ROCKET PLANES, ROCKET TRAINS AND ROCKET AUTOMOBILES

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,

Fritz von Opel in the RAK-2 rocket car [Opel],

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

Von Opel drives his RAK-2 rocket car [Opel].

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.

An Ente replica in the German Sailplane Museum [Martin Bergner and Deutsches Segelflugmuseum].

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

The two rocket motors can be seen at the back of the Ente replica [Martin Bergner and Deutsches Segelflugmuseum]

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

Trajectory of Valier’s transatlantic rocketplane shown in the November 1931 issue of the magazine Science and Mechanics [Science and Mechanics

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

In 1999 Opel produced this replica of the rocket-propelled aircraft RAK-1 [Opel],

Sli

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.

Fritz von Opel in front of the wrecked RAK-1 rocketplane [Opel].

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’

William Swan flies the American rocketplane [Modern Mechanics],

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.