Author Topic: Starship On-orbit refueling - Options and Discussion  (Read 711591 times)

Online TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2600 on: 07/15/2024 04:49 pm »
Ot may be OK to let methane freeze while it’s being stored in the depot. But you’ll definitely need to condition the prop to be liquid before it’s transferred, to say nothing of using it in an engine start.

Offline OTV Booster

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2601 on: 07/15/2024 06:56 pm »
Ive got a rich suite of background processes working on orbital refueling and two concerns have popped out of the que. The current discussion on propellant temperature needs is one. QD sealing is another.


Cryogenic seals are a tough problem. Adding in multiple sealing cycles can only make it tougher. Our discussion has focused on using either the current QD layout or something similar. This puts the oxidizer and fuel seals in close proximity. If both start leaking liquid, local conditions determine the possibility of vaporization. If vapor is present only the lack of an ignition source prevents ignition.


I suspect the current state of material science is inadequate for perfecting the seals. I will gladly defer to wiser minds on this.


AIUI, the flow of the propellants themselves will build up triboelectric potential differences. This isn't insurmountable but it has to be dealt with.


Up until now I've been a strong advocate of working with current QD layout but I'm having second thoughts. Might it be best to keep the LOX connection where it is and having a second QD plate up by the methane tank? This would keep leakage separated and make an ignition source a non problem. It's a PITA but so is unintended consequences. Could this arrangement be worked to ease some of the temperature concerns?


Or am I just pissing in a tea pot and creating a tempest? (Did I get that right?  :o )
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Offline Robotbeat

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2602 on: 07/15/2024 07:57 pm »
Ignition is basically impossible in a non-pressurized volume. Any gas dissipates at approximately the speed of sound into the vacuum of space, so you’ll not have enough gas around. Additionally, you need BOTH fuel and oxidizer to get combustion in space. This is one way where refueling in space is actually easier than on the ground where there’s ambient pressure and the atmosphere already has oxygen in it, so oxygen alone is sufficient.

The materials science of sealing isn’t really any harder than on the ground (vacuum welding is a thing, but it’s actually rare for this to happen). But there’s additional advantages in that you don’t have to worry about ice (or even liquefied air) condensation, which at cryogenic temperatures is as strong as like epoxy. On the ground, condensation and frost can build up which interferes with a seal or makes it difficult to disengage. Not so in vacuum.
« Last Edit: 07/15/2024 07:59 pm by Robotbeat »
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Offline OTV Booster

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2603 on: 07/15/2024 11:17 pm »
Ignition is basically impossible in a non-pressurized volume. Any gas dissipates at approximately the speed of sound into the vacuum of space, so you’ll not have enough gas around. Additionally, you need BOTH fuel and oxidizer to get combustion in space. This is one way where refueling in space is actually easier than on the ground where there’s ambient pressure and the atmosphere already has oxygen in it, so oxygen alone is sufficient.

The materials science of sealing isn’t really any harder than on the ground (vacuum welding is a thing, but it’s actually rare for this to happen). But there’s additional advantages in that you don’t have to worry about ice (or even liquefied air) condensation, which at cryogenic temperatures is as strong as like epoxy. On the ground, condensation and frost can build up which interferes with a seal or makes it difficult to disengage. Not so in vacuum.
What you say about leaks into vacuum supporting combustion is comforting. I will now go and clean out that teapot.


I agree with what you say about in atmosphere freezing but have to disagree on the materials science not being any harder in space than on the ground. On the ground seals can be changed out after every use if necessary and it's no big thing. Depot would be another story.


Extrapolating from seals I've worked with, rubber or rubber like seals need to be warm to be pliable enough to seal. At low pressure and starting warm they will allow a good cryo seal. How high can the pressure go? IDK. Teflon works fine on a cryo ball valve but I don't know how well this translates to a face seal.


