Category Rocketing into the Future

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The He 176 was by no means the end of rocket plane development in Nazi Germany. Alexander Lippisch, the brilliant aerodynamicist who had designed the Ente sailplane which formed the basis of von Opel’s first rocket plane, had in the late 1930s been working on the design for the DFS 194. This was to be a delta-wing airplane without horizontal tail stabilizers, driven by a pusher propeller (at the rear rather than in the nose). The lack of a tail on an airplane not only decreases aerodynamic drag but also results in greater effective lift: a normal wing has the tendency to push the nose of a plane downward, which in conventional planes is compensated by the horizontal stabilizers in the tail. But for them to push the nose up they must push the tail down, which requires negative lift. The result is a balanced airplane but the negative lift of the tail must be made up by additional positive lift from the wings. A well-designed tailless airplane does not need such negative-lift stabilizers. The delta shape of the DFS 194 meant that the wing ailerons could be placed close enough to the rear of the plane to give sufficient leverage for pitch control when both were moved in the same direction, thus eliminating the need for separate horizontal stabilizers with elevators. When moving in opposite directions they would still be able to roll the plane. Due to their dual aileron and elevator functions these control surfaces are called elevons.

Although the DFS 194 is being developed at the Deutsche Forschungsanstalt fur Segelflug (DFS; the German Institute for Sailplane Flight), the RLM considers it to be a good basis for a rocket propelled fighter plane. The work begins at the DFS in cooperation with Heinkel but apart from valuable tests using models in a wind tunnel and in the open air, progress is too slow for Lippisch’s liking. When in late

1938 he is told that his group is to be disbanded because the DFS believes there is no need for a tailless military rocket plane, Lippisch and many of his staff decide to leave the DFS. After negotiations with both Heinkel and Messerschmitt, in January

1939 the project is transferred to the Messerschmitt factory in Augsburg. The decision to join Heinkel’s main competitor may have reflected the fact that Willy Messerschmitt was a more diplomatic salesman than Ernst Heinkel and he had good ties with Hermann Goring.

At Messerschmitt the prototype DFS 194, which the DFS permits Lippisch to take with him, is further developed as ‘Project X’. Late in 1939 the completed airframe is sent to Peenemunde West to be mated with its Walter RI-203 engine. This is similar to the one used earlier in the He 176 but has the advantage that its thrust can be adjusted by the pilot, whereas the previous engine had a fixed thrust. The first gliding flight is made in July 1939 piloted by Heini Dittmar, and by October the aircraft is undergoing engine ground tests. The successful engine tests are followed by the first powered flight in August 1940, when Dittmar flies the DFS 194 to a top speed of 550 km per hour (340 miles per hour). As he later recalled: “With nearly 1,500 pounds of thrust, and as I pulled up steeper and steeper without losing one knot of airspeed, I knew that we were beginning a new era in flight.” Combining a graceful glider with the raw power of a rocket engine results in excellent flying characteristics, and the DFS 194 proves to be a good basis for an operational rocket fighter.

The RLM consents to the production of a series of five prototypes of an improved design, which is named the Messerschmitt Me 163A. The project name ‘Me 163’ had earlier applied to the development of a short-take-off, slow-speed aircraft; it is hoped that using the same name will deflect unwanted attention from the new, secret rocket plane project. In line with this subterfuge, the first prototype is named the Me 163A V4 to suggest a follow up to the three prototypes planned for the slow-speed plane. The Me 163A uses the Walter RII-203 motor in which ‘T – Stoff composed of 80% hydrogen peroxide with some stabilizing additives is decomposed by a liquid sodium or calcium permanganate solution catalyst called ‘Z-Stoff. It produces a maximum thrust of 7,400 Newton. The propellants are injected into the combustion chamber by pumps which run on T-Stoff sprayed over a cement-like solid Z-Stoff ‘stone’. For stability of the aircraft, the bulk of the engine mass must be placed near the center of the airplane, close to the center of gravity of the airframe and the place where the lift from the wings is concentrated. The combustion chamber with the nozzle is placed in the tail, linked to the rest of the engine by a long thrust tube. The vivid violet color of the permanganate causes the Me 163A to leave a huge purple exhaust trail in flight.

The Y4 prototype makes its first towed flight at Augsburg on 13 February 1941 under the control of Dittmar, but the first powered flight does not occur until August due to delays in the development of the Walter engine. When the plane finally flies under rocket power the results are spectacular: once airborne, Dittmar sets off on a horizontal trajectory close to the ground in order to gain speed, then pulls up the nose and makes an incredibly steep ascent: from the moment it leaves the runway the Me 163A needs only 55 seconds to climb to an altitude of 4.0 km (13,000 feet)! It also proves to have excellent flying characteristics. On 2 October Dittmar sets the incredible new world speed record of 1,003 km per hour (623 miles per hour) in level flight, making the Me 163A V4 prototype much faster than anything else in the sky at that time. For the occasion it was towed to an altitude of 4.1 km (13,500 feet) in order to maximize the amount of propellant available for the record attempt. Because of the project’s top secret status, the flight was not officially recorded internationally and was disclosed only after the war. (It would not be surpassed until August 1947, well after the war, when the experimental Douglas D-558-1 Skystreak turbojet aircraft in the US flew slightly faster.)

