Starship On-orbit refueling - Options and Discussion

[DanClemmensen]

milli g thrust can work even if the motive force is gas pressure/ vacuum.  It's just a matter of controlling/throttling and guiding the flow, like they move nitro glycerine in the old westerns.

The acceleration just makes sure that over the longer term, there's a "bottom" for the liquid to pool into.

Even with milli-gee settling, jarmumd's scenario of requiring backing or any kind of overshoot thrust is almost certainly going to be happening at an order of magnitude or higher acceleration, which will overwhelm the settling.

And of course prox ops and docking in an accelerated frame is... interesting...  I guess an orbit is definitionally an accelerated frame, but it's not quite the same thing.

The good news is that you almost always have one Ship with only a few tens of tonnes of prop, and the other with hundreds.  So making the heavy Ship the passive one helps somewhat.  But a few tens of tonnes isn't exactly trivial.

Even though this is the refuelling thread, recall that we have the same problem with any docking, including docking the HLS to Gateway, where HLS will have a bunch of fuel and Gateway presumably is not equipped to take the actively-maneuvering role. Presumably, if you can solve the problem for HLS-to-Gateway, you can use the solution for docking for refuelling. This may also rule out clever solutions using highly-conformable docking systems like long flex hoses. Gateway requires IDSS.

[meekGee]

I like this method also. But the ullage thrust in parallel will probably be done first.
So:
1. Imagine a disk about 7m in diameter give it some thickness maybe .5 m
2. Have docking holes around it
3. Robotic arms to extend down to the fueling ports.
4. Always keep at least 2 ships docked. Maybe 3.
5. 0.07rpm gives you .0015 g at 20m from the center out. 3.5m for disk, 17m down cargo to top of methane.
6. thanks one rev in 14min. You could probably dock while its rotating at that speed.
7. ship power from solar(they need it anyways) will provide power for pumps and robotic arm movement

A VERY slowly spinning hub - so why not just two nose to nose ships?

Spin very slowly (milli g, very low RPM) to prevent slosh and settle propellant to the sump, and then use differential pressure to pump.  EZ PZ.

The mistake is trying to use the inertial component for fuel transfer.

Use it only for settling, eliminating voids, in order to make pressure based transfer predictable.

[Coastal Ron]

OK, bear with me on this 

If you have a "depot", which is one filled Starship tanker, the challenge is that if you want to with centrifugal force to move propellent, then no matter how you mate the two ships, the heavier one tends to have the center of rotation within its envelope, and propellant transfers don't happen without thrusting.

What if you added another Starship that was maxed out on payload weight. Doesn't matter what it is, just as long as it doesn't boil off while spending long periods of time in orbit. Call this the "Counterweight" Starship.

So the sequence would be that the Starship Tanker and the Counterweight would be in orbit together, and they are joined by the Starship that needs to be fueled (i.e. the Receiver).

The Tanker and Counterweight dock together, nose to nose. Then the Receiver docks to the Tanker tail to tail. Then a rotation is started, and the center of rotation will be somewhere in the propellant storage area of the tanker, so propellant will startup moving "down" into the Receiver. Hard to know where the center of gravity starts at, but if some propellant is on the wrong side of that center, then some small thrusting could be used to "settle" the propellant in the tanker so that it stays on the Receiver side of the center of gravity. And actually, the mating sequence could be reversed, with the Receiver and Tanker nose to nose, and the Counterweight tail to tail with the Tanker. Don't know...

When the transfer is done, the rotation is stopped, and all of the ships disengage. The Tanker has to be refilled at some point, so it may be the Receiver in the situation sometimes, but the Counterweight would not change (though it may need to be refueled at some point).

This proposal has the advantage of using gravity to force the propellant to move instead of primarily using thrusters to provide the force. It does add another ship, but if we can't figure out how to dock and maneuver multiple vehicles in space, then we'll never be able to expand out into space anyways.

Any merit to this idea?

[catdlr]

I am not attempting to infringe upon your space, Ron; I merely wish to share this information here, as it was posted today on X. Although it is related to BO, it serves to illustrate what competitors are planning in this regard. My intention is not to dominate or restrict this discussion thread; rather, I aim to keep us informed about the strategic plans of our competition.

https://x.com/blueorigin/status/1976666017293074742

[Vultur]

Presumably, if you can solve the problem for HLS-to-Gateway, you can use the solution for docking for refuelling. This may also rule out clever solutions using highly-conformable docking systems like long flex hoses.

Aren't they planning 4 rigid connectors for Starship to Starship docking and refueling, rather than anything like that?

[Vultur]

In this case, it might be ‘ideal’ for SpaceX not to lead in the initial validations/tests (at least in 1H 2026) using Starship, as the smaller tests planned with NG etc. (ESA I think is also working toward a test and as well ULA if I am not mistaken?) might provide good data.

