[Enzian with RI-203 Motor]



Walterwerke were commissioned in September 1943 to design and produce a new model of motor for the Messerschmitt Flak Rakete FR-1. The commission received the RLM designation HWK 109-739.

In a departure for Walterwerke, the HWK 109-739 motor used a reaction between SV-Stoff (principally nitric acid) and Ergol ("brown-coal benzene") and based on their experience of working with rocket fuels, for the total impulse required, Walterwerke proposed a unit with pumped propellants and liquid cooled combustion chamber with graphite-lined venturi.

The first Flak Rakete airframes were completed in early 1944, but in common with a number of Walter projects, the airframe was ready for testing before the motor. Still under development, the 109-739 motor units were not projected to be ready for flight testing before January 1945.

Under pressure from the Enzian project, Walter modified an existing motor, their RI-203 (known by the RLM appelation, "HWK 109-501"), to be fitted as an interim power unit for Enzian so the flight trials could continue.

Originally produced as an assisted take-off unit with a greater thrust than the HWK 109-500, the RI-203 was a "hot" motor, combusting a hydrocarbon fuel in the hot products of hydrogen peroxide decomposition.


Preserved Unit.

Considering the unusual application of a Walter motor, the student is very lucky that one of these installations has survived the war. Shown on this page is a Walter RI-203 unit, appearing to be in original condition, fitted in the Enzian preserved in the Australian War Memorial museum. Only visible through the cut-away portions of the Enzian fuselage, it does nevertheless show the motor and all its features. This page compares images from that motor alongside images from original Allied sources, drawn shortly after the war.

By referring between the individual images below and the principal cut-away above, it should be possible to see how the installation operates.

The Enzian has sometime been on display in the museum and sometime in storage, so it would be wise to enquire directly with the museum before making a special visit. They do however, have an enormous range of interesting exhibits, so each visit will be worthwhile. To be able to show these images, I am indebted to historian Alan Schenkenbach and his colleagues, with the kind assistance of the Australian War Memorial Curatorial staff.



The decomposition products of hydrogen peroxide (T-Stoff) by a liquid catalyst (Z-Stoff) in a combustion chamber are joined by petrol, which ignites in the hot exhaust, producing a thrust of approx. 3300 lbs. for 30 seconds.

The original RI-203 take-off assister needed only to provide thrust for a limited time, so there was no requirement for a complex pumped delivery system. Propellants were fed to the combustion chamber with compressed air.

In the Enzian, the three RI-203 integral compressed air bottles are supplemented by another four in the nose of the airframe. These provide the motive power for the propellants. Storage pressure is reduced to the operational pressure via a series of valves and regulators.

A large spherical aluminium tank for the T-Stoff is mounted forward of the motor proper, in the centre section of the Enzian fuselage. A small steel tank mounted near the top of the motor contains the Z-Stoff catalyst. A further steel tank contains the petrol, together with a small quantity of B-Stoff (92% Hydrazine Hydrate 8% Water). The B-Stoff and Petrol are not miscible, and the B-Stoff settles out as a separate layer at the bottom of this tank.

Each propellant tank has its flow governed by a change-over valve, and there is a master control unit directing the compressed air to drive propellants towards the combustion chamber. Pipes connect the compressed air bottles to the pressure reducing valves and master control valve, while outflow pipes connect the change-over valves to the combustion chamber.

At the rear of the unit is a stainless steel combustion chamber with an inner tube to act as the decomposition space. This is surrounded by a tubular jacket to make a cooling space, cooling being provided by T-Stoff. The combustion chamber head is a burner block drilled with propellant roses to give impinging jets of T-Stoff and Z-Stoff, into the hot decomposition products of which the petrol is added.



[Australian War Memorial Enzian]

When the Enzian is ready to launch, a hand-operated valve is opened, allowing the compressed air via the pressure reducing valve to charge the master control valve. At the moment of launch, a starter button operates a solenoid closing the vent valve and allowing air to pass towards the T-Stoff tank.

[Air Pressure Reducing Valve] [Air Pressure Reducing Valve]

Pressure Reducing Valve


The air pressure at the master control valve opens the change-over valve at the Z-Stoff tank, which passes air to the peroxide cylinder of the master control valve, causing the T-Stoff change-over valve to open.

[Master Control Valve] [Master Control Valve]

Master Control Valve


Pressure at the change-over valves switches them from venting the tanks to allowing air pressure to drive the propellants forward.

[T-Stoff Change-Over Valve] [T-Stoff Change-Over Valve]

T-Stoff Change-Over Valve


Propellant flow is sequenced such that the Z-Stoff arrives at the combustion chamber first, followed by the T-Stoff, where decomposition occurs. The petrol follows, initially proceeded by the plug of B-Stoff which had settled by the fuel tank outlet. The hydrazine hydrate initiates combustion and the petrol will then burn smoothly in the hot, oxygen-rich decomposition products without risk of explosion.

[Combution Chamber] [Combustion Chamber]

Combustion Chamber



The photographs shown here are from the Enzian on display at the Australian War Memorial. They were supplied by Alan Schenkenbach, with the kind co-operation of the Australian War Memorial Curatorial Staff.

Web Master Shamus Reddin   [SR Logo]
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