However, the record flight of the Y4 reveals that such high transonic velocities are pushing the Umits of its aerodynamic design, because at its maximum speed the plane suddenly pitches nose down. Dittmar is subjected to a negative acceleration of 11 G (acceleration results in positive G, what pilots call ‘eyeballs in’; negative G is deceleration, or ‘eyeballs out’) but he quickly manages to throttle back. The plane

slows down and becomes controllable again. As the aircraft approached Mach 1, he had encountered a phenomenon called ‘Mach tuck’, which is a disruption of the air flow over the wing that causes a plane to nose over (as will be described in more detail later). The outer limit of the speed envelope of the Me 163 had been found.

The five Me 163A prototypes are followed by eight pre-production examples that are designated Me 163A-0, then by an improved and significantly larger operational combat version called the Me 163B. It is officially named the ‘Komet’ (Comet), but the pilots name it ‘Kraftei’ (Power Egg). The main advantage of the ‘B’ is its much more powerful engine. The He 176, DFS 194 and Me 163A all had engines in which hydrogen peroxide was decomposed to produce a reasonable thrust at relatively low temperatures; the Me 163A engine exhaust temperature was about 600 degrees Celsius (1,100 degrees Fahrenheit). These types are often referred to as the ‘cold’ Walter engines. In contrast, the Me 163B is powered by a ‘hot’ Walter HWK 109- 509A (initially the 509A1 but later the improved A2 version) rocket that burns concentrated ‘T-Stoff (hydrogen peroxide) and ‘C-Stoff consisting of 57% methyl alcohol (made from potatoes), 30% hydrazine hydrate and 13% water. The T-Stoff decomposes under the influence of a dissolved copper salt catalyst mixed in with the C-Stoff, while the hydrate helps the alcohol fuel to ignite spontaneously with the hot oxygen produced by the T-Stoff decomposition. The exhaust temperature of the ‘hot’ engine is 1,800 degrees Celsius (3,300 degrees Fahrenheit). The new engine gives the Me 163B more than double the thrust of the Me 163A. It also produces less smoke, making it harder for enemy planes to spot the Komet.

The 16,000 Newton that the new 509A1 engine provides (17,000 Newton for the later 509A2) is high in comparison to the force of gravity on the plane when it is nearly empty (with a weight of 1,900 kg, or 4,200 pounds, corresponding to about 19,000 Newton) and therefore towards the end of its powered run the Me 163B can climb almost straight up. The maximum speed of the operational, armed fighter plane is 960 km per hour (600 miles per hour), which corresponds to Mach 0.91 at 12 km (40,000 feet) altitude; sufficient to enable it to “fly circles around any other fighter of its time”, according to Me 163B chief test pilot Rudolf Opitz.

The thrust of the Me 163B’s engine can be throttled in four stages (idle and then levels 1 to 3) by varying the number of propellant lines that are opened into the combustion chamber. The Walter engine is simple to build, simple to maintain and readily accessible because the entire tail section of the plane can be removed. And rapid replacement is possible because the engine is mounted to the thrust frame within the fuselage using only a few connections.

Two 163B prototypes are built for flight tests. In addition, a number of unpowered 163S glider trainers are built which have an additional instructor cockpit behind and above the usual cockpit. The first tow-glide with the engineless 163B airframe occurs on 26 June 1942 but there is then a long wait for the new engine to become available. The first powered flight is not until 24 June 1943. In part because of this the Komet is unable to enter squadron service until 1944, by which time large fleets of bombers are swarming over Germany and its revolutionary capabilities are desperately needed to stem the tide. Thirty Me 163B-0 are produced armed with two

20-mm Mauser MG 151/20 cannon and about 400 Me 163B-ls with two even more destructive 30-mm Rheinmetall Borsig MK 108 cannon.

The bat-shaped Me 163B is a remarkably small aircraft, with a wingspan of only 9.3 meters (31 feet), a length of 5.8 meters (19 feet) and a maximum take-off weight of 4,300 kg (9,500 pounds) of which about half is propellant. To save battery weight it has a little windmilling propeller on its nose that will turn in the air during flight and drive an electricity generator in order to power the onboard radio. In spite of the emphasis on weight minimization, the pilot is protected by armor plating behind him, a thick section of armored glass immediately above the instrument panel, and an armored nose cone (it was expected that the biggest risk for a Komet pilot was being hit by defensive fire from the bombers it attacked, rather than from an Allied fighter getting on his tail). Due to the lack of metals in Germany late in the war, parts of the wings, which blend beautifully with the fuselage, are made of wood. The short nose and the single-piece blown canopy, which blends aerodynamically with the aft fuselage, gives the pilot good visibility forwards, but he cannot really see what is at his six o’clock (straight behind him); on the other hand, nothing is able to chase him.

The plane takes off at a speed of about 280 km per hour (170 miles per hour) with a much longer run than the distance conventional fighters require. It remains just above the ground until it has achieved the optimal climbing speed of around 680 km per hour (420 miles per hour), whereupon it pulls up into a 70 degree climb and within 3 minutes reaches the bombers at 7 to 10 km (23,000 to 33,000 feet). Movies of its take-off show an amazingly abrupt change from level flight into a steep climb that appears spectacular even today. To spectators used to the gradual

Me 163В cutaway diagram.

Me 163В taking off.

climb of propeller fighter aircraft of that time, it must have been awe-inspiring. Also apparent in these movies is the large amount of smoke that escapes from the belly of the Me 163B: this is hot steam from a gas generator in which a small amount of T-Stoff is decomposed (according to the ‘cold’ principle) to drive the turbine which powers the pumps that feed T-Stoff and C-Stoff to the rocket’s
combustion chamber. (This same turbopump system was used by the ‘cold’ engine of the Me 163A.)

