Walter HWK 109-507

The HWK 109-507 was a liquid-propellant rocket engine developed by Germany during World War II. It was used to propel the Hs 293 anti-ship guided missile.

It was produced by Hellmuth Walter Kommanditgesellschaft (HWK). Like other Walter engines it used hydrogen peroxide as a propellant.

The Hs293 has been variously described as a missile or as a boosted glide bomb.^[1] It consisted of an SC500 bomb casing, fitted with wings, engine and radio control. Control equipment was housed in a rearward extension of the bomb casing but the motor was mounted in a separate housing beneath. It had originally been developed as an unpowered glide bomb, "Gustav Schwartz Propellerwerke", and the engine was added later. After flight tests, a visible tracking flare was also added, in a further rearward extension.^[2]

As the engine was mounted below the missile fuselage, the exhaust nozzle pointed downwards at 30°, so as to align the line of thrust with the centre of gravity of the missile.

The engine had a burning time of around 10 seconds. After this the missile glided to the target, taking up to 100 seconds for a range of 8.5 km.^[3]

As it was intended for attacking lightly- or unarmoured targets, it did not require an armour-piercing high impact speed.^[i]

The same engine was also used for the planned Hs 294, Hs 295 and Hs 296 missiles. As these larger missiles weighed twice the Hs 293, they used a pair of the engines, one under each wing root.^[4]

This engine was a development of the HWK 109-500 Starthilfe (rocket-assisted take-off) engine. The 109-500 was pod-mounted and parachuted back to earth after takeoff. Engine pods were serviced and re-used.

The 109-507 was developed from the 109-500. As a missile engine, it was only required to work once, and for a short duration. It was thus simplified in both its features and in its construction materials. Rather than the complex centrifugal turbopumps used for most Walter engines, a simple gas pressurisation system was used to feed the propellants. A wartime British report expressed surprise that the engine's combustion chamber was made of mere mild steel, rather than anything more refractory.^[5]

The engine's fuel chemistry used 80% high test hydrogen peroxide or 'T-Stoff'. This was a 'cold cycle' engine; the peroxide acted as a monopropellant and was decomposed by a catalyst into superheated steam and oxygen.^[ii] The catalyst used was a consumable liquid solution of calcium permanganate or 'Z-Stoff'. As this catalyst is consumed, the engine is regarded as a bipropellant engine.

Propellants are forced into the combustion chamber by compressed air, stored at 200 bar (2,900 psi) in two steel vessels. This pressure is released through an electrically-fired cartridge that opens a valve with a blow-out disc. This is the full extent of the electrical control system. Once fired, the valve does not close again. A pressure regulator delivers air at 33 bar (480 psi), through a shuttle valve that pressurised first the catalyst tanks and then the propellant tank. This delay ensures reliable ignition in the combustion chamber. A non-return valve ensures that no catalyst can flow backwards into the air or propellant plumbing, with an explosive result. A rubber diaphragm, broken as propellant pressure builds, ensures that there is no backflow through the combustion chamber either.^[5] Z-stoff was known for problems of clogging injectors and so an inline filter was used.^[iii]

The propellant injector in the combustion chamber is a simple light alloy casting, cooled by the propellant flow. The combustion chamber is single-walled mild steel, with no provision for cooling. A steel mixing cup is downstream of the injector, with the radial Z-Stoff 6mm pipe leading into it. One 3 mm diameter injector nozzle points into the cup, thirty smaller radial 2 mm nozzles deliver most of the propellant along the walls of the chamber. Helical swirl baffles in the chamber promote good mixing and decomposition of the peroxide.^[5]

Thrust varied through the boost phase, as air pressure and propellant flow fell, dropping from 600 kgf to 400 kgf.^[6]

The engine pod had a dry weight of 517 kg, carrying 68 kg of propellants when full.^[5]

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