German rocket-powered interceptor prototype
The Bachem Ba 349 Natter () is a World War II German point-defence rocket-powered interceptor, which was to be used in a very similar way to a manned surface-to-air missile. After a vertical take-off, which eliminated the need for airfields, most of the flight to the Allied bombers was to be controlled by an autopilot. The primary role of the relatively untrained pilot was to aim the aircraft at its target bomber and fire its armament of rockets. The pilot and the fuselage containing the rocket engine would then land using separate parachutes, while the nose section was disposable.
The first and only manned vertical take-off flight, on 1 March 1945, ended in the death of the test pilot, Lothar Sieber.
Development.
Background.
In 1943, "Luftwaffe" air superiority was being challenged by the Allies over the "Reich" and radical innovations were required to overcome the crisis. Surface-to-air missiles appeared to be a promising approach to counter 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.
The "Julia" was the front-runner for the contract. 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 jet reconnaissance bomber.
Bachem's proposal.
The Natter was first conceived by Dr 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 machine 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 10 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 55mm R4M rockets. Later, 28 R4Ms or a number of the larger, 73mm 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 had a concrete nose; 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. However, owing to the potential dangers for the pilot inherent in the operation of this precarious aircraft, the Natter is sometimes listed as a suicide craft. 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 Me 163 rocket aircraft had clearly demonstrated, made an aircraft extremely vulnerable to attack by Allied fighters.
Modifications.
Heinrich Himmler became interested in Bachem's design. The Reichsführer-SS granted Bachem an interview and fully supported the project. In the middle of September 1944, the Technical Office of the Waffen-SS made an order for Bachem to develop and manufacture the Natter at his Waldsee factory. The Reichsluftfahrtministerium (RLM/German Aviation Ministry) officially designated the BP-20 the Ba 349A and construction began. During December 1944, the project came largely under the control of the SS and Obergruppenführer Hans Kammler. This decision is said to have been the only time the SS significantly interfered with aircraft design and air fighting strategy. Early-on in the project, the RLM undertook an engineering assessment of the Natter, which it reported on 28 October 1944.
The Natter was designed to be built by unskilled labor with poor-quality tools and inexpensive material. Various stringent economies were imposed on an already frugal design. The Natter had no 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 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. Further model tests were carried out at the "Luftfahrtforschungsanstalt Hermann Göring" (LFA) facility in Völkenrode-Braunschweig, at speeds close to Mach 1. In March 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. The pilot was Erich Klöckner, who made all four documented "Tragschlepp" ("towed") flights.
After carrying out the test programme of the M1, he bailed out and the machine crashed into the ground. It was found that the towing cable, and in the case of the M3, the undercarriage interfered with the flight characteristics of the gliders and consequently the results were difficult to interpret. To resolve doubts about the Natter in the glider mode, Hans Zübert made a free flight in the M8 on 14 February, and showed that the Natter was indeed a good flying machine.
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. Up to and including 1 March 1945, 16 prototypes had been used, eight in glider trials and eight in VTO trials.
Manned test flight.
By January 1945, Bachem was under pressure from the authorities in Berlin 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 machine as possible with the Walter HWK 109-509 A1 engine installed for the first time. A dummy pilot was in the cockpit. Lift-off from the tower was perfect. The engineers and ground crew watched as the M22 ascended under the combined power of the four Schmidding boosters and the Walter engine, an estimated total thrust of . The nose separated as programmed and the dummy pilot descended safely under its personal parachute. The remainder of the fuselage came down under its two large salvage parachutes, but when it hit the ground the Walter liquid-propellant rocket engine's residual hypergolic propellants ("T-Stoff" oxidizer and "C-Stoff" fuel) exploded and the machine was destroyed.
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. The aircraft was equipped with an FM transmitter for the purpose of transmitting flight data from various monitoring sensors in the machine. 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 1100 am, the M23 was ready for take-off. Low stratus clouds lay over the Ochsenkopf. The Walter liquid-fueled rocket engine built up to full thrust and Sieber pushed the button to ignite the four solid boosters.
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