How Nazis' Walter Engine Pioneered Manned Rocket-Craft

by Roy Healy,
Former President,
American Rocket Society
Revealed here are construction and operational details of this unique pilot-controlled rocket engine-— symbolizing advanced German thinking on the makeup of power units for high-speed interceptors — together with design features of those craft in which it was installed.

It may reasonably be said that at the time of V-E day the Germans were approximately one year in advance of this country in the development of rocket power plants for aircraft.

This was not in any measure, as is frequently inferred, the result of superior engineering ability on the part of Nazi research staffs but, rather, attributable to their pressing need for a high-speed fast-climbing interceptor to defend strategic areas from the almost daily attacks of the 8th Air Force bombers. Lack of a tactical requirement for an airplane of this type in our Air Forces — where emphasis was on long-range escort fighters — prevented rocket power plant projects from receiving the high priority which they received in the program of the German Air Force.

Nonetheless, several groups in our own country, working under Army and Navy sponsorship, succeeded in evolving powerful rocket engines. Visitors to the recent AAF Fair at Wright Field were shown the 6,000-lb thrust XCALT-6000 and the 3,000-lb thrust X40ALD-3000 units developed by the Aerojet Engineering Corp for the AAF. Burning a spontaneously combustible mixture of aniline (C6H5NH2) and red fuming nitric acid (HNO3), these power plants may well play a large part in future military aircraft design.

The Me-163, and some of its subsequent models, were the only rocket-propelled combat craft to see service during the War. The Japanese Baka — while carrying a pilot and powered by solid-fuel rockets — falls into the guided bomb category along with the Hs-293. The Bachem BP-20 Natter (German for Viper) was still in the experimental stage of development at war's ending. Both the Me-163 and the Natter were powered by a bi-fuel rocket engine designated as the Walter 109-509, capable of approximately 3,800 lb peak thrust.

The design of the Me-163 is credited to Dr Lippitsch, who gained prominence in German aeronautics for his radical departures from conventional design. First seen in action during the Fall of 1944, the Me-163's usual tactics were to flash up and down through the stream of bombers, alternately attacking from below and then above — relying on its 550-mph speed for evasion of defensive firepower of the bombers. With powered flight time of only 12 min the Me-163 was restricted in range, although by alternately gliding and operating the engine at low power, the pilot could extend endurance to about 40 min. Equipped with 30-mm wing-root cannon and ammunition supply of 60 rounds per gun, this rocket fighter was a serious potential threat, but it never appeared in sufficiently large numbers to hinder bomber operations.

In contrast to the slender lines of most jet craft the Me-163 is characterized by a stubby teardrop fuselage unit, which is combined with sharply-swept-back wings. Nose armor cone contains a wind-driven 200-W generator for electrical supply. Some later models have been found fitted with tricycle landing gear, but the majority of those used in combat took off on a two-wheeled dolly which was then jettisoned. Landing was effected on an extensible belly skid, small tail wheel, and wing tip skids. In an attempt to increase the duration, some of these craft were equipped with towing eyes and were pulled to combat altitude by tow planes. Differentially controlled aileron-elevators — "elevons" — on the wings make possible the elimination of horizontal tail surfaces.

Engine Design Features

The Walter 109-509 — propelling unit of the Me-163 — is also known as the HWK-509. Propellants used in this rocket engine are hydrogen peroxide (H202, 85% concentrate) and methanol (methyl alcohol, CH3OH). A 30% solution of hydrazine hydrate (N2H4.H2O) is contained in the methanol to initiate combustion upon coming in contact with the peroxide. Use of this chemical reaction to cause spontaneous combustion of the propellants eliminates need for an electrical ignition system. Power output is controllable by pilot by a throttle on the left side of the cockpit. Settings of the lever are "Off", "Idling", and three stages of power, varying from an estimated 650-lb thrust, in low, to 3,800-lb thrust in high power setting.

