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I've had an idea floating in my head recently and would love if someone more knowledgeable with physics or engineering could review it/debunk the idea.

There have been many ideas related to using balloons as launch platforms, but what about a small 'rocket' that utilizes the gas from a hydrogen balloon as a fuel to reach orbital velocity, once it's out of the dense atmosphere.

The idea, if at all viable, would only make sense for tiny payloads.

The main challenges in this idea seem to be:

  • Is there enough energy in the hydrogen gas to begin with?

  • Can you convert the gas into a liquid state quickly, in flight? What kind of mechanism could be used and what would be the minimum mass of it?

  • Would you need to carry up liquid oxidizer? I'm assuming a liquid-fuel rocket, but perhaps there are better options? (Something utilizing hydrogen, or maybe helium).

  • Is there a current technology that can use the hydrogen in it's gaseous form, combined with captured atmospheric oxygen, as a propulsion mechanism? (Avoiding the need for conversion all together).

So to those of you who understand the physics and engineering, is this possible -- or practical? Thanks!

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  • $\begingroup$ Why would you need to liquefy the hydrogen at all? $\endgroup$
    – Nathan Tuggy
    Feb 7, 2016 at 22:22
  • $\begingroup$ @Nathan, I was thinking about that just now as well (and you'll noticed I added that to the points). My presumption was that current rocket technologies might require liquid hydrogen. $\endgroup$
    – turnfire
    Feb 7, 2016 at 22:24
  • $\begingroup$ Gaseous hydrogen is perfectly fine. It was even often used in test firings of rocket engines. $\endgroup$
    – SE - stop firing the good guys
    Feb 7, 2016 at 22:28
  • $\begingroup$ By your bullet points: 1) Unclear what you're asking, you mean pressure alone as in cold gas thrusters? 2) See Nuclear Thermal Rocket, 3) Depends, if you want a NTR then no, if you want a chemical rocket then see e.g. Centaur Upper Stage's "balloon tanks", 4) See air-breathing SSTO ascent vehicles like Skylon, but hydrogen used is in liquid form, and obviously that doesn't work well as an upper stage and out of the atmosphere. Combined, your question doesn't make much sense to me as a single system. Individually, it's too broad and you're asking several non-related questions in a single one. $\endgroup$
    – TildalWave
    Feb 7, 2016 at 23:16

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The primary problem is the amount of hydrogen we're talking about. LH2 density is 70.8 kg/m³. Gaseous hydrogen at sea level is 0.09 kg/m³. Take a good high-altitude balloon like Explorer II, it has 100,000 m³ of volume, you're loading up only 9 tons of hydrogen - at sea level! And if you want it to remain buoyant (and not to burst) you must vent hydrogen as you go, so that the density inside the balloon remains lower than density of the surrounding air! (and the pressure not much higher).

And that's for vehicle (payload+oxidizer+balloon+etc.) of 6,800 kg.

Then you need means to vent that hydrogen from the balloon to the engine. You still need oxidizer, optimally in mass ratio of about 6:1 relative to hydrogen. You need some means of driving the hydrogen out of the balloon fast, or it becomes a half-filled rag dragging after your rocket.

...and a whole slew of other headaches. I'm not going to try to dig any deeper - what you have already should be enough to show how impractical it is.

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  • $\begingroup$ As far as I can tell, just the mass ratio is enough to doom the idea. You can't lift 6 tons of oxygen with 1 ton of hydrogen. $\endgroup$
    – Nathan Tuggy
    Feb 7, 2016 at 23:37
  • $\begingroup$ @NathanTuggy: That just means most of the carried hydrogen would need to be discarded/wasted. But the size of the balloon to the rocket mass starts looking quite ugly. Reaching 8km/s with under 6 tons of rocket (hydrogen mass aside) is going to be challenging, and yield pathetic payload to orbit. $\endgroup$
    – SF.
    Feb 7, 2016 at 23:55
  • $\begingroup$ Great information SF, thank you for your answer. It certainly doesn't sound like it's practical if the propulsion requires oxidizer that has to be carried up. $\endgroup$
    – turnfire
    Feb 8, 2016 at 1:10
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For a better use of that hydrogen, you can use a Nuclear thermal rocket. They can use both hydrogen and helium, without the use of any oxidizer. They even gives you a better efficiency than a chemical engine.

Keep in mind that escaping the atmosphere is only a tiny fraction of the way to an orbit, so the rocket has to be substantial even with your proposed scheme. Over all, you only get a small gain from a far more complicated design.

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  • $\begingroup$ Actually, hydrogen and helium make about the worst propellants for NTR. The higher "compressed" density of the propellant the more impulse can be obtained from it. Water makes much better propellant for NTR. Mercury would be even better if you're willing to deal with the nastiness. Heavy, low melting point metals are something worth considering too. Gold-powered rocket anyone? $\endgroup$
    – SF.
    Feb 8, 2016 at 0:42
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    $\begingroup$ @SF. Hydrogen is bad for density-impulse, but the ISP is fantastic. The main use of NTRs is for upper stages, and there almost only the ISP counts. Density impulse is most important for the first hundred m/s. Also, density impulse is a measure of the volume efficiency, and you have a lot of storage volume in a balloon. Here the limiting factor is mass, and then hydrogen is the best possible option. $\endgroup$
    – SE - stop firing the good guys
    Feb 8, 2016 at 0:45
  • $\begingroup$ @Hohmannfan, thanks for adding that in. While I may not know if it's actually possible from a technical standpoint, the idea is that if you can use the 'fuel' (ie. hydrogen gas) to lift through most of the atmosphere with buoyancy then use the same fuel for propulsion - it would significantly reduce the initial mass/fuel needed. This would only be meant for something very tiny with a high thrust to weight ratio -- perhaps some form of mini satellites. P.S I bet a gold-powered rocket would look pretty spectacular at launch! $\endgroup$
    – turnfire
    Feb 8, 2016 at 1:16

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