Starship On-orbit refueling - Options and Discussion

[Vultur]

The initial propellant test will be done even earlier at flight rate 10 per year, using two tankers launched several weeks apart. The first tanker will be subject to a significant boil-off so it is going to be a nice long term loiter experiment.

Boiloff is not a problem during the transfer demo. The pad will almost certainly be able to launch the ships on two consecutive days: that's what it's built to do. Commodities are the limiting factor initially.  Even if the second Ship (prototype Tanker) is long delayed, the first ship (prototype Depot) only needs to retain enough propellant to de-orbit if the Tanker fails to orbit and transfer.

Given that even Ship 39 (which isn't supposed to go to orbit at all) seems to have the "receiving" hardware, I would kind of expect the initial ship to not even be really a prototype Depot.

Call it whatever you like. It will have the four leg thingees. In my universe, only Depot has the leg thingees. All other Ships (tankers, HLS, etc.) have the socket thingees. Thus, that first Ship is a Depot. It does not need to be finalized design, so in my terminology, it's a prototype Depot.

I'm actually not sure if we disagree or if it's a wording issue.

What I'm suggesting is that the passive (socket, Tanker) ship is launched first (and may be a Starlink deployment test which is used for refueling tests afterward). The active (leg, Depot) ship is launched second; thus very little boiloff occurs because rendezvous happens shortly after the launch of the second ship.

In this model the prototype Depot is not first to launch, the prototype Tanker/HLS/passive ship is.

[DanClemmensen]

The initial propellant test will be done even earlier at flight rate 10 per year, using two tankers launched several weeks apart. The first tanker will be subject to a significant boil-off so it is going to be a nice long term loiter experiment.

Boiloff is not a problem during the transfer demo. The pad will almost certainly be able to launch the ships on two consecutive days: that's what it's built to do. Commodities are the limiting factor initially.  Even if the second Ship (prototype Tanker) is long delayed, the first ship (prototype Depot) only needs to retain enough propellant to de-orbit if the Tanker fails to orbit and transfer.

Given that even Ship 39 (which isn't supposed to go to orbit at all) seems to have the "receiving" hardware, I would kind of expect the initial ship to not even be really a prototype Depot.

Call it whatever you like. It will have the four leg thingees. In my universe, only Depot has the leg thingees. All other Ships (tankers, HLS, etc.) have the socket thingees. Thus, that first Ship is a Depot. It does not need to be finalized design, so in my terminology, it's a prototype Depot.

I'm actually not sure if we disagree or if it's a wording issue.

What I'm suggesting is that the passive (socket, Tanker) ship is launched first (and may be a Starlink deployment test which is used for refueling tests afterward). The active (leg, Depot) ship is launched second; thus very little boiloff occurs because rendezvous happens shortly after the launch of the second ship.

In this model the prototype Depot is not first to launch, the prototype Tanker/HLS/passive ship is.

That will work. I chose to launch Depot first and a Tanker second, but your sequence also works. Whichever ship launches first must keep enough propellant to keep itself cooled by boil-off or otherwise) until the second Ship shows up, plus enough to de-orbit itself if transfer fails.

[TheRadicalModerate]

In this model the prototype Depot is not first to launch, the prototype Tanker/HLS/passive ship is.

I wonder if the leg thingies are steerable and extendable.  That would make the capture look more like berthing than docking:  Finish prox ops with the active Ship close enough, then have the leg thingies steer and extend/retract to the appropriate socket thingy. After soft capture, the legs could apply forces to null out velocity residuals and misalignments, then retract until hard capture latches kicked in.  Seems like you could do it with three degrees of freedom (azimuth, altitude, and extension/retraction).

I don't think this necessarily requires the leg thingies to be on the active ship.  Essentially, the distinction between active and passive would disappear once the "prox ops" phase ended and the "berthing" phase began.  That would mean that the chaser could have the socket thingies and still be active for prox ops, with the target (what will eventually become the depot) then steering the leg thingies to soft capture.

That actually jibes pretty well with how berthing was done between ISS and CBM-based spacecraft, doesn't it?

[StraumliBlight]

Update on NASA’s Human Landing System (HLS) Program: Public-Private Partnership Advancing Artemis Sustainable Lunar Exploration [Aug 26]

NASA Marshall’s CFM Portfolio Project Office generated a best-practices report outlining broad guidelines for in-space cryogenic propellant transfer. The report is intended to help commercial companies in the early phases of concept of operations (conops) development and includes general procedures for tank to-tank transfer of cryogenic propellants for settled and unsettled conditions, as well as guidelines for cryocoupler construction, and safety considerations.

