STEAM LTA - PAST, PRESENT, AND FUTURE

Thomas J. Goodey

The Flying Kettle Project86 Whitestile RoadBrentford, MiddlesexEngland TW8 9NL07759-656-559www.flyingkettle.comflyhigh@flyingkettle.com

ABSTRACT

Steam as LTA lift gas: its advantages and disadvantages. A ground boiler is required for initial filling. Possibility of mounting insulation upon the envelope. The double envelope concept. Advantages of size; the square/cube law. The "dribble" and "reboiling" flight modes for a steam balloon. Review of previous proposals: Cayley - Erdmann - Papst - Alcock - Giraud - Domen. Our experiments, and the condensation rates established. Our Giraud-type ground steam generator. Future plans: a large "dribble" balloon, and a "reboiling" balloon carrying a flight boiler. Airship possibilities. Combination with a steam engine for propulsion. Vane motors for maneuvering. The Besler steam aircraft engine. The steam airship mission: limited but effective.

THE CONCEPT OF USING STEAM AS LIFT GAS

The idea of using steam (H2O in its vapor phase) as LTA lift gas - either for a balloon or an airship - has been suggested many times. These suggestions all 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. Yet the idea of using steam as lift gas is attractive, although there are some obvious difficulties.

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, is fairly cheap, and is non-explosive and quite easy to transport and supply, but it is 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

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varies between 100ºC and 120ºC, and thus the lift provided is between 2.7 N/m3 and 3.2 N/m3.

Steam as lift gas, either for an airship or a balloon, will have the following characteristics.

First, to remain gaseous at sea level pressure, steam must of course 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. This is about 60% of the lift of helium and more than twice the lift of hot air. Steam is non-corrosive, non-poisonous, very cheap indeed, and odor-free. It cannot ignite and can be easily produced anywhere.

GAS M.W. Temp.(°C)

Density(kg/m3)

Lift (N/m3)in ISA

Safety Cost Ease of provision

Buoyancycontrol

H2 2 15° 0.084 11.19 bad fair fair no

He 4 15° 0.169 10.36 good veryhigh

verybad

no

CH4 16 15° 0.676 5.39 bad low fair no

NH3 17 15° 0.718 4.97 fair low fair no

hot air 29(avg)

110°

(avg)0.921(avg)

2.98(avg)

good verylow

good yes

steam(H2O)

18 100° 0.587 6.26 good verylow

good yes

As compared to the highest-lift gases - hydrogen and helium - the advantages of using steam would appear to be that it is safe, and also that steam is 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 indefinitely continued flight this condensate needs to be continually re-boiled, and the weight of the boiler required, and of its fuel, can be expected to be 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 or hydrogen 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 may well be much cheaper, and of course the steam itself is extremely cheap.

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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 seven 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 large and heavy ground-based boiler of very high water boiling capacity will be required. This important point was first realized by Andre Giraud. It presents a substantial barrier to practical implementation of a steam balloon or airship.

INSULATING THE ENVELOPE

Heat insulation is not conventionally provided upon hot air balloon or hot air airship envelopes, because the areas are so great, and the lift provided by hot air is so weak, that even very light insulation would be a losing proposition except in the case of an extremely large craft (square-cube law). But with a steam balloon or steam airship envelope whose area is halved as compared with that of a hot air craft of similar lift, it becomes practicable to provide an insulation layer, and this will confer a dramatic reduction in heat loss. Nevertheless the areas involved are very large, and only very light insulating systems can be considered. Various expedients are possible. Making the envelope from a material which has low heat emissivity (Mylar or metal foil) on the outside is very effective. The idea of a double envelope has been suggested, for example by Alcock (as shown), but the best means for stabilizing the outer jacket in position and keeping it inflated is by no means obvious.

