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STEAM BALLOONS AND STEAM AIRSHIPS

Yours truly in the Graf Zeppelin Haus, Friedrichshafen


ABSTRACT

Steam filling an envelope provides about 60% the lift of helium. Condensed water drains to a boiler, whose re-boiling performance is regulated to control lift. Vented steam lift gas can be replaced during flight by boiling stored water ballast. Steam is safe, very cheap, and easy to provide in the field. A layer of light insulation on the envelope keeps fuel consumption very low. The lift can quite safely be further enhanced by mixing in hydrogen at 12% by volume. A steam airship can be deflated after each flight, which facilitates ground operations. Ballonets can be dispensed with by condensing and re-boiling the steam lift gas to manage envelope pressure, if the envelope is sufficiently elastic. An airship using steam lift gas may be propelled by a light high-performance steam engine which exhausts spent steam into the envelope for condensation, and drives a large, slow, and efficient propeller with no gearbox.

LIFT GASES

Hydrogen, helium, methane, ammonia, and hot air have conventionally been used as lift gases. Hydrogen offers superb lifting performance of 11.19 N/m3 in the ISA, but is politically unacceptable nowadays due to its high flammability. Helium provides 10.36 N/m3 lift and is completely safe, but is very costly and difficult to supply in the field. Methane provides only 5.39 N/m3 lift and has no particular merit because it is just as flammable as hydrogen. Ammonia provides 4.97 N/m3 lift and is cheap, non-explosive, and quite easy to transport and supply in the field, but is somewhat toxic, corrosive, and malodorous, and has not found favor in practice.

Hot air must be continually reheated, and buoyancy control can be exerted by varying the reheating rate. Hot air is very cheap and easy to supply in the field and is completely safe, but provides rather poor lift. In practice the average temperature of the hot air within an envelope varies between 100C and 120C, which provides lift between 2.7 N/m3 and 3.2 N/m3. For a powered airship, a disadvantage to the use of hot air lift gas is that it is impractical to keep the envelope at positive pressure, since typically the flame from a gas burner is projected directly into the envelope from underneath. This means that the envelope is very floppy, so that high airspeed is not possible, and the fin and gondola attachments are not properly rigid.

We propose using steam as lift gas, both for free balloons and for airships. In the airship case, we propose also using a steam engine for propulsion.

To remain gaseous at sea level ISA, steam (the vapour phase of H2O) must be maintained at a minimum temperature of 373(K. The lift provided is 6.26 N/m3. This is about 60% of the lift provided by helium and more than twice the lift provided by hot air. Steam is non-corrosive, non-poisonous, cheap, and odour-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

Buoyancy
control

H2

2

15C

0.084

1.140   11.19

bad

fair

fair

no

He

4

15C

0.169

1.056  10.36

good

very
high

very
bad

no

CH4

16

15C

0.676

0.549  5.39

bad

low

fair

no

NH3

17

15C

0.718

0.507  4.97

fair

low

fair

no

hot air

29
(avg)

110C
(avg)

0.921
(avg)

2.980.327  2.2.98
(avg)

good

very
low

good

yes

pure H2O

18

100C

0.587

0.638  6.26

good

very
low

good

yes

90% H2O
10% H2
(by vol.)

16.4
(avg)

97.2C

0.539

0.716  6.73

good

low

fair

yes

85% H2O
15% H2
(by vol.)

15.6
(avg)

95.5C

0.515

0.716  6.96

good

low

fair

yes

 

An interesting further possibility, tangential to the main thrust of this paper, is that the lift provided by steam can be enhanced by mixing in a certain proportion of hydrogen, which of course must be kept within safe limits. A wealth of data on flammability of hydrogen-air-steam mixtures has been provided by work on the safety of nuclear plants. It has been established (reference 1) that if an initial mixture of 10% H2 / 90% H20 (by volume) is diluted by addition of air, in any proportion, the resultant mixture is never flammable. However, as an initial mixture of 15% H2 / 85% H20 is progressively diluted by addition of air, at one stage the mixture becomes marginally flammable. We consider than an initial admixture of 12% hydrogen by volume to steam will be quite safe.

GENERAL PROS AND CONS OF STEAM AS A LIFT GAS

As compared to the highest-lift gases - hydrogen and helium - the advantage of steam as lift gas is that it is safe and also 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 liquid water which will trickle downward to the lowest point of the envelope. For indefinite flight this water of course needs to be continuously re-boiled, and the weights of the required boiler 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 that of a helium LTA craft. But this negative appraisal may not hold true when craft of similar cost - capital and operational - are considered, because for a steam craft the envelope fabric will be cheaper since absolute impermeability is not required, and of course the lift gas is very much cheaper. With a steam airship, as will be discussed later, ground handling problems are also simplified.

