Background In 1943, the
air superiority of the
Luftwaffe was being challenged by
Allied aircraft
over the Reich; seeking to overcome this crisis, German officials urgently sought out radical innovations and unconventional solutions to respond to the threat posed by Allied airpower. The emerging field of
surface-to-air missiles appeared to be a promising approach against the
Allied strategic bombing offensive; a variety of projects were started, but invariably problems with the guidance and homing systems prevented any of these from attaining operational status. Providing the missile with a pilot, who could operate a weapon during the brief terminal approach phase, offered a solution. Submissions for a simple target defence interceptor were requested by the
Luftwaffe in early 1944 under the umbrella of the , literally "
Emergency Fighter Program". A number of simple designs were proposed, including the
Heinkel P.1077 Julia, in which the pilot lay prone (on his stomach), to reduce the frontal area. Following a series of evaluations by the
Reichsluftfahrtministerium (RLM, Ministry of Aviation), the
Julia would become the front-runner to fulfil this requirement in autumn 1944. The initial plan was to launch the aircraft vertically, but this concept was later changed to a conventional horizontal take-off from a tricycle-wheeled trolley, similar to that used by the first eight prototypes of the
Arado Ar 234 Blitz jet reconnaissance bomber.
Bachem's proposal The Natter was first conceived by the aeronautical engineer
Erich Bachem in August 1944, after witnessing an American bombing raid on a major German city. He believed there ought to be a way to break up these large formations of bombers. His
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-plated
bulkhead and bulletproof glass
windshield at the front of the cockpit. The initial plan was to power the aircraft with a
Walter HWK 109-509A-2 rocket engine; however, only the 109-509A-1, as used in the
Me 163, was available. It had a sea level thrust variable between at "idle" to at full power, with the ''Natter's'' intended quartet of rear flank-mount
Schmidding SG34 solid fuel rocket boosters used in its vertical launch to provide an additional thrust for ten seconds before they burned out and were jettisoned. The experimental prototypes slid up a -tall vertical steel launch tower for a maximum sliding length of in three guideways, one for each wing tip and one for the lower tip of the ventral tail fin. By the time the aircraft left the tower it was hoped that it would have achieved sufficient speed to allow its
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 an 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 nineteen
R4M rockets. Later, 28 R4Ms or a number of the larger,
Henschel Hs 297 Föhn rockets were suggested, with either variety of unguided rocket fired from the Natter's nose-mounted cellular launch tubes. The Natter was intended to fly up and over the bombers, by which time its Walter engine would probably be out of propellant. Following its one-time attack with its rockets, the pilot would dive his Natter, now effectively a glider, to an altitude of around , flatten out, release the nose of the aircraft and a small braking parachute from the rear fuselage. The fuselage would decelerate and the pilot would be ejected forwards by his own momentum and land by means of a personal parachute. In an early proposal in August 1944, the Natter design featured a
concrete nose; it was suggested that the aircraft might
ram enemy bombers, 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 glider at an airbase, which, as the history of the Me 163 rocket interceptor had clearly demonstrated, left an aircraft extremely vulnerable to attack by Allied fighters.
Modifications Bachem approached
Adolf Galland, a
Luftwaffe general and
flying ace, after which Galland personally recommended its development to the
RLM. The RLM officially designated the BP-20 as the
Ba 349A and construction began. This decision has been claimed to have been the only occasion that the SS would significantly interfer with aircraft design and air fighting strategy. At an early stage if the project, the RLM undertook an engineering assessment of the Natter, the report for which was issued on 28 October 1944. The Natter was designed to be built by unskilled labour with poor-quality tools and inexpensive material. Various stringent economies were imposed on an already frugal design. The airframe of the Natter was almost entirely composed of wood. It also lacked
landing gear, which saved weight, expense and construction time. Consequently, one of the most unusual features of the machine was the escape of the pilot and recovery of the machine. The proposed sequence of these events was as follows: After the attack, the Natter might dive to a lower altitude and flatten out into level flight. The pilot would then proceed with a well-practised escape sequence. He would open the cockpit canopy latch, which would allow the canopy to flick backwards on its hinge in the airstream. Next, the pilot would undo his seat belt and remove his feet from the rudder pedal stirrups. By squeezing a lever mounted on the control column, he would release a lock at the base of the column, which would allow him to tilt the column forwards where it could engage in and undo a safety latch for the nose release mechanism. He would then lean a little further forward and pull a lever hinged near the floor at the front of the cockpit, freeing the nose section, which self-jettisoned as a result of the reduced aerodynamic pressure at the front of the fuselage. As the nose section separated, it was intended to briefly pull on two cables that released a small ribbon parachute stored on the starboard side of the rear fuselage. The parachute subsequently opened and decelerated the Natter. The pilot would be ejected from the cockpit by his own momentum and as soon as he was clear of the fuselage, he would open his personal parachute and descend to the ground. A parachute was intended to eject the valuable Walter rocket engine from the rear, which would decelerate the aircraft and eject the pilot with inertia, however, associated problems with this mechanism were still not fully resolved prior to the conflict's end.
