Starship On-orbit refueling - Options and Discussion

[OTV Booster]

IIRC, the biggest complaint about geysering was back when we were discussing connecting the ullage spaces. It was feared that geysering would pass propellent back to the originating ship. Maybe it's no big deal after all but I've got to admit that props slamming around doesn't feel like a good idea. Just a gut reaction.

I'm not sure how instead venting that liquid propellant is any better. It seems like it would be worse, in fact.

Is there some device that makes sure that only gas can pass through? If such a device exists, it seems like it could solve the problem in both cases.

Maybe a molecular sieve? I think a simple mesh would stop a lot of it. Not 100%. Thinking about it, we don't know the severity of the problem. If a ton of propellant goes through a second time does it really matter all that much?

[Paul451]

If you mean side-mount and "spinning end-over-end", then you are rotating around the intermediate axis and it is instantly unstable.
If you mean side-mount and spinning flat like a frisbee, then you have massively altered the way the propellant will settle, and it makes the CoM issue vastly worse. The complexity of this configuration makes other suggestions look like child's play.

I assume by "side-mount", you mean dorsal-to-dorsal with noses pointed in the same direction, as in the artwork?

Yes. Or ventral. Or side-by-side. Facing the same way (as in the pics), or facing the opposite. Any of the "parallel, but not in a line" configurations. I wasn't being specific; my objections apply to any of them.

Another issue to consider:  As prop moves from one to the other, the CoMs of the individual ships will move, which will put shear stresses on the docking mechanisms.

Given the extremely low g-levels being discussed in the thread, I don't think that will be a significant issue. They'll need to handle multiple-percentage-of-1g loads during actual RPOD (including from having a surge of propellant bouncing off the walls when the ships contact), so I don't think the sustained (shifting) loads during prop-transfer will be larger than that.

I think this also tilts the axis (really axes) of inertia, which makes the rotational instability question soooooo much more complicated.

I was thinking more that (at least for the side-by-side plus frisbee version) changing CoM changes where the propellant sits, sufficiently that you might not have a single area for a pump inlet. Which changes the design even more than just "put a second sump where you expect it to sit".  To the point of having to redesign the shape of the tanks.

Since nobody picked up on it, I'm gonna pound the table on prop management devices down in the sumps.

I don't know enough about prop behaviour in micro-g. I do note that most cryo liquids have much lower surface tension than water. Like order-of-magnitude lower. That seems to make any "blob" based solution harder.

[Twark_Main]

You propose docking two ships together in a line, and spinning them end over end.

I actually envision dorsal-to-dorsal docking, the same as existing renders.

You mentioned "nose-to-nose or interstage-to-interstage", and "spinning end-over-end". If you meant something else, you should have been clearer, we can't read your mind.

That's why I clarified.  You're welcome.

If you mean side-mount and "spinning end-over-end", then you are rotating around the intermediate axis and it is instantly unstable.

Nope.

If you mean side-mount and spinning flat like a frisbee,

We have a winner!

If you do know what "primary axis" means, why the confusion? I explicitly said "rotating around the primary axis" multiple times... 

then you have massively altered the way the propellant will settle

"Massively" is one of those content-free, unnecessary intensifier words people use when their technical argument is weak.

Yes, you might need some pipes (emphasis on "might"; I think if you're clever you can eliminate them). Fortunately these are short pipes that are entirely contained within the propellant tanks, so you're not adding additional heat gain.

and it makes the CoM issue vastly worse.

There go those intensifier words again...

The complexity of this configuration makes other suggestions look like child's play.

Not really.  Argument By Flowery Exaggeration isn't a thing. 

[Twark_Main]

When we're talking about accelerations, is the intent to transfer fuel using the inertial pressure head, or is the intent to just use it to ensure settling

The second one. Team "pump" all the way. 

Once stable, I think you don't need to continue acceleration, since surface tension will want to keep the boundary layer perpendicular to the ship axis (for minimal surface area) - unless disturbed by slosh in the depot.

Disturbances will cause waves, and if waves start to break, that's where trouble starts.  So have to flow slow.

Exactly. This is what leads to the mathematical invariant that kills linear ullage.

If you have low ullage gravity, you need extremely slow flow. Momentum = thrust x time, so going slower doesn't actually reduce the propellant required. It just spreads it out over a longer burn.

[Twark_Main]

Another issue to consider:  As prop moves from one to the other, the CoMs of the individual ships will move, which will put shear stresses on the docking mechanisms.

There's some non-zero sheer stress (which I note everyone was just hand-worrying about this for linear ullage too  ).  Fortunately such attachments (flat plates in direct contact connected by pins) are naturally strong in sheer (so even if it needs some strengthening it won't be heavy), and any bending or twisting moments are taken up by the struts.

