Bachem's Proposal
Erich Bachem's BP-20 (Natter) was a development from a design he had worked on at Fieseler, the Fi 166 concept, but considerably more radical than the other submissions. It was built using glued and nailed wooden parts with an armour plate bulkhead and bulletproof glass windshield at the front of the cockpit. The initial plan was to power the machine with a Walter HWK 109-509 A2 rocket motor, however, only the 109-509 A1 unit was available as used in the Me 163 rocket aircraft. It had a sea level thrust of 1,700 kg. Four Schmidding SG34 solid fuel rocket boosters were also used at launch to provide an additional thrust of 1,200 × 4 = 4,800 kg for 10 seconds before they were jettisoned. The experimental prototypes slid up a 20 m high vertical steel launch tower for a maximum sliding length of 17 m in three guideways, one for each wing tip and one for the lower tip of the ventral tail fin. By the time they left the tower it was hoped that the aircraft would have achieved sufficient speed to allow their aerodynamic surfaces to provide stable flight.
Under operational conditions once the Natter had left the launcher it would be guided to the proximity of the Allied bombers by an autopilot with the possibility of added beam guidance similar to that used in some V-2 rocket launches. Only then would the pilot take control, aim and fire the armament, which was originally proposed to be a salvo of 19 R4M rockets. Later, 28 R4Ms were suggested, with another source stating that the similar Henschel Hs 297 Föhn unguided rocket was meant to be used in the nose-mounted launch tubes. The Natter was intended to fly up and over the bombers, by which time its Walter motor would probably be out of propellants. The pilot would dive his Natter, now effectively a glider, to an altitude of around 3,000 m, flatten out, release the nose of the Natter and a small braking parachute from the rear fuselage. The fuselage would decelerate and the pilot would be ejected forwards by his own inertia and land by a personal parachute.
In an early proposal in August 1944, the Natter design had a concrete nose and it was suggested that the machine might ram a bomber, but this proposal was subsequently withdrawn in later Project Natter outlines. Bachem stated clearly in the initial proposal that the Natter was not a suicide weapon and much effort went into designing safety features for the pilot. The design had one decisive advantage over its competitors – it eliminated the necessity to land an unpowered gliding machine at an airbase, which, as the history of the Me163 rocket aircraft had clearly demonstrated, made an aircraft extremely vulnerable to attack by Allied fighters.
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