Famous German female test pilot Hanna Reitsch was mightily impressed by its power. In her memoirs (published in EngUsh with the title The Sky My Kingdom) she describes operating the plane during an engine check: “To sit in the machine when it was anchored to the ground and be surrounded suddenly with that hellish, flame – spewing din, was an experience unreal enough. Through the window of the cabin, I could see the ground crew start back with wide-open mouths and hands over their ears, while, for my part, it was all I could do to hold on as the machine rocked under a ceaseless succession of explosions. I felt as if I were in the grip of some savage power ascended from the Nether Pit. It seemed incredible that Man could control it.” The mechanics near the plane had to keep their mouths open because otherwise the pressure differences caused by the rocket engine would rupture their eardrums.

Germany managed to equip only one operational fighter wing, Jagdgeschwader (JG) 400, with the Me 163 before the war was over. The first operational Komet mission was flown by commander Major Wolfgang Spate on 14 May 1944. For the occasion the ground crew painted his entire plane bright red, just like ‘Red Baron’ von Richthofen’s famous aircraft of the previous war. It shows the high expectations for the Me 163 as an invulnerable wonder machine capable of outclassing all other aircraft in the sky. But Spate did not appreciate being made such a distinctive target and he had the color removed after the flight. In any case, the paint added 18 kg (40 pounds) of unwanted weight. Nevertheless, the red Komet has become a classic and many illustrations and scale models show the aircraft in this fiery livery. The planes also often had funny squadron artwork on their nose, including a depiction of Baron von Miinchhausen flying through the air on a cannonball, or a drawing of a rocket propelled flea with the text “Wie ein Floh, aber oho!” (like a flea, but wow!)

The operational history of the different squadrons that belonged to JG 400 and the numbers of losses and successes is unclear, with different sources reporting different numbers since the claims of both Komet and Allied pilots about who shot down how many of whom are not always consistent. Komets often attacked bomber formations together with conventional Luftwaffe fighters and in the chaos of air battles mistakes were easily made about which planes were shot down by Komets and which by other aircraft. AlUed bomber crews sometimes reported being under attack by Komets at times and places where none were flying. It appears that JG 400 Komets drew first blood on 16 August 1944, when they damaged several B-17 bombers and killed two American gunners. However, a gunner on another bomber managed to shoot down a Komet and a P-51 Mustang fighter claimed another. The pilot of the first Komet was able to bail out but the other pilot did not. Allied bomber formations were attacked intermittently up until May 1945, resulting in seven American B-17s and one British Lancaster being shot down. But several Komets were lost due to defending fighters and bomber gunners. In addition to attacking bombers, the Me 163B was also used to intercept the high altitude, fast reconnaissance planes that were usually hard to catch. By the time these were spotted it was normally too late to get conventional fighters airborne and up to the right altitude, but the extremely high climb rate of the Komet gave it a better chance of intercepting them. One Mosquito reconnaissance aircraft was indeed intercepted and shot down in November 1944. In March 1945 a Spitfire Mk XI photo reconnaissance plane was attacked by a pair of Me 163Bs but managed to escape by rapidly diving from 10.7 to 5.5 km (6.7 to 3.4 miles), and in the process reached a speed of about 800 km per hour (500 miles per hour). Later that month two Komets intercepted a Mosquito P. R. Mk XVI photo-reconnaissance aircraft near Leipzig. Nicknamed the ‘Wooden Wonder’ because it was mostly made of plywood, the Mosquito was an especially fast and rather agile two-engine plane, but it was not able to outrun an Me 163B. Although the Komet’s cannon shot out the Mosquito’s starboard engine, Canadian Pilot Officer Raymond Hayes managed to escape by very skillful maneuvering. Hayes and his navigator set for home in the crippled plane, but on the way were attacked by another fighter (not a Komet). Hayes managed to evade this second attacker but his aircraft sustained further damage and his navigator was badly injured in the leg. They finally crash-landed in Lille in France. Hayes received the Distinguished Flying Cross for his feat.

By the end of 1944 a total of 91 aircraft had been delivered to JG 400, but due to a severe propellant shortage most never left the ground: the two main factories which produced its special propellant had been demolished in bombing raids in September of that year. Many Komets were destroyed when their bases were bombed. Attacks from the air, and the approach of Allied ground troops, repeatedly obliged the JG 400 pilots to change airfields and this further hampered their operations. When they did manage to find an intact Komet and sufficient propellant, a typical tactic was to shoot upward through a bomber formation in a 70 degree climb, fire a few rounds at the enemy planes, and then turn and dive through the formation at zero thrust while shooting again. Sometimes this could be done twice before running out of propellant. The plane was remarkably agile and docile even at these high velocities, so it could easily both outrun and outmaneuver Allied fighter aircraft. Moreover, it would zoom by so swiftly that the gunners on the slower bombers hardly had time to swing their guns towards the small Komets.

