Author Topic: The best method to transfer Methalox Fuel in space?  (Read 9700 times)

Offline ChrML

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Re: The best method to transfer Methalox Fuel in space?
« Reply #20 on: 08/06/2017 12:36 AM »
Quote from: jak Kennedy link=topic=43522.msg1710332#msg1710332
The problem I see with this is rockets tanks are usually not empty. The are stringers and other supports inside otherwise the wall thickness would have to increase to give the tank enough rigidity.
Good point.

Hence separate tanks for fuel to be transferred on top of the craft and non-pressurised should help reduce the load. Cylindrical shape will increase strength of outer wall, with the biggest weakness being the bottom, so it should probably be rounded in bottom (even if not all fuel can squeeze out). Bottom will have to sustain ~200 ton at 5-10G, which can be reinforced from below.

Offline Torbjorn Larsson, OM

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Re: The best method to transfer Methalox Fuel in space?
« Reply #21 on: 08/06/2017 02:41 AM »
Not optimized for transfer time but mass use and allowing the ITS side by side image: can you divert flow after the rocket engine pump but before the combustion chamber? (I assume you do not need acceleration for pumping these propellants, were they not self-pressurized?) Would also have much of the machinery well tested before (re)use in orbit. One assumption could be respectively pump flows balanced for combustion use in the Raptor engines.

EDIT: More precise language.
« Last Edit: 08/06/2017 02:46 AM by Torbjorn Larsson, OM »

Offline Ionmars

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Re: The best method to transfer Methalox Fuel in space?
« Reply #22 on: 08/06/2017 02:42 AM »
Quote from: jak Kennedy link=topic=43522.msg1710332#msg1710332
The problem I see with this is rockets tanks are usually not empty. The are stringers and other supports inside otherwise the wall thickness would have to increase to give the tank enough rigidity.
Good point.

Hence separate tanks for fuel to be transferred on top of the craft and non-pressurised should help reduce the load. Cylindrical shape will increase strength of outer wall, with the biggest weakness being the bottom, so it should probably be rounded in bottom (even if not all fuel can squeeze out). Bottom will have to sustain ~200 ton at 5-10G, which can be reinforced from below.
If I were a tank designer, I would want to place nibs at strategic locations around the interior tank wall. I would install logerons and stringers between the nibs, leaving a small gap between the structural supports and the wall. By this means, propellant fluid could flow along the smooth tank wall in any direction.
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Online douglas100

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Re: The best method to transfer Methalox Fuel in space?
« Reply #23 on: 08/06/2017 04:21 PM »
I don't think spinning is necessary. As already pointed out it causes sloshing complications. The prop can be settled by ullage thrusters then transferred using differential pressure. Essentially same the way ULA planned to refuel ACES (but scaled up somewhat!)
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Offline Jim

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Re: The best method to transfer Methalox Fuel in space?
« Reply #24 on: 08/06/2017 07:39 PM »
Thrusting is the way, that was determined long ago.

Offline DreamyPickle

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Re: The best method to transfer Methalox Fuel in space?
« Reply #25 on: 08/07/2017 08:27 AM »
Thrusting is the way, that was determined long ago.

Can you please elaborate? Why is thrusting so much better than other options?

As far as I know transfer of cryogenic propellants in orbit has never been done before so the engineering is not well established.

Online savuporo

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Re: The best method to transfer Methalox Fuel in space?
« Reply #26 on: 08/07/2017 08:33 AM »
As far as I know transfer of cryogenic propellants in orbit has never been done before so the engineering is not well established.

