Summary.

The picture here shows a preserved Walter 109-739 Enzian Rocket Motor at the RAF Museum in Cosford, England.

Walterwerke were commissioned in September 1943 to design and produce a new model of motor for the Messerschmitt Flak Rakete FR-1. With an initial production batch of fifteen units, the commission was to propel a large missile carrying a 500kg warhead up to 50,000 feet.

In common with a number of Walter's designs, the proposed motor used an oxidant into which was pumped a hydro-carbon fuel, combustion being initiated by an accelerant.

In this instance, the oxidant is SV-Stoff (88.5% Nitric Acid, 11% Sulphuric Acid and 0.5% water) the fuel being Ergin ("Brown Coal Benzene"), with combustion initiated by the introduction of "Fantol" (Furfural Alcohol) into the combustion chamber.

To keep the all-up weight of the motor down, Walters employed a small steam driven fuel pump to deliver the fuels for combustion.

The steam for powering the fuel pump was via decomposed oxidant, T-Stoff, impinging upon a solid catalyst (D76).

Description.

The new motor received the designation HWK 109-739. Acting as the main power source, with variations in power not required, the motor was designed to deliver a constant thrust from launch. However, because the Walter motor took a few seconds to come to full power, Enzian was also equipped with four Rheinmetall-Borsig solid fuel booster rockets to propel the missile off the launch ramp to full flight during the time the Walter spun up to flight speed.

As can be seen from the diagramme below, the Walter 109-739 is a self-contained motor mounted on a welded tubular steel frame, which contains all the unit's components except the fuel tanks.

Units comprise a compressed air cylinder, T-Stoff container, steam generator, control valve, fuel pump, combustion chamber and associated pipework.

The motor is designed for reliable, but single use, and the materials used in construction are alloy castings and mild steels, all cheaply made components.

Operation.

The peroxide in the on-board tank is free to vent fumes to the atmosphere through a valve, during periods when the missile is fuelled but stored. Compressed air is stored in its forged steel tank at 150 atmospheres, prevented from passing into the system by a valve having a bursting disc.

When the missile is launched, an electrical impulse detonates two cartridges which pierce the bursting disc, allowing air to escape through a filter. Pipes lead air to the flow valves for both oxidant and fuel, activating them. Compressed air is then led to the peroxide tank to pressurise it, whilst also closing the vent valve.

The T-Stoff is retained in its tank also by a bursting disc, and the pressure build-up causes the peroxide to rupture this, pass through the control valve and into the steam generator. Two pieces of mesh gauze in the welded steel container form a space into which the D-76 catalyst stones are placed. Peroxide sprayed onto the stones generates steam which exhausts into the fuel pump.

In common with Walter designs, the steam entering the fuel pump impinges on the blades of a turbine, the common shaft of which carries both the fuel and oxidant pumps. The pumps are simple centrifugal types, only containing three radial vanes on the impellers, designed to produce 40 atmospheres of pressure at 16,000 rpm (there is a 20 atmosphere drop in pressure between the pump and combustion chamber).

The Ergin fuel is held back in the system by a shearing disc, and when compressed air bursts this, fuel is drawn in to the fuel pump and driven around the system. Downstream in the fuel flow is an expansion bend and a small tank which contains the Fantol. The Ergin/Acid mixture is not self igniting, so the increasing fuel pressure is designed to burst the discs holding the Fantol, driving it into the combustion chamber. The Fantol ignites when it comes into contact with the acid, initiating combustion of the main propellant flow.

The fuel control valve is in the circuit between the Fantol tank and the combustion chamber. This is a piston carrying fuel pressure at one end and oxidant pressure at the other. Thus, an increase in the pressure of the oxidant causes a greater flow of fuel. An increase in fuel pressure causes the valve to close, reducing fuel flow.

The Oxidant system operates on a larger scale, but similar way to the fuel system, up to the pump impeller. From the pressure side of the fuel pump the SV-Stoff is led by large diameter alloy pipe to a bifurcating steel pipe taking acid around the inside of the cooling jacket surrounding the combustion chamber. After circulation, the SV-Stoff is then led to the centre of the combustion chamber burner.

In the combustion chamber, oxidant is carried into a double ring of orifices which spray at 45^o, to strike each other in a fine atomised mist. The Ergin fuel is then carried to a single ring of a similar number of jets which are aimed at the intersection of the oxidant jets. At the point where all the sprays coincide is a target ring plate, upon which the fluids mix. Combustion then occurs a short distance off the surface of the plate, and within the burner, the plate itself thus being liquid cooled.

Reports show that 95% complete combustion was acheived, and the exhaust gas contained only traces of nitric oxide, generally at the start. For complete combustion, a 5% excess of fuel was required.

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