However, the huge difference in airspeed also made hitting the enemy difficult for the Me 163 pilots, even though a bomber was a larger target to aim for and just a few hits from the powerful cannon could cripple even a large multi-engine plane such as a B-17 or a Lancaster. The Komet had to close within a range of about 600 meters (2,000 feet) to have any chance of hitting its target, but had to break off when at 180 meters (590 feet) to avoid a collision. That left less than 3 seconds for shooting the twin MG 151 cannon. Although powerful, these had a rather slow firing rate of 650 rounds per minute. Per attack, the Komet’s two cannon could thus fire only some 65 rounds at a particular bomber. This may sound like a lot, but within that same time a conventional Fw 190D fighter could fire a total of 165 rounds using its four (smaller cahber) cannon. Furthermore, the slower fighter could hold its target for far longer. Another issue was that if the Komet fired its cannon while it was maneuvering, the ammunition belts often jammed. JG 400 First Lieutenant Adolf Niemeyer came up with the idea of installing 24 R4M ‘Orkan’ (Hurricane) missiles under the Komet’s wings. When fired in brief barrages these would be able to knock out a bomber much faster and far more effectively than the twin cannon. Nobody knew how the Me 163 would react to the disturbance of the airflow around its unusually shaped wing while carrying rockets but because the situation for Germany was desperate there was no time for wind tunnel testing and careful analysis. Niemeyer took off in an Me 163A test vehicle with a set of dummy missiles under its wings to find out. Apparently the flight went reasonably well because he survived the experiment (as well as the war). Had Niemeyer been able to mount real rockets on his Komet, he would have become the first man in history to fly a missile-equipped rocket propelled airplane. Such an Me 163B would have been a very lethal weapon indeed, but no Komet ever carried missiles in combat.

One Lancaster bomber did however fall victim to an Me 163B with an experimental SG500 ‘Jagerfaust’ (Hunter’s Fist) weapon system consisting of a set of eight upward-firing, short-barreled guns triggered by a photocell able to detect a bomber’s shadow as the Komet flew underneath it. The system was designed to make it easier for Me 163B pilots to shoot down targets because it did not require careful aiming and timing. The guns were built into the wings, four on each side, and they had to be placed rather far out towards the wingtips and be fired with minute intervals to preclude the explosive shock waves from shattering the canopy. To prevent the recoil from disturbing the aircraft’s flight, the barrel of each gun was ejected downwards as the shell was shot upwards.

Sitting in an Airbus 320 reading about the exploits of the Komet pilots in Stephen Ransom and Hans-Hermann Cammann’s Jagdgeschwader 400, it dawns on me that I am comfortably flying at a speed and altitude at which Me 163B and Allied bombers were slugging it out over northern Europe some 65 years ago. Pilots and crews did not have pressurized cabins, and the engines used were either reliable but slow as in the case of the bomber’s piston engines, or dangerous but fast rocket motors. What at the time were rather extreme flight speeds and altitudes have been standard cruising performance for commercial airliners since the late 1950s: a Boeing 747 typically flies at around 900 km per hour (560 miles per hour) at 11 km (35,000 feet) altitude. The very rapid developments in post-war jet aircraft technology meant that what in 1945 was attainable only by an experimental and extremely dangerous fighter aircraft was perfectly normal for the average tourist-class airhne passenger a mere 15 years later. It is also interesting to note that since then up to the Airbus that I am flying in, airliner operating speeds and altitudes have hardly changed apart from the fantastic Concorde, which is unfortunately no longer in service.

The Me 163B’s performance was revolutionary, but the design was not perfect for operational military use. From the long list of in-flight explosions, engines quitting, weapons jamming, crash landings and aircraft being shot down while gliding back to base, it is clear that the Komet was an experimental aircraft rushed into operational service. On combat missions it was expected that one-third of the aircraft and pilots would not make it back to base. Of those lost, some 80% crashed during take-off or landing, 15% burst into flames or went out of control in flight, and the remaining 5% were shot down. Abysmal numbers for what was supposed to be an operational fighter plane. Another major issue was that the Komet guzzled its propellant at such a rapid rate that it had a total of just 7.5 minutes of powered flight. This meant the fighter could only operate as what we now call a point-defense interceptor, needing

to be stationed close to where the enemy bombers were expected to pass over. A German animation movie of the time shows how the plane would take off when a bomber formation came within a range of 42 km (26 miles), flying towards the Me 163B airbase at an altitude of 7 km (4.4 miles). By the time the bombers had halved that distance, the Komet was in the right position to attack.

Once it ran out of propellant, the Me 163B had to glide back home. AlUed pilots noticed this and tried to attack them during this unpowered descent. This was still a difficult task because even whilst gliding back down the little rocket plane was much more maneuverable than any Allied fighter, and with an unpowered diving speed of over 700 km per hour (440 miles per hour) it was very fast. Shortly before landing, however, the Me 163 was an easy target since it had to fly straight and slow. Allied fighters would circle just outside the defensive perimeter of the airfield’s anti-aircraft guns and from there mount quick attacks on Komets making the final approaches. To counter this tactic, experienced Me 163 pilots would rapidly dive into the protected area at about 800 km per hour (500 miles per hour), then fly circles within the range of the anti-aircraft guns while they bled off excess speed prior to lining up to make a landing approach.

