Operation Kirschkern

cherry stone

'Kirschkern' was the German programme to develop and manufacture the V-1 flying bomb (June 1941/May 1945).

More properly known as the FZG 76 and Fieseler Fi 103, the V-1 flying bomb was almost as feared as the A-4 (V-2) ballistic missile, and was the first long-range pilotless missile to be used in large numbers.

During the years between World Wars I and II, several nations experimented with pilotless bombers flown either by radio control or by an automatic pilot. The first proposals in Germany for such weapons were made by Askania and Siemens in the 1930s. An airframe suitable for the flight testing of such autopilots was designed by the Deutsche Versuchsanstalt für Luftfahrt, which was a government agency, and several examples were built by the Schwarz propeller company in 1937. Uncontrolled launchings of these aircraft proved unsuccessful: the controlled version was never flown and the remaining airframes were transferred to the guided weapons study group at the Henschel Flugzeugwerke.

Another proposal for a long-range missile was made by the Argus Motorenwerke shortly after the outbreak of war in 1939, but was rejected by the Reichsluftfahrtministerium, the German air ministry, on the grounds that it was inaccurate and that it would be unable to distinguish between military and civilian targets at its extreme range. A further objection was raised to its use in a short-range role as it used a relatively expensive piston engine as its powerplant. Most of these objections were overcome by the changes in the strategic situation during the next 30 months.

During the spring of 1942 a series of heavy British raids by Air Chief Marshal Sir Arthur Harris’s RAF Bomber Command on German cities provoked a corresponding series of reprisals against the UK by the Luftwaffe as the so-called Baedecker raids, but the RLM soon realised that the limited bomber force at its disposal was inadequate for the maintenance of a large-scale offensive against efficient fighter and anti-aircraft artillery defences. The pilotless missile thus came to be seen as an attractive alternative to the expensive bomber for future operations, while the German occupation of the north coast of France brought targets in southern England within fairly short range. There was no longer any need for expensive high-powered radio transmitters to guide the flying bombs as a simple automatic pilot was accurate enough over the distances involved, and the piston engine of the earlier project could be replaced with a cheap and expendable pulse-jet motor. The Argus proposal was resurrected, and the engineers responsible were ordered to start design studies, provided that these could be completed without taking funds from existing projects.

In March 1942 the project was mentioned to Robert Lusser of the Gerhard Fieseler company, which was also interested in the possibility of a flying bomb. A staff engineer, Rudolf Bree of the Heinkel company, who was the co-ordinating authority at the RLM, therefore brought Fieseler into the project to build an airframe and Askania to construct a suitable control system, while Argus retained the responsibility for the power unit.

On 10 June 1942, an RLM conference chaired by Generalfeldmarschall Erhard, ordered the highest priority to be given to the development and production of a flying bomb. Fieseler assumed overall responsibility for design and development. The factory designation Fieseler Fi 103 was replaced by the official Luftwaffe classification FZG 76 and the whole project was given the codename 'Kirschkern'. (The letters FZG were an abbreviation of Flak Ziel Gerät, or anti-aircraft target apparatus, in order to confuse Allied intelligence about the true purpose of the missile. As it was widely assumed, even in the German forces, that they were abbreviated from Fern Ziel Gerät [long-range target apparatus] this intended deception was a total failure.) It has been suggested, probably with some truth, that one reason for Milch’s enthusiasm for the flying bomb project was the fact that he had learned that the A-4 rocket, which was sponsored by the army, was ready for its initial trials, and was anxious to protect the status of the Luftwaffe by providing it with an unconventional weapon of comparable performance.

The first FZG 76 airframes was launched from a Focke-Wulf Fw 200 over the development and test facility on Peenemünde island in the Baltic Sea early in December 1942. The first missile to be powered by the Argus 109-014 pulse-jet was launched from the ground on 24 December 1942. Despite the promising performance of these two early test missiles, the FZG 76 was soon found to be aerodynamically unstable in any cross wind. The fault was eventually traced to errors in the autopilot control during the climb of the missile, and this was cured by slight redesign of the airframe. A further instability, which lead to a succession of missiles crashing during the transition from climbing to level flight was also at first suspected to be attributable to autopilot errors, but was ultimately found to be caused by changes in the wing angles during the high initial acceleration imparted by the launcher catapult.

The missile which emerged from these trials was a simple mid-wing monoplane propelled by a pulse-jet mounted above the fin. It had a 1,367-lb (620-kg) warhead, which could be replaced with a telemetry transmitter for test firings. The missile was controlled in heading, being capable of direction changes of up to 60° immediately after launch, before taking up its straight flight path and was then stabilised to prevent small deviations from this flight path compounding into major errors. The missile was also stabilised in roll and pitch to maintain a fixed altitude. A small propeller in the nose of the missile revolved in flight: a counter recorded the propeller revolutions and so measured the distance flown. When the flying bomb had reached its pre-determined range, the elevators were depressed so that the missile dived steeply on to its target. This sudden dive threw the fuel away from the feed pipe to the motor, so that the engine cut suddenly before the missile crashed. The silence after the characteristic note of the pulse-jet proved a valuable warning of the impending explosion of the missile, and enabled many people to take cover in time to avoid injury.