THE ADVANTAGES OF LARGE SIZE

The lift of a steam balloon or airship is proportional to the cube of its characteristic linear dimension, while both the rate of total heat loss and the envelope weight are proportional to the square. Moreover, the cost of the steam lift gas (whose volume varies as the cube) will not be significant as compared to other costs. This means that it is very advantageous for a steam balloon or steam airship to be large - as large as possible within limitations of plant.

MODES OF FLIGHT

With a steam balloon (but not an airship), there are two possible modes of flight: the "dribble mode" and the more sophisticated "reboiling mode".

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In the "dribble mode", the balloon is filled with steam using the ground boiler and is then released, slightly lighter than neutrally buoyant, carrying the pilot and passengers and a large quantity of ballast water. However, no onboard boiler or fuel is carried. As the steam progressively condenses due to heat loss, the resulting condensate is drained from the bottom of the balloon and ejected into the atmosphere, and simultaneously the pilot discharges ballast water so as to compensate for the lift lost. Thus the duration of the flight is limited by the amount of ballast which can be carried at takeoff; when the remaining ballast gets low, the pilot must land. This type of flying will have a charm all of its own, because it will be utterly silent, and the degree of vertical control will probably be the greatest in all aeronautical history.

An important basic figure is the rate of loss of lift from a steam balloon as the steam condenses into water, in this "dribble" flight mode. If one kilo of steam in an envelope condenses into water, the loss of volume is 1.70 m3, so the gross loss of buoyancy in the ISA is 2.09 kg. If the resulting one kilo of condensed water is retained on board, this 2.09 kg constitutes the net lift loss. However if the condensed water is discharged, the net loss of buoyancy becomes 1.09 kg. Therefore, for buoyancy loss to be compensated by ballast discharge to maintain level flight, 1.09 kg of ballast must be discharged for every kilo of condensate discharged. It is interesting that these two figures are nearly equal....

In the "reboiling mode", the balloon is filled with steam using the ground boiler and is then released, slightly lighter than neutrally buoyant, carrying the pilot and passengers, an onboard boiler/burner unit, a quantity of fuel, and perhaps some ballast. As the steam condenses due to heat loss, the resulting condensate is drained from the bottom of the balloon and is reboiled by the onboard unit into steam, which is supplied back into the envelope. Thus the duration of the flight is only limited by the amount of fuel which can be carried at takeoff. Since the burning of one kilo of hydrocarbon fuel can boil more than 15 kilos of water, it is evident that much longer flights can be made in this "reboiling" flight mode than in the "dribble" mode, even allowing for the weight of the onboard burner/boiler apparatus. The larger the balloon, the lesser proportion of its load will the boiler/burner unit constitute.

STEAM LIFT GAS IN HISTORY

I have conducted a fairly comprehensive review of the literature on this rather arcane subject of steam aviation. We can be on pretty safe ground in believing that the earliest suggestion of steam as LTA lift gas was by that under-appreciated genius Sir George Cayley. In 1815 he wrote, referring to hot-air airships:

"...by using steam in lieu of heated air for inflating the balloon, or at least a great mixture of it with the heated air. The power of steam is greater than air at the usual temperature in Montgolfier balloons in the ratio of 18 to 11, although the first inflation will cost more fuel in the ratio of 2.6 to 1. The resistance to a steam-

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balloon will be only as 1 to 1.38, when compared with one of the same power inflated by heated air; and hence a considerable saving of power would be the result of adopting it. But several inconveniences arise upon the introduction of steam into balloons, the chief of which are the necessity of doubling the structure, so as to suspend the steam balloon within one of heated air or gas, and of the materials being incapable of absorbing water.....

Cayley's acumen summed up in a nutshell several of the obstacles to the use of steam as lift gas. His suggestion clearly relates to a powered airship. Presumably he was contemplating the use of a steam engine for propulsion, since absolutely no alternative (other than muscle power) was conceivable at the time; even electric motors hadn't yet been invented, let alone the dreaded infernal combustion engine!