To keep the plane fight, it was not equipped with a proper undercarriage. For take-off it used a jettisonable dolly with two large wheels which, if dropped prematurely, could bounce back against the belly. The plane landed on grass fields using a single extendable skid which was fight and easy to fit into the plane without disrupting its clean aerodynamic shape. It slowed the aircraft down after landing simply by friction with the ground. The skid had hydraulic shock dampeners but these would not work when the skid activation lever in the cockpit was not properly set after deployment. The resulting lack of cushioning in combination with bumpy fields and high landing velocities caused some pilots to suffer back injuries, including experienced test pilot Heini Dittmar when he landed an Me 163A in November 1943. And Hanna Reitsch was badly hurt when the take-off dolly did not separate on her fifth towed training flight in an Me 163B, forcing her to discontinue and land with the wheels attached. Because the dolly made it impossible to deploy the skid and its shock dampener, the plane hit the ground hard, somersaulted and threw her head against the gunsight. She spent several months in hospital and lost her chance to fly the Me 163B under power.

The Me 163B was aerodynamically very clean, and showed its glider heritage by its very flat gliding angle: while unpowered, it would only lose one meter of altitude per 20 meters of distance. Whilst this was great for gliding back to base, it made the plane rather difficult to land: just above the ground the air would become compressed between the wings and the field, forming an air cushion. This ‘ground effect’ could keep the Me 163B floating just above the ground for a long distance, with the merest updraft being sufficient to make it ascend back into the air again. As a consequence, pilots found it difficult to put the Komet down quickly. Since on approach the plane would be unpowered there was no opportunity to circle around for another landing attempt, and a delayed touchdown could easily result in a crash amongst the trees at the periphery of the airfield. And even after touching down the plane needed a run of about 370 meters (1,200 feet) on dry grass to halt, and nearly

twice that on wet grass. With the high landing speed of around 200 km per hour (120 miles per hour) this made landing an Me 163B very challenging. A set of landing flaps, to increase drag while still allowing sufficient lift, provided a modicum of control but it was almost impossible to put an Me 163B down at a precise landing point. Even if the landing were successful, the pilot was still not safe because the Komet had no power to taxi, and was anyway not easy to move on its single skid. It had to be retrieved by a small tractor pulling a trailer equipped with two lifting arms which fitted under the wings, and while waiting out in the open the aircraft was very vulnerable to attack by enemy planes.

Take-off could also be dangerous for an inexperienced pilot because the absence of a propeller-driven airflow over aircraft meant that the aerodynamic controls only became effective at about 130 km per hour (80 miles per hour). Steering during the taxi run was therefore done using the tail wheel, which was coupled to the rudder and so could be operated using the foot pedals. However, it meant that the position of the stick (and thus elevons) was initially immaterial and a pilot might accidentally put it in a turn during the take-off run which could seriously ruin his day when the control surfaces suddenly became functional. A crash soon after an aborted take-off would inevitably lead to a violent explosion of the volatile propellants. The long take-off run of the Komet was also an issue because it required a lot of room and, moreover, used a lot of propellant. A rocket propelled launch rail system was developed to help the plane on its way. This was successfully demonstrated using an Me 163 mockup, but the rails would mean losing the operational flexibility of taking off in any direction from any sufficiently large and reasonably level grassy field.

UnUke modern airplanes, the cockpit of the Me 163 was unpressurized, exposing its pilot to a very quick drop in atmospheric pressure during the fast climb to altitude. Pilots had to eat special low-fiber food to prevent any gas in their gastrointestinal tract from rapidly expanding on the way up. They were also given altitude chamber training to familiarize them with operating in the thin air of the stratosphere without a pressure suit and while breathing through an oxygen mask.

Another major hazard for the Me 163 pilots and their ground crews was that the propellant was very corrosive. The T-Stoff and C-Stoff would explode violently if they came into contact with each other prematurely; even a missed drop could set off a massive explosion of all the propellant in the aircraft. The filling caps on the plane were therefore very clearly marked with a ‘T’ or a ‘C’, and trucks transporting these liquids to the planes were never allowed to get close to one another. After each flight the propellant tanks and rocket engine had to be flushed with water to remove any residual fluids, and the entire aircraft’s surface was washed just to be sure. The pilots wore protective asbestos-mipolam-fiber clothing that would not burn on coming into contact with the concentrated hydrogen peroxide T-Stoff, but the suit was not perfect and if this fluid seeped through a seam it could rapidly react with the pilot’s skin in a chemical reaction that released oxygen, the concentration of which would soon rise to that for spontaneous combustion. In short, the concentrated peroxide would burn a pilot alive, and several pilots were injured by it when propellant lines broke in crash landings. Part of the dangerous liquid was actually put in two small tanks located on each side of the seat in the cramped cockpit, so even a small leak there would have grave consequences for a pilot. The tanks for the T-Stoff had to be kept meticulously clean because even a fingerprint would be enough to cause it to decompose violently into oxygen and hot steam. Because of the dangerous propellant vapors, a pilot had to breathe through his oxygen mask even while the plane was still on the ground, and always had to wear flight goggles to protect his eyes. The Me 163B was not equipped with an ejection seat (the now standard means for escaping from a supersonic or transonic airplane) as this equipment was still very experimental at the time. However, the Me 163B designers did manage to give the pilots some chance of bailing out of a Komet in trouble at 900 km per hour (560 miles per hour): a small drogue chute could be deployed behind the plane to slow it to a velocity at which it was safe to get out of the cockpit, and at which the pilot’s own parachute would not be tom to shreds. But this was not of much use if the engine suddenly exploded upon being ignited for take-off, which frequently happened.