The flying bomb received a priority contract for its production, despite the failure of the two fired at the demonstration of 26 May 1943. The Fieseler works at Kassel, however, were fully occupied with development work, and so the production plant was installed at the Volkswagen works at Fallersleben.

The first experiments with pulse-jets had been made in France during the early part of the 20th century, and the first German experiments, by Paul Schmidt in 1931, appear to have been based, in part, on the earlier work by the Frenchman Caravodine. At about the same time, G. Diedrich of the Argus Motorenwerke started his own experiments independently of Schmidt and produced an operating motor in 1939. There seems some doubt, judging by discrepancies between various authorities, of the degree of co-operation between Schmidt and Diedrich after each had become aware of the other’s work, but development work after February 1940 was shared, at least in part, by both firms. The Argus 109-014, which first flew experimentally suspended under the fuselage of a Gotha Go 145 on 28 April 1941, was an Argus duct fitted with a Schmidt valve.

The pulse-jet, a very simple motor in constructional terms, is theoretically very complicated in operation. In basic terms, the Argus engine relied on the intermittent combustion of aviation fuel within a duct to increase the velocity of airflow sufficiently to provide forward thrust. The air was admitted to the front of the duct through a steel flap or reed valve, and was then mixed with fuel forced from the tanks in the missile body with compressed air. This mixture was ignited with a spark plug, the resulting rise in pressure in the duct closing the flap valve and sealing the front of the duct. The exhaust was thus forced out of the rear of the duct to create the required thrust. When the pressure in the duct fell below the ram air pressure at the front of the motor, the flap valve was forced open once again to admit a fresh charge of air and the cycle was repeated. The whole operation continued at a frequency of 47 cycles per second, and produced a thrust of 529 lb (240 kg). The tank contained 141 Imp gal (640 litres) of fuel, which was sufficient for up to one hour of flight. The missile had a range of 162 miles (260 km) which, in a later version with a smaller warhead and a wooden wing, was increased to 250 miles (400 km).

The FZG 76B was a modification of the standard missile, intended to be powered with the Porsche L09-005 turbojet engine. The higher thrust of this motor would probably have given the FZG 76B performance slightly better than that of the earlier version.

The specification for the control system to be used in the FZG 76 required that it would permit deviations up to 60° from the flight path immediately after launch until the missile had taken up the correct heading, that it would hold the missile on this heading as indicated by a compass for 25 minutes so that the correct course was established, that it would then detect and correct small errors, so that this heading was maintained, and that it would control the missile in pitch to fly at an altitude fixed in advance no lower than 985 ft (300 m) and no higher than 8,200 ft (2500 m).

Askania met this specification with a modification of their standard automatic pilot, designed by one Evers. This was simple and robust enough to withstand the launching stresses imposed by the catapult, and, being already in production, involved no large capital expenditure. It had the additional advantage of being operated pneumatically, for a large compressed air reservoir was available in the missile airframe for pressurisation of the fuel tanks, so no additional power supplies were needed. This automatic pilot provided control by elevator and rudder operation only, and an additional channel was built in to the FZG 76 control system to provide roll compensation signals to the rudder.

The basic control signals were obtained from a gyroscope, which detected missile axis movements (relative to the gyro axes) in pitch, yaw and bank, and directed the high-pressure air line to one side or the other of the servo valves which in turn controlled the air supplies to the rams operating the control surfaces. Heading errors were detected by comparing the missile heading with the pre-set reading of a magnetic compass: if these parameters did not coincide, a differential pneumatic pressure detector operated one of two electrical switches, starting a torque motor to precess the gyroscope until the necessary correction signal was applied to the rudder. The compass was mounted on rubber suspension in a wooden sphere. Compass corrections were made when the missile was delivered to the launching site by the simple expedient of putting the missile on stands in the direction of its desired course and hitting it with wooden mallets until the inherent magnetic field of the airframe was aligned with the earth’s field.

Altitude control was derived from an aneroid capsule, which was mechanically linked so that capsule movements rotated the pitch pick-off of the control gyroscope and demanded a pitch signal until the altitude errors were corrected. Control stabilisation was achieved by two subsidiary rate gyroscopes, which modified the pneumatic signals applied to the servo valves and prevented the control surfaces from overshooting their final positions.

Directional changes during flight were fed in by applying a pre-set voltage to the gyro torque motors in place of the compass bearing. The compass automatically took over control when the turn was completed.

The gyroscopes, the altimeter and the necessary linkages were built into a single unit called the Steuergerät. This was mounted in the tail of the flying bomb, with the servo valves for the rudder and elevator actuators above it. Two compressed air containers, each containing air at 150 atmospheres, were mounted immediately forward of the Steuergerät, a reducing valve in the outlet line from the containers reducing this pressure to 6 atmospheres and a further valve in the Steuergerät reducing it again to between 1 and 2 atmospheres before admitting air to the control system.

