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An interesting question was asked regarding the use of steam as a lifting gas for lighter-than-air vehicles here: What are pro/cons of steam as lifting gas for airships?.

Many of the challenges with steam revolve around the large amounts of power needed to generate enough steam, along with the losses associated with the steam cooling/condensing on the envelope skin.

However, if one were to use a nuclear power source of some kind (let's suspend the safety concerns, as we're only concerned with feasibility), would there be point where a large enough vehicle could theoretically use steam as a lifting gas?

Edit: I'm not equipped to do the sort of math required to answer the question.

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    $\begingroup$ Why do you think it might not be theoretically possible? With enough engineering effort and money all sorts of wild ideas are technically feasible. If you are asking whether a nuclear steam balloon is more financially feasible than current alternatives, then it doesn't take take a detailed cost/benefit analysis to estimate that the answer is likely a resounding no. Don't ask someone to do the math for you... $\endgroup$ – Michael Hall Jul 6 '20 at 14:12
  • $\begingroup$ @MichaelHall Financial considerations would be a whole separate question, I think. I was more thinking about whether it would be completely impossible (it requires an envelope size of 100 miles or something) or merely wildly impractical. It's almost certainly not a good idea, for dozens of very valid reasons. $\endgroup$ – Eric Hauenstein Jul 6 '20 at 14:18
  • $\begingroup$ Well, I think your question needs more focus then. Given that hot steam rises we have already established in the other question that it can be used to lift something. So, the debate becomes one of an efficient heat source to generate the steam. (e.g. BTUs per pound of weight) I'm guessing that a small reactor would pack more potential energy than a bank of propane tanks so I don't think fancy math is required. Just need a little old fashioned research if is important enough for you to confirm what we suspect. $\endgroup$ – Michael Hall Jul 6 '20 at 14:32
  • $\begingroup$ We can build nuclear satellites. We can even build nuclear planes So what makes one think we can't build nuclear.......hot air balloons? $\endgroup$ – Abdullah Jul 6 '20 at 16:25
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So the Convair NB-36H, a prototype US bomber from the 1950's that was designed to eventually be nuclear-powered, carried a 1 MW air-cooled reactor with a mass of 16 Mg (16,000 kg / 35,000 lbs). If we put the reactor from the NB-36H in an airship and use it to heat steam as the lifting gas (let's assume that the reactor has enough power to keep the steam heated), then the volume required for the lifting gas to lift the reactor alone would be 1 m^3 ÷ 6.26 N * 9.87 N/kg * 16,000 kg = 25,200 m^3. The Hindenburg, by comparison, contained 140,000 m^3 of lifting gas. Unfortunately the Wikipedia article for the Hindenburg doesn't give the mass of the airship minus engines, passengers, and cargo, so I can't guess at how much more mass would be required for the rest of our nuclear-powered airship, but let's assume for now that the machine would be light enough to leave the ground.

Unfortunately, there are other problems. First, if there are humans aboard, then they will require shielding. There is no way to make lightweight shielding; shielding inevitably requires mass. The NB-36H had 11 tons of shielding for five crew.

But it gets worse: the airship would leave a trail of radioactive gas everywhere it went, since the reactor is air-cooled. This might have been tolerated in the 1950s, but things are very different now. In a crash, the reactor would break apart into highly-reactive pieces, including the uranium or plutonium fissile material.

To sum up, it may be technically feasible to design an airship that uses a nuclear reactor to heat steam as the lifting gas. However such an airship would be ridiculously expensive, and probably not be able to carry more than a small handful of passengers or crew. It would likely irradiate anything nearby in normal operation, it would leave a trail of radioactive gas wherever it went, and it would cause a major nuclear incident if the reactor were to malfunction, or if the airship were to crash. Such an airship would be highly impractical.

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The other answer you quoted already says that steam as a lifting gas is not feasible no matter what heat source you use. A steam lifting gas nuclear powered airship is not any more feasible because you'd be limited by the amount of water you can carry, and even small nuclear reactors are heavy.

You would get much better results by simply heating air with the reactor, as air is a better lifting gas, and you won't run out of it. Assuming a pressurized water reactor here you would heat the primary coil using your nuclear fuel, and the primary coil would heat air in a secondary loop rather than water as you would when generating power. There's a fundamental problem though, the secondary loop is pretty important in cooling the reactor, and we all know what happens when a reactor overheats. There's probably a way to overcome that through design, this unlikely to happen as nobody wants to spend billions developing a nuclear reactor that could potentially cause a disaster if it were ever to fall to the earth.

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  • $\begingroup$ You don't need a full reactor to generate heat. Radioisotope thermoelectric generators use the decay-heat of elements that are cold enough to work without active cooling. In fact you could put an old rod into some layers of lead, and it would be reasonably hot without ever melting down or blowing up. They power some spacecraft this way. $\endgroup$ – Zsolt Szilagyi Jul 6 '20 at 15:03
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    $\begingroup$ RTGEs are far too heavy for the heat they produce to lift an airship. They create electricity in space through the heat differential between the heat of the reactor and the extreme cold of space @ZsoltSzilagyi. $\endgroup$ – GdD Jul 6 '20 at 15:14
  • $\begingroup$ I lack the skills to run the math, so you might be right. Just keep in mind that I did not propose to use the heat to generate electricity to heat the ship - rather to use the heat directly. I suspect you can get those rods in any (non-military) enrichment/activity level you wish, and as there are some which do need active cooling, some should fall into the Goldilocks zone of not needing active cooling but being hot enough to lift the ship. For those, the output/weight ratio should far surpass traditional zeppelin fuel. $\endgroup$ – Zsolt Szilagyi Jul 7 '20 at 8:57

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