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The idea of using steam (the vapor phase of H2O, i.e. water) as lift gas for a powered airship has been suggested many times. Cayley (1815) was the first, and further proposals have been made by Erdmann (1909), Papst (1969), and Giraud (1991). These projects appear to have remained merely theoretical, although several were quite detailed. It appears that no full-scale trials, or even experiments, have ever been performed.

An obvious corollary is the use of steam as lift gas for an unpowered free balloon, i.e. an aerostat. The intrepid balloonist Brian Boland made an unsuccessful effort to fly in a balloon filled with steam supplied from a geothermal vent in Iceland; I have no documentation on this attempt.

Yet the idea of using steam as LTA lift gas has strong attractions.

In the past, hydrogen, helium, methane, ammonia, and hot air have been used as lift gas. Hydrogen offers the best lifting performance of 11.19 N/m3 in the ISA (International Standard Atmosphere), but its high flammability makes hydrogen politically unacceptable nowadays. Helium provides 10.36 N/m3 lift and is completely safe, but it is very costly, and is difficult to transport and supply. Methane provides only 5.39 N/m3 lift and has no particular merit because it offers no safety advantages over hydrogen. Ammonia provides 4.97 N/m3 lift and is cheap, non-explosive, and quite easy to transport and supply, but it is somewhat corrosive, toxic and malodorous, and has not found favor in practice.

Hot air must be kept hot by burning fuel, and buoyancy control can be performed by varying the fuel burning rate. Hot air is very cheap and easy to supply, and is completely safe, but it provides rather poor lift. In practice the temperature of the air in a hot-air balloon envelope varies between 100oC and 120oC, and thus the lift provided is between 2.7 N/m3 and 3.2 N/m3. For a powered airship, a disadvantage to hot air is that it is very difficult to pressurize the envelope.

Steam as lift gas has the following characteristics.

First, to remain gaseous at sea level pressure, steam must be maintained at a minimum temperature of 373oK, i.e. 100oC. Because the molecular weight of H2O is 18 while the average molecular weight of air is about 29, and taking temperature into account, the lift provided in the ISA by steam lift gas is 6.26 N/m3. As seen from the Table, this is about 60% of the lift of helium and more than twice the lift of hot air. Steam is non-corrosive, non-poisonous, cheap, and odor-free. It cannot ignite and can be easily produced anywhere.





Lift (N/m3)
in ISA



Ease of






1.140   11.19









1.056  10.36









0.549  5.39









0.507  4.97





hot air




2.980.327  2.2.98





steam (H2O)




0.638  6.26






As compared to the highest-lift gases - hydrogen and helium - the advantage of steam as a lift gas is that it is safe and also so cheap that it may be vented without cost concerns. However its lift is not as good. Moreover steam will continually condense upon the inside of an envelope into water droplets which will trickle downward to the lowest point of the envelope. For indefinite-duration flight this water of course needs to be continually re-boiled, and the weight of the boiler required, and of its fuel, are substantial. So, for craft of similar volume, the payload and performance of a steam LTA craft will be much lower than those of a helium craft. But this may not be true when craft of similar cost (rather than volume) are considered, because the material for the envelope of a steam craft will be much cheaper, and of course the steam is extremely cheap.

As compared to hot air, the merit of steam is that its lift is more than twice as great, so that for the same lift the envelope area is approximately halved. (This does not necessarily mean that the rate of heat loss is half, however, although it is less; the situation is more complicated than that.)

To produce the same amount of lift, about three times as much energy is required for boiling water to produce steam lift gas, as for heating air to produce hot-air lift gas. Therefore it is inevitable that, for the initial filling of a Steam Balloon or Steam Airship on the ground before takeoff, a heavy ground-based boiler of very high water boiling capacity will be required.


In order to obtain numerical values for heat loss and for the weight of water trickling down the inside of the envelope, we have performed some experiments by filling small envelopes (about 10 m2, 2.5 m3) with steam. The details are upon our website. In summary, the results were:

Steam condensed per hour:

Envelope colored black both inside and outside, no insulation - 1400 gm/m2

Envelope colored silver outside, black inside, no insulation - 935 gm/m2

Envelope colored black both inside and outside, insulation (30 gm/m2 bubble-wrap) - 700 gm/m2

Envelope colored silver outside, black inside, insulation (133 gm/m2 polyester fiber matting) - 275 gm/m2

Water trickling down the envelope: 85 gm/m2 (at any time)


A steam balloon will be a sort of hybrid between a gas balloon (hydrogen/helium) and a hot air balloon, and it will have some of the advantages and some of the disadvantages of both. Based upon our experimental results, we project the following possible ways of flying a steam balloon (aerostat).

(a) Un-insulated envelope, no flight boiler

In this simple flight mode the condensed water is merely discharged and is not re-boiled, so flight duration is very limited. The only method of lift control is by ballast. We have built a balloon envelope intended to be flown in this manner, of area 400 m2 and volume 600 m3, which weighs 40 kg. With 30 kg for a seat, supporting lines, a load ring, and a large bag for holding water ballast, the total craft weight is about 70 kg. The weight budget is:

380 kg - gross lift (600 m3 X 6.26 N/m3 / 9.81 N/kg)

- 70 kg - craft weight

- 80 kg - pilot

- 30 kg - water trickling down envelope interior


200 kg - net lift

Thus upon takeoff the ballast load will be about 200 kg of water. The rate of condensation will be about 600 kg of steam per hour, so the loss of lift will be about 10 kg per minute. The ballast will therefore be sufficient for about 15 to 20 minutes of flight. Although short, this flight will have its own peculiar charm, since it will be completely silent and the effectiveness of lift control will be very great. It would be possible to increase flight duration up to about an hour by carrying insulation upon the envelope - at the cost of making the envelope much more bulky and harder to handle on the ground.

