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A proposal for lunar development, LunarCOTS, relies on a reusable shuttle running on LH2 and LOX to work. It lands, is refueled on the lunar surface, and then takes off again. Presumably it could sit for days between firings, and would fire many times over the course of months or years. I'm working on adapting that mission architecture, and this part makes me more nervous than usual.

I know the J2s on the Saturn 3rd stages restarted once up to 6 hours after shutting down during Apollo missions. Have liquid hydrogen engines been restarted more extensively since then? Is this a big technical challenge?

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    $\begingroup$ Seems to me storing the LH long-term would be a bigger challenge than restarting the engine. $\endgroup$
    – Hobbes
    Commented Apr 27, 2016 at 19:07
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    $\begingroup$ @Hobbes Yes - in the particular case of the lunar poles, that author refers to the extreme cold within the permanently shadowed craters as being useful to limit boil-off, and also as the fuel is produced on-site, i suppose you just have to have high enough production to account for those losses. $\endgroup$
    – kim holder
    Commented Apr 27, 2016 at 20:40
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    $\begingroup$ If you're producing fuel you have the hardware to liquefy it. Thus boil-off simply becomes an input to your liquifier rather than a loss. $\endgroup$
    – Loren Pechtel
    Commented Apr 28, 2016 at 4:58

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My understanding is that designing for multiple restarts isn't particularly difficult; it's just that most engines don't have the need for it, so they don't bother.

The RL-10 mentioned in the Lunar COTS paper you linked has been restarted at least 7 times in a single real mission, and depending on the model, is rated for 10 starts and 4000 seconds of run time without maintenance. The RL10 is a relatively simple design, with modest chamber pressure and thrust-to-weight ratio; getting longer run times out of a more sophisticated and powerful engine is more of a challenge, but I don't think starts per se are difficult to design for.

Rocketdyne thinks the CECE evolution of the RL-10 will be capable of 50 starts, but I don't know if they've proven that capability. This paper says 27 starts over the course of multiple test series.

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  • $\begingroup$ Great link - i failed to find something like that, although i ended up on the same venerable old yarchive bulletin board. @geoffc has spoken of it before in glowing terms. $\endgroup$
    – kim holder
    Commented Feb 27, 2016 at 2:45
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    $\begingroup$ 50! 50 starts! ha ha ha ha ha! $\endgroup$
    – Russell Borogove
    Commented Feb 27, 2016 at 2:49
  • $\begingroup$ That paper at the end is a real keeper too. $\endgroup$
    – kim holder
    Commented Feb 27, 2016 at 2:56

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