As compared to hot air, the merit of steam as lift gas is that its specific lift is more than twice as great, so that for the same total lift the envelope area is approximately halved. Since the interior gas temperatures are similar, it might be thought that this would mean a halving of the rate of heat loss and of the consequent consumption of heating fuel. However this is not the case, because the barrier to loss of heat from the heated lift gas to the outside atmosphere is not the envelope material itself, whose heat resistance is inconsiderable, but rather dead air layered against the envelope. With hot air, dead air layers providing heat resistance are present against both the inside and the outside surfaces of the envelope, but in the case of steam everything inside the envelope (both water and steam) is at exactly 100C, and thus a dead air layer providing heat resistance is present only against the outside surface of the envelope. Therefore the rate of heat loss per square meter for a (non-insulated) steam balloon is about double that for a hot air balloon, and for the same total lift the rate of heat loss and the consequent heating fuel consumption are similar, since the area is about half.

INSULATING THE ENVELOPE

Heat insulation is not conventionally provided upon hot air balloon or airship envelopes, because their areas are so great that it would be a losing proposition except in the case of an extremely large craft (square-cube law). But with a steam balloon or airship envelope whose area is halved relative to the lift, it becomes practicable to provide an outer heat insulation layer, and this confers a dramatic improvement in heating fuel consumption.

Nevertheless the areas involved are very large, and only very light insulating materials can be considered. A combination of thin metallized plastic film ("Mylar" type) and closed-cell plastic foam offers high insulating performance at weights around 100 to 150 gm/m2. For an airship the insulating material must of course be integrated with the envelope structural material in order to prevent shifting, flapping, and damage during flight, but for a free balloon it is actually more effective to provide a separate exterior insulating bag over the envelope ("tea-cosy" style), because the intermediate air layer is an additional heat barrier.

A commercially available insulation material 5 mm thick and weighing 145 gm/m2, consisting of a single layer of simple bubble wrap and a single layer of Mylar film, is quoted as having a heat transmission of 0.0053 w/m .(K, which translates as 90 w/m2 when it separates steam at 100C from air at 15C. This insulating material is cheap, but is not specifically optimized for lightness. Clearly the development of a custom material for insulating the envelopes of LTA craft would provide further benefits in terms of insulation efficiency and low weight. In the following examples a film/foam/film/foam/film sandwich material, 4 mm thick and weighing 125 gm/m2, is postulated. This should transmit less than 80 w/m2.

A SMALL STEAM BALLOON

We first propose a small one-man free steam balloon of the Cloudhopper type, fitted with an insulating jacket. This envelope, insulating jacket, and boiler (along with burner re-jetting kit) might well be marketed as a retro-fit for a conventional Cloudhopper. Specifications are as follows [with comparison figures for a Cloudhopper in brackets]:

Envelope area: 300 m2 [500 m2]

Envelope volume: 360 m3 [700 m3]

Envelope material: silicone/polyurethane coated nylon: 68 gm/m2 [65 g/m2]

Envelope mass including vent, load tapes, etc.: 25 kg [40 kg]

Insulating jacket mass: 40 kg [none]

Gross lift: 230 kg [215 kg]

Envelope net lift: 165 kg [175 kg]

Mass of flight boiler: 10 kg [none]

Mass of pilot chair, propane burner: 15 kg [same]

Mass of propane fuel: 20 kg [typical 60 kg]

Mass of propane tank (est.): 10 kg [typical 30 kg]

Net lift available for pilot: 110 kg [70 kg]

The rate of heat loss from this balloon is about 24 kw. In an hour, therefore, 86 Mj of heat escapes through the envelope, which results in the condensation into water of about 40 kilos of steam. Re-boiling this water into steam uses about 4 kilos of propane fuel, so that only a small flight boiler is needed. Considering that a typical conventional hot-air Cloudhopper flying under average conditions consumes about 40 kg/hour of fuel, the advantages of using steam lift gas are seen to be notable: even though this steam balloon is initially loaded with only a third as much propane fuel as the Cloudhopper, it can fly for three times as long and has greater lift. The basic gain by using steam as opposed to hot air is that the area of the envelope is reduced, so that insulating it becomes practicable, and therefore the fuel consumption is greatly reduced. Insulating a conventional hot-air balloon envelope is not a viable option; an insulating jacket made of this material for a Cloudhopper envelope would weigh about 70 kg, which would be prohibitive even though the fuel weight required would be reduced.

