Author Topic: The best method to transfer Methalox Fuel in space?  (Read 9646 times)

Offline Peter.Colin

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What do you think is the best method to transfer Methalox fuel from one ship to another in space?
Some options to separate liquid from gas in the tank could be: spinning ships, accelerating ships, making use of a gravity gradient induced by a planet? magnetic force (liquid Oxygen is attracted by magnets), creating an internal vortex, etc.

Offline Coastal Ron

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Re: The best method to transfer Methalox Fuel in space?
« Reply #1 on: 08/05/2017 02:28 PM »
What do you think is the best method to transfer Methalox fuel from one ship to another in space?
Some options to separate liquid from gas in the tank could be: spinning ships, accelerating ships, making use of a gravity gradient induced by a planet? magnetic force (liquid Oxygen is attracted by magnets), creating an internal vortex, etc.

As an example of how a famous space transportation company plans to solve that, look at Elon Musk's ITS presentation from last year.

I would imagine that the simplest near-term method will use some form of artificial gravity, likely generated by rotation.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #2 on: 08/05/2017 02:44 PM »
Do you envision the rotation to settle the fuel at the bottom or at the side of the rocket?
« Last Edit: 08/05/2017 02:45 PM by Peter.Colin »

Offline Coastal Ron

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Re: The best method to transfer Methalox Fuel in space?
« Reply #3 on: 08/05/2017 03:23 PM »
Do you envision the rotation to settle the fuel at the bottom or at the side of the rocket?

Does it matter? It should only matter that where you settle it at you can have an intake pipe for pumping the fuel out. For the vessel receiving the fuel it shouldn't matter what the orientation is, although balance issues will probably need to be considered.
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Offline Nomadd

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Re: The best method to transfer Methalox Fuel in space?
« Reply #4 on: 08/05/2017 03:26 PM »
 Rotation could be a little tricky when your center of mass is constantly changing.

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Re: The best method to transfer Methalox Fuel in space?
« Reply #5 on: 08/05/2017 03:32 PM »
Last year three of us on NSF collaborated on a technical paper that addressed the problems of a large propellant depot in LEO, specifically to expedite the SpaceX Mars architecture for colonization. Many others also assisted.

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Re: The best method to transfer Methalox Fuel in space?
« Reply #6 on: 08/05/2017 04:03 PM »
The paper chose rotation as the method for settling propellants, but now I would favor acceleration.
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Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #7 on: 08/05/2017 04:59 PM »
The paper chose rotation as the method for settling propellants, but now I would favor acceleration.

Why would you favor acceleration over rotation?

Offline rakaydos

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Re: The best method to transfer Methalox Fuel in space?
« Reply #8 on: 08/05/2017 05:32 PM »
Two cyndrelical rockets docking belly to belly, then introducing a minor (sub-1 RPM) rotation to settle the tanks. Within a quarter rotation, all propellant in both tanks should be interacting with a tank wall and gaining the effects of centrifical force, pooling on the side of the tanks opposide the docking port. Then just pump fuel and oxyser into the opposite pressure vessel.

Offline yokem55

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Re: The best method to transfer Methalox Fuel in space?
« Reply #9 on: 08/05/2017 06:15 PM »
Two cyndrelical rockets docking belly to belly, then introducing a minor (sub-1 RPM) rotation to settle the tanks. Within a quarter rotation, all propellant in both tanks should be interacting with a tank wall and gaining the effects of centrifical force, pooling on the side of the tanks opposide the docking port. Then just pump fuel and oxyser into the opposite pressure vessel.
But then you have a shifting center of mass. Eventually you have a mostly full spacecraft and a  mostly empty spacecraft docked together with a center of mass mostly in the full one. It might work but it might put a big strain on the docking interface.

Offline rakaydos

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Re: The best method to transfer Methalox Fuel in space?
« Reply #10 on: 08/05/2017 06:30 PM »
Two cyndrelical rockets docking belly to belly, then introducing a minor (sub-1 RPM) rotation to settle the tanks. Within a quarter rotation, all propellant in both tanks should be interacting with a tank wall and gaining the effects of centrifical force, pooling on the side of the tanks opposide the docking port. Then just pump fuel and oxyser into the opposite pressure vessel.
But then you have a shifting center of mass. Eventually you have a mostly full spacecraft and a  mostly empty spacecraft docked together with a center of mass mostly in the full one. It might work but it might put a big strain on the docking interface.
A strain, yes, but it's proportional to the RPM of the spin. It only has to be enough to settle the fuel.

I see your concern about effectively incresing the arm of rotation by shifting the center of mass, but since it's caused by a reduction of mass on that arm... There's probably a bell curve that fits it. Worst case, you slow the rotation even further during part of the transfer.

Offline wes_wilson

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Re: The best method to transfer Methalox Fuel in space?
« Reply #11 on: 08/05/2017 06:32 PM »
We've had this conversation many times on these forums and there are many viable ideas.

