This motor is a standard 109-509.A-2 motor, with a single combustion chamber, propellant line air ejectors and a gravity fed starter tank.

Known History.

This motor has languished for a number of years in the Rolls-Royce Heritage collection store in Bristol, England. As part of the coalescence of the British aerospace industry during the 1960s, Rolls-Royce took over the De Havilland Aircraft company, also taking over responsibility for the De Havilland aero engine collection. De Havillands had used Walter motors during the post-war period, and it is quite certain that they would have had some of the latest Walter motors for study.

De Havilland's aero engine collection was subsumed into the Rolls-Royce Heritage Trust collection, a vast number of internal combustion, reaction and rocket engines gathered from British aircraft and engine manufacturers of the 19^th and 20^th century. The whole collection is too vast to house together, and has largely been in store at a number of sites around the UK.

The De Havilland collection has been at Bristol for a little while. It is not known when the Walter 109-509.A-2 motor acquired its gaudy paintwork, but unlike the HWK 109-509.C at Cosford, the colours do not seem to be propellant coded; perhaps it was just for preservation purposes.

In 2003, Rolls-Royce Heritage Trust moved the motor up to its restoration facility at Ansty, near Coventry, and a small team of Rolls-Royce volunteers dismantled it and cleaned off all the old gloss paint. It is not known if any original paintwork was found underneath, but certainly on the aluminium parts, the paint layers were so substantial, that vigorous stripping was required.

By June 2004 the 109-509.A-2 was re-painted and re-assembled, awaiting its next move the newly established Rolls-Royce Heritage Trust Museum, which is in the process of establishment in Derby, on the site of the Rolls-Royce aero-engine manufacturer's foundry. The HWK 109-509.A-2 will form part of the rocket collection, which will contain a range of British liquid fuelled rocket motors, some elements of which can be traced back to the Hellmuth Walter technology exhibited alongside.

Pictured left are Eric Devine and David Smith, the principal volunteers who worked on the restoration, guided by Ron Frost and John Scott-Scott.

Description.

The motor is largely well preserved, in near complete condition, bearing little corrosion. There was no manufacturer's plate, not even a position where one might have originally have been fitted.

The design of the "A-2" motor is shown well. It retains the basic pattern of the "A" Series motor; the box frame, large thrust plate, and vertically positioned main control unit, all recognisable as derived from the basic HWK pattern.

However, the differences to look out for are, the repositioned steam generator, dropped down on the thrust plate to a position near the bottom. This is fed by a T-Stoff header tank. Because of this, there is no electric starter motor and the whole port side of the motor frame is open, due to the lack of requirement for the accessories unit, with gearing off for the starter motor.

These are all quite typical of an "A-2" motor.

All photographs © Shamus Reddin
- With thanks to the Rolls-Royce Heritage Trust -

The port side of the motor. The main fuel flow and equalisation unit and fuel control valve are clearly visible at the top of the motor.

The most obvious difference, is that there is no port gearbox casing, leaving the end of the main pump unit visible; there is no electrical starter motor. Obviously, without the end accessories casing, there needs to be a pump end plate, which carries the pump thrust bearing.

Also prominent in the picture is one of the propellant line air ejectors, the rectangular unit bolted to the left of the frame. This is discussed in greater detail later.

At first glance, one might take, in this view, to the opinion that this was a standard HWK 109-509.A-1 motor; but a moment's consideration will show the lack of electrical starter motor.

Of all the features of the "A-2" motor, one of the most obvious is the inverted steam generator with the T-Stoff header tank for motor starting.

The steam outlet (just visible) is repositioned, inverted on the lower face of the motor thrust plate, carrying the steam to the fuel pump turbine around the starboard side of the motor.

The "T" piece in the T-Stoff inlet is visible, carrying the main T-Stoff flow to the steam generator, but also keying off an additional flow to top up the T-Stoff reserve, header tank for motor starting.

In training and operations, Messerschmitt "Komet" pilots experienced a proportionally large number of in-flight motor failures (see later), requiring the motor to be restarted. On the ground, HWK 109-509.A-1 powered "Komets" had a trolley accumulator to provide power for starting. In the air, electrical starting needed power, and also proved to be a tricky procedure for pilots, needing two hands and a cool nerve. Walterwerke's latest design was to replace the electrical starting system with a gravity fed, T-Stoff starter. This saved all the weight of the electrical system, and simplified the starting procedure - once the throttle lever was moved to the starting position, the gravity tank latch was released and T-Stoff flowed onto the catalyst stones in the steam generator. Much simpler for the pilot.

Although test reports do exist which show that this model of motor was flown, there is currently no evidence that production Komets or operational aircraft were fitted with it. Testing was still on-going in the last months of the war, and standard Komet operations were already sporadic at this time.

The most important design feature of the A-2 motor was the introduction of the propellant line air ejector. Reports from operational Komet pilots started coming in with a worrying frequency, describing how the Walter motors would loose power and stall at critical moments in attack missions.

During high altitude combat manouvering this was objectionable enough, but motors were cutting out at low altitude, during take-off. With a fully fuelled Komet losing power close to the ground, the consequences were disasterous.

Initially, it was thought that poor adjustment in the fuel delivery mechanism was causing fuel imbalances. But more detailed analysis of the problem showed that despite the tank baffles, during even modest manouvering, air was being introduced into the propellant lines. As the pressure of one fluid led to the maintenance of flow of the other, an airlock in the line and a consequent reduction of pressure, would shut down the flow of propellants as a safety measure and the motor would stop.

To combat this problem, Walterwerke designed a pair of air ejectors (one per fuel line). The mechanical principal was quite simple. Fuel pressure held a plunger in place. When the pressure fell - at the point when an air bubble in the line arrived - the plunger was driven open and a line tapped from the steam generator drew the air down the ejector until fuel at pressure arrived, driving the plunger home and allowing fuel to flow again.

The system was still under test in late 1944; fuel for operations was short, and opportunities for continued testing infrequent. It is not known whether any of these motors was used operationally, but it is considered unlikely.

This Walter motor design must have been considered successful, as it appears to be the one used by German scientists after the war for testing additional aspects of liquid rocket motor design; review the page for the HWK 109-509.A-2.

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