Wednesday, November 8, 2017
Bachem Ba 349 "Natter"
The Bachem Ba 349 Natter (English: Colubrid, grass-snake) was 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-motor would then land using separate parachutes, while the nose section was disposable. The only manned vertical take-off flight on 1 March 1945 ended in the death of the test pilot, Lothar Sieber.
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 "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.
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-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 motor; however, only the 109-509A-1, as used in the Me 163, was available. It had a sea level thrust variable between 100 kg (220 lb) at "idle" to 1,600 kg (3,500 lb) 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 4,800 kg (10,600 lb) thrust for 10 seconds before they burned out and were jettisoned. The experimental prototypes slid up a 20 m (66 ft)-tall vertical steel launch tower for a maximum sliding length of 17 m (56 ft) 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 19 R4M rockets. Later, 28 R4Ms or a number of Henschel Hs 297 Föhn rockets were suggested, with either variety of unguided rocket fired from the Natter's nose-mount cellular launch tubes contained in its nose. The Natter was intended to fly up and over the bombers, by which time its Walter motor 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 3,000 m (9,800 ft), 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 inertia 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.
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 flights. After carrying out the test programme of the M1, he bailed out and the machine crashed into the ground. It was found that, unfortunately, 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 clear any lingering doubts about the Natter in the glider mode, Hans Zübert made a daring free flight in the M8 on the 14 February, and showed that the Natter was indeed a very good flying machine.
The vertical take-off (VTO) 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 VTO trials. Up to and including 1 March 1945, 16 prototypes had been used, eight in glider trials and eight in VTO trials.
By January 1945 Bachem was under pressure from the authorities in Berlin to carry out a manned VTO 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 motor 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 motor, an estimated total thrust of 6,500 kg (14,300 lb). 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 motor'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 Ocksenkopf. The Walter liquid-fueled rocket motor built up to full thrust and Sieber pushed the button to ignite the four solid boosters. With a roar, the M23 rose out of a cloud of steam and rocket smoke straight up, displaying its camouflage paintwork. At an altitude of about 100 to 150 m (330 to 490 ft), the Natter suddenly pitched backwards into an inverted curve. Initially it climbed at about 30° to the vertical. At about 500 m (1,600 ft) 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 motor stalled about 15 seconds after take-off. It is estimated the Natter reached 1,500 m (4,900 ft), 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.
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 25 cm (9.8 in), hitting the solid wooden rear upper cockpit bulkhead, and either knocking Sieber unconscious or breaking his neck.
This tragedy reinforced Bachem's long held belief that the take-off and flight in the vicinity of the target bombers should be fully automated. The canopy latch was strengthened and the headrest was attached to the backboard of the cockpit. Before the introduction of the autopilot in the test programme, the control column would have a temporary locking device on it, which would allow the machine to ascend vertically to at least 1,000 m (3,300 ft) and then be removed by the pilot. The Walter motor probably ceased operation because the Natter was virtually upside-down and air may have entered the intake pipes in the propellant tanks, starving the motor. Sieber had become the first man to take off vertically from the ground under pure rocket power, some 16 years before Yuri Gagarin's Vostok 1 pioneering, peacetime orbital flight.
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 war. Up to April 1945, 17 aircraft had been used in unmanned trials comprising five gliders, all slung under an He 111 in the Mistelschlepp configuration prior to launch, and 12 VTO examples. Five aircraft were prepared for manned trials, four gliders and one VTO version. The M3 was flown twice, and then rebuilt at which time it was given the new code BM3a but was never flown. The total number of launches to early April 1945 was 22, as was the total number of Natters constructed up to that time. Bachem reported further that there were 14 more finished or almost finished aircraft in April 1945. Four of these were prototype A1 operational Natters built for test launching from a wooden pole launcher, which had been designed for field deployment. 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. The documentation for this third flight may have been destroyed by the SS at war's end. Ten A1 operational Natters, called K-Maschinen, were constructed for the Krokus-Einsatz ("Operation Crocus").
The fate of these 14 A1 Natters was as follows: Three were fired from the pole launcher according to Bachem, four were burnt at Waldsee, two were burnt at Lager Schlatt, Oetztal, Austria, four were captured by US troops at Sankt Leonhard im Pitztal, Austria and one, which had been sent as a sample model to a new factory in Thuringia, was captured by the Red Army. Consequently, the total of 36 test and operational aircraft constructed at the Bachem-Werk can be accounted for. However, Natter carcasses were used for a variety of ground-based purposes; for example, as a static booster rocket, armament and strength testing and pilot seat position tests. Some fuselages were reused after flight testing; for example, the M5, 6 and 7.
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. The remaining machine was possibly destroyed when the CIOS Field Team left the area. Despite being promised one of these Natters, there is no evidence that a machine ever reached UK shores.
In early February 1945 the positions of the centre of gravity for the A1 operational machine during its flight profile were giving the RLM and the SS cause for concern. They wanted these figures to be decided upon for the upcoming construction of the A1 aircraft for Krokus-Einsatz (Operation Crocus), the field deployment of the Natter. The position of the centre of gravity is expressed as a percentage of the chord (distance between the leading and trailing edges) of the main wing. Thus 0% is the leading edge and 100% is the trailing edge. In the manned glider trials the centre of gravity had been varied between 20 and 34%. At a meeting of engineers held on 8 February, the variations in the centre of gravity expected in the A1 Krokus machine were discussed. At take-off with the weight of the four solid boosters, the centre of gravity would be brought back to 65%, but after releasing these rockets it would move forwards to 22%. The free flight by Zübert on 14 February had showed unequivocally that the little Natter had excellent flying characteristics as a glider. The centre of gravity problem was solved initially by the addition of one-metre-square auxiliary tailfins that were released simultaneously with the jettisoning of the boosters. The Krokus aircraft had vanes that would direct the Walter rocket exhaust gases so as to assist vehicle stabilisation at low speed similar to those used in the V-2 rocket.
French forces had captured Waldsee by 25 April 1945 and presumably took control of the Bachem-Werk. Shortly before the French troops arrived, a group of Bachem-Werk personnel set out for Austria with five A1 Natters on trailers. At Bad Wörishofen, the group waited for another squad retreating from Nabern unter Teck with one completed Natter. Both groups then set out for the Austrian Alps. One group with two Natters ended up at the junction of the river Inn and one of its tributaries, the Ötztaler Ache, at Camp Schlatt. The other group went to St. Leonhard im Pitztal with four aircraft. US troops captured the first group at Camp Schlatt around 4 May and the second group on the following day.
At some time during the project, the Bachem-Werk was ordered to give complete details of the BP-20 Natter to the Japanese, but there was doubt over whether they had received them. They were, however, known to have a general knowledge of the Natter and showed considerable interest in the project.
Only one original A1 Natter survives; it is stored in the Paul E. Garber Preservation, Restoration, and Storage Facility in Suitland, Maryland, USA. It is in a poor state of repair and is no longer accessible to the general public. The evidence supports the proposition that this machine was captured by US troops at St. Leonhard im Pitztal, Austria in May 1945. The Natter displayed at the Deutsches Museum is said to have been reconstructed partly from sub-assemblies that survived the end of the war. This machine is of the experimental type as launched from the steel tower and is painted to look like an M17. There are several static reproductions of Natters around the world, for example at the Planes of Fame Air Museum, Chino, California and Fantasy of Flight, Polk City, Florida, US