Thinking it through, maybe seal conditioning (heat) would be the last step before coupling.
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Offline Paul451

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2604 on: 07/16/2024 12:41 am »
Ignition is basically impossible in a non-pressurized volume. Any gas dissipates at approximately the speed of sound into the vacuum of space, so you’ll not have enough gas around.

Not quite. The initial leak will carry away heat, lowering the temperature of the leak-area and leaking fluid until it reaches a new equilibrium. If the leak is very minor, it's not a drama. But if it's enough that the equilibrium is below the freezing point of the leaking fluid/gas, it will start to freeze-boil, building up a slush around the leak. (See the LOx leak on the last F9 launch.) If that slush is a mix of oxygen and fuel, it could form an impact explosive... A boiling/fracturing impact explosive. (The ignition source is free.)
« Last Edit: 07/16/2024 12:41 am by Paul451 »

Online TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2605 on: 07/16/2024 09:13 pm »
Do we know how the terrestrial QD seals are loaded after contact?  Will the same mechanism work in microgravity?

Online meekGee

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2606 on: 10/23/2024 01:37 pm »
With tower catch checked off, I thought it's a good time to revive the thread.

Someone somewhere mentioned the idea of co-rotating nose-docked ships (for propellant settling) and got me thinking...
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Online meekGee

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2607 on: 10/23/2024 02:03 pm »
With tower catch checked off, I thought it's a good time to revive the thread.

Someone somewhere mentioned the idea of co-rotating nose-docked ships (for propellant settling) and got me thinking...
The fueler:
Needs to have a sump that's covered by fuel, and a tube that reaches the docking point.
If it's "vertical", this occurs naturally using a sump at the engine side and existing header tank tubes.
If it's "horizontal", the sump needs to be on a sidewall, which is not natural.

The fuelee, by and large, doesn't care.

So if doing something spinny, it's either nose to nose, or nose to QD port.

Also remember the fuelee is heavier than the fueler, sometimes by a lot, so the center of rotation may not be between the ships.

If relying on ullage thrust, things are a lot simpler in that respect.

--

Also consider the act of docking with loose propellants in the tanks...  Sounds impossible to  me. The fuel banging around weighs comparably to Starship and can cause "perturbations" in 5 if not 6 axes.

But if both ships are co-accelerating (at milli g) along parallel tracks, everything becomes simpler, since all propellants are settled and well behaved.

--

To me this adds up to:
- Parallel docking
- Prolonged ullage thrust along main axis
- An efficient low-force ullage thruster that's zero g restartable
« Last Edit: 10/23/2024 09:45 pm by meekGee »
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Offline Greg Hullender

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2608 on: 10/23/2024 11:21 pm »
I wonder if anyone has taken the time to try to rerun the calculations about how much payload HLS could deliver from the moon back to Gateway assuming Starship V3. I know the last time anyone ran these numbers they got a negative result. That is, it looked as though an HLS fully refueled in LEO, sent to Gateway and then down to the moon would be unable to return to Gateway even with zero payload.

I tried to do this myself, but I couldn't find enough specifics about V3. E.g. what are the wet and dry mass?

Offline Twark_Main

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2609 on: 10/24/2024 07:28 am »
That is, it looked as though an HLS fully refueled in LEO, sent to Gateway and then down to the moon would be unable to return to Gateway even with zero payload.

I thought the consensus was that such a mission plan would require fully fueling in HEEO, not LEO.

Offline Greg Hullender

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2610 on: 10/24/2024 05:44 pm »
That is, it looked as though an HLS fully refueled in LEO, sent to Gateway and then down to the moon would be unable to return to Gateway even with zero payload.

I thought the consensus was that such a mission plan would require fully fueling in HEEO, not LEO.
I thought that was just with the V1 Starship. I still have your "spreadsheet of doom," by the way; I just don't have the numbers to put into it.

Offline InterestedEngineer

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2611 on: 10/24/2024 08:10 pm »
That is, it looked as though an HLS fully refueled in LEO, sent to Gateway and then down to the moon would be unable to return to Gateway even with zero payload.

