STEAM LIFT GAS IN AN AIRSHIP

With an airship using steam as lift gas, insulating the envelope will be even more important than for a free balloon, because the area to volume ratio for an airship is greater than for a free balloon, and also because of the increase in the rate of cooling caused by the airstream when the airship is under way. A loose insulating cover will not do in this case: the insulation must be fitted fairly tightly upon the envelope. The disadvantages of a steam airship as compared with a helium airship are that the lift is of course less, i.e. for a given lift the envelope needs to be larger (about 40% greater in area and 65% greater in volume), and that water which condenses from the steam lift gas needs to be continually re-boiled. However, advantages are that steam is extremely cheap and easy to provide in the field, can be vented freely as required, and can be replaced during flight by boiling stored water ballast. In fact steam is so cheap that at the end of a flight the pilot need feel no concern about deflating the envelope (which is completely impracticable with helium due to cost) or can leave it to collapse gradually by itself as the steam within it condenses. This greatly facilitates ground handling and storage as compared to a helium airship. If desired, however, for a quite modest expenditure a steam airship could be kept inflated on the ground until the next flight by maintaining boiler operation.

Actually simply venting the envelope immediately upon landing might not be advisable, because the crew might be scalded by escaping steam as the envelope settled down around the car. It would be better to perform the deflation procedure in two stages, as follows. First, just before or upon landing, a small opening is formed at the top of the envelope, and simultaneously a high capacity air blower is started up to blow air into the envelope from below. The steam lift gas then vents upwards rapidly and harmlessly with the envelope remaining taut and elevated above the gondola, and the lift drops rapidly to zero as the steam lift gas quickly becomes replaced by air, so that the airship rapidly becomes firmly anchored to the ground. When substantially all of the steam has been discharged so that the danger of scalding has passed, the blower is stopped and a much larger opening is formed in the envelope, which will then deflate quickly and safely.

Propulsion internal combustion engine waste heat could be used for generating steam for the envelope, but rough calculation shows that this concept is only valid for very large craft.

A minor benefit from the use of steam lift gas is that icing upon the envelope ceases to be a problem, and abundant heat is available for de-icing the control surfaces. Let alone the unlimited supply of hot water for cabin heating and crew refreshments!

LIFT AND VOLUME CONTROL

During flight it is in practice impossible to vary the amount of lift gas in a helium airship because helium is too expensive to vent and liquefying it is out of the question. However with a steam airship it is quite easy to vary the volume of lift gas and thus the lift: for lift reduction, the pilot need only reduce the rate at which the water condensed by the envelope is re-boiled to below the break-even rate, so that this condensed water starts to be accumulated as ballast; while for lift increase he need only increase the re-boiling rate to above the break-even rate, so that ballast water is progressively boiled and converted into steam lift gas. This buoyancy control could be of great use in the case of a very large steam airship intended for transport of heavy cargo.

If during flight it is found that the airship is becoming unduly light, presumably due to consumption of fuel, then it is possible to vent some of the steam lift gas and to replace it by pumping a corresponding volume of air into the envelope, so as to reduce the lift while maintaining the pressure differential without adjusting the ballonets (if any). This procedure cannot be reversed during flight, but it may sometimes prove useful.

If the envelope of a steam airship is sufficiently and properly elastic, we consider that it may be possible to dispense with ballonets, especially if the boiler unit is of high capacity to allow quick generation of steam lift gas when descending. Making the envelope fabric more stretchable in the longitudinal direction, than in the circumferential, will help towards this goal.

If the envelope can be made elastic enough for its volume to be varied safely by 10%, this will be sufficient to cope with a rise from sea level to about one kilometre without ballonets, especially bearing in mind the following two helpful effects, as the airship rises: (1) the expansion of the steam lift gas does work upon the external atmosphere and hence removes heat from the steam (in other words, the expansion is not completely isothermal but has an adiabatic component due to the non-zero speed of ascent) which causes an extra quantity of the steam lift gas to condense into water automatically without any heat needing to pass through the envelope; and (2) the increase of pressure differential (relative to the external atmosphere) compresses the steam lift gas to a certain further extent. When combined with the fact that, as the airship rises, the pilot will naturally reduce the rate of re-boiling of steam by the boiler to a very low value, so as to diminish the mass of steam within the envelope as quickly as possible, it seems that such a steam airship should be able to cope with ambient pressure variations due to altitude change, insolation, etc. within a reasonable vertical operational range, without the requirement for any ballonet system.




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