Author Topic: The best method to transfer Methalox Fuel in space?  (Read 22715 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.

Online 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.
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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 »

Online 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.

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.

After landing, Mars pioneers will require our continued support.

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.
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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?

Online 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.

Online 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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Online 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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Online 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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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.


   
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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.




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. 
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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 »

Offline Ionmars

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

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

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

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

Settling of cryogenic propellants on orbit has been done though.

exactly

Offline Peter.Colin

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

Settling of cryogenic propellants on orbit has been done though.

exactly


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


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

Online 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

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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.
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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 »

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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.

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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.

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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 »

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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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Offline 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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Online 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.
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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.
BFS. The worlds first Methane fueled FFORSC engined CFRP structured A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP structured booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C apply. Believe no one. Run your own numbers. So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

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?

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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.

Offline 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.

Offline 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?

Online 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 »

Online 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.

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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.

Offline 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.

Offline 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.

Offline speedevil

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Re: The best method to transfer Methalox Fuel in space?
« Reply #91 on: 03/01/2018 09:43 PM »


The image is 507 pixels across, and though presumably not a final design, I thought it interesting to do some approximate numbers.
Measuring the internal diameter of the two narrow pipes, I came to an approximate figure of 11 pixels, and around 18 pixels for the wider outside.
As these are tapered connectors, 14 pixels seems a reasonable approximation for the ID of the pipe, leading to a diameter of 26.7cm.
For no particularly good reason, I'm assuming it's in fact 25cm.

Only calculating liquid oxygen,and assuming one pipe flow of 18m long, with around 1 bar across the pipe, assuming the receiving tank is vented, this flows around 70 tons a minute. (oxygen viscosity is 188*10-6 kg/ms, density 1.1, http://www.pressure-drop.com/Online-Calculator/ )

For both pipes, this is probably conservative, if the system is properly designed.
I did not bother calculating methane, as oxygen dominates.

This assumes autogenous pressurisation works, and the engines are OK with starting up with less than nominal pressure if you're transferring a lot of fuel, or some autogenous pressurisation device that works without the engines being on.

I was idly wondering in the context of my silly speculations on using BFS SSTO, refuelling suborbitally, and if it only takes two minutes, plus a couple of minutes  to rendevous from close flight, this might be quite plausible.

Offline douglas100

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Re: The best method to transfer Methalox Fuel in space?
« Reply #92 on: 03/01/2018 10:11 PM »

....This assumes autogenous pressurisation works, and the engines are OK with starting up with less than nominal pressure if you're transferring a lot of fuel, or some autogenous pressurisation device that works without the engines being on.

I was idly wondering in the context of my silly speculations on using BFS SSTO, refuelling suborbitally, and if it only takes two minutes, plus a couple of minutes  to rendevous from close flight, this might be quite plausible.

I think it would much longer than two minutes. Propellant would only flow at that rate if the main engines were running and the flow was being driven by their turbopumps. You emphatically would not be doing that during orbital refuelling. (I won't comment on your suborbital proposal.) Pressure differential is all that is required to transfer settled propellant. It doesn't matter in principle whether the pressure differential is caused by autogeneous  pressurisation or a pressurant like helium. In the case of BFS it would be autogeneous. Note that the tanks must be pressurised anyway before any main engine start however it is done. Same for prop transfer.
Douglas Clark

Offline speedevil

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Re: The best method to transfer Methalox Fuel in space?
« Reply #93 on: 03/01/2018 10:19 PM »

....This assumes autogenous pressurisation works, and the engines are OK with starting up with less than nominal pressure if you're transferring a lot of fuel, or some autogenous pressurisation device that works without the engines being on.

I think it would much longer than two minutes. Propellant would only flow at that rate if the main engines were running and the flow was being driven by their turbopumps.

I am assuming the receiving tank is vented to vacuum, and the originating tank is pressurised to somewhere above 1 bar, and propellant simply flows due to difference in pressure.
10" pipes flow quite a lot of liquid at 1 bar head, especially if low viscosity like liquid oxygen, even if 20m long or so.

