from Dulles Airport, accelerate up to 25 times the speed of sound, attaining low earth orbit or flying to Tokyo within two hours”. The design for the X-30 experimental precursor of this National Aerospace Plane (NASP) initially looked very much like the famous Concorde supersonic airliner: delta wings on a long and slender fuselage with a pointy nose. However, while Concorde’s maximum speed record was 2,330 km per hour (1,450 miles per hour, 2.2 times the speed of sound), the X-30 would have to accelerate to an orbital velocity almost 14 times faster. Unlike the Space Shuttle, it would not shed rocket stages and propellant tanks on the way up, but would be a true single-stage – to-orbit, fully reusable spaceplane. It was also to be much safer to fly than the Shuttle, which only a few days prior to Reagan’s speech had experienced its first disaster, resulting in the loss of the Challenger Orbiter, killing seven astronauts and throwing the entire program into complete disarray.

Although the X-30 was initially envisioned to use only (exotic and very complex) airbreathing engines, meaning it wouldn’t really be a rocket aircraft, its development would be founded on a long history of rocket plane and mixed-propulsion jet/rocket aircraft experiments. Prior to the advent of rehable, powerful jet engines, rockets enabled revolutionary Second World War fighters to climb quickly and intercept high-altitude bombers. Soon after the war, rocket planes were the first to break the dreaded ‘sound barrier’ and then the lesser-known ‘heat barrier’, establishing them not to be real barriers at all. Then they became the first (and only) aircraft to reach the edge of space, breaking record after record by flying faster and higher than any airbreathing type of aircraft. And large rocket boosters were used to shoot aircraft straight into the air without a runway. Step by step, often involving considerable danger and consequently some disasters, engineers and pilots thus learned how to design and fly vehicles that were part airplane, part missile and part spacecraft. In fact, up until the early 1960s many people expected the rocket plane rather than the expendable ballistic missile to represent the near future of manned spaceflight. The vehicle envisioned by president Reagan therefore appeared to be long overdue.

However, in spite of the impressive aeronautical and spaceflight heritage and optimistic expectations, only a couple of years after Reagan’s speech it became clear that the X-30 as well as its contemporary European and Japanese competitors (the German Sanger-II, the British HOTOL and the Japanese JSSTO), were technically too ambitious and would be extremely expensive to develop. By the mid-1990s all these spaceplane projects had been killed off by alarmingly rising costs and ever mounting technical difficulties.

It now seems that the concept of the reusable spaceplane has reached a kind of dead end, due to a lack of technology breakthroughs, political will and economic rationale. Work on spaceplanes is still ongoing, but at a rather slow pace and at a relatively modest level, and generally with a focus on hypersonic military missiles rather than orbital crewed launch vehicles. The Space Shuttle was retired in 2011, and its currently planned successors are old-fashioned-looking capsules that will be launched on conventional expendable launchers. Also European and Russian plans for new launchers and crewed vehicles focus on next-generation expendable rockets and relatively simple re-entry capsules. This is a very sad situation, considering the long history of rocket plane and spaceplane development, in addition to the huge amounts of time and money already invested in their development.

There is however also some good news. A new industry of companies developing and marketing suborbital rocket planes has recently emerged. Scaled Composites’ rocket plane SpaceShipOne reached an altitude of just over 100 km (330,000 feet) in June 2004, making it the first fully privately funded human spaceflight mission. Less then four months later it won the $10 million Ansari X Prize by flying beyond 100 km in altitude twice in a two-week period. As per the rules of the X Prize, no more than ten percent of the empty weight of the spacecraft was replaced between these flights, making SpaceShipOne a truly reusable spaceplane, albeit a suborbital one. It is also interesting to note that the flight on 4 October 2004 not only earned Scaled Composites the X Prize, but with a maximum altitude of 112 km (367,441 feet) also the unofficial record for the highest altitude reached by a manned aircraft (unofficial because the plane did not take off under its own power). The previous unofficial record of 108 km (354,199 feet) had been set on 22 August 1963 by the air-launched X-15 rocket plane, and thus had stood for over 41 years!

At the time of writing, commercial space tourism flights onboard the larger SpaceShipTwo plane, also developed by Scaled Composites but marketed by Virgin Galactic, are scheduled to start no earlier than 2012. Several other suborbital rocket planes are under development, and there are even plans to run rocket plane races to boost their development and commercial value.

Nevertheless, suborbital rocket planes are a far cry from the large orbital spaceliners we were promised. We can’t yet book tickets for an orbital cruise around the planet on a luxury spaceplane, or fly from New York to Tokyo within two hours as president Reagan announced over a quarter of a century ago. Even the incredible Concorde supersonic jetliner, which during its 27 years of operations notched up more supersonic flying hours than all the world’s air forces together, is no longer flying and there is no successor in sight! In the second decade of the twenty-first century, astronauts are still launched vertically on top of expendable missiles, and planes with rocket engines are only used for suborbital tourist trips. A true rocket spaceplane, taking off under its own power from a runway, using its wings to fly into orbit and then to glide back to Earth for an elegant landing at an airport seems to a remote vision, as far away from reality as it was in the 1950s.

Why is it so hard to develop a true rocket propelled spaceplane to fly us into orbit “the way it was meant to be”, perhaps even doing a playful roll on the way up? And why has the continued evolution of rocket planes, which has been progressing for over 80 years, seemingly run into a brick wall? Is there any hope left for would-be rocket plane passengers and pilots, and if so, what might an operational spaceplane look like? Is Euro 5 ever going to fly? These are some of the questions I will address in this book. In search of answers, but also just out of curiosity, we will journey through history and discover all the wonderful, exciting and weird rocket planes that people have dreamed of, have designed, and in some cases have actually flown (and crashed). Rocket planes have always been at the forefront of technology, pushing the established boundaries of aviation and spaceflight. Because of this they were often highly secret projects, which just adds to their appeal. Whatever rocket aircraft in history you look at, you will always find that at the time it represented a daring leap in technology, and involved much adventure for their brave pilots. Rocket planes were showing a glimpse of the future, even if that future changed continuously and often did not materialize as expected. Hence the title of this book, Rocketing into the Future.

In this book many key technical issues for rocket planes are described, such as how rocket engines work or how the speed of sound varies with altitude. Books on spaceflight of the 1950s and 1960s tended to explain such things, as it was rightly considered that the general public did not know much about such novel technologies. Nowadays not many books and articles about high-speed aircraft and spaceflight bother to explain the technology and physics involved, assuming that readers are either already familiar with or would be bored by such ‘details’. However, to truly understand the enormous challenges facing rocket plane designers, and the often brilliant solutions that have been implemented, some understanding of the basics of air – and spaceflight is necessary. It is certainly possible to explain these without complicated mathematics, as you will see (but you are allowed to skip the passages concerned of course, since there is no exam at the end).

To limit the scope of this book to a reasonable level, I defined ‘rocket planes’ as manned aircraft that use lift-producing wings and rocket power to fly. Spaceplanes that are launched vertically into orbit on top of conventional rockets, without the use of lift – producing wings, and only truly fly upon return to Earth (such as the Space Shuttle) are only described where appropriate; I consider these mere winged gliding payloads rather than true rocket planes. The ‘brute force’ launchers on which such shuttles are bolted ascend almost vertically out of the atmosphere as rapidly as possible in order to quickly escape aerodynamic drag, rather than exploiting the lift-producing capabilities of the air to fly up to high altitude prior to rocketing into orbit; what is called a ‘lifting ascent’. Conventional aircraft that use added rocket boosters to help them to take off are also mostly considered beyond the scope of this book. Unmanned winged missiles and rockets that have small wings only for steering and stability purposes are also only described when there is an important link to the main topic.

There is of course a grey area associated with this definition of a ‘true’ rocket plane. For example, one could argue that the vertically launched Natter interceptor and the Ohka manned missile of the Second World War are not really rocket aircraft; that the ‘Zero-Length Launch’ jet fighters catapulted into the air with the help of huge rocket boosters represent merely an extreme form of rocket assisted take-off; and that the X-30 was not a rocket plane because it used only airbreathing engines. Nevertheless this book does include relatively detailed descriptions of them, in part because they play a role in the overall story of the rocket plane, but also due to the writer’s fascination with these exotic aircraft. This isn’t a very scholarly approach, but this book does not pretend to be an academic report. Similarly, vehicles like the X-20, the Space Shuttle and the Russian Buran also clearly fall outside the main scope, but it is necessary to discuss them in broad strokes because they represent important intermediate steps that link twentieth century rocket aircraft to possible future orbital rocket spaceplanes; in fact, the Space Shuttle has been mentioned several times in this introduction already. Moreover, several early Space Shuttle concepts were actually true rocket planes.

However, even focusing on rocket aircraft that fit my definition requires further selectivity: there is a vast amount of information ‘out there’, certainly enough for a hefty encyclopedia of rocket aircraft. In particular, the assortment of rocket planes designed during the Second World War in Germany, Japan and Russia is simply bewildering. Selections thus had to be made, and this book is not intended to be an all-encompassing database of rocket airplanes: if you are interested in the diameter of the tail wheel of the Me 163 or have an urgent desire to know the liquid oxygen boil-off rate for the Bell X-l, then you will need to look elsewhere. Whole volumes have been written about the individual rocket planes and pilots that are mentioned in the following pages. The aim of this book is to offer a concise account of the amazing history of rocket aircraft, the perseverance of their designers, the bravery of their pilots, the logic of their technical evolution and how they paved the way to future spaceplanes.

Another thing that I quickly found out is that for early rocket planes, say up to the end of the Second World War, it is possible to concisely describe their development, since it was usually short and involved only a few key individuals. But after the war, aircraft and rocket technology increased in complexity so rapidly that large numbers of people and long development times were required; even the initial design became a team effort rather than the work of a single, brilliant ‘lone wolf inventor. The Opel RAK-1 rocket glider for instance, involved three key people and several technicians, and flew a few months after the project was conceived; in contrast the Space Shuttle took over a decade to develop, involved six main contracting companies and over 10,000 engineers, technicians, managers and support personnel. It is impractical to completely describe the entire history of an extremely complex vehicle such as the Shuttle or even the earlier X-15 in only a few pages, and thus the further we move through time, the more I had to focus on a rocket aircraft project’s most important issues and events.

Researching this story has led me to an astonishing wealth of books, magazine articles, websites and technical papers, many of which are listed in the bibliography. Sometimes different sources provide conflicting pieces of information. Faced with such conflicts, I either sought the original source or used what appeared to be most plausible. My search also often led to surprising finds, such as descriptions of truly weird and suicidal designs that thankfully for the pilots never left the drawing table; and websites selling such must-have items as an intricately detailed scale model of a rocket propelled Opel car, and a wooden made-in-the-Philippines model of a 1920s’ spaceplane design (both of which now decorate my bookshelves). I hope the readers of this book will have as much fun and amazement as I had in writing it. Ready for take-off?

Thanks to Shamus Reddin for information on the Me 163A’s RII-203 engine and also for the interesting information on Hellmuth Walter’s rocket technology on his website (www. walterwerke. co. uk). Bruno Berger of the Swiss Propulsion Laboratory provided some clarifications on the Mirage SEPR 844 rocket boost pack. Alessandro Atzei and Rogier Schonenborg accompanied me on two fruitless efforts to witness a Shuttle launch, during which we nevertheless had a lot of fun and saw much of Cape Canaveral and the Kennedy Space Center. My father allowed himself be talked into making a long car ride to see the Soviet Buran shuttle in a museum in Germany. The European Space Agency team of the Socrates study, of which I was part, I thank for all the interesting discussions and information on the design of a rocket plane (even although it never left the Powerpoint stage). To Georg Reinbold, thanks for all the interesting discussions over many years about the costs of spaceplanes and reusable launchers. I thank Amo Wielders, Stella Tkatchova, Ron Noteborn, Dennis Gerrits and Peter Buist for their suggestions and many interesting ideas concerning space transportation, Ufe, the universe and everything. And David M. Harland, himself an accomplished writer, did all the editing required to put this text into shape. And last but not least, a big thank-you to all those writers who managed to tell the stories of many almost forgotten rocket aircraft projects; without their books, reports, papers and websites the ‘big picture’ presented in this book would not have been possible.

The history of powered flight started on 17 December 1903 at Kitty Hawk, North Carolina, when the Wright brothers were able to keep their feeble plane in the air for some 12 seconds. In that short time pilot Orville Wright managed to cover a distance of 36.5 meters (120 feet); less than the wingspan of a modern Boeing 747 airliner. A bit over 10 years later, airplanes had already become effective military machines and were used for reconnaissance, air defense and attack over war-torn Europe.

The history of rocketry goes back much further, maybe even to thirteenth century China. Gunpowder-filled tubes were initially used for fireworks, but were soon also applied as artillery to rain fiery arrows on the enemy and, perhaps more importantly, to scare them to death. In the early nineteenth century the English inventor William Congreve greatly improved the propellant and structure of powder-based rockets, and his designs were used on European battlefields during the Napoleonic Wars. In 1814 the ship HMS Erebus fired rockets on Fort McHenry during the Battle of Baltimore, and inspired Francis Scott Key to write about “the rockets’ red glare” in the national anthem of the United States of America.

At the beginning of the twentieth century rocketry based on solid propellants like gunpowder was well established, while the golden age of flight had just begun. It was only a matter of time before someone would think of using rockets to propel a plane. The first to suggest it may have been French aviation and rocket pioneer Robert Esnault-Pelterie, who in 1911 proposed a winged, rocket propelled aircraft. The idea was a bit ahead of its time though, as planes were then still rather flimsy contraptions ill-equipped to harness the brute power of rocket motors. But the need to observe hostile armies from the air and shoot down their reconnaissance planes during the First World War soon gave an enormous boost to aircraft development. By the end of that conflict the aeroplane had been transformed from an unreliable curiosity to a sturdy, fast and maneuverable machine exemplified by fighter planes like the Fokker DVII and the Sopwith Camel.

M. van Pelt, Rocketing into the Future: The History and Technology of Rocket Planes, Springer Praxis Books, DOI 10.1007/978-1-4614-3200-5 1, © Springer Science+Business Media New York 2012

In the 1920s and early 1930s science fiction stories became very popular and, inspired by this, rocketry societies were formed in the US, Germany and Russia. These began experimenting with rockets and even speculated about their use for interplanetary travel. It is thus not too surprising that during those years several concepts for rocket propelled planes were conceived. Some we can now proclaim to be highly impractical, such as Fridrikh Tsander’s 1921 self-consuming spaceplane design. According to the Russian’s concept, metallic parts of the vehicle no longer needed during the flight would not be discarded (as in a conventional multi-stage rocket), but be fed into a furnace to be converted into rocket fuel. The structures of the empty tanks and disposable wings would thus actually help to propel the plane. Only an essential part of the structure and a small set of wings would be retained for the return to Earth. Powdered metals do actually burn at very high temperatures, and are often added to the solid propellants of rocket boosters in order to increase thrust. But the exhaust products of the all-metal propellants would certainly have clogged up the engines of Tsander’s aircraft, and of course feeding them with scrap metal at a sufficiently high rate would anyway have been a major challenge. However, there were also some very practical concepts, such as rocket expert and spaceflight enthusiast Max Yalier’s idea to equip already existing airplanes with rocket engines to obtain cheap and relatively reliable rocket plane test vehicles.

Valier is the first to kick off a project that brings airplane and rocket technology together in a real prototype, so let’s start our story with him. In 1927 he approaches Fritz von Opel, grandson of the famous German automobile pioneer Adam Opel. At that time Fritz was director of testing at his family’s car factory and, more importantly for our story, in charge of the firm’s pubhcity. Valier has long been planning to experiment with vehicles that are equipped with rocket motors, and urges von Opel to organize some spectacular demonstrations involving rocket propelled cars. Rockets were able to accelerate a vehicle up to high speed more rapidly than the car engines of the 1920s, and more importantly make more noise and smoke doing so. Von Opel, himself a racing car driver and pilot, quickly recognizes the advertising value for his family’s company. Although several German engineers are developing rocket motors based on liquid propellants that are more potent than solid propellants, these are rather complicated and still experimental. Valier proposes to use the rockets manufactured by Friedrich Wilhelm Sander, based on compressed black powder. In contrast to liquid propellant designs, Sander’s rockets are simple and have already been successfully applied in signal rockets and the line-throwing missiles used to assist ships in distress. The amount of thrust can easily be varied by using and igniting different numbers of rockets in parallel. However, using Sander’s rockets in a manned vehicle has its risks: they are sensitive to storage conditions. In particular, cracks develop in the powder propellant charge as it ages and, following ignition, produce sudden increases in burn rate and therefore pressure, resulting in an explosion. And, of course, it is very difficult to inspect the propellant for defects right before use.