I'm a proponent of nose to end connecting; gentle ullage giving unidirectional settling; the tanker is on "top"; and the ship to be filled is on "bottom"; the fuel simply settles from the tanker into the receiving ship. 


No pumps needed; mitigates risk; no shifting center of gravity issues; only ullage thrusters needed.

I know this isn't what was presented in the ITS presentation.

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Offline Coastal Ron

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Re: The best method to transfer Methalox Fuel in space?
« Reply #12 on: 08/05/2017 07:14 PM »
Rotation could be a little tricky when your center of mass is constantly changing.

Elon Musk was showing side to side docking for ITS, which pretty much makes where the center of rotation is irrelevant.

But then you have a shifting center of mass. Eventually you have a mostly full spacecraft and a  mostly empty spacecraft docked together with a center of mass mostly in the full one. It might work but it might put a big strain on the docking interface.

Aren't you starting with one empty spacecraft docking with one full one? So all that would be happening is that the center of rotation (which could be very slow) would just shift as the mass is pumped into the other vehicle. Sure, there could be large tankers mating with small spacecraft, or large spacecraft mating with small tankers. But the total allowable mass to be mated together would have already been calculated, as well as the allowable RPM for liquid transfer.

I'm not sure I see the concern.
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Offline Coastal Ron

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Re: The best method to transfer Methalox Fuel in space?
« Reply #13 on: 08/05/2017 07:19 PM »
The paper chose rotation as the method for settling propellants, but now I would favor acceleration.

If you're already planning to be accelerating towards a destination then this method would make a lot of sense. And it's not like rotational methods don't use any fuel, since they have to start the rotation of the two spacecraft at the very least, and depending on the fueling methods and docking systems they might have to de-rotate before undocking too. Spinning up and spinning down would consume some of the fuel that otherwise could have gone towards acceleration.

I'm not sure what the percentage of use cases are where that would be preferable, but it's worth remembering as an option.

Nice job on the AIAA paper too - it's one thing to talk about something on a forum, but it's a whole lot harder to write a paper with the intent to be published on AIAA.
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Re: The best method to transfer Methalox Fuel in space?
« Reply #14 on: 08/05/2017 08:09 PM »
The paper chose rotation as the method for settling propellants, but now I would favor acceleration.

If you're already planning to be accelerating towards a destination then this method would make a lot of sense. And it's not like rotational methods don't use any fuel, since they have to start the rotation of the two spacecraft at the very least, and depending on the fueling methods and docking systems they might have to de-rotate before undocking too. Spinning up and spinning down would consume some of the fuel that otherwise could have gone towards acceleration.

I'm not sure what the percentage of use cases are where that would be preferable, but it's worth remembering as an option.

Nice job on the AIAA paper too - it's one thing to talk about something on a forum, but it's a whole lot harder to write a paper with the intent to be published on AIAA.
I agree with your analysis.
In the case of the large depot, it consists of a lightweight framework with up to six spaceships attached to it, all in parallel with one another. Imagine another spacecraft approaching from "behind," intending to attach itself to an empty berth (parking space). As it approaches, it sees the framework and ships (the depot) as revolving around their common center. In order to park, it needs to match the motion of the empty berth. So it begins to spiral towards the depot whereby the radius of the spiral is a little greater than the radius of the spinning berth. Its rotational speed also matches the rotational speed of the depot. It comes to hover just "above" the empty berth. Now it must also revolve  on its own axis slowly, so that its dorsal surface with fuel connectors is always facing the berth. Then it can connect to the berth framework.

This maneuvering is a complex choreography using GNC thrusters. More important, it requires energy (fuel) that could be applied to a better use. So I now think it would be better to not rotate the depot, but allow spaceships to approach and park in a simpler manner. When it is time to transfer fuel, a slight acceleration (10E-5 g) would be applied to the depot, including the ship providing fuel and the ship receiving fuel. The direction of acceleration is important because we want to steer the depot towards a higher orbit that contributes toward TMI for all ships going to Mars. The others (presumably tankers) would return to Earth for reuse.


   
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Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #15 on: 08/05/2017 08:49 PM »
The paper chose rotation as the method for settling propellants, but now I would favor acceleration.

If you're already planning to be accelerating towards a destination then this method would make a lot of sense. And it's not like rotational methods don't use any fuel, since they have to start the rotation of the two spacecraft at the very least, and depending on the fueling methods and docking systems they might have to de-rotate before undocking too. Spinning up and spinning down would consume some of the fuel that otherwise could have gone towards acceleration.

I'm not sure what the percentage of use cases are where that would be preferable, but it's worth remembering as an option.

