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This question is not about rockoons, which are rockets using balloons as launch platforms to start from a greater altitude. This question asks about using the balloon itself as the rocket, like a toy balloon one lets go of at home which flies around for a few seconds, making a farting sound.

High-altitude helium balloons have reached altitudes of over 50km. Those balloons often rip because the light material cannot handle the overpressure at these high altitudes.

Now when I let go of a little toy balloon in my living room they achieve astonishing speeds. Couldn't one use the gas in the pressurized balloon the same way? How fast could one go? Any chance to do a sub-orbital hop? (I'm aware of what is difficult about spaceflight — immortalized by xkcd — but just humor me.)

Upsides:

  • The thin atmosphere does not cause much friction at lower speeds (the returning first stages of the Falcon 9 start their entry burns at 55 km and > 7000 km/h).
  • This is the first time that our propellant lifted us, and not the other way round.

Downsides:

  • I think helium or hydrogen are not our first choices as propellants but it's what we have to work with.
  • The nozzle speed of the propellant is lamentably low.
  • In order to get very high we need a huge balloon with very low density helium. Any chance to utilize an elastic hull material to store energy?

Maybe the numbers indicate something crazy like a 10 km3 volume. Well: Why not? All rockets are crazy. (But we'd probably use hydrogen, not helium.)

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    $\begingroup$ @GdD No -- my question is about using the balloon itself as the rocket. $\endgroup$
    – Peter - Reinstate Monica
    May 16, 2022 at 14:44
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    $\begingroup$ So you're proposing to use the gas pressure alone to produce thrust to get on a sub-orbital lob? That's...not going to work. $\endgroup$
    – GdD
    May 16, 2022 at 15:04
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    $\begingroup$ @GdD Well, since we are floating, there is no time pressure. We can accelerate quite slowly, at least at first. And as I said, think big: imagine a Borg cube made from Graphene... $\endgroup$
    – Peter - Reinstate Monica
    May 16, 2022 at 15:20
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    $\begingroup$ Maybe hard to imagine, but in the mesosphere and the ionosphere the helium or hydrogen inside the balloon will be to heavy for lifting. $\endgroup$
    – Cornelis
    May 16, 2022 at 15:29
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    $\begingroup$ The incredibly low specific impulse makes this idea a non starter. $\endgroup$
    – Organic Marble
    May 16, 2022 at 15:33

2 Answers 2

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There are at least two serious issues:

Ascending to high altitude is useful in reducing air resistance, but does very little to aid the required orbital velocity of around 7500m/s.

Using the gas in a balloon as a propellant would provide a chronically low exhaust velocity of perhaps 500m/s, but even this would rapidly decline even further as the gas pressure (and temperature) decreased. Use of any significant over pressure would not be possible as it would increase the mass of the balloon structure required to hold the pressurized gas and more importantly the mass of the gas it contained rapidly reducing it to a lead balloon.

*In reality an exhaust velocity of 500m/s would not be achievable at altitude as pressure decreases with altitude and 25m/s would be more likely. Also no thrust at all would be available at the point that the pressure inside the balloon was equal to that outside.

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    $\begingroup$ He wasn’t asking about going in an orbit. The question was about getting pass the Karman line. That is a big difference. $\endgroup$
    – The Rocket fan
    May 17, 2022 at 13:19
  • $\begingroup$ Note that 500 m/s means an Isp of about 50s. 25 m/s means an Isp of 2.5s. The first rocket to cross the Karman line had an Isp of over 200s. $\endgroup$
    – Charles Staats
    May 17, 2022 at 14:16
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Well there are a few methods of getting to space with a balloon rocket. If you wanted to build a big balloon and then release it will fly uncontrollably and will probably never reach space. If you wanted to power a rocket with air like a balloon and release is a possibility of getting to space.

To get pass the Karman line you need to reach a speed of 1.4 km/s. Since there is friction from air the required speed would have to be higher. A balloon rocket would blow out all of it's air (fuel) within a very short amount of time meaning that the rocket/balloon would have to reach a high velocity within a extremely short period of time. To make this happen you would need a light balloon/rocket blasted by extreme pressure.

My Idea would be to build a rocket similar to a water rocket just that there would be no water. Here is the formula to calculate the height of a waterrocketenter image description here

According to this formula if you wanted to fly a water rocket to space you would have to make a rocket that only weighs a few grams, have an air pressure of around 6000 bars (600 000 000 pascal) and drop the density of water by half. The best way to do that would be to use 50 - 100 layers of graphene and very bubbly water.

You were asking about a balloon rocket. My idea would be to make a long tube (4m) with around 16L airspace. The walls would be also made out of 50 - 100 layers of graphene and the pressure would be raised to 6000 bars (Since one square meter of graphene only weighs 0.77 milligrams having so many layers really won't make a difference to the height the rocket will fly). In metal factories where they sometimes use water to cut metal, they bring the pressure up to 6000 bars and the water comes out at a hypersonic speed (More info here: https://en.wikipedia.org/wiki/Water_jet_cutter). The required speed to get to space is 1.4 km/s and hypersonic is five times the speed of sound. That is around 1.7 km/s vs 1.4 km/s meaning that this rocket would pass the Karman line.

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  • $\begingroup$ Why was my comment downvoted $\endgroup$
    – The Rocket fan
    May 16, 2022 at 19:00
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    $\begingroup$ What is the source of the image that you're using for the water rocket equation? $\endgroup$
    – notovny
    May 16, 2022 at 21:24
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    $\begingroup$ The various assumptions in the formule make it essentially invalid for your purposes. You can't assume are resistance is negligible, water compresses significantly at 6000 bars, air itself is over 1 kg/L in density at 6000 bars, Your proposed rocket design violates most of the assumptions in the equation you've decided to present, $\endgroup$
    – notovny
    May 17, 2022 at 9:50
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    $\begingroup$ Start with what @novotny wrote. Also an unobtainium water rocket isn't what the question is asking about. And if you want to be taken seriously, don't mix units and number punctuation styles in the same sentence, or give ridiculously over-precise numerical values. $\endgroup$
    – Organic Marble
    May 17, 2022 at 12:01
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    $\begingroup$ For one, the theoretical delta-v of 1.4 km/s really assumes an absolutely instantaneous impulse, and to make it with 1.7 km/s implies ~30 s or less of thrusting (to keep gravity losses under 300 m/s), with absolutely zero atmospheric drag. Second, graphene is not a technologically mature material. $\endgroup$
    – ecfedele
    May 17, 2022 at 17:27

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