Wind tunnel testing on a wooden model, scaled to 40% of full size, was performed at the
Deutsche Versuchsanstalt für Luftfahrt (DVL), the Institute for Aerodynamics at
Berlin-
Adlershof in September 1944 at speeds up to . Results from these tests were reported in January 1945 to the Bachem-Werk. In March 1945, the Bachem-Werk simply received a statement that satisfactory flying qualities should be expected with speeds up to .
Flight testing Construction of the first experimental prototype
Natter, Versuchsmuster 1, was completed on 4 October 1944. V1 was subsequently referred to as
Baumuster1 (
BM1) and later still the "B" was dropped and the machine became known as the M1. Most subsequent prototypes were known by 'M' codes, as the later prototypes of the
Heinkel He 162 were. Manned glider flights began on 3 November 1944. The first glider M1 was towed to around 3,000 m by a
Heinkel He 111 bomber with a cable (
Tragschlepp mode) at
Neuburg an der Donau. After flying at speeds of up to , the pilot successfully bailed out as planned. Amongst the findings reported by the recovered pilot was that the Natter possessed excellent stability and effective controls. The vertical take-off trials were conducted on high ground called the Ochsenkopf at the
Truppenübungsplatz (military training area) Heuberg near
Stetten am kalten Markt,
Württemberg. The first successful unmanned vertical take-off from the experimental launch tower occurred on 22 December 1944. The test machine, the M16, was powered only by the Schmidding solid boosters, as were all the early vertical launch trials. The performance of these solid boosters proved to be erratic, both in terms of thrust output and burn time, while numerous units unintentionally exploded. Up to and including 1 March 1945, 16 prototypes had been used, 8 in glider trials and 8 in VTO trials. By January 1945, Himmler's enthusiasm for the Natter had waned while numerous officials had been advocating for the concentration of resources on the
turbojet-powered
Messerschmitt Me 262 and rocket-powered
Messerschmitt Me 263 interceptors at the expense of less mature competing projects such as the Natter. Furthermore, some figures within the RLM urged for rapid progress to be made on the project, thus Bachem came under considerable political pressure to carry out a manned flight by the end of February. On 25 February, M22 was in the experimental launch tower. It was as complete an operational aircraft as possible with the Walter HWK 109-509 A1 engine installed for the first time and a dummy pilot present in the cockpit.
Manned test flight Despite Bachem's concerns that the test programme had been significantly cut short, a young volunteer
Luftwaffe test pilot,
Lothar Sieber, climbed into the cockpit of the fully fuelled M23 on 1 March. A hard wire
intercom appears to have been provided between Sieber and the engineers in the launch bunker using a system similar to that used in the manned glider flights. Around 11:00 am, the M23 was ready for take-off. Initially, the Natter rose vertically but, at an altitude of about , it suddenly pitched up into an inverted curve at about 30° to the vertical. At about the cockpit canopy was seen to fly off. The Natter continued to climb at high speed at an angle of 15° from the horizontal and disappeared into the clouds. The Walter engine stalled about 15 seconds after take-off. It is estimated the Natter reached , at which point it nose-dived and hit the ground with great force about 32 seconds later, some kilometres from the launch site. Unknown at the time, one of the Schmidding boosters failed to jettison and its remains were dug up at the crash site in 1998. The pilot was likely unconscious long before the crash. Bachem surmised Sieber had involuntarily pulled back on the control column under the effect of the 3 G acceleration. Examination of the canopy, which fell near the launch site, showed the tip of the latch was bent, suggesting it may not have been in the fully closed position at launch. The pilot's headrest had been attached to the underside of the canopy and as the canopy flew off the pilot's head would have snapped back suddenly about , hitting the solid wooden rear upper cockpit bulkhead, and either knocking Sieber unconscious or breaking his neck. The Walter engine probably ceased operation because the Natter was virtually upside-down and air may have entered the intake pipes in the propellant tanks, starving the engine. Sieber had become the first person to take off vertically from the ground under pure rocket power, 16 years before
Yuri Gagarin's
Vostok 1, pioneering peacetime orbital flight. Following Sieber's death, all of the eight subsequent Natter flights were unmanned.
Production The SS ordered 150 Natters, and the
Luftwaffe ordered 50, but none were delivered by the end of the war. Much debate has surrounded the number of Natters built at the Bachem-Werk and their disposition. According to Bachem, 36
Natters were produced at the Bachem-Werk in
Waldsee by the end of the conflict. This new launcher was also constructed on the Heuberg, not far from the experimental steel tower. There is documentary evidence for two pole launches in April but not three as claimed by Bachem in his post-war presentation. Of the four Natters captured at Sankt Leonhard im Pitztal, two went to the United States. Only one original Natter built in Germany in the Second World War survives in storage at the
Paul E. Garber Preservation, Restoration, and Storage Facility in
Suitland, Maryland, under the auspices of the
Smithsonian Institution. The fate of the other Natter brought to the US is unknown. There is no documentary evidence that a Natter was ever flown from
Muroc Field. The tail section of one of the Natters at Sankt Leonhard im Pitztal was broken off while it still rested on its trailer. ==Stability==