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Lusser completed a preliminary design at the end of April 1942. Radar or radio command systems were possible guidance options, but these were quickly rejected since the Allies could develop electronic counter-measures. Instead, the Germans turned to inertial guidance based on gyroscopes, as pioneered by Sperry in 1915. The proposed P35 Erfurt would have a range of 300km (186 miles) with a half-ton warhead and a speed of 700km/hr (435mph). When submitted to the Luftwaffe on June 5th, 1942, the Erfurt found a much more receptive audience than in years past. The attitude of the senior Luftwaffe leadership towards area bombardment weapons had changed due to the declining strategic situation. Britain had begun systematic heavy bomber attacks against Germany in March 1942, and Hitler demanded retaliatory strikes against England. The Luftwaffe lacked a heavy bomber force due to delays in its Heinkel He-177 program. Furthermore, the German Army was muscling in on the strategic bombardment mission with its A-4 ballistic missile program, better known by its later propaganda name, V-2. The Army attempted to win Hitler's support for the A-4 ballistic missile program by pointing out that it would succeed where the Luftwaffe had failed in the 1940 Battle of Britain. This insult was the last straw and, for the prestige of the service, the senior Luftwaffe leadership decided they needed a missile program of their own. The project was approved on June 19th, 1942, and incorporated into the Vulkan ("Volcano") program, which managed Luftwaffe missile efforts. Internally at Fieseler, the P35 was renamed in the standard fashion as the Fieseler Fi-103. The Luftwaffe assigned it the code name Kirschkem ("Cherry Stone"). It was later given another cover-name, FZG-76 (Flakuelgerat 76), linking it to the innocuous Argus FZG-43 target drone.

The first powered launch was conducted from an Fw-200 Kondor bomber over the Baltic on December 10th, 1942, using FM03V7. Note that some early airframes had an unusual vertical rudder in front of the pulse-jet engine. (DAVA)

Argus remained in charge of the pulse-jet engine, now designated as the Argus AS 109-014. The guidance system was entrusted to the Askania company in Berlin which was already involved in the development of inertial guidance systems for other Luftwaffe missiles. Rheinmetall-Borsig developed a rail launcher using a rocket sled to launch the Kirschkem.

The first Fi-103 was completed on August 30th, 1942. It differed from the original Erfurt design in numerous respects including the use of a single rudder instead of a twin tail. A refined version of the Argus pulse- jet was completed in September 1942 and began flight trials. Failure of the high-speed flight trials nearly led to the program's termination, but these problems were gradually overcome after it was realized that many of the anomalous test results were due to the peculiar effects produced by pulse-jets during wind-tunnel tests.

While the problems with the engine were being sorted out, the first flight tests of the Kirschkem were attempted by the Luftwaffe Test Establishment at Karlshagen at Peenemünde-West, near the A-4/V-2 ballistic missile test site. The first unpowered flight was conducted from an Fw-200 Kondor bomber on October 28th, 1942. The missile proved to be well designed and very stable in flight. The first powered test was conducted on December 10th, 1942, with Fi-103V7, the V7 indicating the seventh test (Versuch) airframe.

This test film from the launch of FM03V6 on January 13th, 1943, shows the use of the initial Rheinmetall-Borsig rocket sled. The early missiles had a deeper rudder which extended below the fuselage as seen here. (DAVA)

A launch site for the Rheinmetall-Borsig catapult was established at Peenemünde pointing out eastward along the Baltic coast. The first catapult test was conducted on October 20th, 1942, using a concrete ballistic dummy, followed by the test of a fuselage and Argus powerplant, minus the wings. The first powered launch followed on December 24th, 1942, with Fi-103V12. The flying bomb flew for about a minute and reached speeds of 500km/hr (310mph) before crashing into the Baltic. Although far short of its design specifications, this was an important milestone which led to approval for full-scale development.

The early test missiles had an exposed pulse-jet frame and external engine yoke support as seen here on Fi-103V17. A cowl was later added to channel the airflow. (DAVA)

Due to the urgency of the program, the testing of the airframe, pulse-jet, guidance, and launch catapult were conducted in parallel. This created some significant problems, since in the event of crashes or other failures, it was often impossible to determine which of the sub-systems had been responsible for the problem. Shortcomings of the Rheinmetall-Borsig launcher led to an alternative catapult being designed in early 1943, the Hellmuth Walter Werke (HWW) Schlitzrohrschleuder (split-tube catapult). The Walter design used a gas generator powered by a combination of T-stoff (hydrogen peroxide) and Z-stoff (sodium permanganate) rocket fuel. It worked much like a modern aircraft carrier steam catapult: high pressure gas was pumped into a tube inside the launch rail box that propelled a piston, connected underneath the Fi-103.

The early V-zellen prototype missiles, such as FM03V17 seen here, had a somewhat sleeker shape than the production missiles, and many small detail differences. The probe on the nose contained a flare used in tracking the missile after launch. This particular missile was launched on March 11th, 1943. (DAVA)

The test program was plagued by crashes, many induced by the pulse- jet engine. The fuel detonated inside the engine 47 times per second, creating an enormous amount of noise and vibration that could literally shake the fuselage and wings to pieces. The most common problem was the disintegration of the shutters in the front of the pulse-jet, upsetting the air flow into the engine if enough failed. Through the end of July 1943, 84 Fi-103 flying bombs had been launched, 16 from the air and 68 from ground catapults. Of the catapult launches, only 28 had been successful. About a third of the missiles failed to launch, or after leaving the catapult, rolled and crashed into the Baltic seconds later. None of the missiles had yet been fitted with the full guidance system, which was still in development, nor had any been launched at lull combat weight as the Walter catapult was still not ready. On a more positive note, at least one Fi-103 had attained speeds of 625km/hr (390mph), and had reached distances of 225km (140 miles).

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