Summary.

The HWK 109-507 bifuel rocket motor was one of the first Walter motors to enter massed production. It belongs to the "cold" series of rocket motors, which derives its power from the spontaneous decomposition of a fuel by an oxidising agent.

Walterwerke had already developed the HWK 109-500 cold "Starthilfe" ATO rocket pack, which had been successfully designed and used on service aircraft. The HWK 109-507 motor was a development of this model, designed to fit in an elongated nacelle to be slung beneath the Hs.293 guided bomb.

Description.

Unlike the re-usable HWK 109-500 ATO pack which was recovered by parachute, the HWK 109-507 was a disposable motor, destroyed at the conclusion of the missile's mission with the detonation of the warhead at the target.

As a result, the motor was designed of low value materials, with a minimum of complicated parts. It was not designed to run continuously through the missile's attack, so could carry fuel tanks with a limited volume, and the installation beneath the missile required a modest size. These considerations led to a two fuel system with simple fuel delivery by compressed air, rather than via turbine driven fuel pump used on other Walter motors.

The picture here, shows a complete HWK 109-507 faired unit, removed from the Hs.293 for examination. At the left (rear) you can see the vent for the combustion chamber, and on the top surface, the adjustable, tripod fixing used to sling the motor beneath the missile.

The motor is designed for single use, and the materials used in construction are alloy sheet, castings and mild steels; all cheaply made components. The British examination of a motor in October 1943 reported:-

"The most remarkable thing about the [combustion] chamber is that the whole structure including the internal baffles, is of mild steel... "

The motor comprises the main hydrogen peroxide tank, two compressed air bottles, calcium permangante tank, reducing and control valves and combustion chamber.

Operation.

The HWK 109-507 illustrated earlier is shown above without the streamlined fairing, showing the major components labelled.

Compressed air is stored in forged steel tanks (5) at 200 atmospheres, prevented from passing into the system by a valve with a bursting disc (4). When the missile is launched, an electrical impulse detonates a cartridge which pierces the bursting disc, allowing air to escape. Pipes lead the air to the reducing valve (8) which has an outlet pressure of 33 atmospheres.

From the reducing valve, air is led via a distribution valve (6) to the T-Stoff (peroxide) tank (1) which then drives the oxidant through to the combustion chamber. The valve allows the T-Stoff to flow fractionally after the Z-Stoff to ensure the catalyst is present in quantity in the combustion chamber before the arrival of the peroxide. A rubber diaphragm in this propellant line prevents the premature flow of peroxide.

Simultaneously with the air passing to the T-Stoff tank, it passes through a non-return valve (10) to the Z-Stoff (permaganate) tanks (2). The non-return valve is a precaution designed to prevent the inadvertent mixing of the two fuels outside the combustion chamber.

The Z-Stoff passes through a combined non-return valve with coarse filter, and is then delivered to the combustion chamber.

The combustion chamber itself (3 above) is made from mild steel, and contains the reaction surfaces and two steel swirl baffles and guide vanes. Injected into the chamber in a spray through a light alloy spray head, (9 above) the T-Stoff impinges into a shallow metal cup. Z-Stoff, entering through the side of the reaction chamber via an unrestricted 6mm bore pipe is directed onto a metal anvil in the centre of the cup, thus mixing the peroxide and permanganate in a fine mist.

The fuels are directed down the body of the reaction vessel by a twin helical baffle which acts to effectively lengthen the combustion chamber and promote fuel mixing. The T-Stoff is decomposed into a high velocity flow of steam, and four straightening vanes guide the flow through the combustion venturi throat, with a combustion pressure of approximately 18.4 atmospheres.

The exhaust is directed down from the longitudinal axis of the motor, by an angle of some 30^o.

The motor produced a peak thrust of 600kg, falling to 400kg during the reaction time of 12 seconds. With an empty weight of 517kg, the unit carried 68kg of propellants.

Shown above is the preserved HWK 109-507 motor at the RAF Museum Cosford, in England. This has been coloured and labelled to make the individual parts easy to recognise.

The compressed air tanks are the large white objects, the T-Stoff tank is pale blue just off to the right and the smaller Z-Stoff permanganate tank, dark blue just between the two. Quite clearly shown, the large black reducing valve passes air to the silver distribution valve, which then feeds to the T-Stoff and Z-Stoff tanks. Two red electrical cables attach to the end of the bursting valve.

The same motor seen from the other (port) side also shows the two white compressed air tanks and the T-Stoff tank and smaller Z-Stoff permanganate tanks. Nicely shown, is the large yellow T-Stoff line to the combustion chamber, and the smaller, dark blue Z-Stoff line.

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