Author Topic: The best method to transfer Methalox Fuel in space?  (Read 23203 times)

Offline Lar

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Re: The best method to transfer Methalox Fuel in space?
« Reply #120 on: 03/04/2018 05:38 AM »
pretty sure I'm not exactly sure what is being argued about.
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Offline john smith 19

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Re: The best method to transfer Methalox Fuel in space?
« Reply #121 on: 03/04/2018 09:32 AM »
Because you spent hundreds of dollars per pound getting what you're venting up there. Better to recirculate it back to the tanker to keep THAT pressurized.

Which has to be set against the mass of the pump.
Which are going to be a)Pretty damm big and b)Need a shedload of power to run.
Assuming a O/F ratio of 3.5:1 the LOX pump (with a density of about subcooled of 1200Kg/m^3) pumps 45370 US gallons a minute.

AFAIK filling F9 tanks takes about an hour on the pad, not a minute with electric pumps.

Obviously it depends on how long the rendezvous and docking process but worst case is for a 1 orbit propellant transfer IE up, dock, load, undock, down over a 90 minute period. 30 mins to RV & dock, 40-50 mins to transfer, 10 mins to undock and execute deorbit burn maybe?

I'll also note John Carmacks comment that a delta P of 4psi was enough to move fuel between 2 tanks of a 4 tank cluster on one of Armadillo Aerospaces test vehicles (demonstrating close pressure monitoring is needed, even on small vehicles. Yet another case of "neglecting the details will come back to bite you").

If differential pressure is good enough to get the job done in time that would seem to be the best way to do it.
« Last Edit: 03/04/2018 09:34 AM by john smith 19 »
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Offline Nomadd

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Re: The best method to transfer Methalox Fuel in space?
« Reply #122 on: 03/04/2018 03:24 PM »
 I was thinking of this thread while I was pondering a filling soda cup at the convenience store yesterday, that was foaming so much it was taking forever. People were staring at me. It's probably a good thing they didn't know what was going on in my head.

Offline speedevil

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Re: The best method to transfer Methalox Fuel in space?
« Reply #123 on: 03/04/2018 03:41 PM »
I was thinking of this thread while I was pondering a filling soda cup at the convenience store yesterday, that was foaming so much it was taking forever. People were staring at me. It's probably a good thing they didn't know what was going on in my head.

Foaming and such is a concern if you let the source tank get low enough pressure for it to approach its boiling point.
There is no dissolved gas hopefully!

Other than the extra weight of 10m or so of pipe, there is little reason to fill the receiving tanks from above the liquid level.
Also, though a 25cm pipe seems big a hosepipe into the bottom of a 5 gallon bucket is a much higher relative flow. Though is not of course in miligee, it's a range with a diffusor basket on the end that the output flow is quite gentle.

Offline rakaydos

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Re: The best method to transfer Methalox Fuel in space?
« Reply #124 on: 03/04/2018 04:23 PM »
Hypothetical: One or more of the main raptor turbopumps has an alternateplumbing connection to be used for refueling.

Use: Pump pressurant out of recieving tanks into source tanks to create pressure differential for fuel transfer.

How hard would it be to keep ahead of that kind of pressure differential, with fuel/oxygen flowing out of the tubes we have seen?

Offline speedevil

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Re: The best method to transfer Methalox Fuel in space?
« Reply #125 on: 03/04/2018 04:38 PM »
Hypothetical: One or more of the main raptor turbopumps has an alternateplumbing connection to be used for refueling.

Use: Pump pressurant out of recieving tanks into source tanks to create pressure differential for fuel transfer.

How hard would it be to keep ahead of that kind of pressure differential, with fuel/oxygen flowing out of the tubes we have seen?

The turbopumps are sized entirely wrongly for pumping gas, and will not work at all, if that's what you're meaning.

You need around 100kW if you want to pump pressurant gas.

There absolutely has to be already a system for boiling the pressurant - because the pressurant volume varies by over 10* during the mission, and the propellant will have a substantial amount of pressurant condensing in it as the pressurant cools due to ideal gas law during expansion (even if you were OK with a tiny fraction of the tank pressure as the tanks empty).

Some of this will be taken care of from aerodynamic heating and solar input, but you have to have some base system to maintain the temperature and pressure of the pressurant, and to generate more gas as the mission goes on, or the tank pressure starts to vary lots.

Simply varying the setpoint for this system up on one vehicle and down on the other before docking may be adequate.
Depending on the design, it's quite plausible that the pressure on the receiving tank, once any significant flow and turbulence at all happens will crash to near zero, without any venting, even with nominal flow, if the propellant is subcooled.

Meaning there is no need for venting or losses, to generate even quite large pressures, your concern becomes repressurising the tanks at the end.
But if there are landing tanks, as in BFS, this concern goes away as you can just use those tanks for the initial few seconds of the burn until you bring the main tanks up to pressure again.



Offline Nomadd

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Re: The best method to transfer Methalox Fuel in space?
« Reply #126 on: 03/04/2018 05:06 PM »

Other than the extra weight of 10m or so of pipe, there is little reason to fill the receiving tanks from above the liquid level.

If they're using the same tank ports for both filling and receiving, docked tail to tail and using ullage thrusters to settle the supply end, the receiving end would have the liquid settled at the top of the tanks and would be getting their supply dumped into the gas end. It would be the simplest plumbing wise unless the valves would make it too complicated. Would you rather deal with more valves or more pipes and tank ports?

Offline Semmel

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Re: The best method to transfer Methalox Fuel in space?
« Reply #127 on: 03/04/2018 08:08 PM »
In my option, the best solution is the one that is simplest. And that answer was already given by Musk:
Connect the two ships. Put a small ullage acceleration on it, you dont need much ullage, 1mm/s^2 is enough. The RCS system is easily capable of that. Pressurize the draining tank and vent the receiving tank to produce a pressure differential. Then the fuel will flow right from one tank into the other. No pumps needed. No special hardware needed.

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