I think 1H 2026 is pretty optimistic anyway, since the first flight of V3 (Flight 12) isn't supposed to be until 2026 and is not planned to be fully orbital. So the earliest possible demonstration would probably be Flights 14+15 (leaving 13 as the first full orbit).

If something goes wrong with Flight 12 (eg v3 initial issues) that probably pushes orbit, and thus refueling testing, back another flight. If they want to deploy real v3 Starlinks for testing after the initial orbit but vefore propellant transfer, that would add another.

However, once they're ready, I don't think they'll wait for anyone else - too much, both Artemis and Mars, relies on refueling. If things go really super smooth (flight 12 goes well in Jan-Feb, orbit on 13 a month later) they'd probably be testing by June or so.

[DanClemmensen]

Presumably, if you can solve the problem for HLS-to-Gateway, you can use the solution for docking for refuelling. This may also rule out clever solutions using highly-conformable docking systems like long flex hoses.

Aren't they planning 4 rigid connectors for Starship to Starship docking and refueling, rather than anything like that?

As far as I know, all we have is renders. Do you know of additinal information? Those renders are not detailed enough to show how compliant those connections are. For me, those renders do not preclude telescoping and ball joints.

[crandles57]

Presumably, if you can solve the problem for HLS-to-Gateway, you can use the solution for docking for refuelling. This may also rule out clever solutions using highly-conformable docking systems like long flex hoses.

Aren't they planning 4 rigid connectors for Starship to Starship docking and refueling, rather than anything like that?

As far as I know, all we have is renders. Do you know of additinal information? Those renders are not detailed enough to show how compliant those connections are. For me, those renders do not preclude telescoping and ball joints.

Doesn't the mass of fuel being transferred cause the ships to want to separate unless they are rigidly held in place? Having telescoping would allow momentum to build up prior to reaching the end of the telescoping range making the resisting strength needed higher? That seems like a worse solution than just making it rigid?

[DanClemmensen]

Presumably, if you can solve the problem for HLS-to-Gateway, you can use the solution for docking for refuelling. This may also rule out clever solutions using highly-conformable docking systems like long flex hoses.

Aren't they planning 4 rigid connectors for Starship to Starship docking and refueling, rather than anything like that?

As far as I know, all we have is renders. Do you know of additinal information? Those renders are not detailed enough to show how compliant those connections are. For me, those renders do not preclude telescoping and ball joints.

Doesn't the mass of fuel being transferred cause the ships to want to separate unless they are rigidly held in place? Having telescoping would allow momentum to build up prior to reaching the end of the telescoping range making the resisting strength needed higher? That seems like a worse solution than just making it rigid?

I think they would soft dock while being highly compliant, and then use some mechanism to gradually stiffen. This is conceptually like a gradual version of the soft and hard docking used by IDSS, with a far larger range of motion. Fuel transfer would not start until hard docking is complete.

[spacenut]

Lots of ideas.  Hopefully between SpaceX and others, something will float to the top to become standard procedure. 

[Greg Hullender]

Remember that they're planning to do the prop transfer by opening the upper end of the receiving vessel to vacuum, so there should be quite a bit of force shoving the propellant through the pipes.

Source for this? Pulling ullage all the way down to vacuum sounds like a terrible idea from a boiloff perspective.  To be honest, I think managing any kind of pressure differential sounds much more complicated than a couple hundred watt motor, with the two ullage spaces tied together to equalize the ullage pressures.  Hence my curiosity about the source.

I was just sure I saw that here in this thread. But I guess I just imagined it. I certainly can't find it now . . .

Probably another year before we'll actually know, I guess.

[Roy_H]

   imagine a half full vehicle being thrusted in one direction, setting the fluid to one side.  In this condition, let's say the Force is 10, the Mass is 1000 (10 struct+990 prop), and then the accel is 0.01.  Now you went to far and need to thrust a little in the opposite direction.  Now the fluid is just floating in the middle.  Your Force is still 10, but your mass is 10, so your accel is 100 times higher at 1.  This is obviously extreme and might be completely physically wrong.  If it's not wrong, then your GNC is really going to struggle with the non-linear behavior.

It's never been tested before, and can't be tested on earth.  No space docking has ever had so much fluid mass, unattached to the walls.

After reading this, my perception of the difficulty of docking any Starship has gone from "it's trivial" to "it's really, really hard".  When some of your mass is fluid that can slosh, the acceleration due to your thrusters will have variable and complex delays, and these delays are far more than minor nuisances. The smaller the needed velocity corrections, the more the delay and I suspect the uncertainty in the delay also  increases as the velocity change decreases. This makes the effect on the final stages of docking disproportionately severe.

This affects all Starship docking in zero g. The more propellant, the bigger the problem, but even actively docking Starship to Gateway is likely to be an issue. Docking a Tanker to a Depot will be "interesting".

I'm not a rocket engineer and I have not spent a lot of time on this, but the only obvious solution I can see is to use pistons or bladders. We have discussed those in other contexts (like reducing ullage collapse) and we have mostly concluded that they are infeasible or that they have better solutions but if they are the only way to solve sloshing during docking, maybe SpaceX will be forced to use them.