Moving up to the 20th century, in German Patent 214,019 (1908) Dr. Hugo Erdmann proposed the use of superheated steam as lift gas in a balloon or rigid airship. He suggested eiderdown as an insulating material. (In my opinion the idea of superheating the steam lift gas is not practical.)

The pre-eminent postwar apostle of steam lift gas for airships was Hermann Papst. He patented several concepts relating to the subject in Germany, other European countries, and the USA. In US Patent 3,456,903 Papst proposed a double walled envelope for a steam airship; he believed that such a double walled air-inflated structure would be extremely effective for insulation. His US Patent 3,897,032 relates to a method of adding heated water vapor to lift gas. And his US Patent 4,032,085 relates to a steam airship with pressurized front and rear compartments.

As for the complementary idea of using steam as lift gas for a free balloon, this is outlined in the Erdmann patent, but the first modern and detailed suggestion seems to be due to W. Newman Alcock in a long-defunct ballooning newsletter "Wingfoot" in 1961. He went into the concept at some length. And a couple of notes by David Young appeared in Aerostat in 1973/4. However, the most detailed proposal I have so far found in patent literature is French Patent 2,684,952 to Andre Giraud (1991). It describes the use of a ground boiler for initial filling of the envelope, among other concepts - he seems to have been the first to appreciate that important concept. I don't know if Giraud ever got anywhere with practical implementation ... he was actually the French Minister of Defence under Mitterand during the Cohabitation, so he may have been rather busy with more mundane tasks. Unfortunately he is no longer with us.

Further, there is an ingenious type of low-tech balloon made of black plastic pioneered by the French and called a "bulle d'orage", which is filled on the ground with warm air saturated with water vapor to a proportion of the order of 40 gm/m3. The balloon carries no fuel or burner. When the balloon is released, the adiabatic cooling of the warm wet air lift gas due to pressure drop during the ascent is largely compensated by latent heat released by condensation of the water vapor in the lift gas, and in fact the resultant cooling rate as the balloon rises is less than the rate by which the external atmospheric temperature drops, so that the value of the lift is maintained. With a modest payload a bulle d'orage is capable of attaining very high altitude. There is too little water vapor for any substantial contribution to

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the lift. The great apostle of the bulle d'orage is M. Jean-Paul Domen. His European Patent 524,872 describes a sophisticated type. In 1996 he built a large bulle d'orage which lifted a load of 270 kg to a height of 12 kilometers. He has even performed a brief manned flight, sort of a hop..... flying hanging from a bag made from black plastic garbage bag material taped together may not be everybody's cup of tea!

OUR CURRENT DEVELOPMENTS

For some time I have made it a personal avocation to promote the idea and the practice of using steam as lift gas. It is one of the few simple low-tech ideas which has never been tried, although it is not quite obvious why.

In terms of actual implementation, the use of steam lift gas in a balloon must surely come first in the logical development of the subject, before any airship application. If one can't get steam lift gas to work well in a balloon, it is not likely to work in an airship! Moreover, from the cost point of view, to develop and fly a balloon filled with steam is a project which can be tackled by an individual on a hobby basis, whereas to develop a steam airship promises to be a much more expensive proposition, one scarcely within the reach of a private individual. So I make no apology for the fact that my concrete efforts so far have focused upon the development of a steam balloon, and I shall now presume to describe these ballooning efforts in the context of an Airship Convention.

MY EXPERIMENTS

Previous theoretical proposals to use steam as LTA lift gas have been vulnerable to the criticism of being rather deficient in concrete data. Particularly, the Papst patents and the Alcock article suggest many ingenious concepts, but the numerical values given are extremely speculative and perhaps optimistic, particularly with regard to the all-important question of insulation performance.

Therefore I have undertaken several sets of experiments in order to derive the numerical parameters which are necessary for planning steam flight seriously. Although this is by no means high technology - it's strictly kitchen/garage work - to the best of my knowledge this research is original. The goals of this experimental program were:

(1) Steam filling the envelope of an LTA craft obviously will steadily condense into water and trickle downwards to the low point of the envelope. The first question is: how much parasitic weight is entailed? In other words, at any moment, what mass of water (per square meter) is thus trickling down? This weight is of course a dead load upon the craft, and it cannot be eliminated (although the Giraud patent suggests means for minimizing it).

