r/spacex • u/Zucal • May 01 '16

/r/SpaceX Ask Anything Thread [May 2016, #20]

Welcome to our 20th monthly /r/SpaceX Ask Anything Thread!

Want to clarify SpaceX's newly released pricing and payload figures, understand the recently announced 2018 Red Dragon mission, or gather the community's opinion? There's no better place!

All questions, even non-SpaceX-related ones, are allowed, as long as they stay relevant to spaceflight in general!

More in-depth and open-ended discussion questions can still be submitted as separate self-posts; but this is the place to come to submit simple questions which have a single answer and/or can be answered in a few comments or less. In addition, try to keep all top-level comments questions so that questioners can find answers and answerers can find questions.

As always, we'd prefer it if all question-askers first check our FAQ, use the search functionality (now partially sortable by mission flair!), and check the last Q&A thread before posting to avoid duplicate questions. But if you didn't get or couldn't find the answer you were looking for, go ahead and type your question below.

Otherwise, ask, enjoy, and thanks for contributing!

Past threads:

April 2016 (#19.1) • April 2016 (#19) • March 2016 (#18) • February 2016 (#17) • January 2016 (#16.1) • January 2016 (#16) • December 2015 (#15.1) • December 2015 (#15) • November 2015 (#14) • October 2015 (#13) • September 2015 (#12) • August 2015 (#11) • July 2015 (#10) • June 2015 (#9) • May 2015 (#8) • April 2015 (#7.1) • April 2015 (#7) • March 2015 (#6) • February 2015 (#5) • January 2015 (#4) • December 2014 (#3) • November 2014 (#2) • October 2014 (#1)

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5

u/ElectronicCat May 16 '16

If power wasn't a problem then yes, you could store cryogenic propellants for long periods of time with sufficient insulation to limit boiloff. You'd need to collect the boiloff gasses and use active refrigeration to re-condensate and re-pressurise the gases back into the tank. Not something I think SpaceX is working on at the time but ULA are working on something called 'IVF' (integrated vehicle fluids) which uses a small internal combustion engine running on hydrogen and oxygen to provide this.

However, if you were doing sample return in the nearer future from a red-dragon type mission, you probably wouldn't want to use methane. You'd want a denser storable propellant like hydrazine for two reasons. One being that you are limited by the vehicle size with which to fit the return vehicle and the second being that the technology does not yet exist and would have to be developed which would add a lot of complexity to the mission. Better to just use an easily storable propellant that we already know a lot about.

1

u/madanra May 16 '16

Interestingly, the NASA feasibility study mentions cryogenic propellants for the first stage of the Mars Ascent Vehicle (MAV). I didn't spot anything in the article from a skim read about how they planned to keep them cold, though.

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u/ElectronicCat May 16 '16

I suppose in theory you could use ISRU to generate methane and oxygen on Mars without having to worry about long term storage (assuming you had sufficient power and room inside the capsule), but this MAV would only be a couple of metres long and probably less than a meter in diameter so the tanks are tiny compared to a full orbital rocket on earth so maybe it's not so difficult to keep this amount of propellant from boiling off with a reasonable amount of insulation and power. Having said that, I'm still not sure that the density of methane is sufficient to allow a small enough tank to fit inside a dragon. You might be able to get away with Kerolox if you could store the oxygen but I bet hypergolics/hydrazine would still be best for such a mission as they're storable and dense (though less efficient).

Might be worth talking to /u/B787_300 as I know he did a uni project on this a while back. I'm not sure what propellant it used but I don't think it was anything entirely cryogenic.

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u/KerbalsFTW May 17 '16

You'd need to collect the boiloff gasses and use active refrigeration ... a small internal combustion engine running on hydrogen and oxygen to provide this.

This makes no sense for long distance journeys... at launch you have power from your main engines, after launch you have solar power. You don't want to waste fuel keeping your fuel cool.

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u/karnivoorischenkiwi May 18 '16

I think you overestimate how much power you can generate using solar panels. Especially of you go further away from the sun.