Author Topic: Starship On-orbit refueling - Options and Discussion  (Read 629852 times)

Offline TheRadicalModerate

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1400 on: 09/02/2022 03:18 am »
I doubt that a single set of bellows hoses will support the torques that an almost-full depot can exert against a completely full lift tanker...
I don't think anybody is suggesting bellows hoses would be structural. They just carry fluid. Am I misunderstanding?

Seems to me that there are only three possibilities:

1) The hoses are so flexible that formation-flying works and you just have to figure out how to get the hoses connected.  This has been discussed--I'm skeptical that the station-keeping is good enough to make this reliable.

2) The hoses are rigid enough to bear structural loads that hold the two ships together.  I, like you, think this is unlikely.

3) Some other structure bears the loads and the hoses don't have to worry about bearing any loads other than pressure.  This then decomposes into two sub-cases:
a) The load-bearing members are very close to the hoses.
b) They're farther away from the hoses.

If you had, say, just the QD plate and nothing else, that would be sub-case a), and I'm skeptical that it work work.  If there are structural members within, say, a meter of the hoses, would that work?  I don't know.

On the other hand, if you have one structural member latched onto the QD and one latched several meters away, I'm pretty confident the pair would bear the loads pretty easily.

In my little cartoons above, it's pretty clear the case #4, for all of its nice features, is the worst case in term of torsion.  (Well, nose-to-nose would be worse, but it's also dumb.)  But you can probably construct some set of trusses that will bear loads a meter or so from the hoses.  Is that good enough?


Offline Asteroza

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1401 on: 09/02/2022 04:46 am »
Dumb question, but does use of flexible hoses add the additional problem of both hose pressure stiffening the hose and whipping around both vehicles, and such movement also buckling the hose, altering flowrate?

Offline OTV Booster

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1402 on: 09/02/2022 05:12 am »
I doubt that a single set of bellows hoses will support the torques that an almost-full depot can exert against a completely full lift tanker...
I don't think anybody is suggesting bellows hoses would be structural. They just carry fluid. Am I misunderstanding?

Seems to me that there are only three possibilities:

1) The hoses are so flexible that formation-flying works and you just have to figure out how to get the hoses connected.  This has been discussed--I'm skeptical that the station-keeping is good enough to make this reliable.

2) The hoses are rigid enough to bear structural loads that hold the two ships together.  I, like you, think this is unlikely.

3) Some other structure bears the loads and the hoses don't have to worry about bearing any loads other than pressure.  This then decomposes into two sub-cases:
a) The load-bearing members are very close to the hoses.
b) They're farther away from the hoses.

If you had, say, just the QD plate and nothing else, that would be sub-case a), and I'm skeptical that it work work.  If there are structural members within, say, a meter of the hoses, would that work?  I don't know.

On the other hand, if you have one structural member latched onto the QD and one latched several meters away, I'm pretty confident the pair would bear the loads pretty easily.

In my little cartoons above, it's pretty clear the case #4, for all of its nice features, is the worst case in term of torsion.  (Well, nose-to-nose would be worse, but it's also dumb.)  But you can probably construct some set of trusses that will bear loads a meter or so from the hoses.  Is that good enough?
Well, the only thing I can think of along these lines is the depot QD mounted on a pantograph or possibly some jack screws so it can extend a bit to the tanker QD. I can't see this by itself as structurally sufficient for a safe lashup. I'm addressing two ships dorsal to dorsal facing in the same direction.


A close alternative would be the depot QD mounted on the face of a dog house blistering out a bit from he hull. Might need a retractable cover to protect the connections during ascent and EDL if it's intended to return.


The QD plates have locking lugs to pull them firmly together. The fluid connections themselves might each need a little float to allow self centering. The thermal environment will be all over the place and the dimensions will change.


Speaking of thermal, the whole ship(s) will most likely change dimensions throughout orbit. This will be a challenge for any struts holding the two ships in place. Maybe some intentional slop where they connect. I wonder how much an empty starship would warp with full sun on one side for 50 minutes. Propellant would modify this but it gives an upper limit.


However the two ships get hitched, I don't think they will spontaneously start wild gyrations. More likely little things from transferring propellants or small misalignment from ullage thrust. If something weird does start, a stuck thruster for example, the software will need a fast acting abort mode.