The power unit is comprised of two subassemblies — the rocket engine and its accessory section — separated by approximately 4 ft of connecting tubing running through a sheet metal conduit. Empty of propellants, complete unit weighs about 220 lb.

The engine utilizes an alloy steel combustion chamber of cylindrical section with rounded ends. Interior dia. is 9½" and length is 11". Mounted in the tail of the airplane, the forward end of the chamber contains a 6½" valve disk plate into which are set the propellant inlets. Of these pressure actuated valves, twelve are for hydrogen peroxide and five are for hydrate-methanol solution. Centered in the inlet disk is a pressure tap connected to an automatic fuel cutoff valve — operative when engine chamber pressure exceeds safe limits. To prevent burnout of the engine as a result of high temperatures developed, the methanol is circulated through a cooling jacket, surrounding the motor and nozzle, before being injected into the chamber.

One large fuselage tank and two small cockpit tanks contain 270 gal of the syrupy hydrogen peroxide, and 130 gal of hydrate-methanol solution are carried in the wing tanks. Fuel feed system is very similar to that used in the V-2. A portion of the peroxide is drawn off and decomposed to drive a turbine on a common shaft with two worm-type propellant pumps. The steam producer is a porcelain-lined pressure vessel containing a wire screen on which are distributed pellets of calcium (or potassium) permanganate. Feeding a stream of peroxide over this catalytic agent results in violent decomposition into superheated steam and gaseous oxygen. These resultants are piped to the turbine nozzles and, after spinning the rotor, are exhausted through a rectangular waste nozzle below the fuselage.

The accessory section also contains an electric starter on the end of the turbine shaft, centralized fuel-feed control box with linkage control to pilot's throttle, pressure regulator valve for the steam producer, automatic fuel cutoff valve, and a system filter. Thrust of the rocket power plant is transmitted to a built-up bulkhead in the fuselage by two tubular members which act as the engine mount.

Operating Principles

Starting operation consists in initiating the pumping of the fluids but does not entail operation of the power plant with its voracious appetite for fuel. Movement of the control handle to idling position energizes the starter motor and opens the tank petcocks. The starter drives the pump at low power, causing the feed lines to become filled with propellants, but pressure developed is not sufficient to overcome the valve setting in the main peroxide line to the steam producer. A bypass line feeds back a small quantity of peroxide (from a pickup near the inlet valves) into the steam generator. After a few seconds of rotation the turbine is delivering enough power to the pumps to cause opening of the normal feed to the steam producer, and the bypass cuts off.

Observing sufficient pressure registered on an indicator in the cockpit, pilot moves the throttle to the first power setting. This results in opening of three of the hydrate-methanol valves and three of the peroxide valves in the engine injection plate. Intermingling of the in-spraying fuels results in spontaneous combustion — thereafter continuous so long as propellants are fed into the chamber. Decomposition of the peroxide frees nascent oxygen which burns with the alcohol to form the main source of heat. The violent reduction of the peroxide, under heat and catalytic action, releases both thermal energy (adding to the velocity) and superheated steam (adding to the mass) of the efflux.

Temperature of the engine is registered on a thermometer, visible to pilot, calibrated from 300° to 1,000° C. After the hydrate-methanol solution passes through the cooling jacket, it is returned to the fuel control center where its pressure is adjusted before injection into the combustion chamber. Use of a propellant as the coolant is not a new practice in rocket engines, having originally been suggested in an early design (1903) of Tsiolkovsky — Russian pioneer in this field. It offers the advantage of not only cutting heat losses, by achievement of a regenerative effect, but promotes more efficient combustion by preheating the methanol.

High-Power Operation

Movement of the control into the second power stage opens an additional inlet for the hydrate-methanol and three additional inlets for the peroxide. The high-power setting opens another hydrate-methanol valve (for a total of five) and six additional peroxide inlets (for a total of twelve). The engine operates with a loud roar characteristic of devices of this type and emits a short blue-violet flame.