A scaled-down version of the boiler for the large steam balloon described below is suitable for this small steam balloon. At a weight of about 10 kg, such a boiler is easily capable of boiling about 100 kg/hour of water. Naturally the jets fitted to the propane burner must be much smaller than the conventional Cloudhopper ones.

OPERATIONAL CONSIDERATIONS

The mass of condensed water which at any particular time is trickling down the inside of the envelope is entirely parasitic. However, experiment has proved that with the silicone-coated fabric specified above this weight is relatively low, because the silicone surface rejects water so emphatically that very small droplets slide down it easily and quickly.

In certain circumstances as a steam balloon rises and falls in the atmosphere a small quantity of the steam lift gas may condense into mist, whose mass also is parasitic. Such mist can quickly be eliminated by operating the boiler at a higher temperature than the ambient boiling point of water, so as to superheat the steam delivered into the envelope to some extent. Because the heating fuel consumption of an insulated steam balloon is so low, the flight boiler carried aloft can be very small and light, so that it does not impose any very substantial weight penalty. The flight boiler specified above is of more than twice the boiling capability required for steady state flight, i.e. for continuously re-boiling the water condensing from the steam lift gas as it drains down out from the envelope. The greater the maximum possible performance of the flight boiler and the burner, the greater will be the rate at which stored water ballast can be boiled to increase lift, in other words, the greater will be the operational flexibility in vertical maneuvering. In fact it might well be advantageous to provide a flight boiler of greater capacity than the one specified above in order to enhance maximum upwards acceleration, but of course there would be a weight penalty.

But in any case the flight boiler capacity will inevitably be far lower than is required initially to fill the balloon with steam on the ground. This first balloon envelope contains about 230 kg of steam, so that obviously any boiler suitable for carrying aloft would be quite inadequate to inflate it from empty in any reasonable time. This is not as much of a disadvantage as it might seem, because high capacity fuel oil burners and boilers for producing large quantities of steam at atmospheric pressure are not sophisticated or expensive technology, and a suitable steam ground supply unit can easily be mounted in the rear of the balloon retrieve vehicle, or upon a small trailer. Actually steam is very easy to provide in the field, and is even cheaper than hot air because fuel oil is much cheaper than propane. Boiling water to fill this first balloon with steam consumes about 25 kg or so of fuel oil, the cost of which is negligible.

A LARGE STEAM BALLOON

We next propose a large steam balloon whose linear dimensions are about 2.4 times those of the one-man balloon above. Specifications of the envelope are as follows:

Envelope area: 1,730 m2

Envelope volume: 4,980 m3

Envelope material: same as above

Envelope mass including vent, load tapes, etc.: 145 kg

Insulating jacket material and performance: same as above

Insulating jacket mass: 230 kg

Gross lift: 3,180 kg

Envelope net lift: 2,800 kg

The rate of heat loss from this balloon envelope is about 140 kw. In an hour, therefore, 500 Mj of heat escapes through the envelope, which results in the condensation into water of about 220 kilos of steam. Re-boiling this water into steam requires about 25 kilos of propane fuel. For such a large balloon which provides such great lift, this low rate of heating fuel consumption is absolutely outstanding. Very long duration flight can be envisaged.

The estimated weight for a state-of-the-art lightweight high efficiency boiler capable of boiling 500 kg of water per hour is 40 kg. The figure shows such a boiler in axial sectional view, suitable for gravity feed with minimum of 3 m head. (The labelling of the coils appears somewhat mysterious because their three-dimensional arrangement is very difficult to show clearly in any figure)

Fig. 1 - LIGHTWEIGHT HIGH CAPACITY BOILER

This cylindrical boiler is 40 cm in diameter and 50 cm high and made of stainless steel. There are 30 parallel circuits each about 3 m long. The tube is 12 mm I.D., 22 swg wall. All the tubes are connected in parallel by manifolds, not shown, housed in the conical upper and lower cover assemblies. The inner bank consists of 4 coils, A-B-C-D (31/2 turns); the next bank consists of 5 coils, A-B-C-D-E (31/4 turns); the central bank consists of 6 coils A-B-C-D-E-F (21/2 turns); the next bank consists of 7 coils, A-B-C-D-E-F-G (21/4 turns); and the outer bank consists of 8 coils, A-B-C-D-E-F-G-H (11/2 turns). The coils are pitched at 40 mm centres. Odd and even banks are wound heterochirally. Vertical tie rods, not shown, are provided in the spaces between the coils.

