Author Topic: The best method to transfer Methalox Fuel in space?  (Read 9699 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.
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 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.

Offline Ionmars

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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.

* Mars: a convenient service station for an asteroid-sized spaceship en-route to Ceres. *

Offline Ionmars

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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.
* Mars: a convenient service station for an asteroid-sized spaceship en-route to Ceres. *

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.

@SpaceX "When can I buy my ticket to Mars?"

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.
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 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.
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 Ionmars

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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.


   
* Mars: a convenient service station for an asteroid-sized spaceship en-route to Ceres. *

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.




Offline Ionmars

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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. 
* Mars: a convenient service station for an asteroid-sized spaceship en-route to Ceres. *

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.

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