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Offline TheRadicalModerate

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1403 on: 09/02/2022 06:21 am »
However the two ships get hitched, I don't think they will spontaneously start wild gyrations. More likely little things from transferring propellants or small misalignment from ullage thrust. If something weird does start, a stuck thruster for example, the software will need a fast acting abort mode.

The fast acting abort only works if there's a solution where the ships don't collide post-disconnect.  It's not immediately apparent to me that such a solution always exists--especially if one of the ships is having a thruster malfunction.

The way to avoid problems is with a rigid connection.

Offline TheRadicalModerate

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1404 on: 09/02/2022 06:36 am »
A question:  Where is the LOX downcomer for the nose header tank routed?  Is there an external chine with a line that connects it to the LOX main tank?  Or is it routed through the methane tank?

All of the top of methane tank-to-QD configurations (i.e., #3 and #4 in my diagrams) rely on a LOX line being available to grab in the payload bay.  Without that line, the QD-to-QD configuration (#2) starts to make more sense.

Offline daavery

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1405 on: 09/02/2022 06:44 am »
the LOX downcomer runs along the center of the ventral(windward) side.

Offline john smith 19

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1406 on: 09/02/2022 06:52 am »
I'm very late to this as I was offline for so long.

But wasn't at least some of these problems sorted out when FH was talking about cross-filling during flight?

Once SX decided on specialst "core" stages for FH  that would mean abandoning the idea of gender-neutral connectors

Likewise from Armadillio Aerospace's experience with differential tank pressures we know it can take as little as 2psi to push flow between different propellant tanks of the same type (which played havoc with ending feed).

And of course orbital loading will have a lot more time to play with than the few minutes of an FH flight.
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Offline Asteroza

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1407 on: 09/02/2022 10:08 am »
However the two ships get hitched, I don't think they will spontaneously start wild gyrations. More likely little things from transferring propellants or small misalignment from ullage thrust. If something weird does start, a stuck thruster for example, the software will need a fast acting abort mode.

The fast acting abort only works if there's a solution where the ships don't collide post-disconnect.  It's not immediately apparent to me that such a solution always exists--especially if one of the ships is having a thruster malfunction.

The way to avoid problems is with a rigid connection.

The "Nauka's gone wild" problem. Do you hang on for dear life and fight it, or run.

Offline edzieba

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1408 on: 09/02/2022 12:57 pm »
The bellows are an complete nonissue: the 'problem' of how to have flexible cryogenic couplings on a rigid but moveable linkage without those bellow being load-bearing is not only a solved problem, but there is a solution currently sat on top of the arm halfway up the orbital launch tower, with said solution having already demonstrated the capability to move around, connect to a ship, and transfer cryogenic propellants.
3) Dorsal-to-dorsal nose-to-tail, overlapped docking.  This could be androgynous, but it can also have full plumbing coming out of one payload bay and a stub that just stabilizes things in the other.  (I think I've given up on this one.)

4) Dorsal-to-dorsal, noses in the same direction, but overlapped docking, with the payload bay of one close to the tail QD of the other.  In this case, the single grapple and the plumbing are all that's taking the torsional loads.

OTV Booster seems to be on Team #2.  I think edzieba and I are mostly on Team #4 (although we disagree on whether the torsional loads are a problem).  I used to like #3, but I think #4 does everything it does and is simpler, moments of inertia permitting.
I'm in camp 3, because it means all your drain-propellant-in-orbit-for-transfer plumbing is at the apex of the tanks and out of the way during normal operations, rather than at the nadir where all the drain-propellant-for-propulsion plumbing also needs to be. It also minimises torque about the connection, and minimises the movement of the centre of mass of the system (think of the setup as a heavy glob of propellants that some rigid lightweight shells happen to move around the outside of). It also allows for a 'bumper' to be deployed at the 'unused' (or inactive) nose-to-tail meeting point to aid bracing, if needed.

Offline LMT

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1409 on: 09/02/2022 02:54 pm »
Dumb question, but does use of flexible hoses add the additional problem of both hose pressure stiffening the hose and whipping around both vehicles, and such movement also buckling the hose, altering flowrate?

However the two ships get hitched, I don't think they will spontaneously start wild gyrations. More likely little things from transferring propellants or small misalignment from ullage thrust. If something weird does start, a stuck thruster for example, the software will need a fast acting abort mode.

