The engine delivered 18 months after design started was so compact, that the length of the A4 could be cut in half. Walter Thiel, a gifted and systematic researcher, was responsible for the engine design. He had great difficulties in obtaining stable combustion, and preventing burn-through of the chamber walls. Various injector patterns were studied in a 1.5 tonne thrust chamber. His research finally reduced the combustion chamber length from 2 m to 30 cm, while the exhaust velocity was increased from 2000 m/s to 2100 m/s, and eventually reached 2280 m/s. However the reduction in the cooling area of the chamber also increased problems in preventing hot spots and burn through. This was finally solved by using a conical throat exit and a mixing chamber ahead of the burning chamber. The 1.5 tonne thrust engine was initially run at 15 bar pressure, versus the 50 bar desired. But whenever the combustion chamber pressure was increased, burn-throughs occurred, as well as forcing increases in the mass of the pumps and tanks. Therefore finally the decision was taken to leave the chamber pressure at 15 bar.

The next step was to make a 4.5 tonne thrust by clustering three of the 1.5 tonne engines as preburners. However Thiel still had burn-throughs in test runs. Poehlmann suggested the use of film cooling, which finally solved the problem. For the 25 tonne thrust engine, Thiel simply used 18 x 1.5 tonne thrust chambers, feeding a common mixing chamber. This was on the test stand in early 1939.

The first A5 drop test model is delivered to Peenemuende just weeks after the third A3 test. Production is planned at a rate of 10 per month to define the A4 aerodynamic configuration. Objective of the first tests is to break the sound barrier - in the wind tunnel no configuration of fins had managed to go through the barrier without disintegrating. The only test possibility was to drop the model from a great height, and let gravity accelerate it to supersonic speeds. The model weighs 250 kg and is 1.6 m long and 20 cm in diameter.

In the summer of 1938 the decision is made to go ahead with four A5 tests from Greifswalder Oie without the stabilising system or a parachute. The first missile ascended into a low wind, and reached 8 km altitude, nearing but not exceeding the sound barrier. Maximum altitude reached in the test series is 12 km.

The first rocket fighter, the He-176, powered by a Walther engine, was tested at Peenemuende. In competition, Dornberger's team developed a 120-second duration engine to power the He-122. However loss of control in unpowered flights of the latter resulted in it crashing and being eliminated from further consideration. Dornberger's team left further rocket fighter engine development to Walther, and concentrated on the A4 and follow-on ballistic missiles.

Rocket reached altitude 4920 ft as determined from telescope; from NAA barograph, 3294 ft, but altitude appeared greater visually and by telescope; corrected well, parachute opened at maximum point of ascent. This completed Section C of the L series, conducted from July 1937-August 1938. It had included seven proving-stand tests and 8 flight test attempts, all of which resulted in flights. Average interval between tests 25 days. For the entire L Series, from May 1936-August 1938, there were 13 proving-stand tests 13 and 17 flight test attempts, all of which were successful. Average interval between tests was 22-25 days

The model is dropped from a He-111 bomber from 7000 m. It breaks through the sound barrier at 1000 m altitude at a speed of 360 m/s. The stabilising fins keep the maximum oscillation of the model to within 5 degrees from vertical. The drogue ring parachute then deployed to decelerate the model to 100 m/s, followed by the main parachute which slows it to 5 m/s when it impacts in the ocean.

In order to test the A4's stabilisation system, Walter, Kiel, is subcontracted to build a large number of model A5's. Like the drop test models, these are 20 cm and 1.6 m long. However they weigh only 47 kg gross lift-off mass, with a 27 kg empty mass. The rocket engine burns 85% hydrogen peroxide monopropellant using a calcium permanganate catalyst. The engine produces 120 kgf for 15 seconds, and has an exhaust velocity of 1000 m/s. The design objective is a low cost, reliable, and simple rocket, which will allow a large number of trail-and-error test launches to be made within a tight budget. The fins developed for the A4 as a result of these tests were shorter and wider than those of the A3. They owed nothing to aircraft wing designs of the times, which couldn't withstand supersonic speeds. But they were still too affected by the wind, tending to set the rocket on a rotation around its long axis during ascent.

After the final successful L series flight of August 9, with pressure feed, Goddard turned again to the problem of fuel pumps, which he believed were imperative if very high altitudes were to be attained. This ultimately resulted in the P Series of Tests, which ran from October 17, 1938-October 10, 1941. The work began in the fall of 1938, when he made a thorough study, through more than twenty proving-stand tests, of five models of small, high-speed centrifugal pumps, which had several radically new features.