Summary.

There is little doubt that all anti-shipping operations involving the Hs.293 were conducted with the Walter HWK 109-507 peroxide motor. The initial successes of this missile highlighted the potential for the concept to be used in other areas of defence, noteably to attack the large numbers of bombers which were flying over Germany. To develop the guided bomb to produce an airborne anti-aircraft missile led to the requirement for a more powerful motor.

The HWK 109-507 was effective at bringing the Hs.293 up to its operating speed, following which the missile manouvered down to the target with the assistance of gravity. However, to drive the heavy missile up to a bomber stream would require more power. Generally Hs.293 missiles were not stored fully fuelled as it was the experience of front line units that peroxide stored for long periods was prone to deterioration, and operational missiles gave best results when fuelled close to the take-off time of the aircraft.

There was evidence that the compressed air lines in the HWK 109-507 motor were prone to icing, and for the high altitude operations needed for an anti-aircraft weapon, icing and fuel blockages would be a problem.

The proposed new missile was the Henschel Hs.293H, for which a number of replacement motors were proposed, but the one chosen was the Schmidding 109-513. This motor did not reach development until at least March 1944, and the consensus seems to be that the Hs.293H was not deployed operationally against bombers.

However, the production of this second motor for the Hs.293 seems to have produced an erroneous view which has a degree of currency, in that some authors list the default motor for the operational Hs.293A anti-shipping missile, as the Schmidding motor. This almost certainly cannot be the case. Below is shown an illustration of the Schmidding motor, and it can be seen that although there are size similarities, the HWK 109-507 and Schmidding 109-513 are noticeably different units, difficult to confuse.

Description.

This is a diagramme of a Schmidding 109-513 motor. Although only a drawing, it shows the main features well.

Compare this with the Hs.293 motor at the top of the page. They both clearly show the bulbous fuel tank at the head of the motor, and the sets of oxygen bottles which comprise the main body of the motor. Very different from the HWK 109-507 with its large, cylindrical forward T-Stoff tank taking up nearly half the length of the motor.

Operation.

This diagramme is from an Allied report on Schmidding motors prepared for the British Admiralty in 1943 from captured intelligence (the Admiralty were very keen to learn all they could about the anti-shipping missiles being deployed by the Germans at this time).

Instead of compressed air as in the HWK 109-507 (the RAE materials analysis of the 109-507 suggested that the propellant for the peroxide fuels might be the inert "Nitrogen", which they later learned was incorrect, although air has 78% Nitrogen content) the Schmidding 109-513 motor carries a large store of compressed oxygen stored in four tanks (1) at 220 atmospheres pressure. When the missile is launched, an electrical impulse to a cartridge starter (3) initiates the flow of oxygen, allowing it to pass to the reducing valve (4) which has an outlet pressure of 32 atmospheres.

From the reducing/distribution valve, oxygen is led to the M-Stoff (Methyl Alcohol) tanks (5) which house rubber balloons (6). Oxygen pressure alone inflating the balloons, drives the fuel out of the tanks through to the combustion chamber. In the combustion chamber itself (12) the fuels mix in a sprayhead (11). The fuel combination is not hypergolic, and will not ignite spontaneously, so a three second duration pyrotechnic igniter (10) is used to initiate the combustion which then proceeds due to the heat of combustion, even when the igniter is totally consumed.

The rubber vessels to propel the fuel were resistant to attack from the Methanol, and remained useable down to -70^o centigrade.

The 109-513 was lighter than the Walter unit at 133 kg empty, with a similar performance of 600 kg thrust for 11 seconds, giving the Hs.293H for which it was intended, a higher acceleration and greater maximum speed.

The unit could be filled with fuel at the manufacturing plant, and shipped to the front line charged and ready for use. Due to the stability and non-degradation of the fuels, storage was not a problem and the organisation and support at unit level for using the motors was much simplified.

The Walter HWK 109-507 is a distinctive-looking motor with large peroxide tank, two compressed air bottles and a squat combustion chamber. The Schmidding unit has four compressed oxygen bottles and bulbous, semi-spherical fuel tanks, with a leaner combustion chamber. The mis-labelling of a preserved HWK 109-507 museum at Chino may have added to the confusion surrounding these two motors.

The photograph here, shows a preserved Hs.293 with a Schmidding 109-513 motor - the combustion chamber is highly diagnostic. A number of these combustion chambers have been excavated from site(s) in Germany, and an example is shown below.

Key References.

• King H.F., A.I.2(g) Report 2373
• Pocock R., "German Guided Missiles"
• Rockman S., P.G.11533 UWD, Admiralty
• Stemmer J., "Racketenantriebe"

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