(2) At what rate does the steam condense? That is, for a "naked" steam balloon or airship envelope without any insulation jacket, full of steam, how many kilograms of H2O per square meter of the envelope per hour are condensed from steam to water due to heat loss through the envelope? No attempt seems ever to have been made to quantify this rate of naked steam condensation; published figures for the loss of heat from steam pipes etc. cannot reliably be applied to the cooling of a very large bag of steam in the outdoors,

Steam LTA - Past, Present, and Future Page 6

because of variations with scale in the complex processes of convective cooling. It is quite extraordinary that the steam airship and steam balloon concepts have been repeatedly proposed and discussed, for nearly two hundred years, without any effort being made to determine the value of this fundamental condensation rate experimentally.

(3) How much is this condensation rate reduced by fitting various types of insulation over the envelope? It is evident that in practice the steam balloon and steam airship concept really stands or falls upon the question of whether an insulation jacket can actually be manufactured, sufficiently effective in insulating performance to reduce the rate of condensation to an acceptable value, while still sufficiently light to allow the craft to fly. The normal methods for testing insulation materials are not very applicable for determining their effectiveness in this special context. Published parameters such as "R-values" cannot be relied upon, and the performance of various insulation materials must be evaluated during actual application for insulating the envelope of a steam balloon.

(By the way, it is not necessary to perform any experiments to determine or verify the lifting power of steam - 6.26 newtons per cubic meter. That is a simple matter of basic physics.)

OUR SMALL-SCALE EXPERIMENTS

We first constructed a miniature test envelope of the classic ball-and-cone balloon shape made from twelve gores, of total area 3.5 m2, using a black siliconized nylon balloon fabric.

We managed to inflate this envelope with steam from a small boiler system, and to keep it filled for several hours. This was the first time, as far as I know, that a flexible bag had ever been completely inflated with steam. The steam condensation rate for this small uninsulated black envelope was 1.43 kg/m2.hour. We were also able to measure the amount of water trickling down inside the envelope at any one time: it was about 80 g/m2.

We then insulated this envelope with an inner layer of simple bubble wrap and an outer layer of reflective aluminized polyester film (Mylar):

The steam condensation rate with this quite unsophisticated insulation system was 450 g/m2.hour.

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OUR MID-SCALE EXPERIMENTS

We built a larger test envelope, approximately spherical, from the same material, area about 9 m2. And we glued Mylar over substantially its entire outer surface:

When we inflated this envelope with steam, using no insulation jacket, we found a condensation rate of 935 g/m2.hour.

Thumping hard on this large envelope, inflated with steam and tight as a drum, vividly reinforced our awareness of steam as a real substance.

Then we manufactured an insulating jacket from a sophisticated fluffy insulation material known as Primaloft PL1, nominal weight 133 g/m2. (This material is used in high-quality sleeping bags and outdoor gear.) When this jacket was fitted over the above envelope inflated with steam, the condensation rate was 275 g/m2.hour

We believe that it can confidently be extrapolated that, with a somewhat thicker Primaloft PL1 insulating jacket, quilted in a more sophisticated way and weighing 250 gm/m2 in total, the rate of steam condensation could be reduced to about 200 grams per square meter per hour. Making the jacket still thicker and heavier would probably start to run into the phenomenon of diminishing returns; we consider that 200 gm/m2.hour is the lower limit value in practice that can be obtained for condensation, with any practical form of insulation.

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OUR GROUND STEAM GENERATOR

A high-capacity Giraud-type mobile steam generator is essential for the initial ground filling of a steam balloon or steam airship, so we have tackled the ambitious task of building one. The entire design has been dictated by the requirement to keep the total mass less than 3.5 tons, including trailer, this being the maximum legal weight which can be towed in the UK.