The fast acting abort only works if there's a solution where the ships don't collide post-disconnect.  It's not immediately apparent to me that such a solution always exists--especially if one of the ships is having a thruster malfunction.

The way to avoid problems is with a rigid connection.

The "Nauka's gone wild" problem. Do you hang on for dear life and fight it, or run.

Remember, single-tanker refill would likely be a brief experimental phase.  It wouldn't make sense to dedicate a fleet of Starship tankers to depot duty, long-term.  At fleet scale, inflatable passive cryogenic tanks would be far more mass-efficient and cost-effective. 

If you deployed inflatables on a rigid truss, that truss could also give rigid mounting points for docks, with robust cradling connectors for QD plates and hulls.  A station-keeping engine is needed, but little more.  Such a structure could retire tanker-dock concerns such as crimped hoses and gyration.


Offline OTV Booster

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1410 on: 09/02/2022 02:56 pm »
However the two ships get hitched, I don't think they will spontaneously start wild gyrations. More likely little things from transferring propellants or small misalignment from ullage thrust. If something weird does start, a stuck thruster for example, the software will need a fast acting abort mode.

The fast acting abort only works if there's a solution where the ships don't collide post-disconnect.  It's not immediately apparent to me that such a solution always exists--especially if one of the ships is having a thruster malfunction.

The way to avoid problems is with a rigid connection.
'Tween the devil and the deep blue sea. A tough call. If things go sideways and a tanker and depot are at risk it's one thing. If a crewed ship is involved it's very different. How to deal with it would call for a detailed analysis of potential failure modes and a lot of modeling. Decisions on possible control inputs, yada, yada. If there comes a point where there's nothing to loose, ya gotta go for it. Rigid connection or not.


I agreed with a rigid connection until thermal thoughts crept in. Ever notice that everything on the ISS is linear attachment? I often thought that running two or more rows of modules with cross connect access would give more rigidity. It would be a great help in giving the truss a broader footprint, during boost burns, in reducing moment when changing attitude and even in reducing MMOD armor,  but it's not done. My conjecture is differential heating would cause unmanageable stress.


Assume one standoff up towards the nose about 30m off. One solution might be a pivot at either end of the stand-off that would allow some swing fore and aft. If the ship expands 5mm (example only) in length between the QD and the standoff, and the separation is a nominal 1m, the change in angle between ships would too small to worry about, except for the control software. The latched QD's should be the unmoving origin of the coordinate system. Plug in other numbers if desired.


It's not exactly a rigid connection but maybe good enough?
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Offline OTV Booster

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1411 on: 09/02/2022 03:23 pm »
Dumb question, but does use of flexible hoses add the additional problem of both hose pressure stiffening the hose and whipping around both vehicles, and such movement also buckling the hose, altering flowrate?

However the two ships get hitched, I don't think they will spontaneously start wild gyrations. More likely little things from transferring propellants or small misalignment from ullage thrust. If something weird does start, a stuck thruster for example, the software will need a fast acting abort mode.

The fast acting abort only works if there's a solution where the ships don't collide post-disconnect.  It's not immediately apparent to me that such a solution always exists--especially if one of the ships is having a thruster malfunction.

The way to avoid problems is with a rigid connection.

The "Nauka's gone wild" problem. Do you hang on for dear life and fight it, or run.

Remember, single-tanker refill would likely be a brief experimental phase.  It wouldn't make sense to dedicate a fleet of Starship tankers to depot duty, long-term.  At fleet scale, inflatable passive cryogenic tanks would be far more mass-efficient and cost-effective. 

If you deployed inflatables on a rigid truss, that truss could also give rigid mounting points for docks, with robust cradling connectors for QD plates and hulls.  A station-keeping engine is needed, but little more.  Such a structure could retire tanker-dock concerns such as crimped hoses and gyration.


I'm unclear. Do you propose the inflatables be delivered as cargo with propellant already loaded or empty to be filled by tankers?
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Offline OTV Booster

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1412 on: 09/02/2022 04:49 pm »
Thinking it through, neither way makes sense in the short to medium term. Long term - maybe.


The truss with QD would have the same function as the free flyer tricked out with all the goodies. And it would have a lot more structure to hold the inflatables. The free flyer OTOH, would only need enough structure to hold the goodies and two QD's.