Settling of cryogenic propellants on orbit has been done though.
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Offline Ionmars

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Re: The best method to transfer Methalox Fuel in space?
« Reply #27 on: 08/07/2017 09:40 AM »
From Wikipedia: Orbital Express Project 2007 (DARPA and NASA):
"The project hoped to demonstrate several satellite servicing operations and technologies including rendezvous, proximity operations and station keeping, capture, docking, fluid transfer (specifically, hydrazine on this mission), and ORU (Orbit Replaceable Unit) transfer.......The fluid (fuel) and ORU (battery) transfers were completed successfully at the lowest levels of spacecraft autonomy."
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Offline Jim

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Re: The best method to transfer Methalox Fuel in space?
« Reply #28 on: 08/07/2017 01:43 PM »
As far as I know transfer of cryogenic propellants in orbit has never been done before so the engineering is not well established.

Settling of cryogenic propellants on orbit has been done though.

exactly

Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #29 on: 08/07/2017 03:40 PM »
As far as I know transfer of cryogenic propellants in orbit has never been done before so the engineering is not well established.

Settling of cryogenic propellants on orbit has been done though.

exactly


But not the transfer.
How hard can it be?
Its perceived to be harder in the ITS presentation than producing it on Mars.


Hydrazine is not cryogenic it has a higher boiling point than water.

Offline rakaydos

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Re: The best method to transfer Methalox Fuel in space?
« Reply #30 on: 08/07/2017 03:49 PM »
Is thrust ullage for the entire duration of a multi kiloton fuel transfer really easier than burning up (and down) a minor spin ullage?

From first principles, I would think that spin ullage would be a better fit for slower, longer duration fuel transfers, (thrust the whole time vs thrust once to spin up and once to spin down) which due to square/cube on transfer pipes should scale better to larger craft.
« Last Edit: 08/07/2017 03:55 PM by rakaydos »

Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #31 on: 08/07/2017 05:57 PM »
Is thrust ullage for the entire duration of a multi kiloton fuel transfer really easier than burning up (and down) a minor spin ullage?

From first principles, I would think that spin ullage would be a better fit for slower, longer duration fuel transfers, (thrust the whole time vs thrust once to spin up and once to spin down) which due to square/cube on transfer pipes should scale better to larger craft.

I like your first principles approach. First principals also say that a spinning clinder filled with liquid which is longer than it is wide, does not want to spin on its londitudinal axis.
The liquid will automatically settle on the top and bottom part of the cylinder, and the spin will reverse to a latitudinal one.
Two middle connected cilinders would naturally want to settle the liquid in opposing top bottom positions.
Maybe a nose - bottom "69" position and a spin around the latitudinal axis is better than both liquids can settle on the bottom.
A tethered nose to nose position is also possible but would require a long transfer hose, and making that automatically dock is probably more difficult than a side to side docking, but not impossible.
« Last Edit: 08/07/2017 06:01 PM by Peter.Colin »

Offline Jim

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Re: The best method to transfer Methalox Fuel in space?
« Reply #32 on: 08/07/2017 06:06 PM »
Is thrust ullage for the entire duration of a multi kiloton fuel transfer really easier than burning up (and down) a minor spin ullage?

From first principles, I would think that spin ullage would be a better fit for slower, longer duration fuel transfers, (thrust the whole time vs thrust once to spin up and once to spin down) which due to square/cube on transfer pipes should scale better to larger craft.

Thrusting is easier.  No attitude changes, no additional stresses, no worry about changing moments of inertia, etc.
Boil off gases can be used for the thrusting.

Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #33 on: 08/07/2017 06:09 PM »
Is thrust ullage for the entire duration of a multi kiloton fuel transfer really easier than burning up (and down) a minor spin ullage?

From first principles, I would think that spin ullage would be a better fit for slower, longer duration fuel transfers, (thrust the whole time vs thrust once to spin up and once to spin down) which due to square/cube on transfer pipes should scale better to larger craft.

Thrusting is easier.  No attitude changes, no additional stresses, no worry about changing moments of inertia, etc.
Boil off gases can be used for the thrusting.

You do realize that for maintaining the same acceleration constant for longer time you need to throttle up the trusters exponentially.

The thrusters can be shut off while spinning.