I think this also tilts the axis (really axes) of inertia, which makes the rotational instability question soooooo much more complicated.

It offsets the axis, but it doesn't tilt it. So the analysis isn't complicated, just the parallel axis theorem.

[Twark_Main]

I was thinking more that (at least for the side-by-side plus frisbee version) changing CoM changes where the propellant sits, sufficiently that you might not have a single area for a pump inlet. Which changes the design even more than just "put a second sump where you expect it to sit".  To the point of having to redesign the shape of the tanks.

I still doubt this is necessary. If it is, you might change the tank shape or add a couple valves. So the real "penalty" (again, if even necessary) would be whichever of those two options works out better in the engineering trade analysis.

[TheRadicalModerate]

Another issue to consider:  As prop moves from one to the other, the CoMs of the individual ships will move, which will put shear stresses on the docking mechanisms.

Given the extremely low g-levels being discussed in the thread, I don't think that will be a significant issue. They'll need to handle multiple-percentage-of-1g loads during actual RPOD (including from having a surge of propellant bouncing off the walls when the ships contact), so I don't think the sustained (shifting) loads during prop-transfer will be larger than that.

Unless it moves the moment cross-terms enough that the system suddenly tumbles.  Speaking of which, let me interleave Twark in here:

I think this also tilts the axis (really axes) of inertia, which makes the rotational instability question soooooo much more complicated.

It offsets the axis, but it doesn't tilt it. So the analysis isn't complicated, just the parallel axis theorem.

No, you're changing the moments of all those pesky cross terms, because prop is moving in at least two different dimensions (from ship to ship, and prop level to prop level).  That means that the actual axes rotate, so it's more than the parallel axis theorem at play.

That said, when somebody says the words "eigenvalue" or "eigenvector", I run screaming from the room.

I was thinking more that (at least for the side-by-side plus frisbee version) changing CoM changes where the propellant sits, sufficiently that you might not have a single area for a pump inlet. Which changes the design even more than just "put a second sump where you expect it to sit".  To the point of having to redesign the shape of the tanks.

If you presuppose stability around either the y- or z-axis, I don't see that being a problem.  However, there are cases where prop gets pinned on the wrong side CoM, and can't be accessed.  (I keep wondering if there are cases where it gets pinned early in a spin and, by so doing, it changes the CoM enough that it can't later be unpinned.)

That said, I'm not particularly worried about this, because the idea of spin-settling prop seems bat-guano crazy.

Since nobody picked up on it, I'm gonna pound the table on prop management devices down in the sumps.

I don't know enough about prop behaviour in micro-g. I do note that most cryo liquids have much lower surface tension than water. Like order-of-magnitude lower. That seems to make any "blob" based solution harder.

I'm pretty ignorant in this area as well.  I do know that NASA has played with some cryo PMDs, but I don't know whether they've been nudging them up the TRL ladder or not.

Water isn't really the standard; MMH/NTO is.  But even there, the cryo surface tensions are likely lower.  That said, you don't have to get very clever with a PMD when you have Starship-sized mass margins to play with.  I'd guess that the mass of a pretty fine-grained screen with some ports in it would be more than offset by the amount of prop you'd save in settling acceleration.

[Twark_Main]

I think this also tilts the axis (really axes) of inertia, which makes the rotational instability question soooooo much more complicated.

It offsets the axis, but it doesn't tilt it. So the analysis isn't complicated, just the parallel axis theorem.

No, you're changing the moments of all those pesky cross terms, because prop is moving in at least two different dimensions (from ship to ship, and prop level to prop level).  That means that the actual axes rotate, so it's more than the parallel axis theorem at play.

The dorsal-to-dorsal "frisbee" configuration is the lowest energy configuration, and that doesn't change with different propellant levels. So the axis can't rotate.  The primary axis will always be perpendicular to the "frisbee" plane.

That said, I'm not particularly worried about this, because the idea of spin-settling prop seems bat-guano crazy.

That description seems more apt for the idea of burning 10-20% of your entire propellant upmass burning ullage thruster for hours, when you could spin + despin with the same thruster in....  [math]... <10 minutes. 

That said, you don't have to get very clever with a PMD when you have Starship-sized mass margins to play with.

Ick.  More of this resting-on-SpaceX's-laurels, "they made yesterday's good decisions so I can make tomorrow's inefficient one" attitude. We know SpaceX doesn't work that way!   

This too shall be optimized.  SpaceX is still going to find the best overall strategy, and within that find the best implementation. I've placed my bet.

[aporigine]

snip~
Ick.  More of this resting-on-SpaceX's-laurels, "they made yesterday's good decisions so I can make tomorrow's inefficient one" attitude. We know SpaceX doesn't work that way!   

This too shall be optimized.  SpaceX is still going to find the best overall strategy, and within that find the best implementation. I've placed my bet.

