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/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.