# What are pro/cons of steam as lifting gas for airships?

After quick research, I found this home made experiment of steam used as the lifting gas. This website even gives a comparison with other lifting gasses:

+-------------+----------+------------+-----------------+--------------------+--------+-----------+-------------------+------------------+
|     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           | 11.19              | bad    | fair      | fair              | no               |
| He          | 4        | 15         | 0.169           | 10.36              | good   | very high | very bad          | 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   | very low  | good              | yes              |
| steam (H2O) | 18       | 100        | 0.587           | 6.26               | good   | very low  | good              | yes              |
+-------------+----------+------------+-----------------+--------------------+--------+-----------+-------------------+------------------+


(source: http://flyingkettle.com/jbfa.htm)

Given the conclusion of this website, steam should now be more widely used as a lifting gas:

• it is available since the first lighter-than-air (plenty of time to develop the technology)
• it has a good lifting power
• it is cheap and safe

Yet, on 2020, almost all airships and balloons use helium or hot air, and no other lifting gas. So there must be drawbacks I fail to imagine. I hope finding those drawback will orient my research for my first question ("why didn't they use exhaust steam as lifting gas on airship fitted with steam engines?"). Moreover, there may be advantages I also fail to imagine.

Thus, what are pro and cons of using steam as a lifting gas?

• Maybe someday we'll be allowed to use HTML tables in StackExchange sites... Jul 6 '20 at 14:14
• @rclocher3 This was discussed many years ago and I see no reason for this to change. You may still contribute on the meta post linked in this comment. Jul 6 '20 at 16:22
• I've seen other markdown formats that support tables nicely. An announcement on meta.SE dated June 1st, 2020 gives reason for hope: "Once the dust has settled and we’re all comfortable with the new Markdown renderers under the hood, we can re-evaluate if the time is right to bring table support back to the (drumroll) table!" Jul 6 '20 at 16:39
• @rclocher3 nice one, I just upvoted the meta.SE announcement. Jul 6 '20 at 17:33

The main disadvantage is the energy required just to get off the ground. Let's do a back-of-the-envelope calculation, comparing this to a hot air balloon.

• Let's heat 1m³ of air. To get equivalent lift, we need about 0.5m³ of steam.
• The density of air is about twice the density of steam. So, for a given amount of lift, we need to heat approximately the same mass of lifting medium.

• To heat 1kg of air to 100°C above ambient, we need about 100kJ (specific heat of air is about 1kg/(kJ K)).

• To heat 1kg of water to 100° above ambient, we need about 420kJ just to heat the water. To vaporize the water, we need an additional 2250kJ, for a grand total of 2670kJ.

The most used hot air balloon size is 2800m³. For that we need to heat 1400kg water. Multiplying gives about 4GJ of energy or 75kg of propane just to get off the ground. Since we established steam can lift twice as much per unit of volume, we can maybe get away with 2GJ or 35kg of propane.

Some more negative thinking... Condensation on the outer shell seems like a very efficient mechanism to transport heat energy to the environment, only partially offset by the reduction in surface area compared to a hot air balloon due to the cube-square law. This may require more energy in flight than a hot air balloon. Closed-cycle operation (only heat condensed water, not 'new' water) is preferred due to the high energy requirement, so descending means you have to wait for enough water to condense or be willing to pay the price in fuel. Superheated steam is not safe (despite what your table says); the high heat capacity and enthalpy of condensation can result in serious scalding burns compared to hot air. All of these effects are of course much more pronounced when comparing to a helium-filled blimp.

Still, it's a really neat idea, and sometimes you don't need a practical reason just to build something neat.

• Comments are not for extended discussion; this conversation has been moved to chat.
– Jamiec
Jul 8 '20 at 8:52

In short, helium (or hydrogen) remains in the gaseous state at much, much lower temperatures than water does. In particular, helium or hydrogen will remain gaseous in pretty much any part of Earth's atmosphere. Water, on the other hand, will not naturally remain gaseous in any part of the Earth's atmosphere. On the contrary, it will freeze in most of the Earth's atmosphere.

For steam to remain steam, it must be constantly heated to make up for the energy it's losing by heat transfer to its surroundings. You can aid this by insulating the airship really well, but you can't bring the heat transfer down to zero. And this will also add a significant amount of weight, reducing efficiency further.