In April 1928, after a series of secret tests at the Opel factory in Russelsheim, von Opel organizes his first rocket car run for the press. The RAK-1 (Raketenwagen-1,

or Rocket car 1) is basically a standard racing car with the engine removed and a dozen solid propellant rockets fitted in the back. To force the vehicle firmly down onto the track and prevent it from taking off, a pair of stubby, downwards-pointing wings are fitted (this is the first use of such airfoils, which are now customary in Formula 1 and Indy racing cars). Although five of the rockets fail to ignite during the demonstration, the driver, Yalier, nevertheless stuns the journalists and

photographers assembled at the Russelsheim track to witness the test by achieving a top speed of over 110 km per hour (70 miles per hour).

Just over a month later Fritz von Opel himself, wearing an aviator’s jacket and goggles but no helmet, drives the RAK-2 at the high-speed AVUS track in Berlin, watched by some 3,000 guests from show business, sports, science and politics. The RAK-2 car is based on the chassis of the regular Opel 10/40 PS car, fitted with two down-force wings. In contrast to the fixed wings of the RAK-1, these airfoils are connected to a lever in the cockpit that enables the driver to control the amount of down-force by changing the angle of the wings. They are also much larger in order to handle the anticipated higher speed of the new rocket car, which is propelled by two dozen solid propellant rockets, each of which has a thrust of 25 kg (55 pounds). The amount of propellant is impressive: “120 kilos of explosives, enough to blow up a whole neighborhood” von Opel later recalls. The rockets are ignited by a pedal- activated electrical system, with each press on the pedal igniting another rocket. In the event, all of the rockets ignite and make the RAK-2 run a huge success. Von Opel later described the sensation of driving the powerful RAK-2: “I step on the ignition pedal and the rockets roar behind me, throwing me forward. It’s liberating. I step on the pedal again, then again and – it grips me like a rage – a fourth time. To my sides, everything disappears. All I see now is the track stretched out before me like a big ribbon. I step down four more times, quickly – now I’m traveling on eight rockets. The acceleration gives me a rush.” Trailing a thick column of smoke, the RAK-2 accelerates to 238 km per hour (148 miles per hour). Not enough to break the contemporary speed record for cars, which then stood at 334 km per hour (208 miles per hour), but still very fast in a world where normal cars had a top speed of around 110 km per hour (70 miles per hour). In spite of the negative lift of the two large side wings, the high speed of the RAK-2 makes the front end of the car almost leave the ground, but von Opel is an experienced racing car driver and manages to keep the machine on the road. In less than three minutes the spectacle is over. The adrenalin still pumping through his veins, von Opel announces that his next goal is to fly a rocket propelled airplane, and urges the spectators: “Dream with us of the day in which the first spaceship can fly around our earth faster than the Sun.” Fritz von Opel is an overnight sensation. The magazine Das Motorrad (The Motorcycle) reported: “No one could escape the impression that we had entered a new era. The Opel car with a rocket engine could be the first practical step toward the conquest of space.” Silver-screen darling Lilian Harvey confided to a reporter: “I’d like to ride in the rocket car with Fritz von Opel.” Even now, some 85 years later, the RAK-2 run is remembered as one of the most spectacular events in car history. The Technik Museum Speyer in Germany has a beautiful shiny black replica of the bullet-shaped RAK-2, and even today the car looks fast and stunning.

Even faster is the unmanned RAK-3. When mounted on train rails and equipped with ten rockets it manages to accelerate up to 290 km per hour (180 miles per hour) on its first run. The vehicle is retrieved several miles down the track, towed back and prepared for the second sprint. Thirty rockets are fitted this time, but it is too many and almost immediately after ignition the car jumps off the rails and is destroyed. During the first test of the subsequent RAK-4 rail car, one of the motors blows up and sets off the other rockets in a massive explosion. The debacle only kills the cat that is carried as the sole passenger, but the railway authorities prohibit any further rocket vehicle runs because they don’t want to ruin their railroad track. This seals the fate of the already planned RAK-5 rail vehicle. The rocket propelled motorcycle that von Opel has already begun to test is soon also deemed to be too dangerous by the government, which prohibits von Opel from using it to try to break the motorcycle world speed record. However, even without the RAK-5 and the crazy motorcycle von Opel has by then already earned all the publicity that he sought, as well as the nickname ‘Rocket Fritz’.

Rocket cars have no practical use other than publicity stunts and record breaking, and most German rocket pioneers, worried that he is threatening the credibility of rocketry and its potential for spaceflight, frown on von Opel’s stunts. However, for Valier the cars and railcar are an essential part of his plan to achieve spaceflight. As early as 1925 he devised a step-by-step ‘roadmap’ for achieving space travel using rocket propelled aircraft. In his view, it would start with early test stand experiments with rocket motors, then experiments with rocket ground vehicles such as cars, sleds and railcars would open the way for a rocket propelled airplane that would in turn lead to stratospheric flight and ultimately the construction of a rocket spaceship. With von Opel’s assistance Valier has reached the third step of his master plan. So the team starts work on a project with potentially an important future: the announced rocket propelled plane.

In March 1928 von Opel, Valier and Sander visit the Wasserkuppe plateau, then a focal point for glider flying in Germany. During the 1920s and 1930s virtually every German aeronautical engineer and test pilot of note was building, testing, and flying aircraft at the Wasserkuppe. The light planes were launched from the plateau to fly down into the valleys below, gaining altitude by using updrafts caused by the wind rising up the slopes. The Opel team is seeking a suitable glider onto which it can fit rocket motors, and at Wasserkuppe they encounter some of Alexander Lippisch’s revolutionary, tailless gliders. Instead of having a horizontal stabilizer at the back, they have them at the front, giving them a rather duck-like appearance when seen from the ground (hence these types of planes are called ‘canard’ designs, after the French word for duck). This unusual configuration offers sufficient space to mount rockets at the back without the risk of setting the plane on fire. In June von Opel’s team strikes a deal with a local glider society, the Rhohn-Rositten Geselschaft, in which von Opel finances the Sander rockets and the society furnishes a Lippisch – designed aircraft called the ‘Ente’ (Duck, in German). For the Opel company, the flight of a rocket aircraft will be simply another spectacular publicity stunt, but the society’s goal is to develop rocket propelled take off into an alternative for launching glider planes. Normally gliders are either towed into the air by a rope attached to a car, or launched down a rail by a rubber catapult system with an eight-man crew. A rocket assisted take-off would enable a glider to get airborne without assistance.

The plan is to fit two black powder rockets to the Ente and link them electrically to a firing switch in the cockpit. The first is a powerful boost rocket that supplies a thrust of 360 kg (790 pounds) for 3 seconds. The second will fire immediately after the first burns out. It is less powerful but longer burning to keep the plane in the air: 20 kg (40 pounds) of thrust for 30 seconds. However, tests with model aircraft and scaled rockets show that the high-thrust motor would be too powerful for the plane, so it is decided to use a standard rubber-band rail launcher in combination with two of the less powerful sustainer rockets, which will fire in succession to provide one

minute of continuous thrust. To prevent an overexcited pilot from making a potentially deadly mistake, the electric ignition is rigged so that it is impossible to ignite both rocket motors at the same time. The team also devises an ingenious counterweight system that is placed under the cockpit floor, and which automatically adjusts the center of gravity of the aircraft as the fuel of the rockets is burned, for otherwise the center of gravity would continuously shift forward as the rocket propellant in the back is consumed, making the glider unstable and very difficult to

fly-

Fritz Stamer, who has long been a test pilot for Lippisch’s designs, is selected to fly the aircraft. On 11 June 1928 (shortly before the first test of the RAK-3 rail car) and after two false starts, Stamer takes off and in just over a minute flies a circuit of about 1.5 km (1 mile) around the Wasserkuppe’s landing strip. His verdict was that the world’s first rocket plane flight had been “extremely pleasant” and that he “had the impression of merely soaring, only the loud hissing sound reminded me of the rockets”.

The plane appears to be very easy to keep under control, so for the second flight the team decides to increase the thrust by firing both of the rockets simultaneously. Unfortunately, a well-known problem of Sander’s rockets pops up again: one of the rockets explodes, punching holes in both wings and setting the aircraft alight. Stamer reported: “The launching went alright and while the plane took to the air I ignited the first rocket. After one or two seconds it exploded with a loud noise. The nine pounds of powder where thrown out and ignited the plane instantly. I let it drop for some sixty feet to tear the flames off.” He manages to bring the plane down from a height of around 20 meters (65 feet). Just after landing the second rocket catches fire, but it does not explode. Stainer is able to walk away unhurt. However, the Ente is severely damaged, and the fiery crash scares the sponsoring glider society into abandoning the project. In September of that same year the magazine Scientific American rightly tells its readers: “On the whole we are inclined to think that the rocket as applied to the airplane might be a means of securing stupendous speeds for a short interval of time, rather than a method of very speedy sustained flight.”

In spite of its problems, the Ente project has inspired Max Valier to develop a plan for crossing the EngUsh Channel with a rocket plane based on a more sophisticated rocket motor that uses liquid propellants. He expects his harpoon­shaped design to reach a top speed of 650 km per hour (400 miles per hour) and cover the 30 km (20 mile) distance in only three or four minutes. As if this isn’t sufficiently ambitious, he is already thinking about an even bolder plan. This is described in an article in Die Umschau in Germany in 1928 and later in an article by Hugo Gemsback entitled ‘Berlin to New York in less than One Hour!’ in the November 1931 issue of the American magazine Everyday Science and Mechanics. In the same year Harold A. Panne presents a very similar concept to the American Interplanetary Society, with reference to Valier. The plan is to fly a rocket plane across the Atlantic in record time. In the plan published in 1928 the aircraft leaves from Berlin Tempelhof airport, but in the 1931 concepts the airliner would take off from the water to preclude a long, expensive runway (at that time there were not many large airports, so many large aircraft were built as floatplanes). At an altitude of 50 km (30 miles) Valier’s ‘Type 10’ (his tenth rocket plane design) would reach the amazing speed of 2 km/s (7,200 km per hour or 4,500 miles per hour) and cross the ocean in less than an hour. The speed and altitude for his design were absolutely spectacular, as the air speed record in 1929 stood at only 583 km per hour (362 miles per hour), and the altitude record was close to 12.8 km (42,000 feet). By comparison, it had taken Charles Lindbergh 33.5 hours to cross the Atlantic in 1927. Suspecting that the atmospheric drag as his rocket aircraft descended would be insufficient to decelerate to a reasonable landing speed, Valier foresaw the need for forward-firing braking rockets. In spite of the extreme speed and altitude, he didn’t think the flight would be very interesting for the passengers: they were to be kept comfortable in a pressurized cabin, and he expected there would be little to see of the Earth below because of “the vapor and light cloud formations”. During the unpowered coasting phase at high altitude the passengers would be weightless, but apparently Valier didn’t consider that to be a unique selling point. According to him, the richest reward would be the sight of the black sky and the Sun that would appear “surrounded by glowing red protuberances and the silvery corona”, as can be seen during a total solar eclipse. He was wrong about the view of the Sun, which in space appears brighter but otherwise just the same as from Earth. He was also wrong about the appeal of such a trip, as space tourists are currently willing to pay hefty sums of money for a suborbital rocket flight to the edge of the atmosphere; the marvelous view of the Earth and the few minutes of weightlessness being the key selling points. What Valier did foresee was the large amount of propellant an intercontinental rocket plane would have to carry. Of the 80 ton (180,000 pound) total weight of his Type 10 design, 58 tons (130,000 pounds) would be propellant: 73

percent! With today’s fuel prices, such an appetite would be difficult to combine with commercially profitable intercontinental flights (let alone with today’s carbon dioxide footprint minimization demands).

In the meantime von Opel, not having been able to fly a rocket plane himself in public owing to the crash of the Ente, orders a new rocket plane from Julius Hatry, a well-known German glider builder. As Hatry recalled in an interview in 2000: “From 1927 to 1929,1 had been working on models for rocket-fuelled airplanes. I had flown models successfully and had decided to build a manned craft.” Hatry initially refuses Opel’s offer to team up, but relents when he finds that the car magnate is negotiating to purchase one of his airplanes for modification. Hatry delivers a more conventional glider design than the Ente, featuring a high upper wing and a twin tail arrangement which is attached by twin booms. This allows rockets to be fitted on the back of the fuselage, because the tail is far back and high up with respect to the main body of the aircraft, with the booms leaving sufficient clearance for the rocket’s hot exhaust. To increase the thrust and endurance, instead of the two motors of the Ente this plane is fitted with 16 sustainer rockets that will be fired in pairs. Each motor is able to deliver about 24 kg (53 pounds) of thrust for about 24 seconds. Overall, they should get the plane airborne without the use of a catapult launcher. Confusingly, the plane is also called the RAK-1, same as the earlier rocket car.

Von Opel, Hatry and Sander conduct their first tests on 10 September 1929 in a hunting field just outside Riisselsheim. A handful of onlookers, including a New York Times photographer, watch the plane go nowhere while burning up its boosters. The problem seems to be in the initial launching. A second attempt is made the same day, this time using a standard rubber-band launch catapult. About a meter or two in the air von Opel launches his rockets and flies about 1,400 meters (4,600

feet). The paper publishes a photograph of the flight in its Sunday edition on 6 October. But the team is not satisfied, because they want the plane to be able to take off without any catapult assistance. After some adjustments they make another test, secret this time, on 17 September. With Hatry in the cockpit and with the help of a rail-sled equipped with a pair of rockets together delivering 700 kg (1,500 pounds) of thrust, the plane manages to launch itself. It travels roughly 500 meters (1,600 feet), with a maximum altitude of about 25 meters (80 feet). Satisfied, von Opel calls another publicity event. On 30 September the team prepares the plane at Frankfurt’s Rebstock airport. Sixteen rockets are packed into the back of the RAK-1, and two larger ones in the sled. In front of a large crowd and several cameras, von Opel takes the controls and lights the rockets. He knows he is taking quite a risk, considering the rockets have a tendency to explode. On the first and second attempts the rockets on the plane don’t ignite correctly, causing the aircraft to meekly jump off the launch rail and skid over the ground for a short distance. Now they have a problem: Sander has not reckoned on a third attempt and only 11 rockets for the plane are left. Von Opel decides to try anyway, and this time the rocket glider launches successfully from the 20 meter (65 feet) long slide rail and takes to the sky. Igniting one rocket after the other he keeps the plane airborne for 80 seconds. Even although six of the 11 motors fail to ignite, he flies 25 meters (80 feet) above the ground and reaches a speed of about 150 km per hour (90 miles per hour). Due to the relatively high weight (270 kg, 600 pounds) with respect to the size of its wings, von Opel has to land the plane at high speed to enable the wings to generate sufficient lift for a controlled landing. The fast landing ends badly in a crash; he is able to walk away but the plane is a write off.

Von Opel is nevertheless extremely exited about the flight and the future he sees for rocket planes and spaceflight, and euphorically comments: “It is magical, flying like that, powered by nothing more than the combustion gases streaming out of the engines at 800 km an hour. When will we be able to harness the full power of these gases? When will we be able to fly around the world in five hours? I know this time will come and I have a vision of future world travel which will bring together all the people of the Earth to live as one. So I race towards this vision like a dream with no sense of space and time. A machine flying almost by itself I hardly need to touch the controls. I feel only the borderless intoxicating joy of this first flight.”

In December the Denton Record-Chronicle in Texas writes: “The day of 1,000 passenger airplanes propelled by rockets at 5,000 miles per hour was envisioned by Fritz Von Opel, German auto manufacturer and authority on rocket planes. Von Opel, who flew a rocket propelled plane in Germany last October a distance of a mile in 75 seconds, said he expected to see the 5,000 mile per hour airplane in operation within the next generation and possibly the next decade.” Like Valier, von Opel sees a great future for rocket plane transportation, and expects to see his vision come true soon. Unfortunately the single RAK-1 flight proves to be Opel’s last rocket vehicle experiment. One month after his fiery take off the world stock market crashes even worse than his last rocket plane, and the Opel company is prohibited by its majority owner, General Motors, from pursuing further expensive rocketry work. Von Opel quits the company and goes to live in Switzerland, where he dies in 1971, aged 71. Max Valier had been killed prior to the flight of the RAK-1 plane, when an advanced rocket engine that used liquid oxygen and alcohol as propellants blew up during a test run in his laboratory, curtailing his ambitious plans for flying across first the English Channel and later the North Atlantic.

Although ocean-crossing stratospheric rocket planes were still science fiction to many people in 1929, the use of rockets to assist planes take off either with a heavy cargo or from a shorter runway do find immediate practical use. In August 1929 the German Junkers aircraft company launches a W33 Bremen-type floatplane from the river Elbe with the help of Sander rockets.