The originating tank nominal pressure pretty much has to be pressurised significantly over 1 bar in order to handle even atmospheric loads.

No pumps at all.


I note F9 fills in apparently 15 minutes for 200+ tons of LOX, so unless BFS is significantly slower, 70 tons a minute would be a reasonable figure, and indeed quite slow for BFR.

'In real life' performance, even assuming unchoked pipes is complex, and would need to make assumptions about things like pressurant being very non ideal-gas-like as the temperature drops and more wants to condense into the propellant.

At some total amount of transfer relative to the initial tank volume, you're going to need to start actively pressurising, or the gas will just drop out as it rapidly cools. (or boiling will occur, which is bad)

The engines have to be able to cope in some manner with varying tank pressurisation, as it's explicitly mentioned at IAC that they'll dump them to vacuum.

Perhaps the fill lines go to the main tanks, and the 'landing' tanks remain pressurised through some system, while allowing rapid main tank gas heating when the engines are on through heat exchangers.

I consider the suborbital refuelling idea very very unlikely to happen.
« Last Edit: 03/02/2018 10:54 AM by speedevil »

Offline livingjw

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Re: The best method to transfer Methalox Fuel in space?
« Reply #94 on: 03/02/2018 11:17 AM »
The tanks won't be at zero pressure. Main tank pressures are about 30-45 psi. Receiving tank could be dropped 10 to 20 psi below that. Once transfer is complete, pressure can be raised back up using warm GOx and GCH4. Main engines would never be operated until design tank pressures have been reestablished. Small pressure fed vernier motors would use a different tank system, probably pressurized to a few hundred psi.

John
« Last Edit: 03/02/2018 11:24 AM by livingjw »

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Re: The best method to transfer Methalox Fuel in space?
« Reply #95 on: 03/02/2018 04:10 PM »
The tanks won't be at zero pressure. Main tank pressures are about 30-45 psi. Receiving tank could be dropped 10 to 20 psi below that. Once transfer is complete, pressure can be raised back up using warm GOx and GCH4. Main engines would never be operated until design tank pressures have been reestablished. Small pressure fed vernier motors would use a different tank system, probably pressurized to a few hundred psi.

John

When dealing with tanks partially filled (separately, of course) with super-cooled liquid methane and with LOX, at what temperature and pressure does each freeze into a solid?  I get the feeling that it's a sliding scale, depending on the amount of fluid vs. gas in each tank.

I'm assuming that the freezing point goes higher as pressure decreases.  At a vacuum and super-cooled temps, I'd think you'd flash-freeze a lot of each liquid...
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Re: The best method to transfer Methalox Fuel in space?
« Reply #96 on: 03/02/2018 04:29 PM »
When dealing with tanks partially filled (separately, of course) with super-cooled liquid methane and with LOX, at what temperature and pressure does each freeze into a solid?  I get the feeling that it's a sliding scale, depending on the amount of fluid vs. gas in each tank.

I'm assuming that the freezing point goes higher as pressure decreases.  At a vacuum and super-cooled temps, I'd think you'd flash-freeze a lot of each liquid...

You would need to let a moderate amount of liquid boil off in order to chill the bulk liquid to freezing point.

The reference to 'vacuum' above was a bit misleading - you wouldn't go all the way to vacuum - it will boil - just 14PSI (or whatever) lower than the other tank.
The concern which makes it somewhat more complex to calculate what happens is that as the volume of gas in the tank changes in the 'sending' tank, if you are not using pumps, it can rapidly cool to below the point at which it becomes liquid as it expands, and pressure collapses.
This means you need some means of maintaining pressure in the tank.

In practice, if the propellant is actually chilled below the boiling point, you in addition have a large flow of pressurant gas condensing in the tanks, so 'steady state' is only occurring with the pressurant boiling system - however it's designed - active.

