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BASIC BACKGROUND PHYSICS
AS APPLIED TO LTA,
PARTICULARLY STEAM LTA

LIFT GASES USED IN THE PAST

Hydrogen, helium, methane, ammonia, and hot air have conventionally been used as lift gases for LTA craft. 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 100°C and 120°C, 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.

THE USE OF STEAM AS LIFT GAS

The Flying Kettle Project proposes 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.


Steam is the vapor phase of water or H2O, which is actually a very potent and corrosive substance, sometimes termed “dihydrogen monoxide” by its opponents (who think that its use should be prohibited) and “hydrogen hydroxide” or “hydric acid” by its advocates (who think that it may be employed for restricted purposes without undue danger).

(yuk yuk...)

-oOo-

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 373°K. (Such steam is not strictly speaking a gas but a vapor, because its temperature is below its critical point.) Because the molecular weight of H2O is 18 while the average molecular weight of air is about 29, and taking the temperature into account, in the ISA as a surrounding medium, the density of steam is 0.587 kg/m3, so that its lift is 6.26 N/m3. As seen from the following Table, 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

15°

0.084

1.140   11.19

bad

fair

fair

no

He

4

15°

0.169

1.056  10.36

good

very
high

very
bad

no

CH4

16

15°

0.676

0.549  5.39

bad

low

fair

no

NH3

17

15°

0.718

0.507  4.97

fair

low

fair

no

hot air

29
(avg)

110°
(avg)

0.921
(avg)

2.980.327  2.2.98
(avg)

good

very
low

good

yes

steam (H2O)

18

100°

0.587

0.638  6.26

good

very
low

good

yes

 

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 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. This does not mean that the rate of heat loss is simply halved, however; the situation is more complicated than that.

A very important basic figure is the rate of loss of lift from a steam balloon as the steam condenses into water, (of course providing that it is not reboiled). It can be worked out from the above data that, 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 force in the ISA (ask Archimedes) is 2.09 kg - which assumes that the one kilo of condensed water is retained on board. However if this condensed water is discharged into the atmosphere, the net loss of buoyancy becomes 1.09 kg. Therefore, if the loss of buoyancy is to be countered by ballast discharge to maintain neutral flight, 1.09 kg of ballast must be discharged for every kilo of condensate. It is interesting that these two figures are nearly equal....

For steady flight, both hot air and steam balloons require continual reheating - the heat that is lost from the envelope must be returned to it, in the one case by reheating the air and in the other case by reboiling the condensed water. A benefit in both cases is that the reheating rate can be varied over the short term to perform vertical maneuvering. A very important question is: in each case, how much heat must be added to get how much lift? That is, how does the vertical responsiveness compare, between steam and hot air? The calculations are here. The answer is that for steam being boiled, if 1 mJ of heat is added, the lift increase is 9.0 newtons; while, for hot air being heated, if 1 mJ of heat is added, the lift increase is 26.5 newtons - nearly three times as much. The consequences are twofold.

First, the vertical sensitivity to a burn of a steam balloon is only a third that of a hot air balloon; it is relatively sluggish. In practice, even when flying in the "jacketed reboiling" mode using a flight boiler, probably it will sometimes be advisable to discharge ballast to get rapid upward acceleration near the ground.

Second, the job of filling a steam balloon on the ground is about six times as hard (that is, requires six times as much heat) as filling a hot air balloon of similar volume.

INSULATING THE ENVELOPE

Heat insulation is not conventionally provided upon hot air balloon or hot air 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 steam 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 consumption of heating fuel. Nevertheless the areas involved are very large, and only very light insulating materials can be considered. More detail can be found in the insulation philosophy page, and in the pages recounting our experiments.



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