I thought the consensus was that such a mission plan would require fully fueling in HEEO, not LEO.
I thought that was just with the V1 Starship. I still have your "spreadsheet of doom," by the way; I just don't have the numbers to put into it.

I believe V2 is 1500t of fuel and V3 is 2300t of fuel.

Each 100t of fuel adds about 1.8t of dry mass, so add that in accordingly (V2 => 5.4t, V3 => 20t)

Suppose the dry mass with V1 is 250t, that gives a deltaV (@3500m/sec exhaust velocity) of

V1: 6.2 km/sec
V2: 6.8 km/sec
V3: 7.9 km/sec

What's the rough deltaV from LEO to the lunar surface?  About 6.2km/sec1.  So V1 was always very marginal, V2 will get to the lunar surface with a full fuel reload in LEO, some cargo.  It won't take off again though, but is that required for the first Artemis proof point?


1 - 4.1km/sec to low lunar orbit, Apollo landing was 2.13km/sec
https://space.stackexchange.com/questions/43214/delta-v-for-landing-on-the-moon
« Last Edit: 10/24/2024 08:17 pm by InterestedEngineer »

Offline Herbie

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2612 on: 10/25/2024 03:34 pm »
I have been following this thread for a while, but have not seen this refueling method discussed. I apologize if I missed the discussion.

Depots and propellant supply ships would both have internal ribcage-like manifold in each tank. Outbound ships would not need the manifold.

Ships dock back-to-back.  Flywheel on both ships, spin axis parallel to the long axis of each ship, spin up, imparting a very slow spin to the joined ships, settling propellant along the belly of each ship.

Increase supply-side tank pressure above that of other vessel, and open valves. Slow rotation keeps pressure head of the fluids low, so tank pressure should dominate. Propellant enters manifold  on the supply vessel from ventral side, and leave from the dorsal side on the receiving ship.

Center of mass, rotational axis, and moment of inertia will all shift during transfer, but perhaps flywheels can compensate.

After transfer, flywheels cancel out spin of the vessels, and vessels unlock.

It may be possible to so something similar using only thrusters instead of flywheels.

Offline eriblo

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2613 on: 10/25/2024 04:24 pm »
I have been following this thread for a while, but have not seen this refueling method discussed. I apologize if I missed the discussion.

Depots and propellant supply ships would both have internal ribcage-like manifold in each tank. Outbound ships would not need the manifold.

Ships dock back-to-back.  Flywheel on both ships, spin axis parallel to the long axis of each ship, spin up, imparting a very slow spin to the joined ships, settling propellant along the belly of each ship.

Increase supply-side tank pressure above that of other vessel, and open valves. Slow rotation keeps pressure head of the fluids low, so tank pressure should dominate. Propellant enters manifold  on the supply vessel from ventral side, and leave from the dorsal side on the receiving ship.

Center of mass, rotational axis, and moment of inertia will all shift during transfer, but perhaps flywheels can compensate.

After transfer, flywheels cancel out spin of the vessels, and vessels unlock.

It may be possible to so something similar using only thrusters instead of flywheels.
Interesting concept, the problem is that any axis not aligned with the maximum moment of inertia is unstable. The two ships will start tumbling until they are rotating in the only stable configuration which should be around an axis perpendicular to the symmetry plane going through both ships long axis (and that plane will want to be parallell to the orbital plane).

That configuration might work with your suggestion instead as most of the propellant would pool on the dorsal top and bottom corners of their respective tanks.

Online meekGee

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2614 on: 10/25/2024 04:39 pm »
I have been following this thread for a while, but have not seen this refueling method discussed. I apologize if I missed the discussion.

Depots and propellant supply ships would both have internal ribcage-like manifold in each tank. Outbound ships would not need the manifold.

Ships dock back-to-back.  Flywheel on both ships, spin axis parallel to the long axis of each ship, spin up, imparting a very slow spin to the joined ships, settling propellant along the belly of each ship.