Nice job on the AIAA paper too - it's one thing to talk about something on a forum, but it's a whole lot harder to write a paper with the intent to be published on AIAA.
I agree with your analysis.
In the case of the large depot, it consists of a lightweight framework with up to six spaceships attached to it, all in parallel with one another. Imagine another spacecraft approaching from "behind," intending to attach itself to an empty berth (parking space). As it approaches, it sees the framework and ships (the depot) as revolving around their common center. In order to park, it needs to match the motion of the empty berth. So it begins to spiral towards the depot whereby the radius of the spiral is a little greater than the radius of the spinning berth. Its rotational speed also matches the rotational speed of the depot. It comes to hover just "above" the empty berth. Now it must also revolve  on its own axis slowly, so that its dorsal surface with fuel connectors is always facing the berth. Then it can connect to the berth framework.

This maneuvering is a complex choreography using GNC thrusters. More important, it requires energy (fuel) that could be applied to a better use. So I now think it would be better to not rotate the depot, but allow spaceships to approach and park in a simpler manner. When it is time to transfer fuel, a slight acceleration (10E-5 g) would be applied to the depot, including the ship providing fuel and the ship receiving fuel. The direction of acceleration is important because we want to steer the depot towards a higher orbit that contributes toward TMI for all ships going to Mars. The others (presumably tankers) would return to Earth for reuse.


 

Another method could be to temporarily rotate the fuel inside the large depot tank with a small electric motor connected to a mechanical stirrer inside the tank.
If there's no rotation of the fuel needed, the rotational energy could be converted back to electric energy trough regenerative braking.




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Re: The best method to transfer Methalox Fuel in space?
« Reply #16 on: 08/05/2017 09:16 PM »
...
...
Another method could be to temporarily rotate the fuel inside the large depot tank with a small electric motor connected to a mechanical stirrer inside the tank.
If there's no rotation of the fuel needed, the rotational energy could be converted back to electric energy trough regenerative braking.
It is not intuitively obvious to me that this would work. The interior motor would begin a rotational motion of the fluid, but it seems to me that its energetic action would also cause some droplets to spray into the empty portion of the tank, adding to fuel dispersion as well as conglomeration. 
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Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #17 on: 08/05/2017 09:50 PM »
...
...
Another method could be to temporarily rotate the fuel inside the large depot tank with a small electric motor connected to a mechanical stirrer inside the tank.
If there's no rotation of the fuel needed, the rotational energy could be converted back to electric energy trough regenerative braking.
It is not intuitively obvious to me that this would work. The interior motor would begin a rotational motion of the fluid, but it seems to me that its energetic action would also cause some droplets to spray into the empty portion of the tank, adding to fuel dispersion as well as conglomeration.

In a normal rotating tank, the wall rotates the gasious fuel and the rotating gasious fuel accelerates the droplets to the tank wall. (Nothing would happen if the droplets where floating in a vacuum inside the rotating tank)
This is similar with a mechanical stirrer, the initial spray into the empty portion of the tank will be accelerated by the gaseous fuel towards the rotating fuel at the tanks inner surface where it is captured.


Because the electric motor is connected to the tank wall, the tank will start rotating, in the opposite direction as the fuel.
To prevent this unwanted outside rotation a synchronized side-connected twin tank could keep the construction motionless in space.


Edit: I think the mechanical stirrer doesn't need to be big either, just a very small one would create a gasious vortex (tornado) which is enough to accelerate the liquid droplets to the side of the wall.
If the small vortex creating gas stirrer is placed more in the top of the cillinder the fluid would collect more at the bottom because of a small gas pressure gradient. (Fluid dynamics is complicated, they should try this at the ISS or in a Falcon 9...)
« Last Edit: 08/06/2017 06:28 AM by Peter.Colin »

Offline ChrML

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Re: The best method to transfer Methalox Fuel in space?
« Reply #18 on: 08/05/2017 11:51 PM »
I believe the easiest and least complex way to do this is by designing the tanker with a separate tank for filling the spacecraft (ie not shared with propulsion tank). This tank is made of some ALU compound and is shaped like a cylinder with a piston inside the tank, not pressurised and with some kind of drivebar in the middle for driving the piston. Motor and mechanics are sealed inside the tank, eg no movable parts that go into the tank. Piston ring seals around and a pressure valve for backfilling the other side of the piston with gas to prevent under-pressure.

The methane is easy as it doesn't burn without oxygen (like a car gasoline pump floating in petrol). The oxygen is perhaps a bit harder due to fire hazard.

The vehicle being filled requires no such device.

Then it's a simple matter of matching orbit, connect a hose and squeezing the content over to the other vehicle.


I believe rotation or acceleration will complicate matters more than wished.


Edit: Actually a much simpler design would simply be to leave the piston loose. Just push it down using pressurised methane or oxygen in gas form from the rear side (from boil-off). No center pole or motor needed.
« Last Edit: 08/05/2017 11:56 PM by ChrML »

Offline jak Kennedy

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Re: The best method to transfer Methalox Fuel in space?
« Reply #19 on: 08/06/2017 12:13 AM »
I believe the easiest and least complex way to do this is by designing the tanker with a separate tank for filling the spacecraft (ie not shared with propulsion tank). This tank is made of some ALU compound and is shaped like a cylinder with a piston inside the tank, not pressurised and with some kind of drivebar in the middle for driving the piston. Motor and mechanics are sealed inside the tank, eg no movable parts that go into the tank. Piston ring seals around and a pressure valve for backfilling the other side of the piston with gas to prevent under-pressure.

