### Author Topic: The best method to transfer Methalox Fuel in space?  (Read 22703 times)

#### JasonAW3

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #40 on: 08/07/2017 08:19 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.

You could try a flywheel system to initiate rotation with no fuel expenditure.

Assuming for the moment that you used a set of flywheels, mechanically disconnected from the space craft, you could use electromagnets to spin up the fueling craft's flywheel to the desired speed, (assuming that one were to use a magnetic bearing or air-bearing system to mechanically isolate it from the space craft themselves) while inducing a counter-rotation of the fueling craft. (Action / Reaction).

Fuel could be then transferred, and once the transfer is completed, the space crafts could be de-spun by using the electromagnets as brakes, killing the spin momentum and generating electricity.

As the mass of fuel is transferred, so to would rotational momentum. The fueling ship would, at first, be the center of rotation between the two ships, and at the end of fueling, the center of rotation would become the other space craft being fueled.  This would require both craft, after detaching from each other, to use their flywheels to de-spin themselves.  As the fueling craft will still have some of the rotational momentum initially induced, it will require less spin down that the fueled ship.  (The fueled ship would have less rotational momentum than the fueling ship, although more than the now empty fueling ship).

I know that this is a very basic explanation of this thought, but I've been slammed before for not fulling explaining my ideas.  (Besides, this isn't a new concept, just a different application of an old concept.
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#### llanitedave

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #41 on: 08/07/2017 08:38 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.

Settling only takes a few seconds at most.  Refueling a huge tank could take hours.  Continuous acceleration during the entire fueling period?  Still seems like a waste of fuel, because at the end of it, the tanker still has to turn around and return to its base.  With rotation, you expend just enough energy to start the motion, you don't have to keep expending fuel to maintain it.
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#### Patchouli

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #42 on: 08/07/2017 08:44 PM »
Another way to settle the propellants might be to use gravity gradient with a tether.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19850008697.pdf

Also mentioned here along with other interesting solutions included taking advantage of lox being paramagnetic.
« Last Edit: 08/07/2017 08:45 PM by Patchouli »

#### rakaydos

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #43 on: 08/07/2017 09:31 PM »
The 69 longituninal slow spin seems to me like the most logically sound approach for  long duration fuel transfers. 5 seconds RCS to spin up and spin down, vs 5 hours of ullage thrust, stable rotational moment with a predictable drainable point in all tanks.

#### Jim

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #44 on: 08/07/2017 10:21 PM »
The 69 longituninal slow spin seems to me like the most logically sound approach for  long duration fuel transfers. 5 seconds RCS to spin up and spin down, vs 5 hours of ullage thrust, stable rotational moment with a predictable drainable point in all tanks.

That is neither logical nor sound.  You don't know the actual configuration of the vehicle, size of thrusters, location of the tanks, the rotation moment is not stable.

The transfer of propellants will involve venting, which will be a free source of propulsion. Which makes thrusting more "logical".  It more "sound" because all the previous data.

#### Peter.Colin

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #45 on: 08/07/2017 10:48 PM »
The 69 longituninal slow spin seems to me like the most logically sound approach for  long duration fuel transfers. 5 seconds RCS to spin up and spin down, vs 5 hours of ullage thrust, stable rotational moment with a predictable drainable point in all tanks.

That is neither logical nor sound.  You don't know the actual configuration of the vehicle, size of thrusters, location of the tanks, the rotation moment is not stable.

The transfer of propellants will involve venting, which will be a free source of propulsion. Which makes thrusting more "logical".  It more "sound" because all the previous data.

Should venting occur, which I doubt, the vent opening could be placed to slightly increase the 69 latitudinal spin.
This should increase the settling force a little.
Even if the center of rotation changes towards the being filled ship or tanker, the propellant tanks still maintain a stable and predictable drainage and filling point.
« Last Edit: 08/07/2017 10:56 PM by Peter.Colin »

#### KelvinZero

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #46 on: 08/07/2017 10:52 PM »
Can anyone give some numbers on the efficiency of using acceleration? What would be the delta v over the entire operation? Is this delta-v wasted, or a tiny nudge in the direction you were going anyway? Is it totally negligible?

Until someone gives numbers, for all I know it could be as small as moving the length of the tank during the refueling operation. Some simple numbers might make discussion of spinning and so on obviously not worth the bother.

#### Gliderflyer

##### Re: The best method to transfer Methalox Fuel in space?
« Reply #47 on: 08/07/2017 11:07 PM »
Can anyone give some numbers on the efficiency of using acceleration? What would be the delta v over the entire operation? Is this delta-v wasted, or a tiny nudge in the direction you were going anyway? Is it totally negligible?