We constructed a fire-tube type boiler incorporating more than 700 kg of 22 mm copper pipe, 0.1 mm in wall thickness. The heat exchange area is a whopping 90 m2. This should have a steam production capacity of around 3 tons per hour maximum, granted sufficient firing of course. We used this apparently primitive design because a water-tube boiler with steel tubes of comparable heat exchange area would have been at least twice as heavy.

And we built a firebox and mounted our boiler on it. As an experiment we tried firing this unit with 250 kg of coal, starting it off with 5 liters of diesel oil (which perhaps was a mistake). The firemen were quite amused!

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The white cloud in this last picture was steam, and the smoke (perhaps the fault of the diesel) had pretty well died away, so basically the unit was working well. There are, however, obvious disadvantages to coal.... the control of combustion leaves something to be desired....

We realized that coal-firing was not practical, so the next stage has been to get the unit mobile and to convert it for oil firing. These pictures show the system ready for installation of the oil burner unit and testing:

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Now, fitted with an oil burner unit powered by a small gasoline engine, our ground steam generator is working smoothly on gasoil (heating oil), producing large amounts of steam on demand.

OUR FUTURE PLANS

Next we must build a steam distribution system to convey the steam to the balloon, since various parts of the ground steam generator are at temperatures of several hundred degrees, and obviously the balloon must be kept well away from them. In order to stop it becoming waterlogged, this system needs to have quite sophisticated means for draining

condensate. Then we will be ready to perform a test inflation, and to start actual steam ballooning.

THE FIRST SMALL TRIAL ENVELOPE - "DRIBBLE" FLIGHT

We already possess a small envelope, made from a siliconized nylon balloon fabric similar to the fabric used in our experiments, and built according to a conventional hot-air balloon structure. It is not laminated with reflective Mylar, so it is not optimal for a steam balloon, but it will serve for a first test of the "dribble" flight mode. The area is 400 m2 and the volume is about 600 m3, so the lift will be about 380 kg. And we have built a water ballast dispensing system.

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The weight schedule on takeoff will be roughly:

Envelope weight ...................................45 kgParasitic water.......................................35 kgLoad frame, seat, ballast system...........20 kgPilot.......................................................80 kgWater ballast.........................................200 kg

The condensation rate may be about 10 kg/minute at a maximum, according to our experiments, so the water ballast may be expected to last about 20 minutes. Flight duration should therefore be about 15 minutes before the pilot must come in to land. Even such a modest flight should be quite entertaining - completely silent and superbly controllable - although it will really just be a proof-of-concept.

OUR NEXT LARGE ENVELOPE

The small envelope described above goes against the principle that a steam balloon should be as large as possible. Since we will be able to manufacture about 3 tons per hour of steam with our ground generator, which is a lot, we should aim at a steam balloon of great capacity.

We intend to make our next envelope from a cheap, tough and relatively heavy woven polypropylene tape fabric, similar to that used for common plastic tarpaulins. Such material is unsuitable for hot-air balloons because it is too heavy, but steam offers ample lift. We have located a source for a fabric of this type, laminated with Mylar film on one side and with thin aluminum foil on the other. The aluminum foil has very low heat emissivity and is ideal as the outside of the envelope, while the Mylar on the inside will be an excellent steam and water barrier. The material is quite inexpensive. It weighs about 175 gm/m2.

ENVELOPE DIMENSIONS

The plan is to make a nearly spherical envelope, diameter 23 m, area 1,660 m2, and volume 6,370 m3, which when filled with steam will provide gross lift of about 4,060 kg. At full blast, our steam generator should take two to three hours to fill this balloon. Envelope weight will be about 310 kg and parasitic water about 150 kg, so the envelope net lift will be about 3,600 kg.

This envelope offers the following flight possibilities.