Seeing how long a Falcon 9 takes to fill up, it can take up a few hours to transfer the propellant
« Last Edit: 08/07/2017 06:20 PM by Peter.Colin »

Offline Jim

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Re: The best method to transfer Methalox Fuel in space?
« Reply #34 on: 08/07/2017 06:20 PM »

You do realize that for maintaining the same acceleration constant for longer time you need to throttle up the trusters exponentially.

There is no need for the same acceleration rate, just a constant acceleration.

But your point is wrong anyways.  There is no mass loss or gain.  The "system" is the two spacecraft and they are joined and the mass transfer is internal to the "system".  So the same thrust is the same acceleration.


The thrusters can be shut off while spinning.

No, they can't. A rotating system will have changing moments of inertia and likely require a need for additional thruster firings for control.
« Last Edit: 08/07/2017 06:26 PM by Jim »

Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #35 on: 08/07/2017 06:37 PM »
Same trust is not same acceleration (= not same g-force)


An object moving twice as fast has four times the energy.
going from 50km/h to a 100km/h takes 3 times more thrust than than going from 0km/h to 50km/h.
This adds up exponentially.
« Last Edit: 08/07/2017 06:45 PM by Peter.Colin »

Online Basto

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Re: The best method to transfer Methalox Fuel in space?
« Reply #36 on: 08/07/2017 06:46 PM »

You do realize that for maintaining the same acceleration constant for longer time you need to throttle up the trusters exponentially.

There is no need for the same acceleration rate, just a constant acceleration.

But your point is wrong anyways.  There is no mass loss or gain.  The "system" is the two spacecraft and they are joined and the mass transfer is internal to the "system".  So the same thrust is the same acceleration.


The thrusters can be shut off while spinning.

No, they can't. A rotating system will have changing moments of inertia and likely require a need for additional thruster firings for control.


Also a rotating craft would need additional thruster firings to stop the rotation after the transfer is complete. 

It seems like we are trying to solve a problem that has already been solved, as fuel transfer from two docked craft does not seem that different from fuel transfer to the engines. 



Offline Ictogan

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Re: The best method to transfer Methalox Fuel in space?
« Reply #37 on: 08/07/2017 07:23 PM »
Same trust is not same acceleration (= not same g-force)


An object moving twice as fast has four times the energy.
going from 50km/h to a 100km/h takes 3 times more thrust than than going from 0km/h to 50km/h.
This adds up exponentially.
No. The same amount of thrust will always cause the same amount of acceleration on a vehicle of the same mass, regardless of it's speed(ignoring relativity because you're not going to get anywhere near those speeds). Heck, your claim would mean that in different frames of reference, a vehicle would produce different amounts of thrust.

Edit: and your first sentence is literally saying that F=ma isn't true.
« Last Edit: 08/07/2017 07:25 PM by Ictogan »

Offline Jim

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Re: The best method to transfer Methalox Fuel in space?
« Reply #38 on: 08/07/2017 07:41 PM »
Same trust is not same acceleration (= not same g-force)

An object moving twice as fast has four times the energy.
going from 50km/h to a 100km/h takes 3 times more thrust than than going from 0km/h to 50km/h.
This adds up exponentially.

Doesn't matter for the reasons I stated.  The mass is not changing and there is no need to change thrust

Offline Coastal Ron

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Re: The best method to transfer Methalox Fuel in space?
« Reply #39 on: 08/07/2017 07:48 PM »
Also a rotating craft would need additional thruster firings to stop the rotation after the transfer is complete.

While that certainly could be true, it's possible that you could create fueling systems that undock while still rotating.

Quote
It seems like we are trying to solve a problem that has already been solved, as fuel transfer from two docked craft does not seem that different from fuel transfer to the engines.

I agree. We have enough history on this already to understand some of the solutions, and I'm sure there will be more than one way to do this safely and fuel efficiently.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

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