Reports reaching the United States that Germany is about to launch one of her "secret weapons" in the form of an invisible bomber made of transparent plastic material were received here naturally with utmost reserve, even though this news was endorsed by the Aeroplane. It is possible this was powered by steam, which would not be the first time that the steam engine was used in aviation. One of the first airplanes ever designed, the Avion by Clement Ader (which is exhibited in a museum in Paris) was equipped with a steam engine. In more recent aviation it was mostly this country which tried to give the steam engine a chance.
The steam engine for this particular purpose was promoted in Germany for economic reasons, and war and military uses. These units use comparatively cheap fuel oil which is much easier for Germany to obtain than the expensive high-octane gasoline for airplane motors.
One very important feature of the steam engine from the military angle is its silent performance; even though the din created by the propellers could hardly be eliminated, this fact constitutes an additional difficulty for anti-aircraft defense. Other advantages of the steam engine in aviation are the decreased fire danger due to the kind of fuel oil used. These ultramodern steam engines on which Henschel experimented have very little in common with the standard machines. The fuel used, as mentioned above, is a special, high grade oil. It is induced under high pressure into the steam generator; a special kind of carburetor adds air in a certain ratio and this oil-air mixture is ignited by a plug. The steam is gradually generated in a spiral pipe of very small diameter which is sometimes more than 500 feet long and should reach its highest tension exactly at the end of this spiral just before it enters the cylinders. The exhaust from the cylinders leads to a condenser and the same cycle starts all over again. The loss of steam and/or water is extremely low on these new models. It might be mentioned here that fluids other than water were also used in some experiments and that the water has first to undergo a special treatment. The revolutions per minute can be stepped up to 2000 and more.
The schematic drawing shown just below is based on an automobile steam engine. However, the principal is identical with that for airplanes up to point 5 where either power unit can be installed.
The basic part of the unit is the boiler (1) where the steam is gradually generated in a seamless spiral pipe, which has a very small diameter and often is more than 500 ft long. The steam should reach its highest pressure at the extreme end of this spiral just before it enters the cylinders (5) through the regulation valve (4). Its temperature rises there to 450° Centigrade and the pressure reaches 100 atmospheres or more. The exhaust from the cylinders is connected with the condenser (radiator) (8) where the steam is cooled, and converted into water which is led to the container (9). The exhausted steam drives the small turbines (6) and (7). Turbine (6) operates the air pump (10) and the electric generator (12). The first one provides the heated air for the carburetor, the second one fires the spark plug, which operates the plug continuously and charges the battery. Turbine (7) drives the fan of the radiator.
The container (9) provides for water replacement, acting as a reservoir. The water is pumped (3) into the boiler and the same cycle is repeated. The fuel is induced over the pump (11) in the fire box (2). All these constructions have a special, sometimes three-way, safety device, because an over heating of the steam beyond a certain degree must be prevented as it would lead to an explosion. The steam engines of the latest constructions of this kind have 2 or 4 cylinders. The revolutions per minute can be stepped up to 2000 or more. It is conceivable that Henschel managed to produce a steam turbine for the airplane in question even though the high rpms presented a seemingly unsolvable problem in this connection.
The problem of these steam engines is twofold. First of all it is extremely difficult to find the correct dimensions of the various apparatus to bring them in accordance with the heat value of the unit which must work entirely automatically. Secondly, even though the weight of such an engine is comparatively very low, it is difficult to create an engine which is small enough to be used in an airplane. The weight per horsepower is in practically all cases below 2 lb, and further research should bring this well below this figure.
This short article is excerpted from the "Aviation Engineering" column of the May, 1941 issue of Aviation magazine, vol 40, no 5, p 107.
Note: While the objection to a steam turbine design is justifiable because of the well-known issue of decreasing efficiency with small units, a closed-cycle steam engine could in principle be functional as an aircraft engine, since the technology of shedding waste heat had been very well developed along with the development of liquid-cooled engines.
The limiting factor for most small-scale closed-cycle steam engine applications has been radiator size.