The methane is easy as it doesn't burn without oxygen (like a car gasoline pump floating in petrol). The oxygen is perhaps a bit harder due to fire hazard.

The vehicle being filled requires no such device.

Then it's a simple matter of matching orbit, connect a hose and squeezing the content over to the other vehicle.


I believe rotation or acceleration will complicate matters more than wished.


Edit: Actually a much simpler design would simply be to leave the piston loose. Just push it down using pressurised methane or oxygen in gas form from the rear side (from boil-off). No center pole or motor needed.

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.

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 »

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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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Offline 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.

Offline 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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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 »

Offline 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?

Offline 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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Offline 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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Offline 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.
http://www.ulalaunch.com/uploads/docs/published_papers/extended_duration/propellantdepots2009.pdf
« Last Edit: 08/07/2017 08:45 PM by Patchouli »

Offline 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.

Offline 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.

Offline 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 »

Offline 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.

Offline 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:
http://www.ulalaunch.com/uploads/docs/Published_Papers/Extended_Duration/SettledCryogenicPropellantTransfer.pdf

 ~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

Offline 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.

Offline 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 »

Offline 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.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline 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 »

Offline 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 »

Offline 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.

Offline 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.

Offline 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.

Offline 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 »

Offline 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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Online 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.

Offline 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 »

Offline Kenp51d

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Re: The best method to transfer Methalox Fuel in space?
« Reply #60 on: 08/08/2017 02:53 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.
Thanks for the correction. Micro g= micro flow. Duh the flow needs a bit of help.
I got super busy with multiple things at once and did not think it all the way through. Got hung up attempting to describe alignment.

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Offline Coastal Ron

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Re: The best method to transfer Methalox Fuel in space?
« Reply #61 on: 08/08/2017 05:39 AM »
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)

You have a point, I may have inferred that. In which case using ullage as many have suggested may be the what SpaceX is planning.

Also, no pumps are needed.  Just pressure.

For a simple system I can see how this would be an advantage, but it would appear to require venting gas from the receiving tank so that the gas in the supply tank can pump liquid into the receiving tank. Is that what you mean?

However if you connect the supply tank to the receiver tank with two connections (liquid-liquid, gas-gas), couldn't that be a closed loop system where the liquid and gas could be moved between tanks via pump? Maybe it would require active cooling, but it would seem to reduce gas loss. Not a lot of fuel stations in space, so just trying to see if there is a less wasteful way of refueling.

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.

I could be wrong (see top), but if the two bodies are not rotating on a fixed point, meaning the center of rotation changes as mass shifts between the two bodies, then my assumption was that because the total mass is the same that the rotational forces at the docking points should be the same too.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline Nathan2go

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Re: The best method to transfer Methalox Fuel in space?
« Reply #62 on: 08/08/2017 06:19 AM »
... 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?
Yes.  For two reasons:

1) the tanks must be made to withstand a fair amount of pressure to survive launch.  Just sitting on the pad, a 10 m tall water tank will have 1 atmosphere of hydro-static pressure at the bottom (i.e. the weight of the water), in addition to the pressure at the top.  Rocket tanks are also pressure stabilized, which means that extra pressure is applied to add structural rigidity.  I suspect the flight pressure is around 1 atmosphere, before adding the hydro-static pressure and the ambient outside pressure (the ambient pressure isn't felt on the pad since it's balanced, but the tank feels the lack of it in space).  If you listen to the technical announcers during a launch, they'll usually call out tanks being at flight pressure, just minutes before ignition.

2) tanks pressure must be actively managed on-orbit, via venting, to control propellant temperature (conversely pressure becomes a side-effect of temperature control, if a cryo-cooler is used).  For a liquid stored at it's boiling point, the boiling temperature is a direct function of the pressure (between the limits of T_freeze and T_critical).  So in order to sub-cool the propellants, they must be stored at somewhere between 0 and 1 atmosphere of pressure.  Of course larger pressures can be applied for a short time, but the propellant will gradually warm up (unless a cryo-cooler or really good passive cooling is used).

So you've probably got over 1 atmosphere of pressure differential to work with, without adding any additional equipment or cost (just software).  If your ullage acceleration is 1000 ugees, you can make the propellant flow uphill for about 29000 feet, with no pumps.