Also in 1929 another German by the name of Gottlob Espenlaub, an experienced glider designer and pilot, attempts to follow up on von Opel’s rocket flights. The literature is confused with different sources giving different details on his flights; what follows is my reconstruction. On 22 October Espenlaub prepares his ‘Espenlaub Rakete-Г rocket glider for its first flight at the Diisseldorf-Lohhausen airfield. Painted on the nose is the snout of an angry looking monster bearing large fangs, which is appropriate considering the dangerous nature of the machine. A pair of Sander’s black powder rockets, each with 300 kg (660 pounds or 3,000 Newton) of thrust are installed, then the glider is towed into the air by a propeller plane. At a height of about 20 meters (65 feet) Espenlaub disconnects from the tow plane and ignites one of the rockets. A long stream of fire explodes from the back of the plane with a tremendous roar, giving it a powerful push. But the asbestos put around the tail of the plane turns out to be insufficient protection against the rocket’s exhaust, and soon the rudder catches fire. Fortunately, he manages to land the burning plane safely and without injury. In May 1930 Espenlaub tries again at Bremerhaven, this time, wisely, with a tailless glider design, the Espenlaub-15. He equips the plane with two Sander solid propellant boosters each delivering 300 kg (660 pounds) of thrust and ten sustainer rockets, each with 20 kg (44 pounds) of thrust. For additional boost at take off, he places the E-15 on a catapult sled. When he hits the igniter switch, the combined power of the catapult and one of the large boosters launches him quickly to a height of about 30 meters (100 feet), whereupon he ignites the first of the small sustainer rockets to stay in the air. He decides to fire the second powerful booster to gain the speed required to make a turn, but the big rocket explodes, almost throwing Espenlaub out of his seat. As the little E-15 dives to the ground, he jumps out, strikes his head, and falls into a soft bog. Rescuers take him to a hospital, where he remains unconscious for two days. He never attempts another flight. After the war Espenlaub and his company go on to produce streamlined cars, which are much less dangerous than his high-powered rocket plane.

Inspired by the developments in Germany, daredevils in other countries also take to the skies in gliders fitted with black powder rockets. In June 1931 Ettore Cattaneo flies the ‘RR’ at the airport of Milan in Italy. A 280 kg (620 pound) rocket propelled glider built by the Italian company Piero Magni Aviazione, it resembles von Opel’s RAK-1 with a single wing mounted above an enclosed fuselage and a tail with two vertical stabilizers set high enough that the exhaust of the rockets won’t ignite them. It remains in the air for 34 seconds, covering a distance of about 1 km (0.6 mile). In 1928 the American aviation pioneer Augustus Post publishes a design for a rocket plane but it is never built. It was to have been propelled by liquid air. The air would be heated so that it would expand as a high-speed gas through nozzles in the tail of the aircraft to deliver thrust. Post designed the plane to operate in the stratosphere, where there would be little air to provide lift for flight and maneuvering. To generate lift at high altitude, some of the air would be expelled from a series of smaller outlets on the upper part of the wing’s leading edge, delivering a stream of fast flowing air over the wing. Nozzles in the wingtips would help to steer the plane (an idea much later applied in the X-15). The air would be stored as a liquid rather than as a gas in order to minimize the size of the propellant tanks. The passengers would travel in a pressurized cabin, something now standard in airliners but in 1928 still a novelty (the first experimental plane to have a pressurized cabin, the German Junkers 49, flew in 1931). The first American rocket aircraft to actually fly is rather less ambitious than Post’s design. William Swan’s ‘Steel Pier Rocket Plane’ is a simple high-wing glider with an open-framework fuselage – a design that we would nowadays call an ultra­light. It takes off from Atlantic City, New Jersey, on 4 June 1931, powered by a single solid propellant rocket motor with about 23 kg (50 pounds) of thrust (nine more motors are installed but not fired for this test flight). The plane rises bumpily to an altitude of 30 meters (100 feet) and covers a distance of about 300 meters (1,000 feet). A day later “resort stunt flier” Swan employs twelve of the same motors. Now the little glider and its pilot are propelled to an altitude of 60 meters (200 feet) and remain in the air for eight minutes, demonstrating how rockets can launch a glider into the air without the need for a ground crew and a catapult or tow plane.

The experiments with rocket propelled gliders in the late 1920s and the first half of the 1930s show that rockets can indeed be used to propel an airplane: the ‘instant’

high thrust rockets can certainly make a plane take off and ascend very rapidly, and in principle they can also enable a plane to fly very fast. Moreover, because rockets don’t need oxygen from the atmosphere, rocket planes can potentially fly very high. In 1928, Popular Mechanics Magazine correctly tells is readers: “Opel’s rocket-car and rocket-plane experiments are important because the rocket offers the one method yet discovered for navigating space at high altitudes, above the Earth’s envelope of air. All existing motors depend on air for their operation, and all existing types of propellers screw their way through the selfsame air. The rocket, on the other hand, can be shot out into space and attain tremendous speed by escaping the resistance of the air.” And rockets are much simpler than gas turbine engines, which existed only as concepts when the Ente and RAK planes were already flying (the first turbine jet aircraft, the Heinkel 178, only took off in 1939, also in Germany, which was at that time leading the world in advanced aircraft propulsion). Because of their simplicity, rocket motors can be added to existing aircraft, as demonstrated with the gliders that formed the basis of the Ente, RAK-1, Espenlaub E-15 and Swan’s Steel Pier Rocket Plane, as well as the Junkers floatplane.

However, rocket engines are not very useful for sustained atmospheric flight. Jet engines scoop up air to collect the oxygen needed to burn the fuel, but rockets need to carry both their fuel and their oxidizer with them. As a result, rocket planes need relatively large tanks and are heavier than jet or propeller planes of a similar range. In 1931 the earher-mentioned article ‘Berlin to New York in less than One Hour!’ remarks upon this: “With the present proportions between the weight of the fuel and that of the rest of the flyer, the former is so great that you need over 90 percent of the available space for fuel, and have left only 10 percent for cargo. This makes the venture economically unprofitable and, for that reason, no big machine has, as yet, been constructed.”

Max Valier had already foreseen that most of the weight of his intercontinental rocket plane would be propellant, even when flying at the edge of the atmosphere in order to limit aerodynamic drag for an important part of the journey, and even when using liquid propellants which, by giving more thrust per unit of propellant weight, are much more efficient than solid powder rockets. Valier even suggested that in the denser atmosphere an intercontinental rocket aircraft could limit fuel consumption by using retractable propellers. It was self evident that major steps would be required in propulsion development, and not only in terms of efficiency but also in safety, since the primitive black powder rockets used in the early German rocket planes were not suitable for a trustworthy vehicle. Other major issues with powder rockets were that it was impossible to control their thrust during flight, let alone extinguish them, and they had the disturbing tendency to blow up or set the plane on fire. Rocket motors using liquid propellants would be much more controllable and therefore safer, and much better suited for propelling aircraft. Until these were available, it was hard to conceive of an aeronautical assignment that could not be performed more efficiently by conventional aircraft.

In the 1930s rocket motors, and especially those using liquid propellants, saw rapid development. In Russia the work of a deaf schoolteacher, Konstantin Tsiolkovsky, formed the basis for various experiments with rockets. He laid down the theory of rocket flight and derived the most important mathematical formula used in rocket design: the Tsiolkovsky Equation a. k.a. the Rocket Equation. In 1903, the same year as the Wright brother’s first powered flight, this visionary man had already invented the method of ‘staging’, whereby a series of rockets are stacked on top of one another and discarded once their tanks run dry, in order that the remainder of the vehicle can fly on without their now useless empty weight; a method that has been applied to all the world’s space launchers starting with the one which placed Sputnik into orbit in 1957.

Under the Russian Tsar’s regime Tsiolkovsky never enjoyed any government support, but after the 1917 Revolution the Soviets saw in him a perfect example of the working class genius, and in 1919 he was even made Member of the prestigious Academy of Sciences. Apart from pure rockets, Tsiolkovsky also considered rocket propelled planes. Around 1930 he developed a fourteen-point plan to conquer space. The first step would be to build a low-altitude rocket plane that could fly to a height of about 5 km (3 miles). Step two would be a similar plane, but with higher thrust and shorter wings to limit the air drag at the higher velocities it would attain. Next a rocket plane capable of climbing to an altitude of 12 km (8 miles), equipped with a pressurized cabin, would be required. The subsequent steps would involve wingless rockets that could get into space, space suits, orbital stations, and even colonies on asteroids. Tsiolkovsky died in 1935 without personally building any rockets, but he
inspired a number of young Russian rocket experimenters who, in the 1930s, began to build small experimental rockets based on liquid propellants. As a testimony to his vision, Tsiolkovsky’s most famous words can be found on his grave: “The Earth is the cradle of mankind, but one cannot stay in the cradle forever.”

In parallel, but on the other side of the ocean, the reclusive American Robert H. Goddard was designing, building and launching a series of ever larger rockets. By 1918 he had already constructed and flown a solid propellant rocket that was 1.70 meters (5.1 feet) in length and weighed 20 kg (44 pounds). In 1919 he published a now famous report called ‘A Method of Reaching Extreme Altitudes’, in which he described how a rocket could reach the Moon and signal its arrival by use of flash powder that ignited at impact. However, in spite of their scientifically sound basis,

Goddard’s proposals were met with disbelief and ridicule. In response he became secretive about his plans, worked only with a small team of trusted people and hid his experiments from the public. Often not even his close associates knew exactly what he was up to. Like Tsiolkovsky, Goddard reahzed that rockets based on liquid propellants have several advantages over the simpler solid propellant (gunpowder) rockets used until then for artillery and fireworks. Liquid propellant rockets have a lower weight relative to their thrust and, unlike solids, can be throttled up or down, and even stopped entirely without blowing up. So Goddard began to experiment with liquid propellants and in 1926 flew the world’s first liquid propellant rocket (running on gasohne and liquid oxygen); an event now recognized as historic even though at the time virtually no one knew about it.

Goddard also gave some thought to rocket planes, leading in 1931 to a patent for an especially innovative design. Observing that rocket motors are not very efficient in the lower atmosphere, he proposed that his rocket plane would start off by using the exhaust from its rocket engine to drive a pair of turbines each of which would be connected to a conventional propeller. At higher altitudes the turbine blades would be withdrawn from the path of the hot gas and the plane would fly on rocket thrust alone. The turbines would have had to be made from heat-resistant materials that far exceeded the capabilities of the time, but the general idea was sound and similar to what we call today a turboprop engine. The concept of dual-mode propulsion was also very innovative and would become a feature of advanced spaceplane concepts half a century later (although not involving propellers). In his book The Conquest of Space, the first non-fiction book on spaceflight in English, American David Lasser foresaw fleets of Goddard’s planes connecting London, Paris and Berlin with New York by 1950. Flight time: 1 hour! The general public nevertheless kept ignoring Goddard’s important work, probably wrongly associating it with the fanciful stories that featured in the popular science fiction pulp magazines of the time. After one of his rocket experiments in 1929, a mocking headline in a local Worcester newspaper read “Moon rocket misses target by 238,799 1/2 miles”. By 1935 Goddard’s rockets had exceeded the speed of sound and reached altitudes of 1.5 km (0.9 miles), but the US government did not seem to appreciate the possibilities of this novel technology. During the Second World War Goddard was only assigned a contract to develop rockets to assist propeller aircraft take off from carrier ships.

In Germany the situation was very different. The rocket experiments of a small club of hobbyists, inspired by their own rocket pioneer Hermann Oberth, attracted serious attention from the military. German Army leaders saw the development of powerful rockets as a means of circumventing the ban on the use of large cannon as stipulated in the Treaty of Versailles that Germany had been forced to sign after its defeat in the First World War. Under the technical leadership of the German Space Society’s Wemher von Braun, who was only 20 years old at time, the Army began an enormous rocket development effort. It soon became the Third Reich’s most expensive development project. A dedicated and huge development and launch center was built in Peenemiinde, a remote and sparsely inhabited place on the Baltic shore, involving laboratories, wind tunnels, test stands, launch platforms and housing facilities for the 2,000 rocket scientists, 4,000 supporting workers and their

families. Peenemtinde’s vast resources enabled the team ultimately to develop the infamous A4 rocket; a 14 meter (46 feet) tall monster capable of throwing a 738 kg (1,630 pound) warhead a distance of 418 km (260 miles). In 1942, on its first flight, and A4 (without an explosive cargo) climbed to an altitude of over 80 km (50 miles) and thus became the first man-made vehicle to reach the edge of space. In a speech afterwards, Walter Dornberger, head of the rocket development program, observed: “This third day of October, 1942, is the first of a new era in transportation, that of space travel…”

But spaceflight was not what the rocket was developed for. It was a weapon and, renamed Y2 for ‘Vergeltungswaffe 2’ (Retaliation weapon number 2), at least 3,000 were fired at London and Antwerp towards the end of the war, blowing away entire blocks of houses in an instant. Although impressive and scary, the A4 was however not a huge success as a weapon. For one, it was a very expensive means of dropping a 738 kg bomb on a city in a neighboring country, and not a very precise one at that. The ‘terror’ effect of the rocket striking without warning (because it flew faster than sound it could not be heard before impact) did not have the effect that Hitler hoped for either: Londoners did not panic en masse, and did not desperately demand their government negotiate an armistice with Nazi Germany. In retrospect it would have been better to use the money that went into developing and producing the V2 to buy advanced jet fighters like the Messerschmitt Me 262. Although the A4/Y2 failed to change the outcome of the war, it demonstrated the future of big rockets as ballistic missiles. More importantly for von Braun and other space enthusiasts, it proved that a rocket could reach space and with further development would be able to launch a satellite into orbit in the foreseeable future.

By the end of the war rockets had reached such a high maturity that they could be used to propel operational planes. The rocket engines of the time were powerful and yet less complicated than turbojet engines, so if you needed to get an airplane up and high as fast as possible with a relatively simple power plant, a rocket engine was the obvious choice. In America the Air Force attached rocket pods to heavy cargo planes to help them to take off, but in Germany real rocket fighters were put into operation to counter high flying bombers. While it took a conventional high altitude propeller interceptor such as Focke Wulf s Fw 190D-9 some 17 minutes to climb to bombers at altitudes of up to 10 km (6 miles), the revolutionary Me 163 ‘Komet’ (German for Comet) rocket plane could do it in just under 3 minutes! Even the Me 262 jet fighter took 10 minutes to climb that far. The Me 163 was the fastest aircraft of the Second World War: at top speed, the little rocket interceptor closed in on the Allied bombers at about 960 km per hour (590 miles per hour), allowing the defending gunners no chance to take aim at it. The Komet also outran the propeller – driven escort fighters, so there was essentially no defense against the little rocket plane during its powered attack. Only when it ran out of propellant and had to glide back to its base could the Me 163 be intercepted and destroyed.

Serious plans for launching rocket planes into space also began to be developed during the Second World War, starting with Eugen Sanger’s design for a bomber to strike New York. This ‘Silbervogel’ (Silverbird) would begin its mission by riding a large rocket sled on a rail track over a distance of about 3 km (2 miles). On firing its own rocket engine it would climb into space, its altitude peaking at around 145 km (90 miles). It would fly below orbital speed, slowly descending into the stratosphere until the increasingly denser air would generate sufficient lift to ‘bounce’ the plane up again. Thus the Silbervogel would hop around the planet in the same manner as a stone skipping across a pond. It was calculated that the rocket plane would be able to take off from Germany, cross the Atlantic, drop a small bomb on the USA and land somewhere in Japanese occupied territory in the Pacific. It wouldn’t be a real orbital spaceplane, but pretty close. Fortunately, Germany had no time to develop anything like this before the war ended, and even if they had it would not have worked: later analysis showed that the heat generated by re-entry into the atmosphere would have destroyed the Silbervogel in its original design. Additional heat shields might have saved the concept, but the associated extra weight would have cut the bomb load to zero. Certainly it would have been difficult to justify the vast effort and expense to develop the Silbervogel simply to drop the equivalent of a hand grenade on the US. Even its designer reckoned it would take decades to get the Silbervogel operational. But the idea of a rocket propelled space bomber would return later, even leading to concern in the Soviet Union that the NASA Space Shuttle was actually a disguised orbital bomber with heat shielding that would enable it to make shallow dives into the atmosphere to deploy nuclear bombs!

During the 1940s and 1950s many technological fields experienced radical and rapid advances, in particular aviation: jet aircraft replaced piston-engined propeller planes, and ballistic missiles quickly made strategic bombers all but obsolete. The amazingly fast progress in rocket and aircraft development led to series of rocket aircraft that repeatedly broke altitude and speed records. In October 1947 test pilot Chuck Yeager managed to get the little Bell X-l rocket plane to fly faster than the speed of sound (Mach 1), thereby breaking the so-called ‘sound barrier’ (or rather, discovering there was no such barrier). In 1953 Scott Crossfield became the first to exceed Mach 2 in the Douglas D-558-2 Skyrocket. Three years later Milburn ‘Mel’ Apt pushed the Bell X-2 to the next magic number of Mach 3. Shortly before Apt’s flight, Iven Kincheloe became the first pilot ever to climb above 100,000 feet, as he flew the X-2 to a peak altitude of 126,200 feet (38,5 km, or 23.9 miles).