[...]

In 2026, SpaceX is expected to demonstrate the large-scale cryogenic propellant transfer test mentioned earlier, as well as testing the Starship HLS on-orbit for a long duration (see Figure 13). It is possible that these two tests may be performed back-to-back in Earth orbit. The objectives for the long-duration test would be performed first, with the large-scape propellant transfer test performed at the end of the orbital duration. The long-duration cryogenic propellant transfer test is expected to generate valuable data to help reduce the risk associated with orbital cryogenic propellant transfer.

Update on NASA’s Human Landing System (HLS) Program: Public-Private Partnership Advancing Artemis Sustainable Lunar Exploration [Aug 26]

[DanClemmensen]

In this model the prototype Depot is not first to launch, the prototype Tanker/HLS/passive ship is.

I wonder if the leg thingies are steerable and extendable.  That would make the capture look more like berthing than docking:  Finish prox ops with the active Ship close enough, then have the leg thingies steer and extend/retract to the appropriate socket thingy. After soft capture, the legs could apply forces to null out velocity residuals and misalignments, then retract until hard capture latches kicked in.  Seems like you could do it with three degrees of freedom (azimuth, altitude, and extension/retraction).

I don't think this necessarily requires the leg thingies to be on the active ship.  Essentially, the distinction between active and passive would disappear once the "prox ops" phase ended and the "berthing" phase began.  That would mean that the chaser could have the socket thingies and still be active for prox ops, with the target (what will eventually become the depot) then steering the leg thingies to soft capture.

That actually jibes pretty well with how berthing was done between ISS and CBM-based spacecraft, doesn't it?

I think it's still called "docking". "Berthing" is performed by crew using a general-purpose handling mechanism. It's just a name, though. My own mental model is docking, and the arms perform the soft-docking and mating phase. The are not general-purpose manipulators, and they have a restricted range of motions and capabilities compared to a Canadarm. They support what is really a fairly small capture hypersphere compared to a berthing situation.

Yes, the term "active" has two separate meanings in the docking  nomenclature. For rendezvous and proximity ops. It refers to the maneuvering craft (visiting craft, chaser) and distinguishes it from the non-maneuvering craft (target).  After the two are close enough and actual docking commences, "active" refers to the docking mechanism. The craft using its active soft-docking components is "active". This ambiguity is re-enforced by the fact that in the cases we usually see (Dragon, Starliner) the VV is active in both senses of the word.

In the case of fuel transfer, I think the craft with lower mass will perform the proximity ops, and I think the Depot (with arms) will perform the active docking. If thrusters are used at all during the actual docking phase (i.e., after the first initial physical contact) I suspect the two Ships will be under control of a single command system. For example the non-Depot might slave to the Depot.

[TheRadicalModerate]

I think it's still called "docking". "Berthing" is performed by crew using a general-purpose handling mechanism. It's just a name, though. My own mental model is docking, and the arms perform the soft-docking and mating phase. The are not general-purpose manipulators, and they have a restricted range of motions and capabilities compared to a Canadarm. They support what is really a fairly small capture hypersphere compared to a berthing situation.

I don't want to get hung up on the name either, but there are two different models, and the difference is extremely important:

1) One where the active vehicle rams (very gently!) the passive vehicle, and something soft-captures as a result.  I think that almost everybody will agree that this is "docking".

2) One where the active vehicle comes to as close to a full stop, both translationally and rotationally, as possible, very near the passive vehicle.  Then, one side or the other reaches out, grabs something (or 4 somethings, in this case), and effects the soft-capture.  I call this "berthing", but I accept your argument that it's not exactly the same as ISS-style berthing.  I'm open to a new word.  "Bercking"?  "Dorthing"?

I don't see how you can dock four leg thingies into four socket thingies close enough to simultaneously that weird torques don't ruin your day.

[DanClemmensen]

I think it's still called "docking". "Berthing" is performed by crew using a general-purpose handling mechanism. It's just a name, though. My own mental model is docking, and the arms perform the soft-docking and mating phase. The are not general-purpose manipulators, and they have a restricted range of motions and capabilities compared to a Canadarm. They support what is really a fairly small capture hypersphere compared to a berthing situation.