Note that as the propellant is transferred from the higher pressure tanks into the receiving tank with lower pressure, some "ullage" gas must be vented in the receiving tank to make room for the incoming propellant.  If you want to do no-vent propellant transfers, then you either have to use a cooler to condense the extra gas in the receiving tank, or pump it into the sending tank (thru more pipes, valves, and connectors).  Remember that ullage gas is 200x less dense than LOX, so throwing away a tank of ullage gas for each transfer is really only wasting 0.5% of your propellant.
« Last Edit: 08/08/2017 06:40 AM by Nathan2go »

Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #63 on: 08/08/2017 06:51 AM »
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:
http://www.ulalaunch.com/uploads/docs/Published_Papers/Extended_Duration/SettledCryogenicPropellantTransfer.pdf

 ~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.

This 10^-5 g is sufficient for settling and pumping out liquid.
Pumping out liquid infers almost no flow to the liquid in the vessel, pumping in liquid does.
For settling pumped in liquid there is no data, and that's probably why they purpose to do
an cryogenic transfer experiment.

Suppose 10^-2 g is the limit for keeping pumped in cryogenic Methalox settled (viscosity is important for every pumped in liquid, to slow down flow).

Than:

Acceleration of 10^-2 g for 5 hours is a delta V of 1.8 km per second.
if this is done by thrusters with low ISP the propellant loss is significantly more than with high ISP Raptor engines.

Until the lower g limit for pumping-in cryogenic Methalox is comfimed, discussions about rotation vs linear acceleration are relevant.
 
« Last Edit: 08/08/2017 06:59 AM by Peter.Colin »

Offline douglas100

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Re: The best method to transfer Methalox Fuel in space?
« Reply #64 on: 08/08/2017 08:05 AM »

According to this paper, it doesn't look that bad:
http://www.ulalaunch.com/uploads/docs/Published_Papers/Extended_Duration/SettledCryogenicPropellantTransfer.pdf

This 10^-5 g is sufficient for settling and pumping out liquid.
Pumping out liquid infers almost no flow to the liquid in the vessel, pumping in liquid does.
For settling pumped in liquid there is no data, and that's probably why they purpose to do
an cryogenic transfer experiment.

Suppose 10^-2 g is the limit for keeping pumped in cryogenic Methalox settled (viscosity is important for every pumped in liquid, to slow down flow).

Than:

Acceleration of 10^-2 g for 5 hours is a delta V of 1.8 km per second.
if this is done by thrusters with low ISP the propellant loss is significantly more than with high ISP Raptor engines.

Until the lower g limit for pumping-in cryogenic Methalox is comfimed, discussions about rotation vs linear acceleration are relevant.

You are assuming that the ullage thrusters burn continuously for 5 hours. This is totally unnecessary. They only pulse as required to keep the prop settled. Note also, it's only necessary to keep the prop settled in the tanks of the tanker during the transfer to ensure that it remains over the outlet. It doesn't matter if the prop is not settled in the receiving tank. It can be settled later after the spaceship and tanker have separated.

Also, cryogenic prop has already been kept settled for the order of five hours on Delta IV GSO missions. I think all the basic technology for the mass transfer of cryogenic prop is already in place. It requires development of course. But I don't see any obvious showstoppers. And I don't see the need for rotation.
Douglas Clark

Offline Req

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Re: The best method to transfer Methalox Fuel in space?
« Reply #65 on: 08/08/2017 08:37 AM »
... 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?
Yes.  For two reasons:

1) the tanks must be made to withstand a fair amount of pressure to survive launch.  Just sitting on the pad, a 10 m tall water tank will have 1 atmosphere of hydro-static pressure at the bottom (i.e. the weight of the water), in addition to the pressure at the top.  Rocket tanks are also pressure stabilized, which means that extra pressure is applied to add structural rigidity.  I suspect the flight pressure is around 1 atmosphere, before adding the hydro-static pressure and the ambient outside pressure (the ambient pressure isn't felt on the pad since it's balanced, but the tank feels the lack of it in space).  If you listen to the technical announcers during a launch, they'll usually call out tanks being at flight pressure, just minutes before ignition.

2) tanks pressure must be actively managed on-orbit, via venting, to control propellant temperature (conversely pressure becomes a side-effect of temperature control, if a cryo-cooler is used).  For a liquid stored at it's boiling point, the boiling temperature is a direct function of the pressure (between the limits of T_freeze and T_critical).  So in order to sub-cool the propellants, they must be stored at somewhere between 0 and 1 atmosphere of pressure.  Of course larger pressures can be applied for a short time, but the propellant will gradually warm up (unless a cryo-cooler or really good passive cooling is used).

So you've probably got over 1 atmosphere of pressure differential to work with, without adding any additional equipment or cost (just software).  If your ullage acceleration is 1000 ugees, you can make the propellant flow uphill for about 29000 feet, with no pumps.

Note that as the propellant is transferred from the higher pressure tanks into the receiving tank with lower pressure, some "ullage" gas must be vented in the receiving tank to make room for the incoming propellant.  If you want to do no-vent propellant transfers, then you either have to use a cooler to condense the extra gas in the receiving tank, or pump it into the sending tank (thru more pipes, valves, and connectors).  Remember that ullage gas is 200x less dense than LOX, so throwing away a tank of ullage gas for each transfer is really only wasting 0.5% of your propellant.