The complexity of the aircraft had increased enormously in the quarter century that separated the first rocket propelled gliders to the Bell rocket research aircraft: while the Opel RAK-1 had required only a few simple readiness checks shortly in advance of the launch, the Bell X-l A pilot had a checklist of 197 points to tick off prior to flight. By 1962, test pilots were flying at speeds over Mach 6, and earning their ‘astronaut wings’ by reaching altitudes of 80 km (50 miles) and even higher in the incredible North American X-l5; the US Air Force designates people who travel above an altitude of 50 miles as astronauts, although the International Aeronautical Federation (Federation Aeronautique Internationale, FAI) defines the boundary of space at 100 km (62 miles). Many therefore saw the rocket spaceplane as the logical progression of aviation into space, as had Valier, Tsiolkovsky and Goddard thirty years earlier.

But based on the design of the A4/V2 and insight gained from the Peenemunde experts captured after the war, the US and the USSR had by then both developed an arsenal of intercontinental ballistic missiles. The multi-stage rocket technology used to lob a nuclear warhead halfway around the Earth had reached a very high level of maturity, and laid the basis for a much faster and easier road into orbit which all but suspended the development of rocket spaceplanes. Simply put, the warheads were removed and replaced with satellites and manned capsules. Of course these missiles were not reusable, but in the Moon Race of the 1960s money and sustainability were not the most important issues. Instead of the Silbervogel or similar rocket plane, the ultimate space machine thus became the enormous Saturn V rocket. It boosted Neil Armstrong and Buzz Aldrin to the Moon, in the process beating the Russians in the technological and political space race that had started with the launch of the world’s first satellite by the Soviets in October 1957. Although the ultimate launch vehicle in terms of its capability, the Saturn V was also an extremely wasteful transport vehicle:

excluding the actual payload, the total weight of the 111 meter (363 feet) rocket was some 2,896,000 kg (6,384,000 pounds), 94 percent of which consisted of propellant. At 47 tons (104,000 pounds), the Apollo spacecraft and lunar lander represented less than 2 percent of the entire vehicle that lifted off from the launch pad, and 187 tons (412,000 pounds) of precious hardware was lost by the time the astronauts arrived in lunar orbit; the Saturn Y first stage fell into the ocean, the second stage burned up falling back into the atmosphere, and the third stage went into an orbit around the Sun or crashed on the Moon. Corrected for inflation, in 2011 economic conditions a Saturn V would have cost $2.9 billion per launch, which translates to nearly a billion dollars per astronaut. And for that they were only flying tourist class, with poor food and hardly any leg room. It got you to the Moon and made an impressive amount of noise and smoke doing so, but a Saturn Y was clearly not an economical means of transportation. Good for winning a race, but not for the large-scale economic use of Earth orbit and beyond.

By the mid-1960s the age of the rocket fighter planes and experimental supersonic rocket aircraft had essentially ended, only some 30 years after it started. Jet engines could now also get a plane to high velocities and high altitudes in a short time, and most importantly were much more fuel-efficient than rocket engines. About the only advantage that rocket planes still had over jet aircraft was that they could operate at extreme altitudes and even in the vacuum of space.

In the 1970s the focus of spaceflight in the US turned to a vision of regular, easy and cheap access to low orbit, which seemed to mean good news for rocket plane development. The ideal launch vehicle would be completely reusable, reliable, safe, low-maintenance, efficient and require little work and time between flights (what aircraft operators call a short ‘turn-around’). This sounds just like the description of a regular airliner, and so designs to address these requirements often resembled aircraft.

Normally rocket propulsion would need to be incorporated, because jet engines do not work in space due to a lack of oxygen. But in the 1970s the technology to build a single-stage orbital rocket plane did not exist. NASA therefore sought a multi-stage design. It initially envisaged a combination of two rocket planes in which a massive winged booster would release a smaller vehicle at sufficient altitude and speed for it to insert itself into orbit. The booster would fly back to the launch site. In due course the spaceplane would also land and be prepared for another mission. This two-stage design meant less severe vehicle empty-weight minimization challenges, while both stages of this combination would be fully reusable. But a combination of technical and budgetary constraints mandated a compromise in which the winged first stage vehicle was replaced by a pair of reusable solid propellant rocket boosters, and the orbital spaceplane got an external propellant tank that would be discarded on each flight. This became the Space Shuttle that NASA flew from 1981 to 2011. An ideal spaceplane it was not, because its long launch preparations, complex maintenance, partial reusability and relative fragility led to high costs and high risks. It turned out to be far less cost effective and a much more dangerous means of gaining access to low orbit than its designers envisioned. By the mid-1980s there was a sense in the US, Europe, Russia and Japan that it was time for a fully reusable spaceplane with aircraft-like operations. To limit the propellant load, and thus the overall size and weight, the vehicle would have to combine airbreathing and rocket engines: using oxygen in the atmosphere as oxidizer as long as practical, prior to switching to rocket propulsion when the air density fell below the minimum level required to operate a jet engine. It was believed that materials, flight control and propulsion technology were sufficiently matured to make the development of such a space plane possible.

In a perfect world, a trip in an ideal spaceplane that adheres to the constraints of known physics and near-future technical possibilities could go something like this: On waking up at home you put on your simple flight overall, have a pleasant and relaxed breakfast, then get into your (flying?) car and head for the spaceport. Upon arrival you check in, but don’t need to put on a cumbersome spacesuit, you simply proceed to the plane. In contrast to the early days of spaceflight, there is no doctor checking your fitness, as the flight is extremely benign in terms of accelerations and shocks. Boarding the plane is similar to embarking a normal airliner. Soon after you have settled in, the vehicle starts to rumble down the runway. Just like any airliner, it takes off horizontally and initially ascends at a shallow angle. Apart from requiring a rather long runway there has been nothing extraordinary about the flight up to this point. Even when, two minutes after take-off, the pilot announces that the plane is going supersonic (faster than sound) you don’t notice anything peculiar other than that noise of the engines diminishes because the spaceplane is now outrunning its own sound waves in the air outside. A bit more than another two minutes later you are at an altitude of 12 km (8 miles) and flying at twice the speed of sound. Normal airliners don’t go higher than this, but your spaceplane keeps on climbing. Soon the sky goes dark and the stars become visible. The curvature of the horizon becomes noticeable, confirming the Earth to be a sphere rather than the flat surface it seems through the window of a normal airplane.

At over 28 km (17 miles) and a velocity exceeding five times the speed of sound the engines switch from their airbreathing to pure rocket propulsion mode, and the acceleration, which has been hardly noticeable, suddenly increases and pushes you deeper into your seat. The airplane has become a rocket spaceplane, independent of the oxygen and lift generating capabilities of the atmosphere. Sixteen minutes after take off you are 80 km (50 miles) high and accelerating more than three times more rapidly than a free-falling sky diver. At that moment the engines are shut down, and immediately all sensations of gravity and acceleration vanish and you feel yourself floating in your seat, held in place only by the safety harness. The spaceplane is now in an elliptical orbit around the Earth. You have not yet reached the highest point of your orbit, so you are still going up even although there is no sense of acceleration. Soon you cross the theoretical border of space at 100 km (62 miles) altitude. If you weren’t one already, you’re now officially an astronaut. At 400 km (240 miles) the spaceplane reaches the highest point and the rocket engines are reignited briefly for the boost required to achieve a nice circular orbit. By using small rocket engines, the spaceplane carefully maneuvers towards the space station that is your destination. A couple of hours later the vehicle docks at the large collection of cylindrical modules. You unstrap from your seat and simply float out of the cabin, through the docking tunnel into the space station. You stay there a couple of weeks to work in the biology experiments laboratory module.

When it is time to go home, you board a docked spaceplane of the same type you came up with. It is not the exact same vehicle, because spaceplanes arrive and leave every couple of days transporting people, food, oxygen and experiments to the space station. After undocking, the vehicle slowly drifts away. A half-minute burst of a set of relatively small tail engines in the direction the spaceplane is orbiting, slows the plane down just enough to change its orbit from a circle into an ellipse. The highest point of the orbit is at the altitude of the space station and its lowest point penetrates the atmosphere. As the altitude of the vehicle drops, the aerodynamic drag from the thin atmosphere slows the plane even more. Half an hour after the de-orbit burn you still appear to be in orbit as if nothing had changed, but you are actually falling slowly back to Earth.

The plane was flying backward when firing its rocket engines to break out of its circular orbit, which is okay in the emptiness of space, but it’s not a healthy attitude for entering the thicker layers of the atmosphere. So the pilots rotate the spaceplane through 180 degrees to make it fly nose first. They also pull the nose up, to align its heat resistant belly with the wall of air it is about to encounter. This maneuvering is done using the small attitude control rocket thrusters, because in the near vacuum of space the rudders, elevators and ailerons on the wings and tail of the plane are totally ineffective. The heat shield protects the spaceplane from the extreme heat generated as it slams into the atmosphere. The edges of the wings glow red hot as they reach temperatures of 1,600 degrees Celsius (2,900 degrees Fahrenheit); greater than the melting point of steel. However, unlike the heat shields of the old space capsules the metallic shield on your spaceplane does not slowly burn up, and can thus be reused. It is also not as vulnerable as the old Space Shuttle’s thermal protection tiles, which were reusable but also rather fragile: if it had to, the spaceplane could fly through a hailstorm without damaging its heat shield. And to minimize the vehicle’s take-off weight no propellant was loaded as a reserve for the return flight, so the spaceplane glides back unpowered. As with the Space Shuttle, there is no option of aborting the landing and flying around to make another approach. This might appear to be risky, but actually it isn’t because if the return conditions weren’t perfect and for instance the weather at the airport was likely to be unfavorable, the spaceplane would have just waited in orbit or targeted another landing site well before the execution of the de-orbit burn. As usual the automatic flight system, supervised by the pilots, flies a perfect approach and landing. A bit shaky on your legs, because while you were in space your body adapted to weightlessness, you disembark from the plane. Even as you head home, the vehicle is being refueled, and after a short maintenance check it is declared ready for its next flight.

While this perfect reusable launcher does not yet exist, the above description is based on modern spaceplane concepts such as Skylon, currently under development

at the British company Reaction Engines Ltd. This machine only exists on paper, but realistically illustrates what a near-future orbital rocket plane may look like. What you experience as a passenger may not be too different from what it is like to fly in a high-performance jet aircraft, and while the spaceplane does superficially look like one, there is a big difference in the amount of propellant that it has to carry. While 40% of a large airliner’s take off weight may be fuel (51% in the case of the Concorde), a mixed-propulsion spaceplane such as Skylon will consist of 80% propellant. A pure rocket spaceplane that does not use any airbreathing engines at all would be over 90% propellant, a problem already accurately foreseen by rocket plane pioneer Max Yalier in the late 1920s. Such percentages are similar to those of existing expendable launchers but are much more difficult to attain for aircraft with wings, wheels and a cockpit, that must also be able to survive atmospheric re-entry. In addition, the liquid hydrogen that Skylon uses for fuel has a density that is much lower than that of the kerosene used by normal airplanes: where 1 Uter (0.3 gallons) of kerosene weighs 800 grams (1.80 pounds) the same volume of liquid hydrogen is 70 grams (0.15 pounds). The same amount of fuel thus takes much more room. The passenger cabin onboard a spaceplane will therefore be tiny compared to that of an airliner since most of the vehicle’s volume will need to be filled with fuel (as well as additional liquid oxygen to bum with the hydrogen during the rocket propelled flight phase).

This difference between airplanes and spaceplanes is not so much a matter of their operating altitudes, rather it is the result of their vastly different velocities. Airliners fly at about 950 km per hour (590 miles per hour), whereas to achieve a low orbit a spaceplane will have to achieve a velocity of about 7.8 km per second (4.8 miles per second), which translates to 28,000 km per hour (17,400 miles per hour)! This means the spaceplane’s velocity needs to be about 30 times greater than that of the average airliner. Furthermore, the energy needed to attain a given velocity increases with the square of the flight speed. This means a spaceplane needs some 30 x 30 = 90 times more energy than an airliner of the same weight. This energy must be gained by the engines converting the chemical energy of the propellant into kinetic (movement) energy. And this simplified calculation does not take into account the aerodynamic drag during the climb out of the atmosphere, which also increases quadratically with velocity.

When a spaceplane gets above the atmosphere and reaches orbital velocity, it can circle the Earth without any further need to burn propellant. An airliner however, needs to continuously compensate the drag of the atmosphere it is flying through to maintain its velocity. It does that using its engines, which consume fuel during the entire trip of often thousands of kilometers. This is why airliners in reality do not fly with 90 times less propellant than a spaceplane would require, which you would expect if taking into account speed alone. Nevertheless, whereas airliner designers achieve an optimum in terms of velocity, amount of propellant, cabin volume, cargo weight and ultimately cost, spaceplane designers are pretty much stuck with the need to cram as much propellant as possible into their vehicle, and hopefully in the end have some weight capacity left for the cargo that needs to be taken into orbit, which is, after all, the whole reason for the spaceplane’s flight! In other words, the margin between success and failure is very small: if the spaceplane tank structure proves to be a little heavier than anticipated or the rocket engine yields just 1% less thrust than foreseen, you may end up with a very fast but useless suborbital rocket plane with zero payload, rather than a satellite-launching, money-making, orbital spaceplane.

This narrow margin, plus frustration with the Space Shuttle in terms of costs and risks, has made the world’s space agencies and industries developing launchers extremely cautious with regard to spaceplanes. Since the development of the Space Shuttle hundreds of concepts for spaceplane (and other types of reusable launchers) have come and gone. Some hardware was build and tested, and some designs even got as far as flying a sub-scale test vehicle. But none of them has yet resulted in an operational vehicle, largely because the step to develop a full-scale spaceplane was deemed to be too risky and too expensive, and the benefits could not be sufficiently guaranteed.

In 2005 I attended an international conference on spaceplanes and hypersonic systems in Italy, where scientists and engineers from the US, Europe, China, Russia and Japan, and even Australia, India, South Korea and Saudi Arabia presented developments on exciting sounding topics such as pulsed detonation propulsion and aerospike engines, as well as highly specialized issues like ‘Fluctuations of Mass Flux and Hydrogen Concentration in Supersonic Mixing’ or ‘Pseudo-Shock Wave Produced by Backpressure in Straight and Diverging Rectangular Ducts’. It seemed to me that half the world was involved in spaceplane technology development, and there was certainly no lack of concepts. However, at the time of writing none of the designs for large prototypes, let alone operational spaceplanes, have moved beyond the drawing board.

Whereas up until the 1970s advances in rocket plane technology were often soon incorporated in new experimental planes and high-altitude rocket interceptors, now engineers seem to be stuck in their laboratories, able to fly their innovations only on small-scale test models. This is due to the extreme complexity and enormous cost of developing modern high-performance airplanes in general and space launch vehicles in particular. In the Second World War it took the famous P-51 Mustang fighter only six months to progress from the conceptual design to its first flight, with just another 19 months until it entered combat service. At the time the US government purchased them at $50,000 per airplane, the equivalent of $600,000 today. The US Air Force’s latest F-22 Raptor fighter took over 20 years to advance from concept to operational fighter at a development cost of $65 bilhon. Each of these sophisticated planes costs some $143 million: 238 times more than a Mustang! A future spaceplane will have a lot more in common with modern aircraft like the F-22 than with the nuts-and-bolts Mustang. There is no easy and cheap way to develop an operational spaceplane, so the technical and financial risks will be high. Hence, the benefits must also be high and more or less guaranteed.

Looking into the near future, it is clear that the preference for expendable launch vehicles is ongoing. Various new throw-away launchers or updates of existing ones are under development by a number of agencies, while research on reusable launch vehicles is continuing at a very slow pace with much lower levels of funding.

“Perfect as the wing of a bird may be, it will never enable the bird to fly if unsupported by the air. Facts are the air of science. Without them a man of science can never rise.” – Ivan Pavlov (1849-1936)

To be able to understand the possibilities, limitations, history and evolution of rocket planes, we must look at how they work. We start with the ‘rocket-’ part, then explore the ‘-plane’ element, and finally investigate the wonderful and dangerous things that happen when you combine them.

ROCKET ENGINES

The principle of a rocket engine is fairly simple: generate a gas at high pressure by burning propellant in an enclosed space and let it escape through a nozzle. The resulting thrust has nothing whatever to do with the rocket ‘pushing against the air’, but is purely a consequence of Isaac Newton’s famous principle: for every action there is an equal and opposite reaction. If you stand on a skateboard and throw rocks away, then you will move in the other direction: the ‘action’ is throwing rocks backward and the ‘reaction’ is you moving forward. Pushing the rock away also means pushing yourself away from the rock. Another good example is a fire hose: as lots of water spews out at high speed you feel the thrust trying to push you back. The hose sprays water in one direction, and in reaction the hose itself is pushed in the opposite direction. In essence this is a rocket engine working on water, and if you stood on the skateboard holding the fire hose, then you would have basically created a rocket propelled vehicle. Instead of throwing rocks, you would be throwing out water continuously. The principle that Newton derived works because the rocks and the water have mass, and the greater the mass and the higher the velocity at which you throw them away, then the higher will be the velocity that you achieve in the opposite direction. You can imagine that throwing something with a small mass relative to yourself, such as a feather, won’t have much effect. Throwing away a bowling ball with very little speed, essentially letting it fall out of your hands, will also not achieve much. Only if you throw away objects of substantial mass at a

M. van Pelt, Rocketing into the Future: The History and Technology of Rocket Planes, Springer Praxis Books, DOI 10.1007/978-1-4614-3200-5 2, © Springer Science+Business Media New York 2012

significant speed will the resulting thrust be enough to push you away on a skateboard. If the rock that you throw away is one-tenth of your own weight, then you will attain that proportion of the velocity at which the rock is flying out (ignoring the friction of the skateboard’s wheels with the ground). If you want to go faster, you can either throw out a larger rock at the same velocity, or the same rock at a higher velocity, or indeed a smaller rock at even higher speed.

