Walter HWK 109-509.E
Walter HWK 109-509.A-2E
A Preserved HWK 109-509.A-2E ?

The well-developed, low weight Walter bi-fuel rocket motors delivered high thrust - ideal for driving a small interceptor (for example the Messerschmitt Me.163) up to the height of an incoming bomber stream in a fraction of the time which a conventional interceptor would take. The Bachem Ba.349 "Natter" was a small piloted interceptor, launched from a vertical ramp.

Introduction.

The RLM issued a requirement for a small target defence interceptor in the Spring of 1944. Following evaluation, the two proposals which were pushed forward for development, were the Heinkel P1077 "Julia", with the Junkers EF 127 "Dolly" coming second. However, a new proposal was proposed by its designer, Erich Bachem, which was a semi-expendable target defence interceptor. Half way between the original requirement and a flak missile, Bachem's BP 20 could be deployed in a similar fashion to the "Julia" but was less complex for the pilot. The BP 20 was certainly a low cost aircraft, designed from non-strategic materials which could be assembled by semi-skilled workers. By virtue of the plan to launch from vertical ramps, the BP 20 could be sited around strategic targets and launched against approaching bombers to break up attack formations.

As there was a shortage of trained and skilled pilots, the BP 20 was to be launched automatically, powered to height under autopilot with radio guidance, the "semi-skilled" pilot only bringing the aircraft under his control towards the final phase of the mission to direct the on board rocket battery onto the bombers.

From launch to attack, the mission was only to last a few minutes, after which the pilot jettisoned the forward fuselage, deploying a parachute for the rear of the aircraft. The parachute saved the rocket motor to be re-used later. The sudden extra drag of the parachute would detach the pilot from the fuselage, and he would descend under his own parachute.

The Heinkel P1077 and Junkers EF 127 were designed around the Walter 109-509.C motor. The initial proposal for the BP 20 was for a single combustion chambered motor. Because of the vertical launching method, in common with Enzian, the Walter motor for the BP 20 needed four Schmidding 553 solid fuel booster rockets in the initial phase of the launch to provide sufficient power to initiate flight.

Initially, even though it had been forwarded by General der Jagdflieger Adolf Galland, as the BP 20 proposal was outside the design requirements of the target interceptor it was rejected. However, Bachem was convinced of the viability of his design and sought an interview with Heinrich Himmler - the BP 20 was reconsidered within hours, and allocated a high development priority with the official designation Ba.349. The development of the Ba.349 is outside the scope of this page, and there are a number of books on the market, although Brett Gooden's "Projekt Natter" is by far the most definitive, and is required reading for the subject. For an internet resource, try The Bachem Natter page which contains a number of pages and useful illustrations.

Development.

In common with many other rapid deployment defence weapons systems (see the missiles page), a small, light-weight, powerful motor was required. The liquid fuel rocket motors fitted the bill well. Walterwerke's latest development, the more efficient HWK 109-509.A-2 and the dual chambered HWK 109-509.C motor were still under development, and so the production model of the Ba.349A was worked around the Walterwerke HWK 109-509.A series motor.

However, there were a number of consequences of the vertical launch of the Natter which required modifications to the basic Walter motor layout.

Walterwerke had developed their "A" series to a more refined and reliable motor, the A-2. Instead of the electrical starter, the HWK 109-509.A-2 motor had a T-Stoff starting system, initiated by feeding T-Stoff under gravity into the steam generator. Even though subsequent T-Stoff flow was backed by an increasing pressure as the motor turbine gathered speed, the initial flow was due to the influence of gravity. Walterwerke therefore undertook a redesign to enable the motor to function even though the aircraft was mounted vertically on the launch ramp. This was to be designated the HWK 109-509.E.

As you can see from the picture above, the motor is basically an "A-2" motor with the principal design change being the setting of the T-Stoff header tank, and the steam generator both at an angle of 45^o. Another point to note about this motor, is that the combustion chamber support tube is attached on the upper side of the combustion chamber. This arrangement would be inappropriate for the Messerschmitt Me.163, as the motor sits high in the fuselage, and the positioning of this support is quite diagnostic of a motor for a Natter.

Following its capture by the British, the two photographs above were taken from Walterwerke in Kiel. The picture on the left shows a 109-509.E pump and fuel control unit on test. There is no combustion chamber fitted, so the unit could be undergoing fuel flow testing; the steam generator driving the pump, and the flow of the fuels through the pump being measured.

The picture on the right is the correct way up (!), but is interesting, in that it shows the top of a 109-509.E unit, which has fallen over on its stand (possibly) following the explosion of another motor in the same test room at Walterwerke. It shows the layout of the motor, and is a good comparison between this and the production HWK 109-509.A-1.

To the best of my knowledge, the Walter rocket motor designed for the Natter was the "HWK 109-509.E". Flt. Lt. Beeton (see references) suggests that there was an "HWK 109-509.A-2E" model, which was an interim design tooled up to power test flights of the Natter before production began proper. Contrast the motor above with what I show on the following page;   109-509.A-2E.

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