Great post.  Thanks for going to the trouble of laying it all out.

Offline john smith 19

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Re: The best method to transfer Methalox Fuel in space?
« Reply #66 on: 08/08/2017 10:14 AM »
A few notes. There's quite a lot of history around this.

Propellant settling was done on Apollo on the LH2/LO2 upper stages. Later tests by ULA on Centaur stages confirmed you could get settling with thrust in the 10 micro g range and it didn't need to be constant, but could be pulsed. 

IIRC they said settled propellant could cut propellant venting 50%, greatly increasing on orbit life, even without improved sun shielding.

Propellant transfer by pressure difference was extensively discussed when SX were planning to do cross feeding for FH. I think thre are a good few posts on the subject.

John Carnack's team at Armadillo had trouble with this when they had 4 pressurized tanks on one of their vehicles. They found even quite small differences (<  a few psi) were enough that instead of all tanks emptying evenly one would flow into another, instead of the engine.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline Jim

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Re: The best method to transfer Methalox Fuel in space?
« Reply #67 on: 08/08/2017 02:11 PM »

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?

At launch complexes, LH2 is only moved using pressure (the tanks are loaded only by pressure).  There are no pumps involved.

Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #68 on: 08/08/2017 03:46 PM »

According to this paper, it doesn't look that bad:
http://www.ulalaunch.com/uploads/docs/Published_Papers/Extended_Duration/SettledCryogenicPropellantTransfer.pdf

This 10^-5 g is sufficient for settling and pumping out liquid.
Pumping out liquid infers almost no flow to the liquid in the vessel, pumping in liquid does.
For settling pumped in liquid there is no data, and that's probably why they purpose to do
an cryogenic transfer experiment.

Suppose 10^-2 g is the limit for keeping pumped in cryogenic Methalox settled (viscosity is important for every pumped in liquid, to slow down flow).

Than:

Acceleration of 10^-2 g for 5 hours is a delta V of 1.8 km per second.
if this is done by thrusters with low ISP the propellant loss is significantly more than with high ISP Raptor engines.

Until the lower g limit for pumping-in cryogenic Methalox is comfimed, discussions about rotation vs linear acceleration are relevant.

You are assuming that the ullage thrusters burn continuously for 5 hours. This is totally unnecessary. They only pulse as required to keep the prop settled. Note also, it's only necessary to keep the prop settled in the tanks of the tanker during the transfer to ensure that it remains over the outlet. It doesn't matter if the prop is not settled in the receiving tank. It can be settled later after the spaceship and tanker have separated.

Also, cryogenic prop has already been kept settled for the order of five hours on Delta IV GSO missions. I think all the basic technology for the mass transfer of cryogenic prop is already in place. It requires development of course. But I don't see any obvious showstoppers. And I don't see the need for rotation.


How do you vent gas if the prop isn't settled in the receiving tank?

Offline Jim

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Re: The best method to transfer Methalox Fuel in space?
« Reply #69 on: 08/08/2017 03:58 PM »


How do you vent gas if the prop isn't settled in the receiving tank?

it is settled and vented

Offline tdperk

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Re: The best method to transfer Methalox Fuel in space?
« Reply #70 on: 08/08/2017 04:30 PM »


How do you vent gas if the prop isn't settled in the receiving tank?

it is settled and vented

Or, a screened off volume within the has a small heat source and the vent aperture inlet inside the screened volume.  With thermal equilibrium in the tank as a whole, there will be no liquid within the screened off volume.

Offline MP99

Re: The best method to transfer Methalox Fuel in space?
« Reply #71 on: 08/08/2017 08:33 PM »
ISTR it's possible to use a liquid / gas separator - a rotor flings any liquid outwards, and any gas which might be left within the chamber can be released. I'd guess this relies on waiting until there is sufficient gas inside the chamber to be vented.

Cheers, Martin

Sent from my Nexus 6 using Tapatalk


Offline TomH

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Re: The best method to transfer Methalox Fuel in space?
« Reply #72 on: 08/13/2017 02:55 AM »
Will the prop be offloaded from the same tanks that provide fuel and oxidizer to the tanker's own engines (like siphoning gasoline from one car's tank to another) or would the prop which is to be delivered be in separate tanks? (E.g. when diesel fuel is delivered to a fuel station, the semi tractor draws from its own tanks and the diesel payload is separate in the trailer.)

If the answer above is that it will be in a separate payload tank and if multiple tanker launches are already required, this begs a new question. Would it be more efficient to bring fuel and oxidizer on every flight (thus requiring two payload tanks, two sets of pumps/transfer lines, two pumping events, etc. on every flight, or would it be better to have specialized and separate fuel and oxidizer tankers, which would mean only one payload tank, one set of pumping equipment, one pumping event, etc. per flight?