Increase supply-side tank pressure above that of other vessel, and open valves. Slow rotation keeps pressure head of the fluids low, so tank pressure should dominate. Propellant enters manifold  on the supply vessel from ventral side, and leave from the dorsal side on the receiving ship.

Center of mass, rotational axis, and moment of inertia will all shift during transfer, but perhaps flywheels can compensate.

After transfer, flywheels cancel out spin of the vessels, and vessels unlock.

It may be possible to so something similar using only thrusters instead of flywheels.
I think the mass of flywheels and ribcage manifold is a lot more than propellant/gas required to do exactly the same thing but with forward thrust.

If it really takes only cm/sec2, it's by far the simplest solution.
« Last Edit: 10/25/2024 04:39 pm by meekGee »
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Offline Herbie

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2615 on: 10/25/2024 07:45 pm »
I have been following this thread for a while, but have not seen this refueling method discussed. I apologize if I missed the discussion.

Depots and propellant supply ships would both have internal ribcage-like manifold in each tank. Outbound ships would not need the manifold.

Ships dock back-to-back.  Flywheel on both ships, spin axis parallel to the long axis of each ship, spin up, imparting a very slow spin to the joined ships, settling propellant along the belly of each ship.

Increase supply-side tank pressure above that of other vessel, and open valves. Slow rotation keeps pressure head of the fluids low, so tank pressure should dominate. Propellant enters manifold  on the supply vessel from ventral side, and leave from the dorsal side on the receiving ship.

Center of mass, rotational axis, and moment of inertia will all shift during transfer, but perhaps flywheels can compensate.

After transfer, flywheels cancel out spin of the vessels, and vessels unlock.

It may be possible to so something similar using only thrusters instead of flywheels.
Interesting concept, the problem is that any axis not aligned with the maximum moment of inertia is unstable. The two ships will start tumbling until they are rotating in the only stable configuration which should be around an axis perpendicular to the symmetry plane going through both ships long axis (and that plane will want to be parallell to the orbital plane).

That configuration might work with your suggestion instead as most of the propellant would pool on the dorsal top and bottom corners of their respective tanks.
I was wondering about the rotational stability, but the tennis racket theorem suggests that rotation is stable about the principal axis associated with the minimum moment of inertia. Of course, that result assumes fixed moments of inertia, which wouldnot apply given the dynamic nature of the mass distribution in the system.

Offline eriblo

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2616 on: 10/25/2024 08:04 pm »
I have been following this thread for a while, but have not seen this refueling method discussed. I apologize if I missed the discussion.

Depots and propellant supply ships would both have internal ribcage-like manifold in each tank. Outbound ships would not need the manifold.

Ships dock back-to-back.  Flywheel on both ships, spin axis parallel to the long axis of each ship, spin up, imparting a very slow spin to the joined ships, settling propellant along the belly of each ship.

Increase supply-side tank pressure above that of other vessel, and open valves. Slow rotation keeps pressure head of the fluids low, so tank pressure should dominate. Propellant enters manifold  on the supply vessel from ventral side, and leave from the dorsal side on the receiving ship.

Center of mass, rotational axis, and moment of inertia will all shift during transfer, but perhaps flywheels can compensate.

After transfer, flywheels cancel out spin of the vessels, and vessels unlock.

It may be possible to so something similar using only thrusters instead of flywheels.
Interesting concept, the problem is that any axis not aligned with the maximum moment of inertia is unstable. The two ships will start tumbling until they are rotating in the only stable configuration which should be around an axis perpendicular to the symmetry plane going through both ships long axis (and that plane will want to be parallell to the orbital plane).