As a rough number, this system needs to provide in a normal launch, 1000m^3 over 5 minutes, 3m^3/s. At STP, this volume of pressurant gas would be around 4.5kg/s, at 100K, ideal gas law says around 15kg/s, and a quick google says 200kJ/kg, so the heater needs to provide about 3MW, equivalent to burning about a hundred grams of methane a second, if the heater were to be an isolated system.





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Re: The best method to transfer Methalox Fuel in space?
« Reply #97 on: 03/02/2018 06:44 PM »
Why would they NOT use turbopumps to accelerate fuel transfer? Spend a little fuel via turbopump, save a bit of fuel in ullage thrust, save time because the pumping is faster, and hot partially burned methalox makes for more efficent ullage.

Offline envy887

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Re: The best method to transfer Methalox Fuel in space?
« Reply #98 on: 03/02/2018 06:49 PM »
Why would they NOT use turbopumps to accelerate fuel transfer? Spend a little fuel via turbopump, save a bit of fuel in ullage thrust, save time because the pumping is faster, and hot partially burned methalox makes for more efficent ullage.

Tank pressure is more than enough to empty the tanks in minutes. How do you think the booster tanks empty in less than 2 minutes during ascent? Hint: the turbopumps are not "sucking" it out...

Offline Steve D

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Re: The best method to transfer Methalox Fuel in space?
« Reply #99 on: 03/02/2018 08:38 PM »
As the liquid flows from the tanker why would you not just transfer gas into the tanker from the BFS? I thought there were 4 connections between tanker and BFS. 1 supply and 1 return line for o2 and methane.

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Re: The best method to transfer Methalox Fuel in space?
« Reply #100 on: 03/02/2018 08:41 PM »
Why would they NOT use turbopumps to accelerate fuel transfer? Spend a little fuel via turbopump, save a bit of fuel in ullage thrust, save time because the pumping is faster, and hot partially burned methalox makes for more efficent ullage.

Tank pressure is more than enough to empty the tanks in minutes. How do you think the booster tanks empty in less than 2 minutes during ascent? Hint: the turbopumps are not "sucking" it out...
In anything except a pressure fed rocket, yes, in fact, the turbopumps ARE sucking it out.

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Re: The best method to transfer Methalox Fuel in space?
« Reply #101 on: 03/02/2018 09:10 PM »
As the liquid flows from the tanker why would you not just transfer gas into the tanker from the BFS? I thought there were 4 connections between tanker and BFS. 1 supply and 1 return line for o2 and methane.

You would need a modestly large pump - of the order of 100kW or so power in order to maintain the pressure difference while pumping the return gas.
And of course a power system for this to run from.

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Re: The best method to transfer Methalox Fuel in space?
« Reply #102 on: 03/03/2018 07:27 AM »
Why would they NOT use turbopumps to accelerate fuel transfer? Spend a little fuel via turbopump, save a bit of fuel in ullage thrust, save time because the pumping is faster, and hot partially burned methalox makes for more efficent ullage.

Are you in a hurry to go somewhere? It doesn’t matter if it takes an hour or two. the ship will need to stay in orbit for several hours anyway before it can land. (Takes a while to wait for the orbit to pass over the launch site again while in the proper position)

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Re: The best method to transfer Methalox Fuel in space?
« Reply #103 on: 03/03/2018 09:13 AM »
As the liquid flows from the tanker why would you not just transfer gas into the tanker from the BFS? I thought there were 4 connections between tanker and BFS. 1 supply and 1 return line for o2 and methane.

You would need a modestly large pump - of the order of 100kW or so power in order to maintain the pressure difference while pumping the return gas.
And of course a power system for this to run from.

Probably not worth the extra complexity and mass. As long as the receiving tanks are vented down sufficiently to ensure the prop is transferred, there's no need to return the gas.
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Re: The best method to transfer Methalox Fuel in space?
« Reply #104 on: 03/03/2018 11:38 AM »
(Takes a while to wait for the orbit to pass over the launch site again while in the proper position)
Try about a day before they come back over the launch site.