It seems that separate specialized tankers might require fewer separate transfer events and potentially be less risky. The tankers themselves would be a simpler design than a dual payload tanker. OTOH, it would mean different loading procedures on the ground and tankers which may be different sizes to accommodate liquids of differing densities.

A tanker which delivers both fuel and oxidizer on each flight would mean only one method of loading for each launch, one tanker design, one docking method, etc., rather than two.

Thoughts?

Online guckyfan

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Re: The best method to transfer Methalox Fuel in space?
« Reply #73 on: 08/13/2017 06:09 AM »
A tanker that uses its main tanks is more flexible.

It needs only one tanking procedure on the ground.

It can be used as a depot. Fill it up in orbit and then transfer the propellant to a departing ship.

It can be sent to lunar orbit to refuel a moon lander.

Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #74 on: 08/13/2017 11:28 AM »
Will the prop be offloaded from the same tanks that provide fuel and oxidizer to the tanker's own engines (like siphoning gasoline from one car's tank to another) or would the prop which is to be delivered be in separate tanks? (E.g. when diesel fuel is delivered to a fuel station, the semi tractor draws from its own tanks and the diesel payload is separate in the trailer.)

If the answer above is that it will be in a separate payload tank and if multiple tanker launches are already required, this begs a new question. Would it be more efficient to bring fuel and oxidizer on every flight (thus requiring two payload tanks, two sets of pumps/transfer lines, two pumping events, etc. on every flight, or would it be better to have specialized and separate fuel and oxidizer tankers, which would mean only one payload tank, one set of pumping equipment, one pumping event, etc. per flight?

It seems that separate specialized tankers might require fewer separate transfer events and potentially be less risky. The tankers themselves would be a simpler design than a dual payload tanker. OTOH, it would mean different loading procedures on the ground and tankers which may be different sizes to accommodate liquids of differing densities.

A tanker which delivers both fuel and oxidizer on each flight would mean only one method of loading for each launch, one tanker design, one docking method, etc., rather than two.

Thoughts?

I see your point, about specialized tankers. it might be that a tanker with a disproportionally large 
LOX tank and another with large methane tank would simplify things. Also a specialized tanker for water only could be possible (water can also be used for shielding, so you might need a lot of it)

I do not see the need for a separate tank for Methalox, because the regular tanks already have a separate tank inside them, for slosh free landing.
Also using a separate and thus smaller tank eliminates the possibility of one tanker being completely filled up by multiple other tankers. (= Depot option stated in above post(s))
If you do this, you can have all (but one) tankers completely fueled up in space, before all the spaceships with people are launched. Having all fuel already in space, saves a lot of waiting time for the passengers, and also less risk to the spaceship because there would be only one fueling event instead of 5 or 6 (per liquid).
« Last Edit: 08/13/2017 02:12 PM by Peter.Colin »

Offline Norm38

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Re: The best method to transfer Methalox Fuel in space?
« Reply #75 on: 09/29/2017 03:15 PM »
So Musk confirmed tanker and spaceship will mate end-to-end and use ullage thrusters to settle the propellant down into the empty tanks of the ship.

He said the plan is to reuse the existing plumbing from the booster to transfer fuel.  But in the image here, if say CH4 is on the left and O2 is on the right, when the ship and tanker dock end-to-end, then those connections don't line up.  They might if the tanker rolled over, but that's not what's shown.

Discuss.

Online RonM

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Re: The best method to transfer Methalox Fuel in space?
« Reply #76 on: 09/29/2017 03:26 PM »
So Musk confirmed tanker and spaceship will mate end-to-end and use ullage thrusters to settle the propellant down into the empty tanks of the ship.

He said the plan is to reuse the existing plumbing from the booster to transfer fuel.  But in the image here, if say CH4 is on the left and O2 is on the right, when the ship and tanker dock end-to-end, then those connections don't line up.  They might if the tanker rolled over, but that's not what's shown.

Discuss.

The plumbing on the tanker can be on opposite sides compared to the spaceship. It would match when docked end-to-end and when rotated 180 degrees on the booster.

Offline Norm38

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Re: The best method to transfer Methalox Fuel in space?
« Reply #77 on: 09/29/2017 03:38 PM »
That's a good point.  If the booster has rotational symmetry, then the tanker could go on 180.  That's different from the 12m booster which had a flange at the top and would not allow the ship to rotate.  I didn't see anything that showed the tanker could not rotate.

Online DanielW

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Re: The best method to transfer Methalox Fuel in space?
« Reply #78 on: 09/29/2017 03:40 PM »
Isn't it easier to assume that the picture is wrong for the sake of illustration and that the spacecraft will be rotated with respect to each other in reality?