(perspective view of similar unit)

One of the features of the use of steam lift gas on this scale is that fuel oil rather than LPG could be used as the flight heating fuel. It is not very clear that this would be advantageous for a free balloon. Fuel oil is much cheaper than LPG and has a higher heat content per kilogram, but a fuel oil burner would inevitably be heavier than an LPG burner. Moreover, the fuel oil would need somehow to be supplied to the burner. One possibility would be to use a vaporizer type burner (like a "Primus" stove), and in this case pressure feed and a hand pump would suffice. The burner would be started and warmed up using propane, and could then be switched over to burn fuel oil. As an alternative to fuel vaporization, if sufficient power were available, the fuel could be fed by a powered pump to a spray burner, and a blower fan might also be provided. However a gasoline-powered onboard generator would be required, since electricity is the only practical power option, and this would entail complication, weight, and expense. Of course onboard electrical power might be useful or even required for other applications. One aspect is clear: with the steam lift gas technology we propose, there will be no shortage of boiling water for hot drinks and for keeping the crew warm!

STEAM LIFT GAS IN AN AIRSHIP

Steam lift gas may also be used in an airship, and in this case insulating the envelope is 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 great increase in the rate of lift gas 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 integrally layered upon the envelope. The disadvantages of a steam airship as compared with a helium airship of course are that the lift is 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 ripping 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. 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 ripping the envelope immediately upon landing might not be advisable, because as it settled down the crew might be scalded by escaping steam. It would be better to perform the ripping procedure in two stages, as follows. First, just before or upon landing, a short rip 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 grounded. When substantially all of the steam has been discharged so that the danger of scalding has passed, the blower is stopped and a much longer rip 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 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.

LIFT AND VOLUME CONTROL

During flight it is in practice impossible to vary the gross lift of a helium airship because helium is too expensive to vent and cannot easily be liquefied. 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 starts to be boiled and converted into steam lift gas. This buoyancy control is an important benefit, and 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 progressive 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 atmospheric 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.

A NON-ISOTROPIC ENVELOPE MATERIAL

If the envelope of a steam airship is sufficiently and properly elastic, we consider that it is possible to dispense with ballonets, especially if the boiler unit is of high capacity to allow quick generation of steam lift gas when descending.

This concept is worth exploring in more detail. An airship envelope is typically made from longitudinally extending gores, and these gores are cut from bolts of fabric with the warp running along the longitudinal axis of the airship and the weft running around its circumference. Conventionally the warp and the weft are made of the same fibre material, so that the longitudinal and circumferential elasticities of the envelope material are the same. However this does not result in an envelope which has the greatest possible safe expandability. The shape of an airship envelope is quite different with respect to its longitudinal and circumferential directions, and therefore at any particular pressure differential the maximum longitudinal envelope stress and the maximum circumferential envelope stress will differ. Suppose that the pressure differential increases steadily, so that the envelope fabric stretches more and more both longitudinally and circumferentially. In order not to damage the envelope, the operational upper limit for increase of the pressure differential must be set at the point at which that portion of the envelope fabric which is suffering the greatest strain reaches the safe elastic limit for the fibre material. With a conventional envelope fabric this pressure differential safe upper limit will be dictated by a strain, either longitudinal or circumferential, at some particular critical longitudinal position along the envelope, presumably at or near its fattest portion. However the other strains in the envelope fabric will not yet be at, and most will not be near, the safe elastic limit. This means that the maximum safe stretching potential in both directions of the envelope fabric material is not being taken advantage of.

For absolute maximum safe expandability of the envelope, ideally the linear density Cm2 of fibre cross section per meter) of the warp and of the weft at each point on the envelope would continuously vary along the envelope (while of course being the same around it) so that when the pressure differential reached its safe upper limit each and every one of the fibres of the envelope reached its safe elastic limit simultaneously. At this point the entire mass of the material of the envelope would be simultaneously stretched to its maximum safe amount for full restitution, and the increase in the envelope volume would be at its absolute maximum possible value for the particular type of fibre material. However it is impracticable to manufacture a woven material in which the thickness or density of the warp fibres or of the weft fibres varies along the warp, so that this ideal must be foregone. But it is quite possible to manufacture a material in which the thickness and/or density of the warp fibres and of the weft fibres are uniform, but mutually differ.

