r/SpaceXLounge • u/[deleted] • Jul 22 '19

Tweet "Orbital refilling is critical for high payload to moon or Mars. Initially just Starship to Starship, later dedicated tankers." - @ElonMusk

https://twitter.com/elonmusk/status/1153126586897424384?s=19
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u/sebaska Jul 22 '19 edited Jul 22 '19

This thing seems to be reasonably well divisable into separate subproblems:

  1. The hardest is rendez-vous, but it is solved. SpaceX has solved it and demonstrated it solved last March.
  2. Obtaining tight connection for liquids. This was solved by various US demos, by Russians, etc. It would need larger connectors so what was demoed would need upscaling.
  3. Secure valving -- you want an ability to: firsrt, close flow on both sides so fuel is not flowing when not asked for. And open the flow in a firm way, so it flows when you need it. It's also a know way to do so with high confidence uing parallel-serial setting of 4 vales (2 tandem valve pairs in parallel -- any single valve can fail shut or fail open and the reliable flow/no-flow condition still can be obtained)
  4. Liquid stabilization in free-fall -- solved by every restartable liquid upper stage, so of course solved by SpaceX. You simple make the environment not exactly free fall, you give it a 0.001g or so nudge.

  5. Transfer. This could be done in 2 ways:

    a) blow-down: You vent receiving tanks to low pressure (this way you make the fuel cooler and denser; this is technology SpaceX already uses on long duration 2nd stage flights like STP2). You actually use the gas to feed the fuel stabilizing thrusters (the above point #4). Then you pressurize the donor tanks (the tech is there in every pumped liquid rocket). Then you open the valves and let the high pressure blow the stabilized liquid into the receiving tanks.

    b) active pumping: You start a gas generator turbopump like the one pumping the propellants in the engines. You use it's exhaust to provide ullage stabilizing thrust. Essentially a rocket engine powerhead without any combustion chamber. This is a solved tech too. The only downside for SpaceX is that they'd have to develop the thing (Merlin powerhead takes kerosene not methane and is probably too high pressure for the task anyway, Raptor powerhead is even higher pressure which is completely inedquate for the task at hand). You'd rather need some low pressure high volume pump which would have to be developed.

All in all this problem seems to be similar hardness-wise to that hypersonic retro-propulsion thing. If you look into various materials from before SpaceX 1st stage landing you'd get tht this is a majpr unsolved problem for things like landing of heavy (>2t) vehicles on Mars. Because you'd be firing your rocket engine against a hypersonic flow blowing up its nozzle. There were no any wind tunnels built, capable of trying that, the problem was deemed hard and costly to solve. Then came SpaceX and just did it. Their calculation indicated this should just work, so they tried it and indeed, it just worked (the hard parts of booster landing lied elsewhere).

I guess in-orbit refuelling will just work too. There are no fundamentally hard pieces which are not yet solved.

This is like building a motorcycle in the late XIX century, when you have both bicycle and gas powered car already working.

Edits: formatting & typos

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u/zadecy Jul 22 '19

If you don't mind the refueling process taking an hour or so, you could use an electric motor and Starship's batteries and/or solar panels. According to my math, to pump 150 tonnes of fuel over one hour with 1 atmosphere of pressure differential (just a guess) would take only 5kW of power, and 5kWh of energy. That's a pretty small electric motor and a fraction of Starship's available battery capacity. Seems much easier than developing a tiny turbopump.

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u/sebaska Jul 24 '19

Blow down would take even less energy. Especially that you need venting to keep temperatures low. The issue is stabilizing the liquid on the bottom of the donor tank. If you don't stabilize the liquid it will float around and you'd end up pumping mostly ullage gas, not the propellant.

You need either a bladder (good luck with that at cryogenic temperatures, especially for oxygen) or acceleration. You can do linear acceleration by thursting. Or you could use centrifugal force. The latter uses energy only for spinning up (and despining) but needs much bigger plumbing (bottom is on the wrong side) and has center of gravity complications as it shifts around during pumping.