The ground track of the vehicles moves X degrees for every orbit. If you site is at the poles every orbit puts you over it. At the Equator its once a day. Everywhere else is somewhere in between.

Whatever site you want to land on (or rather get back to) from a given orbit depends on your cross range.

The more orbits away from launch you are (to a maximum 180degress ) the more cross range you need. 

The design goal to return to launch site in 1 orbit (which is the easy case) consumed 10s of 1000s of wind tunnel hours in Shuttle design.

This also brackets your propellant transfer time and hence rate. Maximum rate is about 225tonnes in < 90 minutes if BFS has the designed cross range of the Shuttle.

Or you ignore designing for RTLS in one orbit and just wait till it comes back round, which gives you one less design constraint for BFS to meet. If other factors lead to a design with that much cross range, great. If not it takes as long as it takes.
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Offline livingjw

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Re: The best method to transfer Methalox Fuel in space?
« Reply #105 on: 03/03/2018 01:39 PM »
Why would they NOT use turbopumps to accelerate fuel transfer? Spend a little fuel via turbopump, save a bit of fuel in ullage thrust, save time because the pumping is faster, and hot partially burned methalox makes for more efficent ullage.

Tank pressure is more than enough to empty the tanks in minutes. How do you think the booster tanks empty in less than 2 minutes during ascent? Hint: the turbopumps are not "sucking" it out...
In anything except a pressure fed rocket, yes, in fact, the turbopumps ARE sucking it out.

You are both right. Tanks are at about 30 psi in orbit so they would empty out quite fast on there own. The pumps do suck the pressure down below 30 psi, but not so much that the propellant would boil / cavitate.

John

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Re: The best method to transfer Methalox Fuel in space?
« Reply #106 on: 03/03/2018 01:53 PM »
(Takes a while to wait for the orbit to pass over the launch site again while in the proper position)
Try about a day before they come back over the launch site.

The ground track of the vehicles moves X degrees for every orbit. If you site is at the poles every orbit puts you over it. At the Equator its once a day. Everywhere else is somewhere in between.

Whatever site you want to land on (or rather get back to) from a given orbit depends on your cross range.

The more orbits away from launch you are (to a maximum 180degress ) the more cross range you need. 

The design goal to return to launch site in 1 orbit (which is the easy case) consumed 10s of 1000s of wind tunnel hours in Shuttle design.

This also brackets your propellant transfer time and hence rate. Maximum rate is about 225tonnes in < 90 minutes if BFS has the designed cross range of the Shuttle.

Or you ignore designing for RTLS in one orbit and just wait till it comes back round, which gives you one less design constraint for BFS to meet. If other factors lead to a design with that much cross range, great. If not it takes as long as it takes.

Isn't it twice a day. One on the ascending orbit one on the descending orbit.
With ELV best efficiency was the paradigm. The new paradigm is reusable, good enough, and commonality of design.
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Offline speedevil

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Re: The best method to transfer Methalox Fuel in space?
« Reply #107 on: 03/03/2018 01:55 PM »
(Takes a while to wait for the orbit to pass over the launch site again while in the proper position)
Try about a day before they come back over the launch site. <snip>

This also brackets your propellant transfer time and hence rate. Maximum rate is about 225tonnes in < 90 minutes if BFS has the designed cross range of the Shuttle.

My initial guess was 70 tons or so for a minute, and I was assuming implicitly that close to this would never happen in real life.
On re-looking at this figure, I'm not sure that is a good assumption.

To even be comparable with F9 fill times, BFS needs to fill exactly at this rate, and BFR some four times faster.

It seems reasonable to assume that being able to de-tank at the same rate is a likely design capacity, to ease testing if nothing else.

This means it's quite likely IMO that BFS is designed-in with the capability to take around 70 tons a minute on and off, perhaps with two of the pipes normally being gas return, perhaps if it's subcooled both of them being flow.