The free flyer is intended to mate up with an unmodified tanker carrying the first load of propellant, which would take on the role of accumulator. After other tankers offload into it and it transfers its load to the receiving ship, it's free to return earthwards and continue life as a plain ol tanker. The free flyer might stay on orbit for another refueling campaign or return. Too many hypothetical moving parts to know for sure.


The tanker/accumulator serves as both tankage and structure, removing the need for the inflatables and the structure to hold them. The tanker/accumulator would be out of tanker service for only one campaign and it would start off the campaign delivering the first load of propellant. If the whole system is honed and oiled to where a tanker can be launched every 12 hours AND a returning tanker can be turned around fast enough to participate, one tanker staying on orbit for the entire campaign makes little difference.


The only advantage obvious in a truss with inflatables is the ability to hold more than one tanker/accumulator worth of propellant. Even this advantage is questionable because the ship design is expected (not a done deal) to evolve. Once a tanker design has been superseded it becomes a candidate for one last flight and hulk status as tankage.


The truss itself is not a bad idea. The core of a free flyer is a box with two QD's. It might or might not have tankage beyond that needed to deploy onto the accumulator. It would be sandwiched between the accumulator and another ship and it would have some combination of PV/radiator/cryocooler and MLI. It needs some structure for all of this and a (probably) telescoping truss seems a natural fit. Again, a lot of hypothetical moving parts.
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Offline TheRadicalModerate

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1413 on: 09/02/2022 06:24 pm »
the LOX downcomer runs along the center of the ventral(windward) side.

Where does it go when it gets to the LCH4 dome?  Through it?  It can't get routed to the outside of the vehicle if it's on the ventral side, because it would interfere with the TPS.

Offline edzieba

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1414 on: 09/02/2022 06:29 pm »
the LOX downcomer runs along the center of the ventral(windward) side.

Where does it go when it gets to the LCH4 dome?  Through it?  It can't get routed to the outside of the vehicle if it's on the ventral side, because it would interfere with the TPS.
Through it, inside. Flyover images of previous ships in the process of being scrapped show where it penetrates the forward and common domes, in the middle of the ventral side.

Offline LMT

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1415 on: 09/02/2022 07:25 pm »
The tanker/accumulator serves as both tankage and structure, removing the need for the inflatables and the structure to hold them.. 

The only advantage obvious in a truss with inflatables is the ability to hold more than one tanker/accumulator worth of propellant. Even this advantage is questionable because the ship design is expected (not a done deal) to evolve. Once a tanker design has been superseded it becomes a candidate for one last flight and hulk status as tankage.

No, the $ advantage of inflatables is a depot needing very few tankers. 

If a tanker had 12-hour turnaround, and it delivered 1/5 of a load, it could fill an inflatable depot for ~ 300 crews each synod, by itself. 

Otherwise, a fleet of tankers would be needed.  E.g., given a 14-day window and preloading of depot tankers at synod start, at least 46 tankers would be needed. 

Scale up ~ 10x for settlement cargo flights.
« Last Edit: 09/02/2022 07:31 pm by LMT »

Offline TheRadicalModerate

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1416 on: 09/02/2022 08:15 pm »
The bellows are an complete nonissue: the 'problem' of how to have flexible cryogenic couplings on a rigid but moveable linkage without those bellow being load-bearing is not only a solved problem, but there is a solution currently sat on top of the arm halfway up the orbital launch tower, with said solution having already demonstrated the capability to move around, connect to a ship, and transfer cryogenic propellants.

The arm can handle a small amount of translation, from wind and thermal expansion.  But the amount of torsion on orbit in an emergency is much, much larger.  There's no way that the arm hardware could withstand that.
 
Quote
3) Dorsal-to-dorsal nose-to-tail, overlapped docking.  This could be androgynous, but it can also have full plumbing coming out of one payload bay and a stub that just stabilizes things in the other.  (I think I've given up on this one.)

4) Dorsal-to-dorsal, noses in the same direction, but overlapped docking, with the payload bay of one close to the tail QD of the other.  In this case, the single grapple and the plumbing are all that's taking the torsional loads.