I don't want to get hung up on the name either, but there are two different models, and the difference is extremely important:

1) One where the active vehicle rams (very gently!) the passive vehicle, and something soft-captures as a result.  I think that almost everybody will agree that this is "docking".

2) One where the active vehicle comes to as close to a full stop, both translationally and rotationally, as possible, very near the passive vehicle.  Then, one side or the other reaches out, grabs something (or 4 somethings, in this case), and effects the soft-capture.  I call this "berthing", but I accept your argument that it's not exactly the same as ISS-style berthing.  I'm open to a new word.  "Bercking"?  "Dorthing"?

I don't see how you can dock four leg thingies into four socket thingies close enough to simultaneously that weird torques don't ruin your day.

I agree, let's ignore terminology. As to method: Once the active Ship has completed the proximity ops and placed itself in the capture hyper-elipsoid, One "leg"  will reach out and place its end ball into the socket at zero force and it will get latched by the socket. latching does not constrain rotational movement. It will then do the same with the remaining three end balls, still at zero force. only after all four are latched will it begin exerting forces to bring the Ships to relative rest and to bring them into a hard dock.

This is only my speculation, completely free of any actual knowledge of what SpaceX intends other than the renders and pictures of the sockets.

[TheRadicalModerate]

I agree, let's ignore terminology. As to method: Once the active Ship has completed the proximity ops and placed itself in the capture hyper-elipsoid, One "leg"  will reach out and place its end ball into the socket at zero force and it will get latched by the socket. latching does not constrain rotational movement. It will then do the same with the remaining three end balls, still at zero force. only after all four are latched will it begin exerting forces to bring the Ships to relative rest and to bring them into a hard dock.

This is only my speculation, completely free of any actual knowledge of what SpaceX intends other than the renders and pictures of the sockets.

That's close enough to what I was thinking.

As for what it means to early transfer tests:  It means that which Starship has the leg thingies and which has the socket thingies is irrelevant.  One will sit there, while the other does active prox ops until it stops.  Then whichever one has the leg thingies will probe around, doing disturbing things to the one with the socket thingies.

And that's before all the probing begins for seating the transfer hardware.  Lots of probing going on.

[DanClemmensen]

And that's before all the probing begins for seating the transfer hardware.  Lots of probing going on.

My completely speculative assumption is that the length-control actuators in the four legs are sufficiently precise to align and mate the actual hard docking hardware of propellant transfer interface. This interface may be the SQD interface on the non-Depot. If so the Depot will have its own ordinary SQD interface on its dorsal side and this "pad-side" SQD on its ventral side, or possibly further forward on its dorsal side if the ships dock with the Depot offset to aft.

[SpaceLizard]

If the four leg thingies can extend/retract on command and also pivot, wouldn't that allow stewart-platform-like control?

[DanClemmensen]

If the four leg thingies can extend/retract on command and also pivot, wouldn't that allow stewart-platform-like control?

This has been my assumption all along: each of the four can change its length with high precision and exert high and variable force, and each of them can change its direction in two rotations, and at least two of them can exert a fair amount of force on these directional movements. This is not a pure stewart platform (six purely linear actuators), but It still provides 6DOF within the capture hyper-ellipsoid. The required forces are substantial, because the momentum gets large when a 400-tonne Ship is moving relative to another Ship even at very low speeds.

[steveleach]

Is there any overlap between the force range the leg-thingies would be able to exert and the force range that RCS can? Or would the max force of the leg-thingies be lower than the min force of the RCS?

[jak Kennedy]

Why not have the transfer 'legs' and 'socket' crossed. What I mean by that is one leg and one socket on each side? It looks like the legs extend out so they must be usually stowed inside during assent. By having them crossed then any type of Starship could dock with any other type.

[StraumliBlight]

Why not have the transfer 'legs' and 'socket' crossed. What I mean by that is one leg and one socket on each side? It looks like the legs extend out so they must be usually stowed inside during assent. By having them crossed then any type of Starship could dock with any other type.

If all the complexity is kept on the Tanker side it can be serviced on the ground when a problem occurs.

[steveleach]

Why not have the transfer 'legs' and 'socket' crossed. What I mean by that is one leg and one socket on each side? It looks like the legs extend out so they must be usually stowed inside during assent. By having them crossed then any type of Starship could dock with any other type.

If all the complexity is kept on the Tanker side it can be serviced on the ground when a problem occurs.

I wouldn't be surprised if the depots also return to Earth after they are depleted.