If the propellant is already settled in the tank, and you have reduced pressure in the destination tank, the sending BFS/tanker experiences pretty much an exactly nominal de-tanking.

The other directions BFS is 'upside-down', and you would want to look carefully on if the pipe exit needs to be submerged in the fluid, or if a simple omnidirectional spray will work fine, and you want to ensure that any gas venting vents only gas.

This is not a high pressure spray though, and the tank will be mostly at this temperature anyway.

Simply as that's what the hardware is designed for on earth, and with very, very minimal changes - perhaps a colander type spray-nozzle over the end of the pipe - and hardware you need anyway - to vent the tanks it 'just works'.

Meaning tanking of the 150 tons or so a tanker can do takes a couple of minutes, wholly dominated by docking time.

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Re: The best method to transfer Methalox Fuel in space?
« Reply #108 on: 03/03/2018 03:02 PM »
Why would they NOT use turbopumps to accelerate fuel transfer? Spend a little fuel via turbopump, save a bit of fuel in ullage thrust, save time because the pumping is faster, and hot partially burned methalox makes for more efficent ullage.

Tank pressure is more than enough to empty the tanks in minutes. How do you think the booster tanks empty in less than 2 minutes during ascent? Hint: the turbopumps are not "sucking" it out...
In anything except a pressure fed rocket, yes, in fact, the turbopumps ARE sucking it out.

You are both right. Tanks are at about 30 psi in orbit so they would empty out quite fast on there own. The pumps do suck the pressure down below 30 psi, but not so much that the propellant would boil / cavitate.

John
The big reason you need the pumps is that you're topping off a partially filled tank that's also already at 30 PSI.

Offline speedevil

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Re: The best method to transfer Methalox Fuel in space?
« Reply #109 on: 03/03/2018 03:13 PM »
The big reason you need the pumps is that you're topping off a partially filled tank that's also already at 30 PSI.

Can you explain why simply venting the receiving tank to 15PSI or so does not work?

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Re: The best method to transfer Methalox Fuel in space?
« Reply #110 on: 03/03/2018 03:22 PM »
The big reason you need the pumps is that you're topping off a partially filled tank that's also already at 30 PSI.

Can you explain why simply venting the receiving tank to 15PSI or so does not work?

Because you spent hundreds of dollars per pound getting what you're venting up there. Better to recirculate it back to the tanker to keep THAT pressurized.

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Re: The best method to transfer Methalox Fuel in space?
« Reply #111 on: 03/03/2018 03:44 PM »
Time is money, and in a fuel rich architecture you may not want to worry about perfect recovery if it means you have to stay in orbit a whole day longer to land the tanker back at the launch site.

(Not disagreeing, just saying there are trades)
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Offline speedevil

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Re: The best method to transfer Methalox Fuel in space?
« Reply #112 on: 03/03/2018 03:54 PM »
Because you spent hundreds of dollars per pound getting what you're venting up there. Better to recirculate it back to the tanker to keep THAT pressurized.

Which has to be set against the mass of the pump.

If you are concerned with losses, there has to be some pressure system in the tank heating the pressurant in order to keep the volume right, and this has to work at a level sufficient to keep the pressure at the right level from umbillical disconnect to engine shutdown.

Add in the fact that there must be some tolerance - the engine does not need 30.000PSI to start, and you have all the subsystems you need to without loss set the pressurisation of the first tanker at 32PSI, the receiving one at 28PSI, and flow several tons a minute with no pumps, and no external loss.

From at least the perspectives of BFR/BFS - if you are routinely caring about the mass of around a ton of pressurant in the tanks being important to mission success or failure, things have gone horribly, horribly wrong.

