Germany’s wonder weapons

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

GERMANY GETS SERIOUS

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

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

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

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

Ground testing a rocket engine in an He 112, probably the Walter engine.

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

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

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

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

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

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

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

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

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

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

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

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

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