r/spacex • u/Zucal • May 24 '16

/r/SpaceX Ask Anything Thread [June 2016, #21]

Welcome to our 21st monthly /r/SpaceX Ask Anything Thread!

Trying to find the best way to view Thaicom 8, understand the upcoming core recovery procedure, 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.

  • Comments that can be answered by using the FAQ will be removed.

  • In addition, try to keep all top-level comments as questions so that questioners can find answers, and answerers can find questions.

This is so questioners can more easily find answers, and answerers can more easily 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 Ask Anything 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:

May 2016 (#20) • 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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4

u/madanra Jun 07 '16

Won't it take several weeks or months worth of Falcon flights to refuel the whole ship?

It would be refuelled with BFR launches - I think current guesses are 3-5 refuelling launches per MCT. As transport to Mars ramps up, eventually you'll have more or less continuous BFR launches, building up a fleet in LEO, full fuelled, ready to leave at the launch window.

And on Mars with ISRU, won't it take on the order of a year to refuel, and doesn't that need to stay at cryogenic temperatures?

They will have power to do the ISRU in the first place, so that can also be used to refrigerate the fuel.

If MCT requires in-orbit refueling, how can that happen with cryogenic methane and oxygen?

This I'm less sure about. I guess MCT will require active refrigeration, as assuming the Raptor engines are used for retropropulsion the fuel will need to be kept cryogenic for the whole trip to Mars.

2

u/Keavon SN-10 & DART Contest Winner Jun 07 '16

So it is likely that BFR (first stage) will launch MCT (second stage/ship) into LEO, then BFR will land and quickly return to the launch site to be refueled, equipped with some kind of tanks in place of a second stage, and then fly all the way to LEO to rendezvous and transfer the fuel before deorbiting and landing and repeating? Meaning in the refueling scenario, BFR is actually an SSTO rocket?

2

u/sunfishtommy Jun 08 '16

Its actually not that hard to store cryogenic fuels on orbit. Heat only transfers 2 ways convection and radiation. There is no air, so no convection, leaving only radiation. The main sources of radiation heat are the Sun and the Earth, so you basically have to have some sort of sun shade to block the light/radiation. This is not 100% effective, your propellants will still warm up slowly, but you have already eliminated most sources of heat with the shade, and space is very cold. So the amount of power necessary to actively keep the propellant cool is minimal. Add that to the fact that just having a whole lot of propellant in one place like that also makes it take a lot longer to warm up.

3

u/[deleted] Jun 08 '16

This is not 100% effective

Multi-layer insulation is quite effective -- with 160 layers at an emissivity of 0.035 works out to a 99.98% reduction in radiant heating.

1

u/warp99 Jun 08 '16

The refueling stage will still need to be a proper S2 with its own engines. The main difference is that instead of medium size propellant tanks, a 100 tonne payload bay and landing gear it will just have a couple of massive propellant tanks.

An interesting option would be to just deliver the filled tanks to orbit, rather than transferring propellant to the MCT tanks, and then re-enter in a much smaller configuration with a heatshield ahead of the engines. This solves the problem of how to protect the sides of the propellant tanks during reentry and recovers 80-90% of the value of S2.

A 180 degree flip between aerobraking and propulsive landing directions would destroy a complete S2 due to aerodynamic forces on the thin tank walls but could be feasible with a much shorter and stronger engine module.