The thrust of a rocket engine is measured in ‘Newton’ in the metric system, and in ‘pounds of thrust’ in the US. One Newton is the force that a 0.1 kg mass exerts on a floor on the Earth’s surface. Isaac Newton stated that a force (or thrust) is equal to mass times acceleration. On Earth, if you let something fall it will speed up by about 10 meters per second every second: i. e. after 1 second its velocity is 10 meters per second, after 2 seconds it is 20 meters per second, and so on. This means that on the Earth’s surface the gravitational acceleration is 10 meters per second per second (or to be more precise 9.81 meters per second per second). If you stand on your skateboard again, and every second you throw away a 1 kg rock at 10 meters per second, then you will be creating a thrust of 1 kg times 10 meters per second per second = 10 Newton.

In real rocket engines the necessary high pressures are generated by combustion. The resulting gas expands out through a nozzle at tremendous velocity, and because it has mass this results in a powerful thrust. It is basically a continuous explosion: a single explosion gives a short kick, a series of explosions provides a series of kicks, and continuous combustion and expansion yields a steady thrust. For combustion to occur, a fuel and an oxidizer are required. An oxidizer is a substance that contains the oxygen that makes things burn. In the engine of a car the fuel is gasoline and the oxidizer is ordinary air, which contains some 21 percent of oxygen. Rockets don’t use atmospheric air, but carry their own oxidizer.

In a liquid propellant rocket engine, the fuel and the oxidizer are in the form of liquids, for example alcohol and liquid oxygen. These are stored in separate tanks, from which they are fed into a combustion chamber using pressure in the tanks or powerful pumps. Such pumps are typically powered by a separate gas generator, in which some propellant (which can be the same as those used in the combustion chamber) is burned or decomposed to provide high-pressure gas. This gas is then fed

through a turbine that runs the pumps, and expelled through a separate exhaust; this arrangement is known as a turbopump. In more sophisticated engines some of the rocket’s propellants are burned in a pre-burner and then used to run the turbopumps. However, instead of being dumped directly, the exhausted gas is then injected into the main combustion chamber along with more propellant, in order to complete the combustion; this is called a ‘staged combustion cycle’. Turbopumps run at tremendous rates. For example, the turbines of the main engines of the Space Shuttle spin at 30,000 cycles per minute!

In the rocket combustion chamber the fuel and oxidizer are mixed and burned to create an extremely hot, high-pressure gas. This can only escape through an opening in the combustion chamber that is connected to the rocket nozzle. The gas flows out at high velocity through the nozzle, whose shape permits the gas to expand (and thus accelerate), and flow nicely in the right direction. For high-performance rocket engines the exhaust velocities are around 16,000 km per hour (10,000 miles per hour); much faster than throwing rocks! The nozzle is where the expanding gas exerts its forwards pressure, and the correct shape and length are critical in determining the achievable exhaust velocity. If the nozzle is too short or its shape

does not allow the exhaust to expand properly, then lots of energy that could be used for generating thrust is lost. The nozzle first converges to a narrow throat so that the velocity of the gas stream is increased, just as occurs when water is passed through a narrowing channel. At the throat it reaches Mach one (the gas mixture’s speed of sound) and creates a shock wave, after which the nozzle diverges to allow the high-pressure gas to expand and thereby flow out efficiently at speeds far beyond Mach one. The temperature of this gas stream can reach 3,000°C (5,400°F). The combus­tion chamber and the nozzle must be cooled to prevent them from melting. Their walls are often made hollow, so that rocket propellant can be pumped through in order to act as coolant before being burned inside the combustion chamber. It is also possible to make the nozzle so thick that it can be allowed to slowly erode during flight. These so – called ablative nozzles are relatively simple and cheap, but also heavy and obviously not reusable. They are usually applied in solid propellant boosters, which have no liquid propellants to use as coolants.

The efficiency of a rocket is indicated by its specific impulse, and is measured in seconds. It is one of the most important parameters in the Solid propellant rocket. equations that describe a rocket’s performance. A

specific impulse of 400 seconds means that with 1 kg of onboard propellant the rocket can generate 1 kg of thrust (i. e. 10 Newton) for 400 seconds. (In the US, 1 pound of thrust from 1 pound of propellant for 400 seconds). You can think of the specific impulse of a rocket engine as the number of seconds a certain amount its propellant is able to generate the thrust required to keep itself in the air.

The simplest rockets combine the fuel and oxidizer into a solid propellant like for instance gunpowder. A solid propellant rocket can be viewed as a pipe stuffed with propellant. The propellant grain is usually hollow in order to expose a large burning area, and hence a high thrust due to the high pressure and the resulting large amount of gas flowing out. Firework rockets are of this type. The main advantage of solid propellant rockets is that they are simple, because they do not require any pumps, pipes and valves. As a result, they can provide a lot of thrust for relatively low cost. A big disadvantage however, is that they normally cannot be reused: the throats and nozzles of these motors typically bum away during firing, as there are no liquids to act as coolant. Other very important disadvantages are that, in contrast to liquid propellant engines, solid propellant motors cannot be stopped and you cannot actively control the thmst. Once ignited, the grain will bum away until it is all gone. If something goes wrong along the way, you cannot slow down or stop. In fact, if there is a problem with the motor itself, like a crack in the propellant grain or a piece of material blocking the nozzle, the propellant will keep on burning and the increasing pressure will result in a violent explosion. Solid rockets either work well or they blow up; there is no ‘benign failure’ which merely results in a loss of thmst, as is possible when using liquid propellant rocket engines. Nevertheless, by cleverly designing the shape of the propellant grain it is possible to vary the amount of thrust desired at certain times after ignition. Many solid propellant rockets used for launch vehicles have star-shaped cross sec­tions. At first the burning surface will be large and the rocket will provide a maximum of thmst in order to get the vehicle off the ground quickly. As the pie-shaped sections burn away, the active surface is reduced and the thrust diminishes in order to limit the aerodynamic forces on the vehicle while it is flying at high speed through the atmosphere.

Another important disadvantage is that for any given mass of propellants, solids cannot provide as much thmst as liquids. A rocket engine that uses liquid hydrogen and liquid oxygen can have a specific impulse of 450 seconds but solid propellants can achieve no better than 290 seconds. On the other hand, because of their relative simphcity it is

easier to develop and build big powerful solid propellant rockets than large liquid propellant rocket engines, albeit they are much less efficient in terms of the amount of energy to be derived per kilogram of propellant. Also, the storage of solid propellants (in the rocket motor) is generally easier than of liquid propellants, which often require cooling and can be corrosive or toxic.

Probably the most famous solid propellant rockets are the Solid Rocket Boosters of the Space Shuttle. Each of these huge rockets provides a maximum thrust of 13,800,000 Newton, which means one booster could lift some 1,380 cars. Together the two boosters generate about 83% of the lift-off thrust of the Space Shuttle, with the more efficient but far less powerful liquid propellant engines of the Orbiter vehicle only providing the remaining 17%. The Space Shuttle’s rocket boosters were actually recovered (by parachute, splashing into the ocean) and reused, but this required much cleaning and refurbishment, and was only economical because of the booster’s huge size and production cost.

In contrast to a rocket engine used in an expendable missile, one that is meant to propel an aircraft has to comply with more requirements: it needs to be restartable, reusable, maintainable, and reasonably safe for both the pilot and the ground crew. Ablative cooling, where the rocket’s throat and nozzle lose heat by slowly burning away, is not a viable solution for a reusable system; an aircraft rocket engine needs active cooling which pumps the propellant through cooling ducts in the combustion chamber and nozzle. The engine’s igniter, which was an external piece of ground support equipment for the series of missiles of which the A4/V2 was part, must be incorporated into the rocket motor itself. On the other hand, the required reliability and safety means that performance may require to be sacrificed to improve safety margins, reduce wear and tear and simplify maintenance.

Overall, their non-reusability, lack of a throttle, poor efficiency and explosion hazards make solid rocket motors generally a poor choice for propelling manned aircraft, other than briefly for an assisted take-off. Liquid propellant rocket engines are more controllable, more efficient and can be made reusable, so are generally a better choice for aircraft propulsion, even if they are more complex.

The main benefit of rocket engines over jet engines and propellers, and the reason that they are used in spaceflight, is that they can operate outside the atmosphere. Jet engines uphold Newton’s ‘action equals reaction’, just like rockets, but they depend on oxygen from the air to burn aircraft fuel. Propellers push air backwards to make a plane go forward, and are driven by engines that need atmospheric oxygen. Rockets carry both the necessary oxidizer as well as the fuel. Independence from the atmosphere is rather handy if you are flying at high altitudes or through the vacuum of space, where there is either insufficient or no oxygen available for your engines. Interestingly, rocket engines also offer an advantage in the thicker atmosphere because their operation is totally independent of velocity. Propellers lose efficiency because of aerodynamic shock waves which form when the rotation of their blades approaches the speed of sound. The same holds for the compressor fans in turbojet engines, although these can still be used at supersonic flight velocities because the airflow entering the engine can be slowed down to subsonic speed by the air intakes (but this does cost energy at the detriment of the plane’s velocity, and becomes increasingly problematic at higher supersonic flight speeds).

Rocket engines are also intrinsically less complex than jet engines. This is not to say that rocket motors cannot reach very high levels of complexity (just take a look at a schematic of the Space Shuttle Main Engine) but it is generally easier to build a simple rocket motor than it is to construct a simple jet engine. Basic solid propellant rocket motors are much simpler than any piston engine and propeller combination; an alternative history in which the Wright brothers power their aircraft using not a primitive piston engine but a few simple solid rockets is not completely unrealistic. After all, solid propellant rockets had already been in use for several centuries when the piston engine and – even later – the jet engine were invented.

Rocket engines running on liquid propellants are much more efficient in terms of the amount of energy that can be obtained out of a certain amount of propellant, but the requirement for pumps, valves and cooling makes them more complex than their solid propellant counterparts. Nevertheless, without large compressors and turbines, complex air intakes, supersonic shock problems and intake drag, they were, at least initially, a simpler solution for high-speed, high-altitude aircraft than turbojets. This is why so many of the high-speed, high-altitude aircraft developed during the 1940s and 1950s were propelled by rockets rather than jet engines. For the same thrust, a liquid propellant rocket engine is also much less heavy than a turbojet engine: a large modern rocket engine can produce a thrust that is 70 (the Space Shuttle Main Engine) or even 138 times (the Russian NK-33) as great as its own mass (although this number is much lower for smaller rocket engines), whereas for a turbojet the thrust to weight ratio approaches eight at best. For the same thrust, rocket engines are also considerably smaller than jet engines. However, a rocket engine quickly loses this advantage if account is taken of the weight and volume of the propellants that must be carried. Considering just the fuel flow, since the oxidizer comes from outside, the specific impulse of a jet engine is some 20 times that of a rocket engine, and is thus much more efficient. For a plane that needs to have a considerable range it soon becomes more economical to use an air-breathing jet engine rather than rocket propulsion. In aviation rocket engines have therefore been mostly used for early high-speed, short range aircraft such as experimental planes and high altitude military interceptors.

A rocket can propel a vehicle to high speeds and high altitudes, both of which are required in order to achieve orbit. An orbit represents a delicate balance between a vehicle’s velocity and the Earth’s gravity. Imagine that you toss a ball away at low speed. You will see it follow a curved trajectory and hit the floor some meters away. If you want it to go further, you will need to throw the ball a bit faster. It will still fall and accelerate towards the ground at the same rate as before, because the force of gravity remains the same. However, because its initial horizontal speed is higher, it will cover a larger horizontal distance before landing. Now imagine that you shoot

your ball away with a cannon: watch it fly, all the way over the horizon! Because the Earth is round, its surface drops away under the ball while it is falling. The result is that it takes the ball longer to reach the ground, and as a consequence it will manage to fly farther away than if we were living on a flat world. If you get the ball up to a velocity of about 8 km per second (5 miles per second), the curvature of the ball’s trajectory under the pull of gravity is exactly the same as the curvature of the Earth. In effect, the ball is continuously falling around the world, without ever hitting the ground: it is in orbit! If you shoot the ball eastwards, it will circle around the planet and reappear from the west just under 1.5 hours later.

In reahty, the atmosphere would slow the ball down so much that it would never come back, it would either burn up or fall short. However, at altitudes over 100 km (60 miles) there is hardly any atmosphere left to slow down a moving object. If you can get your ball up to speed there, it will be able to circle the Earth and become a satellite.

To launch a satellite, a rocket is initially fired straight up in order to rapidly climb above the thickest layers of the atmosphere and minimize aerodynamic drag. It then starts to pitch over, so that as the rocket keeps accelerating it increases both altitude and horizontal velocity. On the way up a conventional launcher drop stages to shed the dead weight of the empty tanks and engines that are no longer required. Without this ‘staging’ it would be too heavy to reach orbit. Most launch vehicles consist of two or three rocket stages, one on top of the other, plus sometimes two or more rocket boosters attached to the side of the first stage. Once the last stage reaches the proper orbital altitude and velocity, the satellite is released. The last rocket stage usually stays in orbit as well, but the other stages splash into the ocean, crash on land, or burn up in the atmosphere when falhng at high velocity. In principle it would be possible to design these stages to be recovered and reused, but that would add an enormous amount of complexity and, most importantly, weight. A

recoverable upper stage would need a heat shield to protect during its re-entry into the atmosphere, parachutes, and probably also airbags to cushion the impact on land or more likely at sea. Such a stage could easily become so heavy that it would not be of any use in a launcher. In addition, the recovery and refurbishment would be very expensive. All currently used satellite launch vehicles (except for the Space Shuttle, until recently) are therefore expendable, meaning they can only used once because everything except the satellite payload is discarded during the short flight up. As we will see later, in the case of the Space Shuttle the two Solid Rocket Boosters and the Orbiter itself were reused, but the large External Tank which held most of the liquid propellant for the Orbiter on the way up was still discarded. In operational terms, it would be best to use a fully reusable vehicle requiring only a single stage to get into orbit, just like you would not want to have a commercial airliner dropping off tanks and engines on the way to its destination. Expendable rocket stages are expensive to build, but usually it is more cost effective than providing a soft-landing capability and then retrieving, refurbishing and reintegrating reusable rocket stages. A Single Stage To Orbit (SSTO) vehicle must carry everything all the way up, including the large rocket engines and propellant tanks which will be used only for the ascent. It must also carry into orbit all the propellant, heat shielding, parachutes, wings and so on, that it will need to return to Earth. All the extra weight that an SSTO has to take into space diminishes the amount of cargo, or payload, it can transport, which is the actual reason for the launch. Each 100 kg of empty tankage that an SSTO takes into orbit is at the cost of 100 kg of payload. Since it takes about 30 kg of propellant and rocket hardware to place 1 kg of payload into a low orbit, and the payload typically represents only 3.5% of the total weight that leaves the launch pad, the design of an SSTO can easily result in a vehicle with a zero payload capability if the tank mass is a bit

higher than expected or the rocket engine is slightly less performing. Such a launcher would only be able to put itself into orbit, if at all.

Flying is all about balancing forces: thrust and drag in the horizontal direction, and lift and gravity in the vertical direction. The engines generate the thrust required to pull or push a plane through the air. For a steady speed, this thrust must be equal to the aerodynamic drag on the airplane. If the thrust is lower than the drag, the plane will slow down. If it is higher, the plane will accelerate. However, with increasing speed the drag will also increase, so at a certain moment the drag on the plane will again be equal the thrust. When that happens, the vehicle will continue to fly at a constant speed that is higher than it was at the lower thrust level.

In the vertical direction, the aerodynamic lift generated by the wings must be in balance with the force of gravity pulling the plane down. If the lift is too small, the airplane will descend; if the lift exceeds the plane’s weight, it will gain altitude. To maintain a constant altitude, the lift must precisely balance the weight of the plane. When thrust equals drag, and lift equals weight constant velocity, straight and level flight is possible; but if any of the forces changes, the balance will be lost and the airplane will go up or down, accelerate or decelerate. These changes often occur in combination: for example in a dive, a plane will lose altitude and at the same time speed up.

Aerodynamic drag is a familiar thing: you feel it when you walk against the wind or when you stick you hand out of a car window. The amount of drag depends on the speed of the air (wind) or the speed of the car in the second example. Whether the air moves to you, or you move through the air doesn’t matter: what is important is your relative velocity with respect to the air. Aerodynamic drag increases as a function of the square of the speed, so if you go twice as fast, the drag will increase by a factor of four. If you double your speed again, the drag will become sixteen times what it was originally. You can see where this goes: the drag increases at a much higher rate than your speed, so the higher your velocity, the harder it will be to go even faster.