The Komet was the first airplane to really enter the domain of transonic flight near to Mach 1, where the compressibility of the air starts to be noticeable. When a plane flies at the speed of sound, air no longer flows nicely around its nose and over the wings. Wherever the supersonic air hits a significant obstacle, like the blunt nose of an airplane, or a region in which air is flowing at a lower velocity, it is slowed

down. But the air molecules are no longer able to move out of the way in order to ensure a smooth flow. Instead they collide with each other and are compressed into a shock wave. While the Me 163 could not reach Mach 1, it still encountered compressibility effects because the airflow over part of its airframe would actually go supersonic. We have seen earlier that air is accelerated when flowing over the wings. This means it locally flows faster than the plane itself is flying through the atmosphere, and thus shock waves can occur even if the plane’s airspeed is just below the speed of sound. The flight Mach number at which the local flow over the wing first goes supersonic is called the ‘critical Mach number’ of the airplane. When flying above the critical Mach number but below the speed of sound, the airplane is in the transonic flight regime.

The thicker the wing, the more the air passing over it will accelerate and thus the lower the critical Mach number of that airplane. Several fast conventional propeller planes of the Second World War experienced air compressibility issues in a fast dive because their wings were relatively thick. The P-38 Lightning had a critical Mach number of 0.68, and thus already started to experience transonic aerodynamic effects when flying at 68% of the speed of sound. Pilots of early versions of the Lightning therefore often ran into trouble when they put the plane into a high speed dive, with the controls suddenly freezing, the tail shaking violently, and the plane nosing into an ever steeper dive beyond the pilot’s control.

When the airflow over the surface of a wing goes supersonic, the point where the sum of the lift forces across the wing’s surface can be thought to originate (called the plane’s center of pressure) moves aft. Basically what happens is that the speed of the airflow over the aft part of the wing is faster (supersonic) than the subsonic flow over the front part of the wing. The aft part thus gives relatively more lift and causes the center of pressure to migrate further aft in the airstream. As a result, the aircraft is no longer balanced along its pitch axis. As the plane enters the transonic regime, the lift force will try to push the nose down.

Another nasty consequence of transonic flight can be a sudden loss of lift. When the airflow over an area of a wing goes supersonic, a shock wave can form at the aft boundary of the supersonic-flow region, beyond which is subsonic flow. This shock wave can grow so strong as to cause the airflow behind it to separate from the wing and no longer nicely follow the wing contours. This leads to a serious loss of lift, a condition called ‘shock stall’. On the P-38 this also meant that the airflow behind the wings went more straight towards the tail and gave the horizontal stabilizer more lift, which only worsened the pitch-down problem. If a P-38 flew faster than its critical Mach number of 0.68 the shift in the center of pressure, in combination with shock stall, would inevitably induce ‘Mach tuck’. The pilot would naturally try to get the nose back up by pulling on the stick, deflecting the tail elevators upward. This would actually give the airflow at the elevator hinge line on the underside of the stabilizer more room, making it expand and hence accelerate. The resulting shock wave near the hinge line would make the elevators shock-stall as well, become ineffective, and cause the plane to uncontrollably nose down into an ever-steepening dive. Since the shock stall happens irregularly and is subject to constant change, the aircraft would also shake violently as it plunged towards the ground. Not surprisingly, a pilot would like to bail out at that point, but without an ejection seat to boost him clear of the airplane this would mean certain death. But if he remained in the plane he actually had a chance of survival because when the diving plane encountered the denser, warmer air at lower altitudes the local atmosphere’s speed of sound would increase. As a result the Mach number would diminish, the shock waves would lose their strength, and the elevators would regain their effectiveness. With luck, the pilot would be able to pull up before creating a hole in the ground. A more effective way to solve the Mach tuck problem was to equip the P-38 with dive flaps that could quickly be extended downwards and influence the center of pressure on the plane in a manner that would prevent it from nosing down. In late 1943 these flaps became standard on all new P-38s, and the company issued kits so that they could be retrofitted to already operational planes.

Alexander Lippisch knew about compressibility and Mach tuck, and he designed the Komet with relatively thin delta wings. The thin wings (at least in comparison to other contemporary planes) resulted in less acceleration of the air flowing over them, which contributed greatly to a high critical Mach number. The delta shape, or rather the swept wing in general, was another revolutionary good idea for high-speed flight. The most important factor in creating shocks on a wing is the velocity of the airflow perpendicular to the leading edge. If a wing is swept backwards, the component of the velocity perpendicular to the leading edge is diminished even if the total velocity remains the same. Or in other words, the air is presented with a more gradual change in wing thickness as the wing is swept back. This means that for the same thickness, shock waves will build up at higher speeds and thus the critical Mach number of the wing is increased. Another benefit of swept-back wings for supersonic flight is that they remain out of the V-shaped shock wave that forms at the nose of a plane when it exceeds the speed of sound. Straight wings partly project through this shock wave and this produces high drag. The downside is that the more a wing is swept back the less lift if will generate at any given velocity, reducing its effectiveness, especially at low speeds. The triangular shape of a delta wing combines wing sweep with a stronger structure that is supported by a longer section of the plane’s fuselage. As a result, the wing is more capable of resisting the strong force of high-speed flight. Moreover, at high angles of attack a delta wing creates a vortex of air which is able to stick to its surface much better than a normal straight airflow, thereby delaying wing stall. This is very handy at low flight speeds, where a delta-winged airplane can fly at a higher angle of attack and create more lift than a plane with normal swept wings. A delta wing is therefore efficient for high-speed flight as well as take-off and landing, as demonstrated by the excellent flying characteristics of the Me 163.