Until someone gives numbers, for all I know it could be as small as moving the length of the tank during the refueling operation. Some simple numbers might make discussion of spinning and so on obviously not worth the bother.
According to this paper, it doesn't look that bad:

~10 lb/hr with a 100 mT hydrogen stage at 10^-5 g. The BFS is a lot bigger and uses a different prop, but you are still looking at only hundreds of pounds per hour. Any sort of grappling system and spin up/down propellant is going to weigh more than that.
« Last Edit: 08/07/2017 11:08 PM by Gliderflyer »
I tried it at home

#### intrepidpursuit

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #48 on: 08/07/2017 11:40 PM »
If two craft with dramatically changing mass and centers of mass are connected passively at a single point the stresses on the point would be huge. Now you have to design the whole ship around a new loading problem.

When spinning the tanks the fuel will be on the outside of both ships, so there will have to be extremely powerful dedicated pumps to move the fuel from one to the other. My physics is rusty, but if the fuel is a significant fraction of the two ships together, then the power used to accelerate them shouldn't be much more than the power used to pump the fuel. And, as Jim keeps pointing out, boil-off or gaseous forms of the fuels can be used to provide a portion of that thrust.

I don't see why this wheel needs to be reinvented.

#### Jim

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #49 on: 08/08/2017 12:11 AM »
The 69 longituninal slow spin seems to me like the most logically sound approach for  long duration fuel transfers. 5 seconds RCS to spin up and spin down, vs 5 hours of ullage thrust, stable rotational moment with a predictable drainable point in all tanks.

That is neither logical nor sound.  You don't know the actual configuration of the vehicle, size of thrusters, location of the tanks, the rotation moment is not stable.

The transfer of propellants will involve venting, which will be a free source of propulsion. Which makes thrusting more "logical".  It more "sound" because all the previous data.

Should venting occur, which I doubt, the vent opening could be placed to slightly increase the 69 latitudinal spin.
This should increase the settling force a little.
Even if the center of rotation changes towards the being filled ship or tanker, the propellant tanks still maintain a stable and predictable drainage and filling point.

There is no doubt, there will venting.  It is a given for the receiving system.

Also, no pumps are needed.  Just pressure.

Spin is not going to happen.  The whole eco system:. tanker, receiver, prop depot, etc will be based on low thrust.  It is stupid to rotate the base and tanker, so why rotate the tanker and receiver.

What says tanker, receiver or base are all the same size?. That is another reason for thrusting.
« Last Edit: 08/08/2017 12:16 AM by Jim »

#### Coastal Ron

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #50 on: 08/08/2017 12:16 AM »
If two craft with dramatically changing mass and centers of mass are connected passively at a single point the stresses on the point would be huge. Now you have to design the whole ship around a new loading problem.

Just as a reminder, it appears that the approach SpaceX plans to use with the ITS is to attach two spacecraft side by side and then rotate them. With such a technique you don't have to worry where the center of rotation is, although you would have to use pumps to transfer liquids.
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#### Lars-J

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #51 on: 08/08/2017 12:21 AM »
If two craft with dramatically changing mass and centers of mass are connected passively at a single point the stresses on the point would be huge. Now you have to design the whole ship around a new loading problem.

Just as a reminder, it appears that the approach SpaceX plans to use with the ITS is to attach two spacecraft side by side and then rotate them. With such a technique you don't have to worry where the center of rotation is, although you would have to use pumps to transfer liquids.

Side by side, yes (as seen in video), but where do you get the rotating from? Sideways thrusting would work.

(Spinning it also adds a complication of working against the spin gravity)
« Last Edit: 08/08/2017 12:22 AM by Lars-J »

#### Req

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #52 on: 08/08/2017 12:29 AM »
If two craft with dramatically changing mass and centers of mass are connected passively at a single point the stresses on the point would be huge. Now you have to design the whole ship around a new loading problem.

Just as a reminder, it appears that the approach SpaceX plans to use with the ITS is to attach two spacecraft side by side and then rotate them. With such a technique you don't have to worry where the center of rotation is, although you would have to use pumps to transfer liquids.

I don't remember rotation being explicitly stated or implied.  Is that just an assumption you made or am I not remembering something?  "Attaching two spacecraft side by side" seems like the easiest way to attach them no matter what you're going to be doing, so I think that continually asserting that rotation is obvious by this orientation is a bit off.

If ullage thrusting is used, a couple pumps near the bottom of the tanker is all that's needed, and is likely less weight and almost definitely less complexity than docking the spacecraft in some strange nose-nose or nose-engine orientation to allow for "passive" refueling via ullage thrusting.

Edit - Didn't see Lars-J's post - So some of that is redundant.
« Last Edit: 08/08/2017 12:32 AM by Req »

#### intrepidpursuit

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #53 on: 08/08/2017 12:29 AM »
If two craft with dramatically changing mass and centers of mass are connected passively at a single point the stresses on the point would be huge. Now you have to design the whole ship around a new loading problem.

Just as a reminder, it appears that the approach SpaceX plans to use with the ITS is to attach two spacecraft side by side and then rotate them. With such a technique you don't have to worry where the center of rotation is, although you would have to use pumps to transfer liquids.

Side by side, yes (as seen in video), but where do you get the rotating from? Sideways thrusting would work.

(Spinning it also adds a complication of working against the spin gravity)

Of course you have to worry about where the center of rotation is. The stress is there exists no matter how the vehicles are docked.