A system scavenging arrangement is incorporated, which drains all propellants from the lines upon return of the control handle to off position, also shutting off the tank petcocks. A drain tube, connected to the cooling jacket with a pressure-operated valve in its line, serves to exhaust the alcohol, while the peroxide apparently vents through the turbine and waste nozzle. Excessive chamber pressure will also cause shutting of the petcocks and drainage of the propellants.

Interrogation of GAF pilots familiar with the Me-163 revealed that it occasionally blew up in the air. This is understandable, since highly concentrated hydrogen peroxide is capable of sudden violent decomposition. Regardless of its lack of success, the Me-163 is of considerable interest as the first of the purely rocket-propelled manned aircraft. Experimental versions resulting from modification of the Me-163 were the Me-263, 8-328, and a pilotless model intended for aerial ramming.

A trainer version was powered by the steam jet resulting from the reduction of the peroxide when brought in contact with a calcium permanganate solution — no combustion in the usual sense of the word taking place as in the combat version.

German designation of the propellants are T-stoff for the hydrogen peroxide, C-stoff for the hydrazine-hydrate in methanol, and Z-stoff for the calcium permanganate solution.

Bachem BP-20 Natter

The Natter design typified the German trend in short-range high-speed interceptor aircraft, being, in effect, a manned anti-aircraft projectile with a multi-rocket nose. Launched vertically up a tower with the aid of two powder rockets — each reportedly developing a 1.000-lb thrust for 12 sec — the Natter is said to have a rate of climb of 37,000 ft/min with an initial acceleration of 1.6G. The stub wings were utilized only to extend the range at the apex of flight — the level of the Allied bomber stream.

Also powered by the Walter 109-509 rocket engine, the Natter — because of limited fuel supply — has only 80-sec flight time at full power. At reduced power, endurance may be increased to 120 sec, and by gliding between bursts of power this may be increased to several minutes. No provision for landing is included, since the aircraft is intended — after salvoing its 24 nose-carried 73-mm rocket projectiles — to split into three parts upon detonation of explosive bolts. Pilot would parachute to safety and the engine unit would also be parachuted for recovery and re-use.

Jet Deflector Control Vanes

One of the more portentous features of this unorthodox aircraft is the use of heavy steel deflector vanes, mounted at the rear of the nozzle mouth, to deflect the rocket jet for directional control. Hinged at their outboard ends, these deflectors are linked to the tail elevons and hence are also differentially actuated. It is safe to predict that this type control will see wide usage in both jet- and rocket-powered craft, since indications point to the conventional control surfaces as a source of major difficulty at transonic and supersonic speeds.

Specific Consumption High

Immediate widespread adoption of rocket power plants for aircraft is prevented by the ravenous fuel consumption of these engines. Present average unit has a specific fuel consumption of about 1 lb/sec propellants for each 165 lb/sec thrust developed, and even the most efficient designs give little more than 200 lb/sec thrust per lb/sec consumption. While the engine itself may weigh only about 25 lb/1,000 lb thrust, the huge weight of fuel required for a reasonable duration of flight currently rules out the rocket power plant for any but auxiliary use in commercial operations. Continued research in engine design and propellant composition will eventually improve the specific fuel consumption ratio. And it may also be expected that one of the initial nondestructive applications of atomic power will be in the form of rocket propulsion.

Fundamental Design Data of German Craft
Fitted With Walter 109-509 Rocket Engine
Me-163B  BP-20
Span, ft30.6  13  
Length, ft19.4  20.5
Takeoff wt, lb9,500    3,800  
Powered flight, min12    2  
Max speed, mph550    620  
Climb to 30,000 ft, min2.6  0.8

This article was originally published in the January, 1946, issue of Aviation magazine, vol 45, no 1, pp 77-80.
The original article included a diagram and 3 photos.
Photos are not credited.