UNINSULATED "DRIBBLE" FLIGHT

According to our experiments, without any further insulation, this Mylarized envelope may be subject to a condensation rate of about 975 gm/m2.hour, i.e. about 1,620 kg/hour in total. For a "dribble" flight, allowing a few hundred kilos for basket, pilot, several passengers, and ballast water tankage, it will therefore be possible to carry enough ballast water for two hours flight (3,240 kg). This will be a pretty impressive performance, considering that the only cost for a flight will be that of about 600 kg of gasoil fuel for the ground fill.

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DOUBLE ENVELOPE - "DRIBBLE" FLIGHT

If this envelope is doubled, i.e. another similar envelope is mounted over its outside with an intermediate air gap, which is not a simple construction to implement, the envelope weight will be roughly doubled, and the heat loss is expected to be less than half, so that about four hours "dribble" flight duration could be anticipated.

INSULATED ENVELOPE - "DRIBBLE" FLIGHT

If a Primaloft insulating jacket as described above of weight 250 gm/m2 were fitted over this envelope, the net lift would be reduced to about 3,200 kg, so only about 2,800 kg of ballast water could be carried. But the condensation would now be only about 200 gm/m2.hour, i.e. about 330 kg/hour in total. "Dribble" flight duration would now be about 8 hours.

The problem with this type of insulating jacket would be, not so much its weight, but its bulk. The volume might be 30 m3 or more. This would present something of a problem upon retrieval. Personally, I am not convinced of the practicability of a bulky insulation jacket of this type upon a balloon, which needs to be retrieved after every flight. The case with an airship is quite different; a relatively thick insulation jacket is very practicable.

"REBOILING" FLIGHT MODE POSSIBILITIES

Allowing 200 kilos for an onboard burner/boiler system (which is shown by the Besler boiler data cited later to be ample), and supposing a high boiler efficiency of 80% so that the combustion of 1 kilo of onboard gasoil fuel reboils about 15 kg of water, the following approximate figures for "reboiling" flight performance are easily derived:

UNINSULATED SINGLE ENVELOPE

Fuel load...........................3,000 kgFuel consumption.............120 kg/hourFlight duration..................24 hours

DOUBLE ENVELOPE

Fuel load...........................2,500 kgFuel consumption.............50 kg/hourFlight duration..................50 hours

INSULATED ENVELOPE

Fuel load...........................2,000 kgFuel consumption.............20 kg/hourFlight duration..................100 hours

These flight durations are greater than could ever realistically be required, except in long-

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distance balloon racing. This shows that, as one possibility, a large number of passengers could be carried for an all-day flight. These performance extrapolations are somewhat speculative, but they are supported by our experiments as far as they go, and at least they show that the steam ballooning concept is well worth pursuing.

POSSIBLE AIRSHIP DEVELOPMENTS

Any possible actual development of a steam airship is a long way off, but the general outlines of what might be possible are emerging. Within limits, the picture is encouraging.

First, the obvious disadvantages of using steam lift gas in an airship are relatively low lift, and the necessity for reheating. The advantages are cheapness, and the ability to deflate the airship after each flight, thus obviating the need for a hangar or mast. A rigid airframe would sacrifice this second great advantage of steam lift gas while preserving all its disadvantages, and so I think that the idea of a rigid steam airship is a non-starter.

The problem of the bulk associated with an insulating jacket, which might give trouble with a steam balloon, does not apply to a non-rigid steam airship, because airships virtually always return to base after a mission. So we can assume that a steam airship would be fitted with something like the Primaloft insulating jacket detailed above weighing about 250 gm/m2, and that the rate of steam condensation would be about 200 gm/m2.

In order to minimize condensation the volume of the envelope ought to be maximized relative to its area. The ideal is a sphere, but of course this is not a practical shape for an airship. A lenticular configuration could be interesting. At least a steam airship could not be of the conventional Hindenburg-type shape with fineness ratio about 4:1; there would be too much area to lose heat. Therefore a high-speed steam airship is impracticable. The compromise might be a dumpy shape like a puffer fish, presumably pitch-stabilized by pendulum action.