Drag also depends on the size and shape of an object moving through the air. For similar shapes, an object with a large frontal surface will experience more drag than one with a smaller surface: a small hand out of a car window will feel less drag than a big hand, and a truck will suffer more drag than a small car. Aerodynamic drag on a vehicle can be decreased by using a good shape: the easier the air can flow around an object, the lower the drag will be. This is why sleek aircraft and racing cars have pointy noses. Lower drag means it takes less thrust to attain a certain speed, or that you can reach a higher velocity with the same thrust. Minimizing aerodynamic drag has therefore always been one of the driving issues in airplane design, and has led to continuous improvements in the shape of fuselages, the use of undercarriages which retract and the elimination of high-drag beams and cables.

The aerodynamic force that holds an airplane up is the lift is created by its wings.

The physics behind lift are complicated. I have a book written in 1909 called Flying, The Why & Wherefore, in which several theories on lift generation are presented; six years after the invention of the airplane, at a time when many types of planes were flying around, it was still not really understood what made those machines stay in the air! Even more surprising is that those same theories are all still employed to explain how lift is generated, even today, after over a century of flying experience. First there is the ‘Longer Path’ theory, which is also known as the ‘Bernoulli’ or ‘Equal Transit Time’ explanation. This theory is based on the assumption that air molecules that reach the leading edge of a wing at the same time, then flow either over or under it but all reach the trailing edge at the same moment. As the top surface of a typical asymmetrical wing is more curved than the underside, air molecules going above the wing have to travel a longer distance than those that pass under the wing, doing so in the same amount of time. Air flowing over the wing must therefore travel faster than that under the wing. Bernoulli’s equation, a fundamental of fluid dynamics, states that as the speed of the air increases, its pressure decreases (the air molecules have less time to exert pressure on the surface). Hence the faster moving air on the top surface of the wing develops a low pressure area, while the slower moving air maintains a higher pressure on the underside. The low pressure essentially ‘sucks’ the wing upward (or the high pressure pushes it up, depending on your point of view). The weakness of this theory lies in its assumption that two molecules that become separated by the leading edge of the wing rejoin at the trailing edge at exactly the same moment. Even though you can measure that the air on top of a curved wing does indeed travel faster than under the wing, there is no fundamental reason why the molecules ought to meet again at the trailing edge and reach it at exactly the same time.

Another way to explain why wings generate lift is the ‘Newtonian’ explanation, based on the ‘action equals reaction’ idea which is also the working principle of the rocket engine. Air molecules hitting the bottom surface of an inclined wing bounce off and are deflected downward. In reaction, the wing is not only pushed up (lift) but also backwards (drag). You can ascertain that this is true by sticking your hand horizontally out of the window of a moving car, and slowly rotating it vertically. The

greater the angle with respect to the airflow, the stronger will your hand be pushed up and in the direction of the airflow. This idea explains why airplanes with symmetrical airfoils (symmetrical wing cross sections) or even flat wings (such as those of paper airplanes) can fly, but it does not explain why an asymmetric airfoil with a strongly curved upper surface provides more lift. In fact, the Newtonian explanation leaves the top of the wing completely out of the picture. We also now know that molecules in the dense lower atmosphere, in which aircraft normally fly, do not act as individual particles, they actually interact and influence each other in complex ways. Nevertheless, air is indeed deflected downwards by an angled plate. The Newtonian explanation also correctly predicts that if you increase the inclination of the wing with respect to the airflow (its ‘angle of attack’) it will provide more lift but also experience more drag.

What happens in reahty is a combination of these explanations, plus some more complex fluid dynamics. Air approaching the top surface of a wing is compressed into the air above it as it moves upward near the leading edge. The top surface then curves downward and away from the airflow, creating a low-pressure area that pulls the air above down toward the back of the wing. Simultaneously air approaching the bottom surface of the wing at the leading edge is slowed, compressed and directed downward. When this air nears the rear of the wing, its speed and pressure gradually match that of the air coming over the top. When you sum up all the pressures acting on the top and bottom of the wing, you end up with a net force that pushes the wing upward. However this force is not aimed straight up, it has a component in the backward direction. This is the aerodynamic drag described before. If the angle of attack is increased, the pressure differences between the bottom and top of the wing become larger, resulting in more lift as well as increased drag. There is of course a limit to how steep the angle of attack can be made, because beyond a certain angle the airflow over the wing is no longer able to nicely follow its curved contour; it no longer ‘sticks’ to the upper surface. This detached airflow creates a large turbulent wake that dramatically decreases the lift while increasing the drag. This is called a stall, and if the plane does not have enough engine thrust to compensate for the loss of lift it will fall out of the sky like a leaf falhng from a tree. Normally it will accelerate going down, building sufficient speed to regain lift and control. However, if the plane is turning while falling it can enter what is called a spin. If the aircraft is forgiving and/or the pilot is lucky, this spin will be a normal one in which the nose points somewhat downwards and the corkscrewing descent provides sufficient control to achieve a recovery. The aircraft may be upside down, which is called an inverted spin. Much more serious is a flat spin, where the plane is falling straight down in a horizontal orientation and rotating on an axis perpendicular to its wings. In that case the airflow around the wings and tail is completely useless, and recovery often impossible.

The fact that the upper surface of an asymmetrical airfoil is curved means the pressure effects on the upper wing surface are more pronounced than those on the bottom of the wing. The upper part of the wing therefore contributes most to the generation of lift, which is one reason why most airplanes that use wing-mounted engines have them hanging under the wings rather than attached on the top of the wings: disturbing the air beneath a wing is less detrimental to flight than disturbing the air above a wing. This is also why the aforementioned ‘liquid-air’ rocket plane designed by Augustus Post in 1928 had air being expanded over the wing to create additional lift at high altitudes.

The wings of an airplane are optimized for a certain use in terms of speed and altitude. Airplanes that need a lot of lift at relatively low speeds have wings with strongly curved upper surfaces. This makes it possible to take off and land at low speeds and with heavy loads; very useful for military cargo carriers that have to be able to use short, improvised runways. The downside of such wings is that along with the powerful lift they also generate a lot of drag, which makes it difficult and uneconomical to fly at high speeds. If you want to fly really fast, you require small, thin wings designed to minimize drag while still generating sufficient lift to remain airborne at high speed. But such wings do not provide much lift at low speeds, and consequently planes that have such wings have higher take off and landing speeds and require long, smooth runways. If speed is of paramount importance, as it is for military fighter planes, you will go for the benefits of small, thin wings and accept the disadvantages that go with their use.

Lift not only depends on speed, but also on the density of the air. High up in the atmosphere the density of the air is much lower than at sea level, so that the lift that any given wing creates will diminish with increasing altitude (which is why aircraft have maximum operating altitudes). To be able to fly high, you must either employ larger wings or you must go very fast so that your wings generate more lift (as lift, just like drag, is a function of the square of the velocity of the air). But flying very fast increases aerodynamic drag and so requires large, powerful engines. Using big, slender wings is more economical in terms of engine power and fuel consumption. An example of the long-wing solution to reach extreme altitudes is the famous U2 spy plane, the modem version of which can reach an altitude of almost 26 km (16 miles) but has a maximum speed of only 800 km per hour (500 miles per hour). The SR-71 Blackbird spy plane can reach a similar altitude, but its short delta wings give it a top speed of no less than 3,530 km per hour (2,200 miles per hour). The penalty, of course, is that the SR-71 has a much more voracious fuel consumption.

AEROPLANE ANATOMY

An airplane consists primarily of a fuselage, wings, stabilizers and engines. The fuselage is the body that connects all the other parts and holds the passengers, cargo and the pilots who control the vehicle from the cockpit. The required lift is provided by the wings. Besides these basic elements, much more is however needed to safely control an airplane.

Normal wings are designed to work optimally at the normal, cruise speed of the airplane. At take-off and landing, airplanes necessarily fly much slower, so ideally at those times you would rather have wings that give more lift at low speed. Airplanes are therefore often equipped with mechanical ‘flaps’ and ‘slats’ that can effectively change the shape of the wings. Flaps can be extended rearward and downward from the trailing edge to give the plane more lift at low speeds. Slats do a similar job, but on the front of the wing. In normal flight, when they are not needed, flaps and slats are retracted in order to minimize the aerodynamic drag of the wings. ‘Spoilers’ are door-Uke flaps on top of the wing. When moved up, they disturb (spoil) the airflow over the wing and thereby quickly diminish lift and increase drag. They are used to slow down and reduce altitude in landing, and also to assist with braking as well as to keep the plane firmly on the runway as it rolls after touchdown.

Like any object, an airplane tends to rotate around its center of mass. To steer an airplane, a pilot must control its movements around three axes that can be imagined

to radiate orthogonally from the center of mass, also known as the center of gravity. Rotation around the horizontal axis that runs from one wingtip to the other, making the nose go up or down, is called ‘pitch’. The left-right movement of the nose, in other words the rotation around the vertical axis, is called ‘yaw’. The third axis runs from the nose to the tail and is the line around which an airplane can ‘roll’ to make one wing rise and the other one drop. When driving your car you can only steer it to go left or right, but a pilot has two more rotation axes to take care of. In addition, your car can only go forward or backward, but a plane can also go up and down (although normally not backward). All this makes flying a plane much more complex than driving a ground vehicle.

Conventional airplanes have horizontal tail stabilizers, also called the tailplane. When normal wings are generating lift, they have the tendency to push the airplane’s nose downwards, i. e. make the plane pitch down. To avoid this, the small horizontal stabilizers act as wings that provide a negative lift, pushing the tail down (and hence the nose up) and thereby counterbalance the pitching-down effect of the main wings. They are equipped with moveable flaps called ‘elevators’, which can increase or decrease the lift of the horizontal stabilizers and thereby make a plane pitch up or down. If a pilot pulls on his control stick, the elevators point up, pushing the tail down and therefore the airplane’s nose up. This increases the angle of attack of the wings, increasing the lift and making the airplane gain altitude. Pushing the control stick forward has the opposite effect. By use of the elevators a pilot can control the altitude of the plane.

The vertical stabilizer on the tail gives stability in the horizontal direction, much like the keel of a boat. A moveable flap on its trailing edge, called the ‘rudder’, lets the pilot move the nose of the airplane left and right and thus provides yaw control. The rudder is operated with the pedals at the feet of the pilot: pushing the left foot forward moves the airplane’s nose to the left and vice versa. Roll is controlled by ‘ailerons’ on the main wings. If the pilot pushes the stick to the right, the aileron on the left wing moves down and increases the lift of that wing. On the right wing the opposite happens. The result is the left wing goes up and the right wing goes down. From the pilot’s point of view the airplane rolls clockwise. The spoilers can also be used to roll an airplane. Extending the spoilers on one of the wings will reduce the lift on that wing and make it drop, so that the plane will roll in the direction of the ‘spoiled’ wing.

Many planes also exhibit a so-called dihedral angle in their wings, which means the wings are canted slightly upward to form a weak Y-shape. This helps to prevent unwanted roll, making the plane more stable. When a plane with wing dihedral rolls away from level flight, the lift force on its wings will no longer point straight up but somewhat to the side. As a result, the plane will sideslip, which means it is not only flying forward but also slightly sideways. Because of the wing dihedral the situation of the wings with respect to the airflow, which now comes shghtly from the side, is asymmetrical. The upward tilted wing presents a less favorable angle to the airflow than the wing that is angled downwards, and hence produces less lift than the other wing. This unbalance in lift will automatically roll the plane back until both wings are again at the same angle to the horizon; in essence, the sideslip airflow ‘pushes’ the wings back to the horizontal level.

The undercarriage, normally fitted with wheels, enables a plane to move over the runway during take-off and landing, and to taxi around on the airport. At very low speeds the rudder on the tail of a jet or rocket plane cannot work, because of the low velocity of the air flowing over it. A rotating nose wheel can then be used to steer the airplane. The undercarriage creates a lot of aerodynamic drag during flight, so in most modern airplanes it is retracted when not needed. Although there still are airplanes that have fixed undercarriages with wheels, floats or skids, high-speed

airplanes must always get their undercarriage out of the way, either by retracting it into the wings and/or fuselage, or by dropping it altogether; the last option has the benefit that the plane does not have to drag the heavy undercarriage around in flight, but of course it still needs something to land on. The landing undercarriage can however be less robust and often smaller, because the weight of the plane at landing will be lower than at take-off by the amount of fuel consumed. This may be a good idea for experimental planes and spaceplanes with severe weight limitations, but it reduces the operational flexibility by requiring the airplane to be re-mated to the jettisonable undercarriage prior to each flight.

An important issue in the design of an airplane is whether it should be able to fly faster than the speed of sound. Sound has a speed limit that is easily noticeable: when you observe a flash of lightning in the distance the light reaches your eyes virtually instantaneously, but you may hear the thunder only several seconds later. Clearly, sound travels through the air at a speed that is much lower than that of light. In fact, every three seconds that elapse between lightning and thunder means that the event occurred about 1 km further away from you (or 1 mile every five seconds); if the flash and boom arrive at about the same time, you are in real danger. The speed of sound is defined as Mach 1, after the Czech/Austrian physicist Ernst Mach. The velocity of airplanes able to fly faster than the speed of sound is usually expressed in terms of Mach numbers, with Mach 2 meaning twice that speed. Flying significantly faster than the speed of sound is called supersonic. In contrast, subsonic flight means a speed of less than Mach 1. For example Mach 0.5 means half the speed of sound. Flying at precisely the speed of sound is called sonic flight. The Mach number not only depends on the actual speed of an airplane, but also on its altitude. The higher you go in the atmosphere, the lower the speed of sound because of the diminishing temperature of the air. At sea level and 20 degrees Celsius (68 degrees Fahrenheit), Mach 1 corresponds to a velocity of 340 meters per second (1,130 feet per second), or 1,240 km per hour (770 miles per hour). However, at 11 km (7 miles) altitude it means 1,060 km per hour (660 miles per hour). At altitudes from about 11 to 20 km (7 to 12 miles) the air temperature is constant, and so too is the speed of sound. But above this region up to about 50 km (30 miles) the air temperature and the speed of sound increase again. If it is mentioned somewhere that a plane is capable of flying at Mach 2, you will need to know at which altitude it can achieve this in order to be able to derive its real velocity.

The engines generate the thrust necessary to pull or push the plane through the air. For relatively slow aircraft like commercial airliners that do not exceed the speed of sound, the engines are often attached to the outside of the wings or the fuselage with struts. This makes it easier to maintain and replace the engines, and allows the use of different types of engines without the need to modify the rest of the aircraft. Aircraft that need to be more streamlined, for instance for supersonic flight or high speed at low altitudes, require their engines to be inside the fuselage or attached very closely to the wings, without struts. Typical examples are military fighter jets and the Concorde. Conventional airplanes which fly at relatively low speeds and low altitudes often use propellers, which, in simple terms, are spinning wings. The rotation provides the propeller blades with the necessary speed to generate lift in the

horizontal direction. Helicopter blades operate on the same basic principle. Propellers work best in dense air, so are not suitable for high altitude flight. Another major disadvantage is that a propeller’s performance drops quickly when the blade speed exceeds the speed of sound, because shock waves will form that dramatically increase the aerodynamic drag on the propeller whilst decreasing the thrust that it develops. As the speed of a propeller blade depends on both its rotational speed and the velocity of the aircraft, the blades will reach sonic speed long before the rest of the aircraft. Consequently, aircraft equipped with conventional propellers are only good for flight speeds up to about Mach 0.6. Propellers can be driven by combustion engines similar to those in cars, or by gas turbine engines.

Gas turbine engines (often simply called jet engines) all consist of the same basic components: an inlet for the air, a compressor, combustion chamber(s), a turbine and an exhaust. The compressor consists of rotating rows of fan blades that suck in air through the inlet and compresses it to increase the amount of oxygen available per liter of air. The high-pressure air then enters the combustion chamber(s), where it is mixed with fuel (typically kerosene) and burned. The resulting powerful stream of high-pressure exhaust gas then expands through (and therefore turns) another set of fans called the turbine. The turbine is connected to the compressor via a shaft, so the exhaust turns the turbines which turn the compressors to suck in and compress more air and thereby keep the engine running. In addition to the compressor, the shaft can be connected to a propeller, making the engine a so-called turboprop engine. Since the propeller needs to spin at a much lower speed than the compressor, it is linked to the shaft via a gearbox. Using the same principle a gas turbine engine, now referred to as a turboshaft engine, can drive the propellers of a ship, the wheels of a tank or the blades of a helicopter.