Please examine the following idea for a different way to get cheap, stable spin settling with ventral/ventral or bidorsal docking.

It involves “wasting” one Ship that is tethered to nose of Depot by maybe a mile of cable. The array is slowly spun. I imagine docking Tanker or Ship-taking-fuel with Depot will not be more complex than other RPOD schemes.

The two advantages I see here are
1) sustained settling acceleration for cheap
2) acceleration direction aligns with launch configuration (on both active systems) for which fuel-refuel plumbing is optimized.
3 of 2) Higher settling accelerations are available, expediting transfer ops.

[meekGee]

Please examine the following idea for a different way to get cheap, stable spin settling with ventral/ventral or bidorsal docking.

It involves “wasting” one Ship that is tethered to nose of Depot by maybe a mile of cable. The array is slowly spun. I imagine docking Tanker or Ship-taking-fuel with Depot will not be more complex than other RPOD schemes.

The two advantages I see here are
1) sustained settling acceleration for cheap
2) acceleration direction aligns with launch configuration (on both active systems) for which fuel-refuel plumbing is optimized.
3 of 2) Higher settling accelerations are available, expediting transfer ops.

That's nice, if the dynamics work out.

You could use two depots spinning about their shared c.m. so you don't waste the ballast ship.

[TheRadicalModerate]

I haven't gone through a very thorough audit of these to double-check them, but all of the plausible v3 configurations¹ we'd care about have the center of mass deep in the LOX tank, which means that spinning them along either the y- or z-axis will cause some of the LOX and all of the LCH4 to migrate up to the top of the tanks, where it'll be inaccessible for transfer.  So unless aporigine's scheme for using a third Starship as tethered ballast works out, spinning seems... well, I'll back off of "bat guano crazy" to "extremely unlikely".

____________
¹Numbers attached.  Notes:

1) Everything more-or-less based on a v3 propulsion section.

2) Again, poorly audited so far.

3) I'm assuming that tankers and depots have the common and LCH4 domes pushed forward to carry another 200t of prop to LEO.  Some payload barrels removed .

4) LSSes have lots of barrels removed, so the bottom of the ogive section is only about 3.5m above the garage deck.

5) Note that LSSes with stowed vs. deployed landing legs are different heights.

6) Tilt angle is computed assuming 4 5m-long legs, which, when deployed at a 30º angle to the ship, results in a centerline-to-leg-pad length of 8.8m.

[Twark_Main]

where it'll be inaccessible for transfer.

I'll say it again... if you really do run out of cleverness¹, pipes. 

Still way less mass than linear ullage, and lower risk (and R&D, and probably mass too) than adding a bunch of paramagnetic fluid management pickup hardware everywhere.


_{¹ which I doubt actually, you're just (currently) motivated to make the concept fail rather than succeed}

[meekGee]

where it'll be inaccessible for transfer.

I'll say it again... if you really do run out of cleverness¹, pipes. 

Still way less mass than linear ullage, and lower risk (and R&D, and probably mass too) than adding a bunch of paramagnetic fluid management pickup hardware everywhere.


_{¹ which I doubt actually, you're just (currently) motivated to make the concept fail rather than succeed}

If nobody noticed this little flow flaw before, don't blame the messenger.

Not saying pipes are impossible, but that's a significant kink in the plan to overlook.

[TheRadicalModerate]

where it'll be inaccessible for transfer.

I'll say it again... if you really do run out of cleverness¹, pipes. 

Still way less mass than linear ullage, and lower risk (and R&D, and probably mass too) than adding a bunch of paramagnetic fluid management pickup hardware everywhere.


_{¹ which I doubt actually, you're just (currently) motivated to make the concept fail rather than succeed}

Wanna take a stab at prop losses chilling down those pipes for (and during) transfer?

[Greg Hullender]

I'll say it again... if you really do run out of cleverness¹, pipes. 

It's not cleverness. It's that we've been told repeatedly by people who claim to know that cryogenic plumbing is a big deal, and that if you propose something that needs a lot of new plumbing, it's going to be dead-on-arrival unless there really is no other way to do it.

[Twark_Main]

If nobody noticed this little flow flaw before, don't blame the messenger.

Not saying pipes are impossible, but that's a significant kink in the plan to overlook.

No flaw was overlooked, I just didn't think I had to spell out the obvious.  I mentioned my solution so I thought that was enough hand-holding.

Wanna take a stab at prop losses chilling down those pipes for (and during) transfer?

The pipes that are already inside the cryogenic tank? 

The depot is going to be fully insulated, and the tankers are going to be mostly insulated with the insulated side pointed at the Sun & Earth¹. If you can't manage heat gain into the tanks, you have bigger problems than pipe loss.