For discussion let us consider an airship of volume similar to our large balloon envelope, i.e. 6,370 m3, and with area now about 2,400m2 since it is no longer spherical, made from the same fabric as above and fitted with a Primaloft insulating jacket. The above polypropylene fabric is not strong enough to withstand very high internal pressure, but we are not talking about a high-speed airship. (It is probable that ballonets could be dispensed with, because the volume of the steam lift gas can easily be varied by changing the boiler operating rate.) The total envelope weight is now about 1,060 kg, and the parasitic water will be about 200 kg, so that the net envelope lift will be about 2,800 kg and the steam condensation rate will be about 600 kg/hour. It will take about 40 kg/hour of fuel to reboil this condensate, so fuel for a ten hour mission will weigh about 400 kg, and the burner/boiler should not weigh more than 150 kg. We are therefore left with about 2,200 kg available for the empennage, gondola, engines, engine fuel, pilot and flight gear, and payload. This seems adequate for an airship adapted for advertising or camera platform missions. And the price will certainly be right! In capital and operating cost, such a steam airship would more resemble a hot-air airship than a helium airship.

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STEAM PROPULSION FOR A STEAM AIRSHIP

Such an airship filled with steam lift gas could of course be powered by conventional gasoline or diesel aeronautical engines, but the intriguing possibility arises of using a steam engine. The spent exhaust steam from the engine would naturally be discharged into the envelope to replenish the lift gas. Since a steam airship must in any case carry a boiler for reboiling the condensate, and since the envelope itself would function as the condenser for the steam engine, only the actual steam engine itself would be additionally required. A steam reciprocating engine can be quite lightweight, and its reliability and high torque / low rpm characteristics are very suitable for airship application. Moreover, maneuvering thrusters could be driven by steam vane motors, which are very light indeed. However vane motors could not be used as main airship engines, because their thermodynamic efficiency is poor.

An idea of what is possible can be gathered from the details of the first and only flight of a steam powered aeroplane by Besler in 1933 - a little-known episode in aviation history. The Besler brothers had participated in bringing the Doble steam car to its very high pinnacle of development, before they successfully turned their attention to steam power for aircraft.

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The Besler Corporation worked on an improved version of this steam aircraft power plant in 1958, and the system they produced is now on exhibit in the Smithsonian Institution. Salient points from the test data were (in Imperial units):

Engine and auxiliaries, weight..........................160 lbsBoiler/burner and auxiliaries, weight................170 lbsPower output (max at 2000 rpm).......................160 hpSpecific fuel consumption (cruise)....................0.8 lb/hp.hourSteam production (max)....................................2100 lb/hour

These weights could be considerably reduced with modern practice and materials. The specific fuel consumption is not as good as that of an internal combustion engine, but for the steam airship application this is irrelevant, since the steam is required in any case for lift purposes. The above figures prove conclusively that a boiler/burner unit can be built sufficiently light to be used in a steam balloon or airship, and that a steam engine system weighing about an hundred kilos, fed by such a boiler, can provide more than a hundred horsepower. The entire plan appears quite viable, although outré .

CONCLUSION - THE STEAM AIRSHIP MISSION

Obviously the non-rigid steam airship does not have the potential to displace the helium airship in every application. However I think that it will have its niche. Specifically, I 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:

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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 upon or towed with 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 that of 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 capabilities 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.

—o0o—

References:

Cayley - letter dated 24 December 1815 in the "Philosophical Magazine".Erdmann - German Patent 214,019 (1908).Papst - US Patents 3,456,903 (1969), 3,897,032 (1975), and 4,032,185 (1977).Giraud - French Patent 2,684,952 (1991).Domen - European Patent 524,872 (1993).Alcock - article in "Wingfoot" (1961).David Sarlin - article in Steam Power Club News (April 1981)Besler Corporation - Final Report on Aircraft Steam Powerplant #1843.00 (1958)

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Steam LTA - Past, Present, and Future Page 17