The momentum of the exhaust from a gas turbine engine can also be used more directly to provide thrust by Newton’s ‘action equals reaction’, just like in a rocket engine. In such an engine, called a turbojet engine, it is the flow of gas that pushes

J L

A turbojet engine [Federal Aviation Administration].

the aircraft through the air. This enables very high speeds and is therefore primarily used in military fighters. The principal difference between a turbojet engine and a rocket is that a turbojet uses atmospheric oxygen to burn with the fuel, rather than oxidizer drawn from a tank. The working principle is otherwise basically the same. The powerful thrust of a turbojet can be further increased by using an afterburner, a long tube that is installed behind the jet engine’s exhaust, and into which additional fuel is injected and burned with the unused oxygen remaining in the hot gas coming out of the engine. As per the ‘action equals reaction’ principle, this provides a large boost in thrust. It is however a pretty inefficient means of propulsion and it greatly increases fuel consumption. Afterburners are typically used on fighter aircraft, and even then only for rapid acceleration during brief periods of time. Afterburners are however a great way to attract attention at air shows, because they provide lots of noise and long, dazzling exhaust plumes.

In the gas turbine engines that most modern jet aircraft employ, only part of the exhaust gas is used directly to provide thrust through a fast and hot exhaust jet. An important part of its energy is used to drive a large fan in front of the compressor. Instead of compressing air into the engine for combustion, this fan moves ‘bypass air’ around the actual engine, forming a cold jet that flows out at a lower velocity than the hot gas from the core. Rather than giving a small amount of air a very high velocity as occurs in the hot part of the engine, the fan gives a lot of air a relatively low velocity. Turbofan engines that have relatively large fans are typically used by commercial airliners, because they are ideal for the high but still subsonic speeds at which they fly. In addition, the low-speed air helps to cushion the noise of the hot and fast core exhaust, making the engine quieter than a pure turbojet. Modern

fighter planes also employ turbofan engines, but use smaller fans that are more optimal for supersonic flight.

Gas turbines configured as turboprop, turboshaft, turbofan or turbojet engines, deliver a lot of power compared to the weight of the engine. Their power-to-weight ratio is in fact much higher than for reciprocating engines such as those used in cars. In addition, gas turbine engines are relatively small for the power they provide. One disadvantage is that, because of the high rotation speeds of the compressors and the turbines, and the high temperatures employed, they are relatively compUcated and expensive. Another limitation is that at flight speeds exceeding about Mach 3, the temperatures inside the engine can become so hot that the turbine blades melt and break apart.

How about removing the fragile turbine blades and fans? This is indeed feasible. At speeds over Mach 1, air is rammed into the engine at such high velocity that the shape of the inlet duct will satisfactorily compress the air. Such a ‘ramjet’ typically resembles a tube with a pointy core in the middle of the air intake; rather Uke a gas turbine engine without turbines and compressors. The shape of the tube and the core ensure that the incoming high-velocity air is squeezed into a small area and thereby compressed to a high enough density that it can be burned with the fuel (ramjets are therefore also known as ‘stovepipe’ jets). The good thing is that at around Mach 3, where gas turbine engines start to run into trouble, ramjets not only work well but actually perform more fuel-efficiently than turbojets. In addition, a ramjet is much less complex than a turbine engine, and therefore cheaper and more robust. These advantages make them ideal for use in long-range cruise missiles. A disadvantage is that a ramjet only produces thrust when it moves at high speeds, and only becomes reasonably efficient near supersonic speed; at low velocities the air does not rush into engine fast enough for proper compression and hence combustion. Cruise missiles are therefore usually accelerated up to high-subsonic or low-supersonic speeds using expendable rocket stages, whereupon the ramjet is engaged. But a manned high­speed plane used for reconnaissance or the interception of enemy planes needs to be able to switch from subsonic to high-supersonic velocities at any time. Expendable rocket engines that can only be used once are not very useful for that purpose. A reusable rocket engine or a gas turbine engine could be employed but at the penalty of extra weight. For the aforementioned SR-71 Blackbird, a solution was found by combining a gas turbine and a ramjet engine into a turboramjet. This hybrid engine essentially consists of a turbojet mounted inside a ramjet. For take-off and while climbing to altitude, flaps inside the SR-71 engines force the incoming air into the compressor of the turbojet part of the engine. Just short of Mach 1 the afterburners of both engines are ignited to accelerate the plane to supersonic speed. Then the bypass flaps are moved to block the flow into the turbojet and instead direct the air around the turbojet core and bum it with the fuel only in the afterburner part of the engine. At that moment the engine has been turned into a ramjet, with the air being compressed by the shock cones at the air inlets, without the need for fan compressors. This unique engine enabled the SR-71 to operate from zero speed to Mach 3 +, and to fly at speeds between Mach 3 and 3.5 for long durations. As we will see later, the combination of different types of airbreathing engines with rocket

engines may be the solution for future spaceplanes, something which Max Valier gave some thought to in the 1920s when he proposed designs for planes that would employ propellers in the lower atmosphere and rocket engines at extreme altitudes.

Conventional planes depend a lot on the atmosphere: air is required for the wings to provide lift, to enable surfaces like rudders and ailerons to provide control, and as a supply of oxygen for the engines. However, flying through air also produces drag, which makes going very fast at low altitudes difficult. The higher the altitude, the lower the air density and thus the lower the drag at a certain speed. As a result, it is possible to fly faster at higher altitudes. But at the same time the lower air density Umits the amount of available lift and thrust. Hence an aircraft’s overall maximum speed is linked to an optimal altitude. Above that altitude the maximum attainable speed drops, because the engines are less efficient and the plane may require to fly with a higher angle of attack in order to produce enough lift, which increases drag. Furthermore, below the ideal altitude the greater air density will increase drag and hence also limit the plane’s maximum velocity.

The use of rockets instead of airbreathing engines eliminates the reliance on the atmosphere for producing thrust. Without air obstructing the outflow of the exhaust jet, rockets can actually work more efficient in vacuum than within the atmosphere. With rocket engines, the maximum altitude of a plane is only limited by the lift it requires. And because lift is a function of the square of the velocity, rocket engines can accelerate the vehicle to such high speeds that even small wings can generate sufficient lift. Moreover, since rocket engines do not take in air from outside, they have no velocity constraints such as those that limit the speed at which airbreathing engines can be used. Unlike ramjets, rocket engines do not require a minimum flight velocity, and unlike gas turbine engines they aren’t constrained by a maximum velocity beyond which the inrushing air heats up the engine so much that turbine blades are damaged. Rocket engines thus allow high speeds at high altitudes; in principle even virtually unlimited speeds at unlimited altitudes. Rockets can thus even be used to propel an aircraft into orbit, turning it into a spaceplane!

At high altitudes planes do experience control problems, however: because the efficiency of an aerodynamic control surface depends on the density of the air, it drops as the altitude increases. At very high altitudes and in the vacuum of space a control surface is completely ineffective, no matter how large it is. As a result, the vertical stabilizer and rudder can no longer ensure that the nose remains pointing in the direction of flight, and the elevators and ailerons can no longer control pitch and roll. Small misalignments in the rocket engine’s thrust direction or weak residual aerodynamic forces on the wings and fuselage can cause a plane roll, pitch and yaw uncontrollably. If a plane falls in an uncontrolled orientation, maybe even sideways, the increasingly denser air could easily tear it apart. A solution is to equip the plane with reaction control thrusters: small rocket engines like those used on spacecraft for attitude control. When the aerodynamic control surfaces are rendered ineffective the thrusters can take over, pushing an aircraft around its axes to provide roll, pitch and yaw control.

In addition, the main rocket engine(s) can provide control during the powered phase of a flight by the use of gimbals. These enable a rocket engine to rotate in the horizontal and/or vertical direction in order to provide thrust vector control. When a rocket engine that is mounted in the rear of the fuselage is gimbaled up, its thrust will push the tail down. As the plane rotates around its center of mass, its nose will go up. Vertical gimballing of the engine thus provides pitch control. Likewise, rotating the nozzle in the horizontal direction provides yaw control. Roll control can be achieved if you have two rocket engines off-center from the rotational axis running from the nose to the tail of the plane, for instance one in each wing. In that case you can push one wing up and the other one down to make the plane roll. Rocket engine gimbals are pretty heavy however, and rocket planes are therefore normally controlled using aerodynamic control surfaces and small reaction control thrusters. The Space Shuttle Main Engines could gimbal to help to control the vehicle’s attitude during the ascent through the atmosphere. When watching footage of a Space Shuttle launch, look for the test-gimballing of these three engines shortly prior to ignition. The Solid Rocket Boosters could also vector their thrust, but these massive rockets were discarded about two minutes into the flight.

Although rocket engines enable a plane to fly both higher and faster than any conventional airbreathing vehicle, a downside is of course that rocket planes, apart from fuel, need to carry oxidizer with them. This adds weight, requires additional tanks and takes up more internal volume. An added complication is that while most jet aircraft carry part of their fuel in their wings, this is generally not possible for a rocket plane. That is because rocket engine turbopumps require the propellant tanks to be pressurized to at least several times that of the atmosphere at sea level in order to ensure an efficient flow without cavitation (the formation of bubbles). Generally only tanks that are more or less cylindrical in shape can contain such pressure while remaining relatively light (like a balloon), but the resulting bulky shapes are impossible to fit into the thin wings of a fast plane. If you look at a cut-away drawing of for instance the X-l or X-15 rocket planes, you will notice that indeed most of the available fuselage volume is taken up by propellant tanks.

The need to carry oxidizer means that the flight time of a rocket plane is a lot less than that of a similar jet-propelled airplane that can fill all its tanks with fuel and take its oxygen from the air. Rocket planes are therefore primarily useful for missions that require only relatively short-duration rocket-propelled boosts. Rocket aircraft usually return in an unpowered glide, what pilots call a ‘deadstick landing’, because the rocket propellant will typically have been spent earlier in the flight.

Many experimental rocket planes have been dropped from carrier planes. This saves the propellant that would have otherwise been needed to accelerate the rocket plane to take-off speed and to fly it to the planned test area. The function of a carrier plane is thus similar to that of the first stage of a conventional launch system. The rocket aircraft also has a shorter climb to attain its target altitude, because it begins its independent flight at the altitude of its carrier. Finally, at high altitude the air pressure at the rocket nozzle’s exit is lower, enabling the exhaust gases to flow out
more freely and expand further than at sea level. With a nozzle optimized for these conditions, the result is a higher specific impulse and a higher efficiency without any changes in the rocket engine itself. All this combined, saves considerable propellant weight. For example, the starting weight of the air-launched X-15 (about which you will read more later on) was almost 43% fuel and oxidizer. In other words, the propellant weight was equal to ‘only’ three quarters of the rocket plane’s empty weight, but this was sufficient to enable the plane to climb to altitudes over 100 km (330,000 feet). A similar aircraft capable of doing that by taking off from the ground would have been about 70% of propellant: a propellant weight twice the vehicle’s empty weight. The air launch enabled the X-15 to be a much smaller aircraft with lower structural weight constraints. Of course, the price paid is a large carrier aircraft, although for many applications an existing, slightly modified bomber suffices.

Apart from directly saving on propellant, dropping a rocket plane in the air means that it does not need a robust and heavy undercarriage that can handle the weight of the fully loaded plane prior to taking off. Wheels or skids will still be required to land the vehicle, but these can be small and light because they will only need to support a nearly empty plane, since half of the initial weight was propellant and that has been consumed. Indeed, the undercarriage and structure of the X-15 was only designed to carry the plane on the ground with empty tanks. On one mission in 1959 a small fire in the rocket engine forced pilot Scott Crossfield to make an emergency landing. He was unable to dump all of the propellant before he

touched down, so he landed at a tremendous speed and the heavy load snapped the vehicle in two! The Southern California Soaring Society awarded Crossfield the ‘Order of the Streamlined Brick’ for this flight, as it had set a record for the shortest descent time from 38,000 feet (11.6 km) to the ground as a glider. Fittingly, the trophy was a streamlined brick mounted on a piece of mahogany.

If, like the X-l and the X-15, an air launched rocket plane is small enough it can be carried in the bomb bay or under a wing. When it is released it simply falls away from its mothership and can start its rocket engine once safely clear, so that there is little risk of a collision or damage to the carrier due to the rocket exhaust. A larger (space)plane will need to be put on top of its carrier, requiring it to have sufficiently large wings and initial thrust to quickly fly away without falling back down, yet not scorch its mothership with its super-hot exhaust. That this can be a risky procedure was demonstrated in 1966, when a unmanned experimental drone carried on top of an SR-71 suffered engine problems and struck the carrier’s tail immediately after separation. Both planes were lost, and although the Blackbird’s two crewmembers ejected and parachuted in the sea, one of them drowned.

The X-15 was what is called an ALHL: Air Launched & Horizontal Landing. There are a number of other possibilities, which aeronautical engineers designate with equally puzzling codes. Conventional aircraft are HTHL: Horizontal Take-off & Horizontal Landing. But if the thrust of the engines exceed its take-off weight, the plane could operate as a VTHL: Vertical Take-off & Horizontal Landing. At launch, the Space Shuttle System is a pure rocket and does not use its wings. Only on its way back to Earth does the Orbiter exploit its aerodynamics to fly and land like an airplane. One reason it takes off vertically is because it is attached to the huge External Tank and large Solid Rocket Boosters. It is hard to see how such a collection of bulky rocket stages and tanks could take off horizontally from a runway; imagine the enormous undercarriage required! Structurally, it is also much easier to design something that large and tall to be launched vertically. Just think of a long wooden pole: if you hold it vertically it will remain straight and you could put quite a load on top of it, but if you hold it horizontally its own weight may already be sufficient to bend or perhaps even break the pole.

A vertical launch also ensures that the Space Shuttle clears the denser part of the atmosphere as soon as possible, to limit aerodynamic drag and therefore the amount of propellant needed to achieve orbital speed: spending less time in the atmosphere and not requiring wings which create lift and also drag generally means that vertical take-off launchers need less propellant to get into orbit than a spaceplane taking off horizontally. Even if a rocket propelled spaceplane were to raise its nose sharply for a near-vertical climb immediately after takeoff, it would still use considerably more propellant than a vertical take-off vehicle because it must fire its rocket motor for a longer time due to its less efficient trajectory. But if the spaceplane combines jet engines with rocket propulsion, and spends a considerable time efficiently building up speed using its airbreathing propulsion within the atmosphere, this can more than compensate for the energy required to overcome aerodynamic drag. Moreover, a horizontally flying spaceplane can use its wings to stay in the air, while a vertically launched machine has only its thrust to keep it from falling back to Earth, and thus needs a more powerful engine. A winged launcher taking off horizontally can gently build up speed, while a vertical take-off vehicle needs to accelerate rapidly because otherwise the energy that it loses due to gravity will be too great (costing too much propellant to compensate). The result is that for a HTHL vehicle the acceleration during launch can be low, which is especially beneficial for passenger transport. Such accelerations are called ‘G’ forces. Standing on the ground you experience one G, resulting in your normal weight. In a fast accelerating sports car, the horizontal force with which you are pushed back into your chair may be 0.7 G, i. e. equivalent to 70% of normal gravity. A descending elevator initially causes a bit less than 1 G, making you feel lighter, but once the elevator achieves a constant velocity, only the normal 1 G gravity force remains. And while it is slowing down, you feel a bit more than 1 G. In a free fall you have no weight, i. e. 0 G. This is the situation in orbit, which is merely a continuous free fall around the Earth.

Another benefit of a HTHL is that it potentially has better abort characteristics: a horizontally flying plane that loses thrust can continue underpowered, even glide if necessary (provided it has sufficient speed). A vertically launched vehicle will simply fall out the sky unless it has a sufficient number of redundant engines (‘engine-out’ capability), but this imposes high weight and cost penalties. The sudden shut down of an engine in the first few seconds of a vertical launch is thus also likely to have catastrophic results. However a spaceplane starting horizontally may be able to stop before the end of the runway, or fly around on reduced thrust for an emergency landing in the same way as a normal multi-engined aircraft. The possibility to fly at less than full power also means that HTHL spaceplanes can be test flown, progressively increasing speed and altitude on successive missions. In contrast, the VTHL Space Shuttle’s first powered flight had to be a full-blown orbital mission.

In contrast to vertical take-off vehicles, HTHL planes can, at least in principle, use existing runways and airports for take-off and landing (of which trillions of dollars worth of infrastructure already exists, spread all over the planet); launchers leaving the ground vertically need dedicated launch platforms and towers. On the other hand, horizontal take-off exposes a plane to failure modes which do not apply to vertically launched machines, such as collisions with obstacles or blown tires (for instance, out of the nineteen SR-71 Blackbirds that were lost in accidents, four were as a result of tire failures during take-off, and the only Concorde crash was caused by coming into contact with debris on the runway).

A Vertical Take-off & Vertical Landing (VTVL) vehicle is in principle possible, but generally does not make much sense for a winged plane that is optimized to fly horizontally. The Harrier ‘jump jet’ was equipped to take-off and land vertically in order to be able to operate from small clearings on a battlefield, but that capability adds a lot of complexity to the design of the plane and its engine (which uses four rotating nozzles), and was purely to satisfy the aircraft’s military requirements. There are concepts for VTVL launch vehicles, but these do not have wings and can thus not be considered rocket planes.