OTV Booster seems to be on Team #2.  I think edzieba and I are mostly on Team #4 (although we disagree on whether the torsional loads are a problem).  I used to like #3, but I think #4 does everything it does and is simpler, moments of inertia permitting.
I'm in camp 3, because it means all your drain-propellant-in-orbit-for-transfer plumbing is at the apex of the tanks and out of the way during normal operations, rather than at the nadir where all the drain-propellant-for-propulsion plumbing also needs to be. It also minimises torque about the connection, and minimises the movement of the centre of mass of the system (think of the setup as a heavy glob of propellants that some rigid lightweight shells happen to move around the outside of). It also allows for a 'bumper' to be deployed at the 'unused' (or inactive) nose-to-tail meeting point to aid bracing, if needed.

When I went and actually drew this out, I got horribly confused, but Option #2 starts to look a lot better.  Take a look and see what you think.  Option #3 is really complicated, although it does indeed separate the transfer sources from the engine sumps, and it's the most mechanically stable.  Note that the pros and cons haven't been thought through completely, but are a nice basis for discussion (or the hurling of rotten fruit).

 ¯\_(ツ)_/¯

Offline OTV Booster

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1417 on: 09/02/2022 08:15 pm »
The tanker/accumulator serves as both tankage and structure, removing the need for the inflatables and the structure to hold them.. 

The only advantage obvious in a truss with inflatables is the ability to hold more than one tanker/accumulator worth of propellant. Even this advantage is questionable because the ship design is expected (not a done deal) to evolve. Once a tanker design has been superseded it becomes a candidate for one last flight and hulk status as tankage.

No, the $ advantage of inflatables is a depot needing very few tankers. 

If a tanker had 12-hour turnaround, and it delivered 1/5 of a load, it could fill an inflatable depot for ~ 300 ships each synod, by itself. 

Otherwise, a fleet of tankers would be needed.  E.g., given a 14-day window and preloading of depot tankers at synod start, at least 46 tankers would be needed. 

Scale up ~ 10x for settlement cargo flights.
What size inflatables are we talking about?


What you're talking about would come under what I call long term. Definitely more than 5 years. Probably under 10 but not at the scale you envision. Still, hulk tankers would be a lot less R&D, and the hardware would already be in hand. Has anybody ever played with cryo temp inflatables?


On the number of tankers, we might have a terminology problem here. What I call a tanker is the ship that has no other job than lifting propellant onto orbit, transferring it to an accumulator or a depot, then going home. An accumulator would structurally be a tanker but with a more specialized role. Somewhat the same for depots.


Yes accumulators and depots would be stacking up  and at the scale described, a lot of them. It's an interesting question. How many hulk candidates would be available? Maybe not enough.


Something to consider. There are low energy transits to mars and they are not terribly sensitive to a specific launch window. The downside is that they take longer. This isn't a problem with cargo. I think a year long cargo launch campaign would work, but IANARS. Each launch would be more direct and higher energy than the last.


As an ideal, everything would land before the crew mission lights off but probably not realistic. Anyway, spreading the launches does away with the need for massive amounts of propellant on orbit at one time.


When multiple settler ships are launching the equation changes and this idea starts having traction. We might be talking 12-15 years. By that time we might have space hardened bacteria that can link pseudopods and move in a line to poop out a bio printed tank. Arthur C. Clark! Where are you when I need a story written?
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Offline spacenut

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1418 on: 09/02/2022 08:28 pm »
Why did SpaceX abandon end to end refueling?  To me that would be the simplest and no flexible connections needed. 

Offline LMT

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1419 on: 09/02/2022 09:23 pm »
No, the $ advantage of inflatables is a depot needing very few tankers. 

If a tanker had 12-hour turnaround, and it delivered 1/5 of a load, it could fill an inflatable depot for ~ 300 ships each synod, by itself. 

Otherwise, a fleet of tankers would be needed.  E.g., given a 14-day window and preloading of depot tankers at synod start, at least 46 tankers would be needed. 

Scale up ~ 10x for settlement cargo flights.

What you're talking about would come under what I call long term. Definitely more than 5 years. Probably under 10 but not at the scale you envision...  Has anybody ever played with cryo temp inflatables?

Crew-2029 is "definitely more than 5 years"...

Example:  A basic inflatable depot design:  "Antaios", patented in 2017.


« Last Edit: 09/02/2022 09:28 pm by LMT »

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