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Comment by zuminator | Hacker News Reader

On a something like a gas giant with a hydrogen atmosphere surrounding a rocky core, would it be possible for the vessel to be hydrogen breathing until it reaches the edge of space and then ignite a stage to carry it out of the gravity well? Or if a nitrogen or CO2 atmosphere is thick enough, to fly aerodynamically or even float until it reaches a point where the gravity is appreciably lower than at surface level?

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On rocky planets gravity doesn't get much lower in the orbits we're concerned about. For example the ISS still experiences 90% of the gravity we experience at the surface. Reaching orbit is mostly about reaching a speed where the arc in which you are falling never intersects the surface.

But you can absolutely use an aircraft to gain height and speed, and then launch a much smaller rocket from that aircraft (where the speed is the primary advantage, and is what rockets use most of their fuel for). This setup is used by Virgin Galactic's SpaceShipTwo. There is also Virgin Orbit's LauncherOne, which is a small rocket that launches from a modified Boeing 747. On Earth it's just about not worth the additional complexity, but on planets with stronger gravity but comparable access to powered flight this might be the preferred method of reaching space.

One important factor might be the speed of sound. Subsonic flight is much easier for aircraft than supersonic flight. In an atmosphere with a much higher speed of sound, like say hydrogen, aircraft could reach much higher speeds and thus would be a much more advantageous launch platform for rockets. Assuming you already solved the issue of powering those planes of course.

I've never really understood this, why is it easier for a plane to reach that speed than a rocket? Is it sort of just another rocket stage?

An air-breathing jet engine doesn't need to carry oxidizer, which in a rocket is most of the propellant weight. It also has access to unlimited reaction mass, so it can be much more energy-efficient in producing thrust (it is more efficient to produce thrust by accelerating a lot of mass by a little, than by accelerating a little mass by a lot, but a rocket can't take advantage of this because it would need to carry all that extra mass. A plane can use ambient air for this purpose)

This all adds up to a plane needing to carry many times less mass to gain the same altitude and speed as a rocket, at least within relatively dense atmosphere.

Is there a reason that there isn't a stage 0 to rockets that takes advantage of these properties?

A rocket on a typical orbital launch profile spends less than 60 seconds in air dense enough for jet engines to have good performance, so there is little to gain.

Pegasus is an orbital rocket launched from an aircraft, but it doesn't exactly impress with performance or cost-effectiveness. Just doesn't make much sense to operate a huge aircraft and design your system around it just to improve on the least important 10% of the flight.

The basic physics of https://en.wikipedia.org/wiki/SABRE_(rocket_engine) have been vetted, which is an air breathing rocket engine. I don't know how much difference trying to liquify H2 vs O2 is though.

The oxygen is the majority of the mass (but not volume!) in a stoichiometric hydrogen engine, so the mass savings would be less I think. The RS-25 (space shuttle main engine) runs at a higher fuel ratio. Should work very similar to SABRE in general - the concept of a high speed atmosphere collector and precooler is pretty universal to any gas, and hydrogen has an extremely high heat transmission rate.

On a smaller gas giant you could build a floating platform (like a giant zeppelin) and launch from there. Because gas giants are so large the “surface” gravity at the altitude such a platform would be floating at is not as high as you might expect. On Uranus and Neptune it’s actually lower than 1G.

However, past Jupiter size the mass keeps increasing while the radius doesn’t, so even from a floating platform you’re contending with multiple G’s.

No. To float you would need a gas lighter than hydrogen, which isnt a thing. And powered flight (wings) without oxygen would be trickey, requiring more of a rocket motor than an aeroplane engine.

Yes, you could use a balloon filled with vacuum, but lifting something the size of an orbital rocket in a hydrogren atmosphere would require a vacuum chamber at least the size of a city, possibly the size of a small state. It would probably be easier to build a tower.

> No. To float you would need a gas lighter than hydrogen, which isnt a thing.

The atmosphere gets denser further down. You just need a negative pressure vessel, or to heat the hydrogen, like a hot air balloon. At 1 (Earth) atmospheric pressure the gravity of most Gas giants is quite low.

Sorry if I wasn't clear, but I brought up the question of floating with respect to nitrogen/CO2 atmospheres (thinking Titanlike or Venuslike) not hydrogen.

It's a moot point, but I still want to point out that "a gas lighter than hydrogen" is a thing: it is simply hydrogen at a higher temperature, i.e. a hot-hydrogen balloon, analogous to a hot-air balloon.

I do not want to ride in a hot hydrogen balloon :)

Why not? Hydrogen inside a hydrogen atmosphere is perfectly safe.

It's hydrogen inside an oxygen atmosphere that's the problem.

(So on Jupiter, you wouldn't want to ride in an oxygen balloon, ie you wouldn't want to ride in a hot air balloon there.)

Well, maybe if there's no oxygen nearby.

If you have a planet with a hydrogen atmosphere, it's a pretty good bet there is no (free) oxygen nearby.

Fair point

How do you "fill something with vacuum"?

With a vacuum pump ;)

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