(or right, and people keep buying payloads right at your mass limit and nagging you to fix the small inefficiencies)
« Last Edit: 03/03/2018 04:10 PM by speedevil »

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Re: The best method to transfer Methalox Fuel in space?
« Reply #113 on: 03/03/2018 04:51 PM »
Why would they NOT use turbopumps to accelerate fuel transfer? Spend a little fuel via turbopump, save a bit of fuel in ullage thrust, save time because the pumping is faster, and hot partially burned methalox makes for more efficent ullage.

Tank pressure is more than enough to empty the tanks in minutes. How do you think the booster tanks empty in less than 2 minutes during ascent? Hint: the turbopumps are not "sucking" it out...
In anything except a pressure fed rocket, yes, in fact, the turbopumps ARE sucking it out.

You are both right. Tanks are at about 30 psi in orbit so they would empty out quite fast on there own. The pumps do suck the pressure down below 30 psi, but not so much that the propellant would boil / cavitate.

John
I think people might be getting hung up on the word "suck". It's pretty obvious nothing will flow if the pressure at the pump inlet isn't lower than in the tank. When I think of a pump sucking fluid it would be a pump that has less pressure at the inlet than ambient outside pressure, like a shallow well pump, which would be kind of hard in a vacuum.

Offline Patchouli

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Re: The best method to transfer Methalox Fuel in space?
« Reply #114 on: 03/03/2018 05:13 PM »
The big reason you need the pumps is that you're topping off a partially filled tank that's also already at 30 PSI.

Can you explain why simply venting the receiving tank to 15PSI or so does not work?

Because you spent hundreds of dollars per pound getting what you're venting up there. Better to recirculate it back to the tanker to keep THAT pressurized.

It may be worthwhile to loose some to keep the system simple this may not be completely wasted but used to create thrust for settling the propellant.
But it also may be possible to create a pressure differential by simply placing the vehicle to be refueled in the shade and the tanker in direct sun.
A sun shade on BFS could be useful during it's time in LEO to minimize propellant loss.

As far as mating the two vehicles I think a robot arm of sorts could be helpful.
« Last Edit: 03/03/2018 05:16 PM by Patchouli »

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Re: The best method to transfer Methalox Fuel in space?
« Reply #115 on: 03/03/2018 06:07 PM »
As far as mating the two vehicles I think a robot arm of sorts could be helpful.

In the context of BFR+BFS, I note that for this to work well, you've already worked out reliable docking at over a thousand times ISS-dragon docking rates, with wind!

(Falcon boosters are at around 300m at 5 seconds or so, dragon ISS dockings from 300m take 5000s, and BFR needs to dock precisely into a cradle)

Clearly, docking isn't going to happen at >1G all the way, but it's not going to be happening at centimeter a second rates from 300m.
« Last Edit: 03/03/2018 06:39 PM by speedevil »

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Re: The best method to transfer Methalox Fuel in space?
« Reply #116 on: 03/03/2018 08:18 PM »
Why would they NOT use turbopumps to accelerate fuel transfer? Spend a little fuel via turbopump, save a bit of fuel in ullage thrust, save time because the pumping is faster, and hot partially burned methalox makes for more efficent ullage.

This is very much completely incorrect. Partially burned methalox makes for the worst ullage gas and pressurant you can find.

What is partially burned methalox? It is largely:

[*]unburnt propellant (mostly good, except when it goes in the wrong tank, in which case it's probably a disaster), 
[*]carbon monoxide which is slightly worse for pressurization than argon, but not as good as nitrogen(this makes it an improvement over both oxygen and methane but your production yield in combustion is likely to be low), 
[*]carbon dioxide, which will promptly condense to solid dry ice (it is competing for second worse combustion product, for use as pressurizer, it's vapor pressure graph is only slightly better than N2O, but not quite as good as ethane. 
[*]water, which will promptly condense to solid ice (it is the worst combustion product for pressurization, it's vapor pressure graph is only an improvement over zero pressure).   


Putting partially combusted propellant back into it's tank is not only counterproductive for filling ullage space, it is bad for propellant quality and vehicle safety. Piping partially reacted gas back to oxidizer tank is almost certainly prone to rapid unscheduled disassembly risk.