If you have wings on your vehicle, it is generally best to exploit them as much as possible and thereby minimize complexity and the amount of thrust required. Rocket planes are therefore usually HTHL or ALHL vehicles. But HTVL does not really

make much sense, since if you have the wings and sufficient runway to take off horizontally, it is hard to justify the need for a vertical landing when you get back.

So with this technical background in mind, let’s now see how rocket planes have evolved since they first became a serious business shortly before the Second World War.

“Science is one thing, wisdom is another. Science is an edged tool, with which men play like children, and cut their own fingers.” – Sir Arthur Eddington (1882­1944)

GERMANY GETS SERIOUS

After the experiments of von Opel and Espenlaub with gliders equipped with simple powder rockets, work on rocket planes in Germany continued with the focus on more controllable motors using liquid propellants. Most spaceflight visionaries at the time believed that a reusable spaceplane would eventually be required to make launches into space routine and affordable. Wernher von Braun, the technical genius leading the development of the А-type rockets for the German Army at their Kummersdorf proving ground (the success of which ultimately led to the notorious A4/V2 missile), was no exception. But where others merely came up with ideas and published their hypothetical concepts, von Braun was actively trying to set up a practical program to evaluate an aircraft with a rocket motor propulsion system. The ultimate goal of all von Braun’s efforts was space exploration, not the development of military missiles, but he recognized that in Germany in the 1930s the military was the only source of funding to develop rockets. He hoped later to use the technology for the exploration of outer space. To get the military, which was already funding his missiles, also to pay for rocket plane experiments he once again had to convince them of the novel technology’s possibilities for making war.

The success of von Braun’s rocket missile development program indeed managed to convince the Army High Command and the highest echelons of the Reichs- LuftfahrtMinisterium (RLM; Air Ministry) that a rocket propelled fast interceptor plane was feasible. In May 1935 Major Wolfram von Richthofen, in charge of developing and testing new aircraft for the German Air Force, the Luftwaffe, put forth a proposal to develop a rocket propelled interceptor for use against high flying bombers. He knew that the British were developing strategic bombing as a means of disabling enemy industry, and Richthofen (a fourth cousin of the First World War flying ace Manfred ‘Red Baron’ von Richthofen) proposed to defend German factories against this threat by equipping them with dedicated rocket propelled interceptors. Also, German airplane designer Ernst Heinkel, founder of the Heinkel airplane manufacturing company, decided to support von Braun. Heinkel was passionate about high-speed flight, and very interested in any form of aircraft propulsion which promised higher speeds than could be achieved using traditional piston engines with propellers. To get von Braun started, Heinkel sent Walter Kiinzel, one of his best engineers, to join the development team, and donated the wingless fuselage of a He 112 fighter aircraft for use in ground tests.

Work on the aircraft engine, which burned liquid oxygen and alcohol, began in Kummersdorf early in 1936. The development team came up with a system that was pressure fed, meaning that instead of using pumps the propellant was fed into the rocket motor by using a pressurized gas to force the fuel and oxidizer out of their tanks. This method would deliver less thrust than possible when using a turbopump, but in principle the engine’s relative simplicity would make it easier and safer to operate and maintain. In the particular case of the engine that von Braun intended to use in the He 112 the pressure was created simply by letting the liquid oxygen propellant evaporate, eliminating the need for a separate pressurant gas and tank. To test the effects of acceleration on the propellant injection and combustion, a centrifuge was built consisting of an 8 meter long (26 feet) beam attached to an axis in its middle and with the rocket engine at one end. This allowed the engine to zoom around in circles of 4 meters (13 feet) radius, powered and accelerated by its own thrust. One day the engine didn’t want to stop and the centrifuge brake failed, and the motor started to fly around out of control, faster and faster. The operator, who sat in the middle of the centrifuge, had to run for his life when the engine broke free. Much more than nowadays, rocket development was a dangerous business.

At the end of 1936, having gotten the engine to work as required, the engineers planned to install it in the He 112 fuselage. The tank with liquid oxygen was placed ahead of the cockpit and the tank of alcohol was placed behind the pilot. The engine sat in the tail of the airplane. Ground trials at Kummersdorf began in early 1937. The fuselage was secured to the ground with ropes and cables in order to prevent it from running off under the power of the engine, which had a maximum thrust of about 10,000 Newton. These experiments raised further interest in the RLM and later that year a secret rocket plane test program was established by Heinkel, von Braun and another rocket engine developer named Hellmuth Walter. The RLM also seconded to the program Erich Warsitz, one of their most experienced and technically proficient test pilots. Owing to the biography of Warsitz, The First Jet Pilot (written by his son Lutz), the details of his involvement in the rocket plane and jet plane programs of the late 1930s is now known. Warsitz did not know exactly what he had gotten himself into. He did know that it had to do with flying rocket planes and that his not being married was a major factor in his selection, so clearly it was going to be a dangerous job. Warsitz got some idea of how dangerous when he first arrived at Kummersdorf. He noticed a heap of torn and twisted, container-like metal objects. A mechanic told him they were combustion chambers that had violently exploded during previous tests, and that he, as the test pilot, might end up

among them if he were not careful! He then followed von Braun to the test stand where, kneeling on the wing root of the modified He 112, he watched von Braun confidently ignite the rocket engine from the cockpit. Warsitz was very impressed with the long exhaust flame and the ear-splitting noise that the engine produced. The force of the exhaust even managed to blow away several 1 cm thick metal plates that covered the ground some tens of meters behind the engine. Warsitz learned later on that the engine was normally ignited remotely from behind a thick concrete wall, and for safety reasons it had never before been started from the cockpit. Quite reasonably, von Braun and Kiinzel had been afraid that Warsitz would never get into the cockpit if he saw the engine being operated that way! Warsitz was impressed by von Braun’s expertise and enthusiasm, and stuck with the hazardous project. However, tests did continue with the engine being ignited from the safety of the concrete wall’s protection, and for good reason: one day and engine blew up during a demonstration for officials from the Army Weapons Office, totally destroying the He 112 fuselage. Fortunately Heinkel understood the risks inherent in testing revolutionary technologies and gave von Braun another He 112 fuselage to continue the experiments.

As an alternative to von Braun’s liquid oxygen and alcohol engine, the RLM had also commissioned Hellmuth Walter’s firm in Kiel to supply a rocket engine for the He 112. This engine ran on hydrogen peroxide which, under the influence of calcium permanganate as a catalyst, decomposed into hot oxygen gas and steam (a catalyst is a substance that facilitates a chemical reaction without being consumed by it; i. e. the calcium permanganate was itself not chemically affected by the decomposition of the hydrogen peroxide). Originally developed to turn turbine engines on submarines as “air independent propulsion” (contemporary submarines used diesel engines while on the surface and battery powered electric motors when submerged), the expanding gas could also be used directly to deliver rocket thrust. Experiments with a small Walter engine with a thrust of 1,500 Newton fitted to a Heinkel 72 propeller biplane in the autumn of 1936, and then a Focke Wulf 56 propeller plane with an engine of twice that power had been very successful (even the head of technical development for the Luftwaffe, Colonel Ernst Udet, had dared to make a flight in the latter aircraft). One benefit of the propellant combination used in Walter’s rocket engine was that it did not require an igniter to get started; the fuel produced hot gases spontaneously upon contact with the calcium permanganate when they were simultaneously injected into the combustion chamber. There was thus less risk of the engine not starting, making it in principle simpler and more reliable. The combustion in the engine also occurred at about 480 degrees Celsius (890 degrees Fahrenheit), which was much lower than the 1,700 degrees Celsius (3,100 degrees Fahrenheit) of the rocket that von Braun’s team was using for their project. The lower temperature reduced wear and tear of the engine. Another advantage that Walter’s engine had over von Braun’s rocket motor was that its propellants could be stored at normal temperatures, whereas liquid oxygen had to be chilled down to minus 183 degrees Celsius (minus 279 degrees Fahrenheit) because otherwise it would start to boil and rapidly evaporate. A missile or rocket plane using liquid oxygen could thus only be fueled shortly prior to launch, making it less suitable as a rapid response weapon.

An important disadvantage of the Walter engine was the low specific impulse inherent in decomposing hydrogen peroxide; von Braun’s engine was much more efficient. But the main problem with Walter’s engine was the hydrogen peroxide it consumed. In diluted form hydrogen peroxide (typically a 9% concentration) can be used to bleach hair, but the highly concentrated version used in the engine was able to make various materials ignite spontaneously and eat away human flesh. In 1934 von Braun had three members of his rocket development team killed by a hydrogen peroxide rocket engine explosion, so he was wary of it. Warsitz, who was to fly both types of engine gained firsthand experience of how dangerous Walter’s engine was during a ground demonstration of a modified He 112 in Kiel. To show his confidence in the system in front of observers from the organization funding the program, the RLM, Warsitz planned to ignite the engine from the cockpit. Previous tests in which the engine was started remotely had gone well, but Walter’s senior engineer Bartelsen nevertheless urged Warsitz not to operate the engine from the cockpit. It saved his life because the propulsion system blew up, spraying acid and metal fragments through the cockpit.

For the actual flight tests with modified He 112s (one equipped with von Braun’s rocket engine and the other with Walter’s engine) a suitable terrain had to be found. It had to provide enough room for emergency landings and be surrounded by open space so that crashes would not put populated areas at risk. A good location was found at the large Neuhardenberg reserve airfield, situated some 60 km (40 miles) east of Berlin. This site would enable flight experiments to be performed in secrecy, without locating the team far away from its important contacts in Berlin. However, because Neuhardenberg was only a backup field to be used in the event of war, it had no buildings or facilities. A number of tents were therefore erected to accommodate the technicians and their aircraft.

In addition to equipping He 112s with rocket engines, Heinkel and the RLM were interested in evaluating rocket engines as a way to get heavily laden bombers off the ground. Walter had designed an egg-shaped rocket pod that could be mounted on the He 111 medium bomber, one under each wing. Each pod would give the bomber an extra 3,000 Newton push for 30 seconds and enable it to get into the air with a load which would otherwise have prevented it from taking off within a reasonable ground run length. Flight tests with this system (what in the US would become known as a Rocket Assisted Take-Off or RATO system) begin early in the summer of 1937 with Warsitz at the controls. On his first flight he shows that the rockets work well and that the combined thrust of both rocket pods makes the plane climb very fast. Their exhaust jets disturb the airflow around the plane, but control remains manageable. He makes several further flights. On one of them a rocket pod comes loose just prior to take-off. Warsitz promptly stops the rocket engine and the plane. This demonstrates the advantage of a controllable liquid propellant rocket engine over a simpler solid propellant system that cannot be halted after ignition. Later he flies the He 111 with an overload of sand bags, concrete blocks and water tanks. The plane was too heavy to get off the ground unassisted, but with the RATO pods it manages to get airborne. He gets the plane rolling using the propeller engines alone, then ignites the boosters only after it has covered some 20 to 40 meters of runway. Because the boosters only work for 30 seconds, timing the ignition is crucial in order to gain the extra thrust at just the right time for take-off and the start of the climb. On another flight one of the rockets fails just when the plane gets airborne. The pod on the other wing, situated between the propeller engine and the wingtip, continues and the uncompensated leverage pushes the plane around in a rapid 180 degree turn. Warsitz is tempted to counteract the unbalanced thrust using the aircraft’s rudder, but that would cause too much drag and make him lose altitude over the dense woodland that he is flying over. So he quickly extinguishes the other rocket engine as well. Now the plane has trouble staying in the air, lacking the speed and the thrust for a normal climb. With his wheels clipping the trees, Warsitz manages to stay airborne just long enough to make a safe landing. During a demonstration flight for RLM observers, Warsitz seeks to make a spectacular impression by taking off without any cargo, and with the thrust of both rocket pods the plane goes nearly straight up!

In the meantime work on von Braun’s rocket engines are suffering problems. He too has developed rocket pods for the He 111 bomber. At 5,000 Newton these give even more thrust than the Walter rockets. However, they are not ready for flight

tests. There are also problems with the rocket engine for the He 112: often the combustion chamber splits because of the high pressure inside. More ground tests are performed at the Neuhardenberg airfield. After some of these have been completed successfully, Warsitz presses to start the flight testing. He agrees to have one more standing test of the engine in the actual plane, and ignites it from the cockpit. The explosion not only destroys the engine, but rips the entire airplane fuselage apart! Warsitz is blown out of the cockpit and lands on the ground some 4 meters (13 feet) away, unharmed. It is another timely lesson that rocket propulsion is basically a controlled explosion, with the explosion easier to attain than the control.

Fortunately Heinkel agrees to give the team yet another He 112 to continue their tests; going through the official Luftwaffe channels to get a replacement might have ended the program prematurely because not all military officials are convinced there is any reason for rocket planes. On 3 June 1937 Erich Warsitz makes the first flight with a rocket powered He 112 fitted with von Braun’s engine. The plane still has its standard propeller engine for take-off and landing; the rocket engine will be ignited once the plane is in the air. Planning of the flight is not easy, because von Braun’s engine is pressurized by the natural boil off of some of its liquid oxygen propellant. Ten minutes after tanking, enough of the oxygen has evaporated to supply just the right amount of pressure in the tanks. If the engine is ignited too early, there will be insufficient pressure to force the propellants through the lines into the combustion chamber. But if the rocket is started late, the pressure may be so high that the engine explodes! Warsitz takes the He 112 up to 450 meters (1,500 feet) on propeller power alone, and flying at 300 km per hour (187 miles per hour) he waits until the pressure is just right and then he hits the ignition. Fortunately the engine starts correctly. The rocket gives a fixed thrust of some 3,000 Newton, quite modest for a plane with a weight of almost 2,000 kg (4,400 pounds), but sufficient for Warsitz to feel the kick. Within seconds the He 112 accelerates up to 400 km per hour (250 miles per hour). As Warsitz reported afterwards, at this point he noticed “a strong acrid odor of burning rubber and paint” and “clearly perceptible hot gases flowed under the pilot’s seat”. Because the gases irritate his eyes and hurt his lungs, he opens the canopy for ventilation and puts on his flight goggles to protect his eyes. Looking back, he sees flames in the fuselage! Unlike Walter’s engine, von Braun’s rocket cannot be turned off, and even although it has caught fire the plane continues to accelerate under the combined power of the propeller and the rocket. Warsitz cuts the propeller engine to slow down, and then simply waits for the rocket to consume its 30 seconds worth of propellant. Knowing the dangerous nature of the experimental propulsion unit on his plane he prepares to bail out and land by parachute but then realizes that his altitude has already fallen to about 200 meters (650 feet), which is too low to bail out. Side-slipping the plane to increase drag and hence lose altitude without increasing speed (which would happen if he simply pushed the nose down) he manages to get to the ground quickly. There is no time to deploy the wheels, so he belly-lands, scrambles out and runs for his life. The flames are quickly extinguished by the fire brigade, but the damage to the plane is significant.

Later the team finds out that the source of the accident are some ventilation slits that are fitted the wrong way around: instead of releasing the gases that build up due to the usual leaks in the rocket’s propellant supply system, the slits were sucking the gases as well as jet exhaust forward into the cockpit. Analysis of the engine also shows that the combustion chamber has cracked. While the flight was not a complete success, it proves that rocket thrust from the tail of a plane can work and doesn’t, as some critics had expected, make the aircraft flip over.

Warsitz continues flight tests with the other He 112 equipped with the fixed-thrust

3,0 Newton Walter engine, which performs several flights without blowing up. To protect himself against the dangerous acid fuel in case of a leak, Warsitz wears white clothing of a specially developed type of plastic; even his shoes and necktie are made of it. His normal clothes would act as a catalyst for the hydrogen peroxide, dissolving and burning when coming in contact with the angry substance.

The tests at Neuhardenberg using the Walter engine in the He 112 are completed satisfactory at the end of 1937 and the marquees are dismantled. But flights with the He 112 and the Walter engine are continued at the Luftwaffe’s section of the secret Peenemiinde center: Peenemiinde West, on the other side from Peenemiinde East where von Braun is developing the A4/V2 rocket for the Army. Eventually Warsitz dares to take off powered by both the propeller and the rocket engine: it makes the plane leap almost vertically into the sky. After this he starts it under rocket power alone, with the piston engine running in neutral and the propeller disengaged. This proves to be rather difficult because the rocket’s thrust is never exactly in line with the central axis of the plane, making it veer to the side, while the rudder is not very effective at low speeds without the propeller blowing air over it. While accelerating for take-off he therefore has to steer using the wheel brakes, which costs much energy and speed. A better solution is found in adding a rudder just behind the nozzle, to deflect the rocket’s exhaust jet and thereby help to steer the plane.

Even although von Braun’s engines are in principle better performing, Walter’s simpler hydrogen peroxide engines prove to be operationally more interesting for a rocket fighter plane: they are more reliable and do not depend on extremely cold liquid oxygen which is difficult to store and cannot be kept inside a plane for very long without a special cooling system. Interservice rivalry may also have played a role in the preference for a rocket plane with a Walter engine: the engines that von Braun was developing were primarily for use in the Army’s A4/V2 rocket, and the Luftwaffe may simply have wanted an independent rocket propulsion system.