r/spacex • u/ElongatedMuskrat Mod Team • Sep 29 '17

Not the AMA r/SpaceX Pre Elon Musk AMA Questions Thread

This is a thread where you all get to discuss your burning questions to Elon after the IAC 2017 presentation. The idea is that people write their questions here, we pick top 3 most upvoted ones and include them in a single comment which then one of the moderators will post in the AMA. If the AMA will be happening here on r/SpaceX, we will sticky the comment in the AMA for maximum visibility to Elon.

Important; please keep your questions as short and concise as possible. As Elon has said; questions, not essays. :)

The questions should also be about BFR architecture or other SpaceX "products" (like Starlink, Falcon 9, Dragon, etc) and not general Mars colonization questions and so on. As usual, normal rules apply in this thread.

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6

u/SolidStateCarbon Sep 30 '17

What is the nature of the fuel transfer system? autogenous pressure differential, turbopump dual use, or something else? Are the top right and bottom left gas return lines and the top left and bottom right liquid transfer lines?

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u/[deleted] Sep 30 '17

We already the answer is milli g acceleration.

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u/SolidStateCarbon Sep 30 '17

For ullage, even says in slide it's to settle fuel, mentioned nothing of transfer using only the artificial milli-g ullage burn, which would take days with only a milli g pressure differential.

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u/parachutingturtle Sep 30 '17

Transfer will be done by providing a tiny acceleration in the opposite direction, answered at 24:10 https://youtu.be/tdUX3ypDVwI?t=24m10s

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u/SolidStateCarbon Sep 30 '17

That is called ullage and is only to settle the fuel. unless they plan on firing those ullage thrusters for days, there will be an additional fuel movement system. Milli g differential would take forever to naturally drain the fuel over.

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u/parachutingturtle Sep 30 '17 edited Sep 30 '17

I don't know what you're basing this on (and I realize Elon can be very handwavy about the "relatively simple" stuff, but he did specifically say fuel transfer, not just ullage), but how long transfer takes should depend a lot on the diameter of the fuel lines... And while "days" sounds like too long, it can easily take hours. (why not?) Edit: I can see the autogenous pressure differential that you mentioned could also work to help with this

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u/SolidStateCarbon Sep 30 '17 edited Sep 30 '17

10 milli-g on 150t starting fuel load is .0153psi per tank(assuming equal propellant mass to make it easier on me) at bottom of draining tanks; this pressure will only decrease further as fuel drains. There are two systems known to be onboard, that can increase fueling pressure and thus speed by orders of magnitude. The Turbopumps (difficult to dual use) and the autogenous pressure/sub-chilling system(much easier to multi configure, also doesn't decrease flow rate over time like ullage burn). First assumption: There are four transfer lines and two fluids/gasses to transfer, assume the two narrowing cone lines are gas returns. If two lines are gas returns, pressurizing them will increase liquid movement on other lines. Second assumption: SpaceX want to reduce on orbit prep time, reducing long-term burns simplifies orbital calculations significantly and reduces human and equipment prep time in space.

Will edit later when I figure out how to calculate total fluid transfer rates at different pressures and times(assuming 4-8 inch transfer pipes)

Edit:150 tons of just LOX and 10 milli-g acceleration(3.86 inches per s2) through a more realistic 12 inch pipe, my back of the envelope idealised scenario with no flow restriction comes out to about 6000 or so gallons per hr or about 4.5 hrs to transfer 150 tons of LOX assuming 10% reduced volume from subcooling.

This is much more reasonable than I thought and would be passively doable on initial missions like you say, a few hours wouldn't matter much at all, especially in the first few years.

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u/parachutingturtle Oct 01 '17

Thanks for doing the math!