If that is not a reason not to even consider turbo pumps for propellant transfer, I'm sure there are other reasons

« Last Edit: 03/03/2018 08:30 PM by Hominans Kosmos »

Offline envy887

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Re: The best method to transfer Methalox Fuel in space?
« Reply #117 on: 03/04/2018 12:06 AM »
Why would they NOT use turbopumps to accelerate fuel transfer? Spend a little fuel via turbopump, save a bit of fuel in ullage thrust, save time because the pumping is faster, and hot partially burned methalox makes for more efficent ullage.

This is very much completely incorrect. Partially burned methalox makes for the worst ullage gas and pressurant you can find.

What is partially burned methalox? It is largely:

[*]unburnt propellant (mostly good, except when it goes in the wrong tank, in which case it's probably a disaster), 
[*]carbon monoxide which is slightly worse for pressurization than argon, but not as good as nitrogen(this makes it an improvement over both oxygen and methane but your production yield in combustion is likely to be low), 
[*]carbon dioxide, which will promptly condense to solid dry ice (it is competing for second worse combustion product, for use as pressurizer, it's vapor pressure graph is only slightly better than N2O, but not quite as good as ethane. 
[*]water, which will promptly condense to solid ice (it is the worst combustion product for pressurization, it's vapor pressure graph is only an improvement over zero pressure).   


Putting partially combusted propellant back into it's tank is not only counterproductive for filling ullage space, it is bad for propellant quality and vehicle safety. Piping partially reacted gas back to oxidizer tank is almost certainly prone to rapid unscheduled disassembly risk.

If that is not a reason not to even consider turbo pumps for propellant transfer, I'm sure there are other reasons

Pretty sure he meant methalox ullage settling thruster, not methalox ullage space pressurant.

Offline envy887

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Re: The best method to transfer Methalox Fuel in space?
« Reply #118 on: 03/04/2018 12:24 AM »
The big reason you need the pumps is that you're topping off a partially filled tank that's also already at 30 PSI.

Can you explain why simply venting the receiving tank to 15PSI or so does not work?

Because you spent hundreds of dollars per pound getting what you're venting up there. Better to recirculate it back to the tanker to keep THAT pressurized.

Or use it for RCS/ullage thrusting instead. There will be boiloff from the main tanks during the days or weeks of refueling, and I'm sure SpaceX will plan to do something useful with that boiloff. Without numbers to know exactly how the processes balance over time in different environments, it's impossible to tell what the pressure in any tank at any time will be. The receiving tank could easily be lower pressure without venting.

Or maybe they are going to use venting to subcool the prop for better performance to TMI...

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Re: The best method to transfer Methalox Fuel in space?
« Reply #119 on: 03/04/2018 05:16 AM »
Why would they NOT use turbopumps to accelerate fuel transfer? Spend a little fuel via turbopump, save a bit of fuel in ullage thrust, save time because the pumping is faster, and hot partially burned methalox makes for more efficent ullage.

... Partially burned methalox makes for the worst ullage gas and pressurant you can find....


Pretty sure he meant methalox ullage settling thruster, not methalox ullage space pressurant.

Pretty sure cold gas thruster has lower efficiency for thrust than complete combustion  in Draco2 next generation combustion chamber

Offline Lar

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

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

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

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

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

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

If differential pressure is good enough to get the job done in time that would seem to be the best way to do it.
« Last Edit: 03/04/2018 09:34 AM by john smith 19 »
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Re: The best method to transfer Methalox Fuel in space?
« Reply #122 on: 03/04/2018 03:24 PM »
 I was thinking of this thread while I was pondering a filling soda cup at the convenience store yesterday, that was foaming so much it was taking forever. People were staring at me. It's probably a good thing they didn't know what was going on in my head.

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

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

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

Online rakaydos

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

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

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

Offline speedevil

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

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

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

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

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

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

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

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

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



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

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

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

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

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