That configuration might work with your suggestion instead as most of the propellant would pool on the dorsal top and bottom corners of their respective tanks.
I was wondering about the rotational stability, but the tennis racket theorem suggests that rotation is stable about the principal axis associated with the minimum moment of inertia. Of course, that result assumes fixed moments of inertia, which wouldnot apply given the dynamic nature of the mass distribution in the system.
Indeed, it does specifically not hold for any object that is able to dissipate some rotational energy, the classic example of which is tanks with mixed fluids in them. This experiment can be done at home (perhaps outside) with a half full water bottle.
« Last Edit: 10/25/2024 08:06 pm by eriblo »

Offline Paul451

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2617 on: 10/26/2024 12:42 am »
I was wondering about the rotational stability, but the tennis racket theorem suggests that rotation is stable about the principal axis associated with the minimum moment of inertia. Of course, that result assumes fixed moments of inertia, which wouldnot apply given the dynamic nature of the mass distribution in the system.

Long axis instability is different from intermediate axis (tennis racket) instability. It requires a medium to transfer rotational momentum between long axis rotation and short axis rotation. How fast it happens depends on how much flex is in the system, and there are ways of using the same mechanism to actually passively counter the instability. However, with a prop depot, the overwhelming majority of the mass is mobile, so the instability would be extremely rapid and impossible to protect against.

And that's ignoring that the centre of rotation itself is moving between the ships as you transfer propellants. Especially for the first prop transfer, when the depot- is empty and the tanker is full; most of the combined mass of the system is the propellant in that tanker, and so most of the mass of the system changes location.

Offline Greg Hullender

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2618 on: 10/26/2024 07:59 pm »
I have been following this thread for a while, but have not seen this refueling method discussed. I apologize if I missed the discussion.
Gee, it's only 131 pages over the past five years--read faster! :-)

Seriously, the things that soured me on all rotational approaches in the foreseeable future were:
a) You only need maybe 50 micro-g of acceleration to settle the tanks, so even a very long ullage burn (e.g. hours) won't waste very much fuel.

b) Cryogenic plumbing is the very devil to make work, so anything that needs new plumbing is probably off the table for a long time. Any practical system needs to work with Starships more-or-less as they are today--no new pipes and/or outlets. You definitely want to avoid anything that would add extra mass to each and every tanker Starship--adding mass just to the depot is probably okay.

c) No one has even tried to make a rotating system work in space, much less one that would potentially involve huge changes in moment of inertia from docking, fuel transfer, and sloshing. You could imagine it taking many years to get the kinks worked out.

The ullage-burn approach is so easy and so cheap it's just impossible to beat. Or so it seems, anyway. We'll know more when they start actually trying to do it.

Offline magnemoe

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2619 on: 10/26/2024 08:34 pm »
I have been following this thread for a while, but have not seen this refueling method discussed. I apologize if I missed the discussion.
Gee, it's only 131 pages over the past five years--read faster! :-)

Seriously, the things that soured me on all rotational approaches in the foreseeable future were:
a) You only need maybe 50 micro-g of acceleration to settle the tanks, so even a very long ullage burn (e.g. hours) won't waste very much fuel.

b) Cryogenic plumbing is the very devil to make work, so anything that needs new plumbing is probably off the table for a long time. Any practical system needs to work with Starships more-or-less as they are today--no new pipes and/or outlets. You definitely want to avoid anything that would add extra mass to each and every tanker Starship--adding mass just to the depot is probably okay.

c) No one has even tried to make a rotating system work in space, much less one that would potentially involve huge changes in moment of inertia from docking, fuel transfer, and sloshing. You could imagine it taking many years to get the kinks worked out.

The ullage-burn approach is so easy and so cheap it's just impossible to beat. Or so it seems, anyway. We'll know more when they start actually trying to do it.
This, and as its just over pressure in tanker pushing liquid gas so gas voids is not an issue, just pump gas back to tanker.
Most of ullage is probably boil off anyway and you can simply burn some methane and oxygen gas for more trust once you nailed it. Not needed for the prototype version however, none has done this before.
Its like its 1916 and you are building something called an aircraft carrier. It will not be an very good one but it was unlikely to get into carrier vs carriers fights.

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