The combination of a relatively thin wing and its delta shaped planform gave the Komet a critical Mach number of about 0.84, so that it experienced transonic effects only at a much higher velocity than the P-38 or even the P-51, the latter of which had a critical Mach number of 0.78. This was very important, since even in level flight the Komet could fly much faster than a diving Lightning or Mustang and could more readily run into trouble. In addition a delta-winged aircraft needs no horizontal stabilizers, which saves weight and eliminates a source of drag. But the need for a high critical Mach number meant that the Me 163 had very little ‘washout’. Most wings on conventional airplanes are twisted so that the tip encounters the air at a lower angle of attack than the root. This is done to make sure that when the wing stalls it does so at the root first, leaving sufficient effectiveness for the ailerons near the wingtip (where they have the biggest leverage) to control the plane. The wing’s twist is called ‘washout’, and it is particularly important for swept wings since they more readily stall at the tip, causing the wing to drop and potentially result in a spin. Sufficient washout on the Me 163 would have caused the lower surface of its wingtips to shock stall at high speeds and bring on Mach tuck. To keep the tips from stalling at low speeds and high angles of attack in spite of the lack of washout, Lippisch installed fixed leading edge slots that would suck air from below the wing and pass it over the wingtips, thus delaying their stall.

Because the Me 163 flew much faster than propeller fighters and could surpass Mach 0.84 even in level flight, compressibility and Mach tuck remained a danger in spite of Lippisch’s revolutionary wing design. In comparison to modern supersonic planes, the wings of the Me 163 were still relatively thick. When flying the Me 163A Heini Dittmar was the first pilot to encounter the now familiar problems associated with supersonic flight. In his book Raketenjager Me-163, former Me 163 pilot Mano Ziegler quotes Dittmar regarding his record-setting flight of October 1941: “When I looked at the instruments again, I had gone over the 1,000 km per hour mark, but the airspeed indicator was unstable, the elevator started to vibrate, and at the same time the aircraft plummeted out of the sky, gathering speed. I could not do anything. I immediately turned off the engine and was certain that the end was near when I

suddenly felt the steering responding and managed to get the Me out of her nose-dive relatively easily.” To prevent pilots from exceeding the critical Mach number, a red warning light was installed in the cockpit to tell them when it was time to throttle down. However, while accelerating horizontally just before the sharp climb, or when leveling off in preparation for attacking enemy bombers, pilots tended not to look at their cockpit display very often, so later an alert horn was positioned right behind the pilot’s head to gain their attention.

After the war, other high-speed tailless airplanes with relatively thick wings had similar Mach tuck problems. The British de Havilland D. H. 108 Swallow research aircraft killed three pilots, including Geoffrey de Havilland, son of the company’s founder, because of its nasty Mach tuck characteristics. The American Northrop X-4 experienced similar difficulties during flight tests. Later tailless combat aircraft had thinner wings and better designed elevons that were shaped to prevent the loss of control power.

In his book, Mano Ziegler claims that during a flight in July 1944 Heini Dittmar actually broke the sound barrier in an Me 163B and reached a flight speed of 1,130 km per hour (702 miles per hour) by placing his Komet into a steep powered dive. Several observers on the ground are reported to have heard the sonic boom. If they indeed heard this, it would be positive proof of the plane flying faster than sound. A sonic boom is created by the pressure waves a plane causes by moving through the

air, similar to the bow and stern waves of a boat. These waves travel away from the airplane at the speed of sound, so when the plane flies at Mach 1 or higher the waves are forced together and merge into a single shock wave in the shape of a cone which has the aircraft at its apex. When that wave reaches your ear as a sharp increase and following decrease in air pressure, you hear a bang. However, modern analysis of the Me 163 design shows that it was incapable of surpassing the speed of sound because its wings were too thick and its fuselage not slim enough. As the Komet got close to Mach 1, the resulting strong shock waves would create so much drag that the aircraft would not be able to accelerate further, even in a powered dive (shock waves require energy to form, and this is taken from the aircraft’s speed and creates a form of aerodynamic drag). The speed registered by Dittmar was very likely inaccurate, as indicators designed for subsonic airspeeds cannot measure transonic velocities very well. So the noise heard by the spectators was probably not the fully developed sonic boom associated with faster-than-sound flight.

As with all German ‘wonder weapons’, the Me 163B Komet was too little and too late to change the outcome of the war. Due to enemy raids on their airfields, the lack of rocket propellant, and the constant need to retreat, Komet pilots managed to shoot down only eight bombers and a single Mosquito reconnaissance aircraft. The cost to JG 400 was high, with many aircraft and pilots lost due to exploding engines and rough landings ending in deadly crashes, or because they were shot down. Overall, the Komet represented more of a threat to its own pilots than to the enemy. Nevertheless the Me 163B was the fastest plane in the sky during the Second World War, and had it been deployed earlier it would have made a much bigger impact than it did. The Komet was the first, and last, pure rocket propelled airplane ever to fly in combat.

As the war neared its conclusion, a more advanced rocket fighter, the Me 263, was already under development. Although based on the Me 163B it would have a proper retractable undercarriage with a nose wheel, a larger and better shaped fuselage, a greater propellant load and a pressurized cockpit with a bubble canopy for all­around vision. It would initially be powered by a new HWK 109-509C Walter rocket engine with two combustion chambers and nozzles, mounted one above the other. The larger chamber would provide power for take-off and climbing (and at 20,000

Newton was much more powerful than the engine of the Me 163B) whilst the smaller chamber would provide a lower thrust of 4,000 Newton for cruising and the return to base. In that way the plane would be able to fly under power and spend significantly longer attacking bombers than was possible for the Me 163B. The smaller chamber was also to enable the Me 263 to taxi to a safe shelter, making it less vulnerable on the ground (this smaller part of the engine was tested on two experimental Me 163Bs). The new plane would have the same wings and tail as the Komet, and so have a similar critical Mach number. One vital difference however, was that some of the propellant would be stored in tanks fitted in an empty space in the wings.