We therefore propose a novel fabric for an envelope of a steam airship. The purpose of using this fabric is to ensure that the envelope can be distended by increasing pressure differential to as great an extent as possible without damage, i.e. with such distension remaining fully reversible. First, the strands from which this fabric is woven are made from an artificial fibre substance the elasticity of which is pronounced. A possible choice for this fibre substance is Nylon 6 (which is widely used for fishing line because of its elasticity). It is known that Nylon 6 can be strained 6% and will still return to its original length, even if it has remained stretched for some time. Second, the thickness and/or the number per meter of the warp strands and the thickness and/or the number per meter of the weft strands are set to be different, so as to provide different linear densities of warp material and of weft material. Specifically, when designing this fabric for a particular envelope contour, the ratio of the warp and weft linear densities should be so chosen that (for any particular value of the pressure differential) the maximum value over the entire envelope of the warp fibre strain is substantially equal to the maximum value over the entire envelope of the weft fibre strain. This ensures that the elasticity of the envelope fabric in both the warp and weft directions is best taken advantage of.

Granted this mutual proportion, with the use of Nylon 6 as described above, if the absolute values of the warp and weft linear densities are chosen so that, when the pressure differential in the envelope is at its minimum value for safe flying and the envelope volume is at its base value, the maximum values over the entire envelope of the warp and the weft fibre strains (which are equal) are about 2%, then as the pressure differential increases these maximum strains can increase by about 4% to reach about 6% (simultaneously) before the safe elastic limit of the envelope is attained. If all the fibres all over the envelope were simultaneously strained to the same degree (the ideal case), then the volume of the envelope would have now increased by about 12% over its base value. However, in fact only the fibres at the critical longitudinal position along the envelope (both warp and weft) will be thus fully strained to 6%. This critical longitudinal position is near the fattest part of the envelope which is its portion which contributes most to its volume, and we therefore consider that the volume of the envelope will in fact have increased by about 10%.

This value of 10% possible safe volume increase for the envelope available by stretching will be sufficient to cope with a rise from sea level to about one kilometre, especially bearing in mind the following two helpful effects that have not been considered above: (1) the expansion of the steam lift gas as the airship rises 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 compresses the steam lift gas (relative to the external atmosphere) to a certain further extent. When combined with the fact that, as the airship rises, the pilot will naturally keep the rate of re-boiling of steam by the boiler to a very low value, so that the mass of steam within the envelope progressively diminishes, it will be understood that this airship will be able to cope with ambient pressure variations due to altitude change, insolation change, etc. within a reasonable vertical operational range, without the requirement for any ballonet system.

STEAM PROPULSION FOR A STEAM AIRSHIP

For an airship using steam lift gas, the intriguing possibility arises of employing a steam engine for propulsion. Steam engines were used to power several very early airships. 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 - for example reference (2) - 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 - reference (3) - 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 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 suitable lightweight high pressure boiler is of similar design to the boiler illustrated earlier, but with all the coils connected in series as one long tube. The feed water is forced by an engine-driven feed pump into the outer coil and progresses inwards from coil to coil. It becomes entirely vapourized into steam by the end of the third coil, and is then superheated by the inner two coils. This boiler is capable of producing sufficient steam to drive a reciprocating engine developing 50 hp. It could readily be scaled up as required.

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.

Using a steam engine for airship propulsion yields several advantages. 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 a reciprocating steam engine or a vane motor. These can be constructed with very respectable power to weight ratios, have low maintenance requirements, and operate at high torque and 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.

The figure shows the two-cylinder reciprocating steam engine fitted to the Stanley Steamer automobile which set a land speed record of 127 mph in 1906 (!). 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. 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.

Fig. 2 - LIGHTWEIGHT HIGH POWER RECIPROCATING STEAM ENGINES

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.

Fig. 3 - STEAM VANE MOTOR

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.

CONCLUSION

The steam balloon appears to be an extremely viable proposition; and while the steam airship does not have the potential to displace the helium airship in every application, it appears that it may well find a rewarding niche, perhaps in sports or low operating cost roles. And a steam engine is certainly a competitive alternative to the use of an "explosion engine" for powering a steam airship. The writer is a little surprised that these concepts have never before been seriously advanced. The very idea of a STEAM AIRSHIP brings a smile to the face of Mr. T.C. Mits (The Celebrated Man In The Street), which implies that an exploratory project, as well as having great research worth, would be massively justified from an advertiser's or sponsor's point of view in terms of publicity value.

REFERENCES

(1) "Hydrogen:Air:Steam Flammability Limits and Combustion Characteristics in the FITS Vessel", B. W. Marshall Jr., Sandia Nat. Labs. Report SAND84-0383, Dec. 1986.

((2) "Steam Aircraft Propulsion", E. E. Wilson, N.A.C.A. Technical Note No. 239 (1926)

((3) "Steam Powered Aircraft", J. N. Walton, Light Steam Power (magazine), March-April 1972, p. 96.



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