The German operational plan envisaged a combined bomber and missile assault on the UK both to disrupt Allied invasion preparations and to force a large part of the Allied war effort to be diverted to the defence of the UK. The date of the combined attack was set provisionally as 15 December 1943. Flakregiment 155 (W), commanded by Colonel Max Wachtel, was formed to operate the FZG 76 flying bomb, which was intended to form the missile element of the assault. This regiment was drafted to Zempin for training in August 1943, and at the same time the Todt forced labour organisation started the construction of 64 main launching sites, 32 reserve launching sites and eight supply dumps between Cherbourg and Calais.

Between August and November 1943, the plan for the missile element of the offensive encountered a series of difficulties. The first delay was created by the 'Hydra' major attack on Peenemünde on the night of 17/18 August, though this attack caused more damage to the A-4 development effort than to the Luftwaffe facility at Peenemünde-West. A more serious air raid took place a month later, when the Fieseler works at Kassel were bombed and the plans for mass production at Fallersleben therefore had to be delayed until the end of September 1943. This delay in the supply of missiles exacerbated training difficulties at Zempin, so the first practice shot was not fired until 16 October and the first battery of Flak Regiment 155 (W) was not posted to France until 21 October. This battery, and the others as they came from training, started a combined exercise with the bombers of Generaloberst Otto Dessloch’s Luftflotte III in preparation for the December attack, but an additional delay was then imposed on the flying bomb programme at this stage, when nearly all the launching sites were destroyed in a further series of air raids, and this made it impossible to start the assault as planned. On 21 January 1944, the offensive was started by the bombers alone, while the Todt organisation began the construction of new and better camouflaged launching sites.
The bomber attacks began with raids by conventional weapons on London, Hull and Bristol, followed by attacks on the invasion ships assembling in south coast ports.

The substantial losses suffered by the manned bombers led to the ending of the 'little Blitz', and left the offensive against the UK entirely to the flying bombs. Renewed preparations were carried out more carefully than those of the previous autumn. The new launching sites were built with far more attention to camouflage, so successfully that the damage from bombing was negligible, and the personnel of Flak Regiment 155 (W) were not quartered at the sites until the launchings were due to begin. Once again, the German command hoped for a combined missile and bomber assault by Flak Regiment 155 (W) and Generalmajor Dietrich Peltz’s IX Fliegerkorps, which was planned for the night of 12/13 June 1944. The missile contribution was to consist of two salvoes of about 70 flying bombs each, one being fired at 23.00 and the other at 04.00.

As were so many Luftwaffe plans for the use of guided missiles, this was over-optimistic. As 23.00 approached, one site after another reported itself unserviceable until only seven remained operational, so that the plan for salvo firing was abandoned and the seven sites were ordered to start launching independently. Only 10 flying bombs were launched on this first night of operations, and of these only one reached London, the intended target.

Further operations were delayed for a few days for re-assessment and repairs. Firings started again from 55 sites on the night of 15 June, a total of 244 missiles being launched in the next 24 hours. This firing rate could not be maintained in the following weeks, partly because Allied bombing of the German lines of communication was delaying the delivery of missiles and fuel to the launching sites, and partly because heavy air raids had caused a temporary cessation of missile production. The firing rate was reduced from one missile every 26 minutes to one every 1 to 11 hours. A launching rate of 120 to 190 flying bombs per day was maintained throughout August 1944.

The order to cease firings was given at 04.00 on 1 September, when the sites were abandoned and the German retreat from the Pas de Calais started. Flak Regiment 155 (W) was disbanded on the next day. During its operational life, this regiment had launched against London 8,564 FZG 76 missiles, of which 1,006 crashed soon after launch and 2,340 reached their target, and 53 against Southampton, of which nine crashed and 20 reached their target.

The weight of high explosive carried to London by the flying bombs was about one-sixth of that dropped on the same target by conventional bombers during a comparable period in 1940. The missiles caused about 5,000 deaths, mostly civilians, which compared favourably with the casualty rate of a bomber assault of similar intensity, and had diverted a considerable number of RAF and USAAF bombers to the attacks on the production and launching sites, but the campaign had failed entirely in its primary purpose of delaying the invasion. Indeed, the invasion had taken place before the first flying bomb had been launched.

An attempt was made to continue the bombardment with modified FZG 76 missiles launched from Heinkel He 111 bombers. Some 20 aircraft of Hauptmann Siegfried Jungklaus’s III/Kampfgeschwader 3 were available for the task and nine bombs were launched in the first attack on 5 September 1944.

There followed a lull as the III/KG 3 withdrew from Venlo to Hamburg, and the attack resumed on 16 September when another nine bombs were launched, and further sorties were flown altogether on 52 nights between September and December 1944. As additional He 111 bombers were made available from disbanded units, the flying bomb force expanded and was finally organised into two Gruppen of Oberstleutnant Fritz Pockrandt’s KG 53. The last operational air-launcher missile sortie was made on 14 January 1945.

Altogether, He 111 bombers had launched about 1,200 flying bombs against the UK, and of these 66 had reached London, one had reached Manchester and 168 had fallen elsewhere on land. KG 3 and KG 53 had lost 41 bombers on flying bomb operations, about half shot down by British fighters and the other half lost in accidents resulting from attempts to avoid radar detection by flying low above the sea.