Offline John Alan

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Re: The best method to transfer Methalox Fuel in space?
« Reply #79 on: 09/29/2017 03:43 PM »
Bottom line for me... was SpaceX took all the ideas put forth before yesterday and showed a simple concept that uses existing plumbing and existing thrusters... and can be done unmanned... Brilliant.. 

No pumps... no tethers... KISS at it's finest...  :o  8)

On edit..
I am assuming there is really 4 lines... a gas and liquid line for each tank... a top and bottom feed...
IF you cross them over by backing up back to back like that... the routing is correct...
Draw yourself a sketch and think on it a bit...  ;)

Attached pic down below later in the day...  :)
« Last Edit: 09/29/2017 05:58 PM by John Alan »

Offline Norm38

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Re: The best method to transfer Methalox Fuel in space?
« Reply #80 on: 09/29/2017 04:09 PM »
You mean the fill and vent lines?  And they're bidirectional?  I have no idea if the valves can support that or not.
« Last Edit: 09/29/2017 04:10 PM by Norm38 »

Offline rakaydos

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Re: The best method to transfer Methalox Fuel in space?
« Reply #81 on: 09/29/2017 04:27 PM »


4 pipes for fuel transfer. You can see in the render which side flares and which side narrows for mating the pipes.

Offline John Alan

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Re: The best method to transfer Methalox Fuel in space?
« Reply #82 on: 09/29/2017 05:39 PM »
You mean the fill and vent lines?  And they're bidirectional?  I have no idea if the valves can support that or not.

On the rocket... yes... all liquid rated and capable of flow in either direction... use valves that allow this on the rocket...
Back them together in space... latch the connectors... and open all 4 valve lines...

On edit... added pic of my... 60 second yes that works... paper stand in
Apply thrust and fluid can be transferred in either direction by "gravity"...
« Last Edit: 09/29/2017 05:57 PM by John Alan »

Offline shooter6947

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Re: The best method to transfer Methalox Fuel in space?
« Reply #83 on: 09/29/2017 05:47 PM »
So Musk confirmed tanker and spaceship will mate end-to-end and use ullage thrusters to settle the propellant down into the empty tanks of the ship.

He said the plan is to reuse the existing plumbing from the booster to transfer fuel.  But in the image here, if say CH4 is on the left and O2 is on the right, when the ship and tanker dock end-to-end, then those connections don't line up.  They might if the tanker rolled over, but that's not what's shown.

Discuss.

Just put CH4 on the top, and O2 on the bottom.

Online DanielW

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Re: The best method to transfer Methalox Fuel in space?
« Reply #84 on: 09/29/2017 06:18 PM »
I find it interesting that the spaceship will be fueled by the booster on earth. I would assume that means that all other umbilical connections for power/data etc will also come from the booster.

Offline Jim

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Re: The best method to transfer Methalox Fuel in space?
« Reply #85 on: 09/29/2017 06:25 PM »
You mean the fill and vent lines?  And they're bidirectional?  I have no idea if the valves can support that or not.

They are called fill and drain lines.  The tanks have to be emptied for scrubs.

Online envy887

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Re: The best method to transfer Methalox Fuel in space?
« Reply #86 on: 09/29/2017 06:32 PM »
And they won't use milligees of this to do the actual transfer. The thrust is to settle the propellant. The transfer is driven by pressure differential.

Offline Norm38

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Re: The best method to transfer Methalox Fuel in space?
« Reply #87 on: 09/30/2017 04:10 AM »
You mean the fill and vent lines?  And they're bidirectional?  I have no idea if the valves can support that or not.

They are called fill and drain lines.  The tanks have to be emptied for scrubs.

Got it. So when the separate ships are fueled for launch on Earth, are they filled from the top or the bottom?  From John Alan's sketch, they will fill from the bottom due to ullage thrust. 
« Last Edit: 09/30/2017 04:12 AM by Norm38 »

Offline Lars-J

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Re: The best method to transfer Methalox Fuel in space?
« Reply #88 on: 09/30/2017 05:45 AM »
You mean the fill and vent lines?  And they're bidirectional?  I have no idea if the valves can support that or not.

They are called fill and drain lines.  The tanks have to be emptied for scrubs.

Got it. So when the separate ships are fueled for launch on Earth, are they filled from the top or the bottom?  From John Alan's sketch, they will fill from the bottom due to ullage thrust.

F9 fills its stages from the bottom, as does most (all?) liquid launch vehicles.

Offline Semmel

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Re: The best method to transfer Methalox Fuel in space?
« Reply #89 on: 09/30/2017 11:35 AM »
They could just mount the pipes at the front and back if rotating 180 degrees is a problem. I dont think the tanker will have reversed pipes  because this makes the ground equipment more complicated. Just have them front and back and you can stick the two second stages together as shown in the image.

Offline Nibb31

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Re: The best method to transfer Methalox Fuel in space?
« Reply #90 on: 09/30/2017 11:40 AM »
There is no need to rotate. The male/female connections are symmetrical, as shown in SpaceX's slide. It's quite clear.

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