Because Messerschmitt was totally overloaded with production demands for its conventional fighter planes, the project was assigned to the Junkers aircraft company, which renamed it the Ju 248 and developed it under the leadership of Dr. Heinrich Hertel. Three prototypes were built, and the first one made several unpowered flights under tow by Messerschmitt Bf 110 twin-engined fighter aircraft before testing was halted by a shortage of fuel for the tow plane. It appears no powered flights with the Me 263 were made before the war ended.

After Germany’s capitulation, many Komets were removed to Britain, the US and Russia for analysis and tests. None of these were ever flown under power, mostly due to unfamiliarity with the rocket engine and its dangerous propellants. It is likely that the revolutionary airplane simply scared most of the potential test pilots. Ten of the expropriated planes survive (most of which were once part of JG 400) and are on display in museums in Britain, Germany, the US, Canada and Australia. During the restoration of one in Canada in the late 1990s it was discovered the aircraft had been sabotaged by the French laborers who had been forced to build it. They had wedged a small stone in between the fuselage fuel tank and a supporting strap in an attempt to cause a dangerous leak, and they had also weakened the wooden wing structure by using contaminated glue. On the inside of the fuselage they had written in French “Plant Closed” and “My heart is not occupied”. This particular Me 163B can now be seen in the National Museum of the United States Air Force near Dayton, Ohio. In the mid-1990s a former Luftwaffe pilot built an unpowered glider reproduction of the Komet which flies very well and from a distance looks very much like the real thing (its internal construction and the materials used are very different). The builder, Josef Kurz, had been in training to fly the Me 163B near the end of the war but never flew it. He made the first flight in his replica in 1996, being towed into the air by another plane. He later sold his Komet reproduction to the large European aircraft company EADS, one institutional strand of which could trace its roots back to none other than Willy Messerschmitt.

The original Komet hanging from the ceiling in London’s Science Museum now looks somewhat dusty, silent and inert: no longer advanced and top secret, merely an artefact of a war which was fought long ago. Just like a stuffed jaguar, it is initially hard to imagine that this display item was once an agile and terrifying hunter; a killer that could leap up to attack its victims high in the sky in an unprecedented manner, digesting toxic liquids and spitting fire. Suspended next to the museum’s Hurricane and an early Spitfire fighter, it looks rather diminutive, but it could readily out – climb, outrun and outmaneuver these conventional propeller planes that entered

service only a few years before the Me 163B took to the sky. But if you look more carefully you cannot help but admire its sleek shape, the audacity of its design, and the pilots who dared to strap themselves into those dangerous little machines. They zoomed through the air at what were then utterly amazing speeds, in machines powered by incredibly dangerous propellants. A punctured propellant tank from a bullet or a rough landing would probably be fatal. On the one hand these machines were part of the jet age in terms of aerodynamic sophistication and propulsion technology, yet the construction technology and primitive landing skid clearly belonged to the realm of Second World War propeller aircraft. Its wooden wings and fabric-covered control surfaces would normally even be associated with planes of the First World War! The Komet pilots were the kings of the sky while they had propellant in their tanks (and provided their plane did not spontaneously explode), flying and climbing even faster than the early jets, but once out of power they found themselves in a glider that could be shot down by conventional fighters.

The Me 163’s legacy of advanced aerodynamics based on the delta-wing, tailless design led to such fast vehicles as the Concorde and the Space Shuttle, and it lives on in today’s advanced fighter aircraft. After the war Lippisch, the 163’s chief designer, was moved to the US as part of Operation Paperclip (along with von Braun and his A4/V2 team). There he continued his work at the Convair airplane company, which subsequently produced delta-wing airplanes such as the XF-92, F-102 Delta Dagger, F-106 Delta Dart, B-58 Hustler and the Navy’s F2Y Sea Dart. One Me 263

prototype was taken to the US. After Soviet forces captured the Junkers factory another ended up in Russia, along with some technical staff and engineering documentation. It formed the basis for the Soviet MiG 1-270 (to be discussed later). Walter and his technology were captured by the British, and later gave rise to the powerful engine that pushed the Black Arrow rocket into space, as well as the Spectre engine of the Saunders-Roe SR.53 rocket interceptor (also to be discussed later). And his ‘cold’ rocket pods were developed into the Sprite engine that de Havilland designed to assist aircraft take off from airports located in hot places and at high elevations where the low air pressure prevented the wings from generating sufficient lift. A pair of Sprites were tested on a Comet jet airliner in 1951.

The only Alhed pilot ever to fly the Komet under power was the famous British test pilot Eric Brown, who assessed the astonishing capabilities during a single flight of a captured plane in Germany shortly after the surrender. He later commented: “I was struck by how small it was, and yet how elegant it looked, and at the same time how very lethal. Lethal not only to the enemy, but to those who flew it.” Brown flew no fewer than 487 types of aircraft, not even counting different versions of the same basic type, so his judgment can be trusted.

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.