If the plan is to position the depots where they are needed for each mission (rather than plan for the missions to travel via wherever the depots are), then you'd need to reposition them between missions, and it could be easier to simply return them to Earth before launching them into the next location.

[DanClemmensen]

Why not have the transfer 'legs' and 'socket' crossed. What I mean by that is one leg and one socket on each side? It looks like the legs extend out so they must be usually stowed inside during assent. By having them crossed then any type of Starship could dock with any other type.

If all the complexity is kept on the Tanker side it can be serviced on the ground when a problem occurs.

I wouldn't be surprised if the depots also return to Earth after they are depleted.

If the plan is to position the depots where they are needed for each mission (rather than plan for the missions to travel via wherever the depots are), then you'd need to reposition them between missions, and it could be easier to simply return them to Earth before launching them into the next location.

Depot should not be designed for EDL. EDL capability takes a lot of mass: TPS, control surfaces, header tanks, increased mass to support the catch, and increased mass to support the actual re-entry stresses. But Depot needs to be as light as possible to support efficient propellant transport.

Moving a near-empty Depot to a new orbit is cheaper than EDL and relaunch.

[spacenut]

Ok, why can't the stretch the booster using R3 engines and not stretch the Starship.  Get the Starship higher and faster, like Falcon 9 with it's upper stage.  Then maybe if the booster travels in an arc, could it reach Pad 39A and land there, refuel there and boost back to Starbase.  Then a depot Starship as well as a tanker would have more fuel load left in the depot and the tankers and have fewer tanker launches. 

[DanClemmensen]

Ok, why can't the stretch the booster using R3 engines and not stretch the Starship.  Get the Starship higher and faster, like Falcon 9 with it's upper stage.  Then maybe if the booster travels in an arc, could it reach Pad 39A and land there, refuel there and boost back to Starbase.  Then a depot Starship as well as a tanker would have more fuel load left in the depot and the tankers and have fewer tanker launches.

Booster is not designed to re-enter with that much energy. Look at how hot the engine bay gets during with the return profile from flights 10 and 11. If you want to hop a booster from Starbase to KSC you will need one or more intermediate points. If you add TPS mass to mitigate, you defeat your purpose.

[Twark_Main]

Ok, why can't the stretch the booster using R3 engines and not stretch the Starship.  Get the Starship higher and faster, like Falcon 9 with it's upper stage.  Then maybe if the booster travels in an arc, could it reach Pad 39A and land there, refuel there and boost back to Starbase.  Then a depot Starship as well as a tanker would have more fuel load left in the depot and the tankers and have fewer tanker launches.

Booster is not designed to re-enter with that much energy. Look at how hot the engine bay gets during with the return profile from flights 10 and 11. If you want to hop a booster from Starbase to KSC you will need one or more intermediate points. If you add TPS mass to mitigate, you defeat your purpose.

I think we need actual mathematical analysis to show that, not just asserting it is true.

If you only need to add 1 kg of mass, then that satisfies your antecedent ("you add TPS mass to mitigate") and yet it's pretty clear that it such a small addition of mass wouldn't really satisfy the consequent ("you defeat your point"). It depends on the actual numbers, clearly.

[DanClemmensen]

Ok, why can't the stretch the booster using R3 engines and not stretch the Starship.  Get the Starship higher and faster, like Falcon 9 with it's upper stage.  Then maybe if the booster travels in an arc, could it reach Pad 39A and land there, refuel there and boost back to Starbase.  Then a depot Starship as well as a tanker would have more fuel load left in the depot and the tankers and have fewer tanker launches.

Booster is not designed to re-enter with that much energy. Look at how hot the engine bay gets during with the return profile from flights 10 and 11. If you want to hop a booster from Starbase to KSC you will need one or more intermediate points. If you add TPS mass to mitigate, you defeat your purpose.

I think we need actual mathematical analysis to show that, not just asserting it is true.

If you only need to add 1 kg of mass, then that satisfies your antecedent ("you add TPS mass to mitigate") and yet it's pretty clear that it such a small addition of mass wouldn't really satisfy the consequent ("you defeat your point"). It depends on the actual numbers, clearly.

Indeed yes. I thought I recalled an actual analysis somewhere earlier in this 167-page thread, but maybe it was in the "Ship boosters to KSC" thread.  It was associated with the concept of just putting a disposable nosecone on a Booster and sending it to KSC.