#### Jim

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #54 on: 08/08/2017 12:32 AM »
If two craft with dramatically changing mass and centers of mass are connected passively at a single point the stresses on the point would be huge. Now you have to design the whole ship around a new loading problem.

Just as a reminder, it appears that the approach SpaceX plans to use with the ITS is to attach two spacecraft side by side and then rotate them. With such a technique you don't have to worry where the center of rotation is, although you would have to use pumps to transfer liquids.

I don't remember rotation being explicitly stated or implied.  Is that just an assumption you made or am I not remembering something?  "Attaching two spacecraft side by side" seems like the easiest way to attach them no matter what you're going to be doing, so I think that continually asserting that rotation is obvious by this orientation is a bit off.

If ullage thrusting is used, a pump near the bottom of the tanker is all that's needed, and is likely less weight and almost definitely less complexity than docking the spacecraft in some strange nose-nose or nose-engine orientation to allow for "passive" refueling via ullage thrusting.

No pump is needed.

#### intrepidpursuit

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #55 on: 08/08/2017 12:34 AM »
If two craft with dramatically changing mass and centers of mass are connected passively at a single point the stresses on the point would be huge. Now you have to design the whole ship around a new loading problem.

Just as a reminder, it appears that the approach SpaceX plans to use with the ITS is to attach two spacecraft side by side and then rotate them. With such a technique you don't have to worry where the center of rotation is, although you would have to use pumps to transfer liquids.

I don't remember rotation being explicitly stated or implied.  Is that just an assumption you made or am I not remembering something?  "Attaching two spacecraft side by side" seems like the easiest way to attach them no matter what you're going to be doing, so I think that continually asserting that rotation is obvious by this orientation is a bit off.

If ullage thrusting is used, a pump near the bottom of the tanker is all that's needed, and is likely less weight and almost definitely less complexity than docking the spacecraft in some strange nose-nose or nose-engine orientation to allow for "passive" refueling via ullage thrusting.

It isn't going to use the main engines for ullage  thrusting. It will use RCS engines anyway so it would have side-mounted thrusters. It could fire side-mounted thrusters and the fuel would passively transfer from one vehicle to the other with very little complexity compared to these other methods.

#### Req

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #56 on: 08/08/2017 12:49 AM »
Obviously RCS would be used, has the term "ullage thrusting" applied to main engines ever in anything?

Why are no pumps needed?  It'll either have to fill the tanks from the bottom or run up a tube along the side to fill from the top.  Why doesn't the ullage thrusting cause head pressure which fights against the "passive filling" at some point in the transfer process?

The bottom drilled overflows in my fish tanks won't push water through vinyl hoses if I grab one and hold it's end above the water level in the tank, so what gives?
« Last Edit: 08/08/2017 01:02 AM by Req »

#### Kenp51d

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #57 on: 08/08/2017 02:07 AM »
Is the plan for the "ullage thrusting" to push sideways through both vessels? If so then if the tanker is on top basically because of parallel thrusting through the sides, then the induced micro g would cause the fluids to flow down hill to the recipient ship.

Hope I did not cause anybody's head to hurt like trying to describe this in just words. A picture would make it simple. And That ain't happening from my phone. If anyone picked up on what I meant and draws a pic, Thank you before hand.
ll > the 2 ll's are the parallel ships, and the thrust is this way >

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#### envy887

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #58 on: 08/08/2017 02:13 AM »
Obviously RCS would be used, has the term "ullage thrusting" applied to main engines ever in anything?

Why are no pumps needed?  It'll either have to fill the tanks from the bottom or run up a tube along the side to fill from the top.  Why doesn't the ullage thrusting cause head pressure which fights against the "passive filling" at some point in the transfer process?

The bottom drilled overflows in my fish tanks won't push water through vinyl hoses if I grab one and hold it's end above the water level in the tank, so what gives?

It does if you boil the tank. The transfer is driven by pressure differential.

The ullage thrusting is measured in fractions of a micro-g. It's completely irrelevant for anything other than settling - it does not cause any significant flow.

#### Req

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##### Re: The best method to transfer Methalox Fuel in space?
« Reply #59 on: 08/08/2017 02:28 AM »
Obviously RCS would be used, has the term "ullage thrusting" applied to main engines ever in anything?

Why are no pumps needed?  It'll either have to fill the tanks from the bottom or run up a tube along the side to fill from the top.  Why doesn't the ullage thrusting cause head pressure which fights against the "passive filling" at some point in the transfer process?

The bottom drilled overflows in my fish tanks won't push water through vinyl hoses if I grab one and hold it's end above the water level in the tank, so what gives?

It does if you boil the tank. The transfer is driven by pressure differential.

The ullage thrusting is measured in fractions of a micro-g. It's completely irrelevant for anything other than settling - it does not cause any significant flow.

Gotcha.  Is it completely apparent/proven that maintaining that pressure differential is preferable to some pumps in terms of mass/complexity/risk/cost/etc at the desired flow rates?
« Last Edit: 08/08/2017 02:30 AM by Req »

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