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STEAM PROPULSION FOR
A STEAM-LOFTED AIRSHIP

For an airship using steam lift gas, the intriguing possibility arises of employing a steam engine for propulsion.

A steam engine was used to power the very first airship; see the prior art page. This approach failed because the power to weight ratio available from steam engines at the time was abysmal; the technology of high pressure lightweight steam boilers and engines had simply not yet been developed (it was greatly advanced by the advent of the steam car). But in any case the use of a steam engine in a conventional hydrogen or helium airship is doomed for the reason that has always bedevilled the mobile steam engine: the condensation problem. A closed cycle steam engine needs a very large condenser to convert all its exhaust steam back into feed water, and the condenser weight and size have always been considered to preclude aircraft use. Studies have been made of steam propulsion for aircraft - again, see the prior art page - and the usual conclusion has been that the overhead of the condensing apparatus is unbearable. A steam power plant was once fitted to an aeroplane - again see our prior art page - but the only documented flight lasted a bare ten minutes, undoubtedly due to inadequate condensation.

However, when an airship whose lift gas is steam is fitted with a steam engine, naturally the spent steam from the engine will be discharged into the envelope, and this eliminates the condenser as a separate unit. Probably for the first time in the history of the steam engine, the problem of condensation is neutralized without penalty. Furthermore, the same boiler will be used for supplying steam both to the engine and directly into the envelope, and this is an effective synergy.

The envelope condensing capability increases rapidly with the airspeed, which is very suitable, because the higher the power level at which the steam engine is operating the greater will be the requirement for condensation of its spent steam. Without outer insulation the envelope would have more condensing power than could possibly be required, and the task of the designer, within the constraints of weight, will be to provide the right amount of insulation upon the envelope to match its condensing capability to the requirements of the steam propulsion apparatus. However he must ensure that in all operational conditions of the airship - from loitering on station to progress at speed - the envelope is always capable of condensing more steam than is being exhausted from the engine, because any shortfall of condensing capability would mean that steam would have to be vented in order to avoid stretching and eventually rupturing the envelope. Because of this inevitable design excess of condensing capacity, the boiler will be required always to be operating to boil water into steam at atmospheric pressure for supply directly into the envelope, as well as boiling water into high pressure steam for driving the engine. This dual requirement will not present any problem, because it is easy to convert high pressure or superheated steam into atmospheric pressure steam at the ambient boiling point of water without loss of thermal efficiency simply by expanding it while spraying in water.

A preliminary discussion of the type of boiler that might be suitable for a steam airship can be found on the boilers page.

In the airship case propane is not a viable alternative to fuel oil because of the high quantities of fuel that will be consumed, and accordingly the complication of a power-driven burner, pump, fan, etc. cannot be avoided unless solid fuel such as anthracite is used.

TYPES OF STEAM ENGINE

There are three types of steam engine: reciprocating piston engines, vane motors, and turbines. Although a steam turbine has a very high power to weight ratio, it is complex and expensive and requires a lot of maintenance, and its high rotational speed means that for driving a propeller a heavy gearbox would be required. We think that there is better potential in using reciprocating steam engines or vane motors. These can be constructed with very respectable power to weight ratios, have low maintenance requirements, are reversible, and operate at low rotational speed so that they can be directly coupled to large slowly rotating propellers, which excel in thrust efficiency, especially at the low airspeeds typical of airships. Furthermore, the typically quiet operation of a steam engine eliminates the requirement for any silencer.

RECIPROCATING STEAM ENGINES

This is a diagram of the very effective twin-cylinder reciprocating steam engine which was fitted to the Stanley Steamer motor car:

Images of a record-breaking engine of this type, now in the Smithsonian, can be seen here; warning: these images are 280K in total. This engine weighed about 85 kg and was reputed to develop 250 hp. Modern practice could improve substantially on these figures. A typical rotational speed for such an engine operating non-expansively at high pressure is 1100 rpm. Its high torque is maximum at startup. This type of engine can run for long periods between services and is very straightforward to maintain, and its inherent reliability is much greater than that of an internal combustion engine, especially a spark ignition engine.

STEAM VANE MOTORS

Steam vane motors can be made very lightweight, and can achieve very high power-to-weight ratios, but are not as thermodynamically efficient as the best reciprocating steam engines. Of course, this does not matter if steam is required in any case for the envelope. Vane motors are extremely simple and reliable. The blades do require regular servicing and replacement, but they are very cheap and easy to change. In contrast to reciprocating steam engines, their startup torque is relatively low, but this is not a problem for driving a propeller.

Steam vane motors of this type would be ideal as manoeuvering engines for airships, in view of their minimal weight and immediate reversibility.

The basic reason that, comparatively, very high power to weight ratios are available with steam engines (not counting the boiler or condenser weight) is that metals are extremely strong in tension, but with the internal combustion engine this is not taken full advantage of, since the mean effective pressure is usually less than 600 Kpascals. Moreover for thermodynamic reasons the indicator card is very narrow. However the steam engineer can choose his mean effective pressure to suit the particular circumstances of application.




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