Additionally, as Sanchises's answer mentions, it takes a large amount of energy to turn that water into steam in the first place. Water has a very high specific heat and also a high enthalpy of vaporization. That is, water takes much more energy per unit mass than most substances to heat up and also more energy per unit mass to convert from a liquid to a gas.

So, you're requiring a very large amount of energy to get the water into the gaseous state in the first place and you then need a large and continuous energy source to keep it there during flight.

• Re "Water, on the other hand, will not naturally remain gaseous in any part of the Earth's atmosphere.": As long as the temperature is above the dew point it will naturally remain gaseous. At any one time (on average) 0.4% of the entire atmosphere is water wapor (1% at sea level). Jul 4 '20 at 7:08
• @PeterMortensen Well, yes, but the dew point of a steam-filled airship will be much, much higher than the local atmospheric temperature, especially at altitude. If it's allowed to drop in temperature inside the airship or to vent at any significant concentration outside of it, it will very quickly condense (and, at high altitudes and/or cold climates, subsequently freeze.) I was simplifying things a bit for the purposes of what is relevant to the question that was asked. Jul 4 '20 at 7:50

Lighter than air balloon works by replacing air with some gas that is lighter than air. Your steam airship would work if we can keep steam at 100 C or higher or else you risk condensation, your balloon collapse into some saggy bag and you fall out of the sky. Wikipedia mentions that when water turns into steam it expand 1700 times so imagine, in reverse, your 1700 cu.m. hot steam balloon when cool down and becomes 1 cu.m. bag of water and you lose all the lift. That does not sound safe at all. Consider low temperature up there you need a lot of energy to combat heat loss through conduction over a large surface area of the gas bag.

To sum it up you need lots of energy to keep the balloon in positive buoyancy and a big safety issue to solve when that energy source fail.

• Condensation won't just cause the balloon to "fall out of the sky". The energy required for condensation has to go somewhere, and if the balloon is sealed, radiation and conduction through the balloon is the only way it can exit. As long as enough heat enters the balloon to equilibrate it, the physics work out just like an average hot air balloon. Jul 3 '20 at 19:00
• @BlokeDownThePub and if the flame fails on a standard hot air ballon? It’s still going to be a gradual process for the ballon deflating and falling.
– Tim
Jul 4 '20 at 0:00
• You seem to be thinking that the steam needs to be kept above 100 °C, because if it were to drop to, say, 99 °C, all the water would condense and you'd fall out of the sky. That's completely wrong. Going by the numbers in Sanchises' answer, steam contains an enormous amount of energy compared to hot air. The amount of heat loss which would cool hot air to room temperature (producing, I think, about a 25% decrease in volume) would merely condense 5% of the steam (a 5% decrease in volume). Jul 4 '20 at 13:19
• So, by my calculations, in the event of a heater failure, steam would be much safer than hot air, quite contrary to your assertion that "you risk condensation, your balloon collapse into some saggy bag and you fall out of the sky". Jul 4 '20 at 13:20
• @TerranSwett is absolutely right, the huge heat capacity of water and the even bigger condensation heat (i.e. heat that is produced when condensating, helping to stop further condensation) is a safety feature, not a problem. The problem would be that this huge heat capacity is also dangerous if you come into contact with the gas - you can probably survive for a moment inside a hot air balloon, a steam balloon would lethally burn you in seconds. Jul 4 '20 at 14:45

In addition to the tremendous energy requirements, you'd need a highly specialized design that is not at all bothered by boiling hot water dripping back out of the balloon. Even if it was highly insolated, you'd need to be resupplying steam to keep the balloon inflated because even small amounts of local condensation would pull a lot of water out of the air because water expands so much when it becomes steam.

Even like, a cup of water running back down the balloon over the course of the entire voyage would be over 100 gallons of steam. This in general would be hard to build, fuel inefficient and difficult to control while being wildly dangerous.

They didn't use exhuast steam because the efficiency of a balloon isn't acheived by min/maxing the displacement of air by any means possible. It is acheived by realiability, cost and control. Blasting a second gas as a by product of the steam engine, doesn't improve that in the slightest, and likely would create problems.

• That problem could be mitigated by having a bag that contains water vapor which is captive within the main air gasbag so that it wouldn't need to contain a significant pressure nor temperature gradient. Jul 5 '20 at 23:52