What do you calculate for "prop losses chilling down pipes for (and during) transfer" for the linear ullage approach? Or are you making special objections because you're (as I speculate) "motivated to make the concept fail rather than succeed?" 


_{¹ I already know what someone is going to inevitably say to this, and yes I already addressed it, almost two years ago, in this very thread}

[Twark_Main]

I'll say it again... if you really do run out of cleverness¹, pipes. 

It's not cleverness. It's that we've been told repeatedly by people who claim to know that cryogenic plumbing is a big deal, and that if you propose something that needs a lot of new plumbing, it's going to be dead-on-arrival unless there really is no other way to do it.

You misunderstand me. The "cleverness" I mention is employed to reduce or eliminate the need for plumbing. And yes, I still don't think we've run out of it. 

Word choice is telling. Since you say that "lots" of new plumbing is the problem, you acknowledge that some, non-zero amount of new plumbing is okay, but it should generally be minimized. So in other words, we agree.

[Greg Hullender]

Please examine the following idea for a different way to get cheap, stable spin settling with ventral/ventral or bidorsal docking.

It involves “wasting” one Ship that is tethered to nose of Depot by maybe a mile of cable. The array is slowly spun. I imagine docking Tanker or Ship-taking-fuel with Depot will not be more complex than other RPOD schemes.

The two advantages I see here are
1) sustained settling acceleration for cheap
2) acceleration direction aligns with launch configuration (on both active systems) for which fuel-refuel plumbing is optimized.
3 of 2) Higher settling accelerations are available, expediting transfer ops.

One way to potentially make this work that would even be stable is if your sidereal spin rate is the same as your sidereal period of revolution. That is, from the perspective of the Earth, the depot is always down and the counterweight is always up. (Or vice versa.) The cable might need to be a hundred kilometers long or so (I keep meaning to estimate this, and I keep forgetting to), but that's not a big deal. Your settling acceleration comes from tidal forces, so you don't need to worry about the usual problems of getting a stable rotating system.

However, I couldn't figure out how to cope with the problems that occur when you add/remove propellant to/from the depot. Unless the counterweight is very heavy, the center of mass is going to move a lot and the thing is going to swing. The oscillations will damp out over some period of time (but I haven't worked that out either). Still, the sideways thrust to stabilize are probably a lot less than what's required for an hours-long ullage burn.

[Twark_Main]

Spin-gravity isn't a great example of this (most of the problems are due to people just not reading), but...



This forum has a funny, narrow idea of the design process. Everyone acts like if you don't immediately emit a set of full technical drawings and operations manuals (exaggerating obviously, but only slightly), then it's worthless and must be ridiculed and mocked and "debunked." However this inability to engage in those early, nebulous, "sketching" parts of the design process means we can't produce anything new. We tend to get locked into old approaches from the past. Anything that requires creativity isn't given a chance to be a baby.

NSF prefers instead to murder it in the crib, because it dares to not be a fully-formed adult. 

This makes it impossible to collaboratively produce anything new. People act like we can only ever examine a new idea if some genius emerges from their study and hands us a fully-formed plan, and anything less means instant death. 


Again I think spin-ullage isn't the best example (since it's reasonably fully-formed), but it's just a tendency I've noticed here.

[Twark_Main]

Please examine the following idea for a different way to get cheap, stable spin settling with ventral/ventral or bidorsal docking.

It involves “wasting” one Ship that is tethered to nose of Depot by maybe a mile of cable. The array is slowly spun. I imagine docking Tanker or Ship-taking-fuel with Depot will not be more complex than other RPOD schemes.

The two advantages I see here are
1) sustained settling acceleration for cheap
2) acceleration direction aligns with launch configuration (on both active systems) for which fuel-refuel plumbing is optimized.
3 of 2) Higher settling accelerations are available, expediting transfer ops.

One way to potentially make this work that would even be stable is if your sidereal spin rate is the same as your sidereal period of revolution. That is, from the perspective of the Earth, the depot is always down and the counterweight is always up. (Or vice versa.) The cable might need to be a hundred kilometers long or so (I keep meaning to estimate this, and I keep forgetting to), but that's not a big deal. Your settling acceleration comes from tidal forces, so you don't need to worry about the usual problems of getting a stable rotating system.

However, I couldn't figure out how to cope with the problems that occur when you add/remove propellant to/from the depot. Unless the counterweight is very heavy, the center of mass is going to move a lot and the thing is going to swing. The oscillations will damp out over some period of time (but I haven't worked that out either). Still, the sideways thrust to stabilize are probably a lot less than what's required for an hours-long ullage burn.

If we assume 0.1 mm/s² and rotating once every 90 minutes, that's a radius of ~3 km.

https://futureboy.us/fsp/frink.fsp?fromVal=%280.1+mm%2Fs%2Fs%29+%2F+%281%2F%2890+min%29%29%5E2&toVal=#calc