Helium balloon as a rocket

Question

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.

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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.)

Answer 1

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.

Answer 2

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 waterrocket

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.