(b) Flight boiler and burner provided

In this case a flight boiler is provided to re-boil the condensed water. As with a hot-air balloon, lift control is available by varying the rate of boiler operation. We have started to build a flight boiler (of about 20 m2 heat exchange area) which we believe will be sufficiently powerful to boil about 600 kg of water per hour, using perhaps 45 kg of fuel. It appears that the boiler/burner weight may turn out to be about 60 kg, but it is too early to say definitely. Without insulation on the envelope, with our current balloon carrying this boiler, it appears that it will be possible to start with enough fuel for about three hours flight. This would be an excellent performance for such a small envelope. A somewhat larger version would be capable of very long flights. Addition of an insulating layer upon the envelope would give further efficiency; but the question of the exact insulation thickness which would give the best benefits is complex, involves many trade-offs, and cannot be decided as yet.


The status of our project's actual hardware is as follows.

Envelope (as above) - completed

Seat and ballast arrangements - virtually completed

Ground boiler for initial filling - construction started

Flight boiler - construction started

We anticipate that the first Steam Balloon flight will take place during this year.


In practical terms, it is obvious that we need to get a lot of experience operating a Steam Balloon, before building upon this experience (and the publicity it reaps) by seriously considering the production of a Steam Airship.

However it may be permitted to speculate!

We do not think there is any potential in a steam airship of the rigid type. This is because one of the great advantages of steam lift gas will be in ground handling, since the airship can be routinely deflated after every flight. A steam airship will, therefore, be a non-rigid.

However, the conventional elongated Zeppelin shape involves a hidden danger if steam lift gas is used. That is, water will be continually trickling down the inside of the envelope and accumulating at its bottom, to be drained out and re-boiled. With the conventional shape, if a steam airship assumes a nose-pitched-up attitude for a few minutes, water will start to accumulate in the rear end, and will weigh it down. This condition will get rapidly worse: the situation will be unstable. Therefore we think that a steam airship should be spherical or lenticular, or nearly so; at least, its shape should be much more bloated than the classic airship shape.

Since a steam airship will necessarily carry a boiler to re-boil the condensed water, the intriguing possibility arises of using a steam engine for propulsion.

The first airship that ever flew (Giffard, 1852) was powered by a steam engine. This approach failed because the power-to-weight ratio of steam engines at the time was very poor. (It was greatly improved during the development of the steam car.) But in any case the use of a steam engine for propelling a hydrogen or helium airship (or indeed an airplane) is doomed, because, considering the total weight including the boiler and condenser, a steam engine is much heavier than an internal combustion engine of equivalent power.

However, with a steam airship in which a boiler is required in any case for keeping the lift gas in vapor form, a new situation arises. Excluding boiler weight and condenser weight, a modern reciprocating steam engine can actually be lighter than the equivalent internal combustion engine. In fact, with modern practice, it is perfectly possible to manufacture a reciprocating steam engine which develops 100 ps and can be lifted with one hand. The engine can be expected to be much lighter than the boiler. Moreover, since the airship envelope itself will serve as the condenser, the perennial problem of providing adequate condensation is completely neutralized - possibly for the first time in the history of the steam engine!


Obviously the non-rigid Steam Airship does not have the potential to displace the helium airship in every application. However we think that it will have its niche. Specifically, we think that a Steam Airship will be able to satisfy the demands that hot-air airships try to satisfy but fail. Consider the following mission requirement:

During reasonably fine weather, to fly over a major sporting event and maintain station for a few hours, displaying advertising or carrying a news camera.

A hot-air airship is not able to meet this requirement. Theoretically it might be capable, but in practice the wind is usually too strong - because a hot-air airship is defeated by even a light wind.

At present a helium airship is the only possibility for this mission, and they are extremely expensive to operate, fundamentally because they must be kept inflated more-or-less indefinitely.

I believe that, with development, a Steam Airship will be able, in average good weather, reliably to:

Arrive from base, deflated and packed in a single vehicle, at an unprepared launch site in a park within a few kilometers of the target area;

Be inflated with steam from a ground boiler carried on the same vehicle, by a small ground crew;

Fly to the target area and hold station over it for several hours;

Return to the launch site and be deflated and returned to base.

And I believe that the cost may be perhaps twice that of a hot-air airship, but much less than a helium airship. And I think that the up-wind performance of a steam airship will be sufficiently reasonable for this mission to be possible on, perhaps, 80% of days.

In fact for a limited mission such as the one specified above, the full abilities of a helium airship - such as long-term endurance, high airspeed, and poor-weather flight capability - are not actually needed. The steam airship will have the most important qualities necessary for advertising and camera platform work: hover capability in moderate winds, and large size. And I think that the low cost and the convenience in ground handling of a Steam Airship will, in this restricted operational context, more than compensate for its deficiencies.


The strange thing about this Steam Balloon and Steam Airship project is that the basic idea is so simple and so low-tech.

Often people ask me "If it's such a good idea, why hasn't it been done already?" (Of course this objection could be made against any technical development whatever; it actually means nothing!)

The 19th century was the age for very simple yet world-shaking inventions. 20th and 21st century technology has become very complicated: one usually needs special materials and/or advanced physics to accomplish anything new and wonderful. From this point of view a Steam Balloon or Steam Airship is a technological curiosity, because it could have been built any time in the last 150 years. It requires no advanced materials or delicate or subtle processes, and indeed comparatively little financial investment. I have no idea why it has never been tried in practice before; it is quite strange..... Nevertheless, it may be a very effective development. We shall see!

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