I think this is the most important post I have read on this thread. Mind you I have not read a large portion of the 160+ pages. But I do see  there are several proposed solutions involving settling the fuels like having both units under very low acceleration while docking. Also Dan introduced me to the concept of soft docking. Not really sure how that would be carried out, but sounds promising. The other solution is with mechanical arm(s) that would grab the spaceship at some distance and absorb any sloshing effects as it drew them together for docking.

One other point I have probably missed is the issue of a permanent depot. I don't see how this concept would work as I can't imagine an orbit suitable for all missions, Moon, Mars, asteroids, GEO. Am I wrong? I think it is only practical to transfer fuel directly to mission specific rockets (in their most optimum orbit) and not have an intermediate host.

[DanClemmensen]

One other point I have probably missed is the issue of a permanent depot. I don't see how this concept would work as I can't imagine an orbit suitable for all missions, Moon, Mars, asteroids, GEO. Am I wrong? I think it is only practical to transfer fuel directly to mission specific rockets (in their most optimum orbit) and not have an intermediate host.

Alas, you are yet another victim of the incorrect name "Depot", for this spacecraft. The name causes us to think by analogy with a depot on Earth, which is in a fixed location. Wrong. It's a propellant accumulator/transport. You are correct: for any given mission, it will accumulate propellant in a very low orbit in the appropriate plane from Tankers. It will then move that propellant up to the appropriate orbit in that plane to meet with the Ship to be refuelled. This is much more efficient than having the Tankers rise up to the refuelling orbit because the Tanker is much heavier relative to the fuel mass than is the "Depot". That's because the "Depot" carries a lot more fuel and because the Tanker carries all the hardware needed for EDL.

[catdlr]

Remember that they're planning to do the prop transfer by opening the upper end of the receiving vessel to vacuum, so there should be quite a bit of force shoving the propellant through the pipes.

Source for this? Pulling ullage all the way down to vacuum sounds like a terrible idea from a boiloff perspective.  To be honest, I think managing any kind of pressure differential sounds much more complicated than a couple hundred watt motor, with the two ullage spaces tied together to equalize the ullage pressures.  Hence my curiosity about the source.

I was just sure I saw that here in this thread. But I guess I just imagined it. I certainly can't find it now . . .

Probably another year before we'll actually know, I guess.

Post:

"The standpipes are actually the contingency transfer pipes. If one of the two cryo fluid + ullage gas connectors fails, they can simply purge the connector to vacuum, throw a couple valves in each ship, and use the other connector to transfer the stranded propellant.    It takes over twice as long, but this way the entire tanker mission won't be a waste. Genius!"

source: https://forum.nasaspaceflight.com/index.php?topic=50157.msg2477388#msg2477388

[Roy_H]

One other point I have probably missed is the issue of a permanent depot. I don't see how this concept would work as I can't imagine an orbit suitable for all missions, Moon, Mars, asteroids, GEO. Am I wrong? I think it is only practical to transfer fuel directly to mission specific rockets (in their most optimum orbit) and not have an intermediate host.

Alas, you are yet another victim of the incorrect name "Depot", for this spacecraft. The name causes us to think by analogy with a depot on Earth, which is in a fixed location. Wrong. It's a propellant accumulator/transport. You are correct: for any given mission, it will accumulate propellant in a very low orbit in the appropriate plane from Tankers. It will then move that propellant up to the appropriate orbit in that plane to meet with the Ship to be refuelled. This is much more efficient than having the Tankers rise up to the refuelling orbit because the Tanker is much heavier relative to the fuel mass than is the "Depot". That's because the "Depot" carries a lot more fuel and because the Tanker carries all the hardware needed for EDL.

Thank you for your explanation. So what happens to the propellant accumulator/transport after it has done it's job? Is it discarded? Also, I envisioned my Moon missions proposals to be based from a 500km circular LEO, and hadn't really considered your scenario. I chose 500km because it is still well below the Van Alan belts and above most space debris and satellites orbiting Earth, while requiring slightly less energy for TLI. So I take it that your propellant accumulator/transport. would work from about 200km to 500km? Or did you have larger differences in orbits in mind?

[DanClemmensen]

One other point I have probably missed is the issue of a permanent depot. I don't see how this concept would work as I can't imagine an orbit suitable for all missions, Moon, Mars, asteroids, GEO. Am I wrong? I think it is only practical to transfer fuel directly to mission specific rockets (in their most optimum orbit) and not have an intermediate host.

Alas, you are yet another victim of the incorrect name "Depot", for this spacecraft. The name causes us to think by analogy with a depot on Earth, which is in a fixed location. Wrong. It's a propellant accumulator/transport. You are correct: for any given mission, it will accumulate propellant in a very low orbit in the appropriate plane from Tankers. It will then move that propellant up to the appropriate orbit in that plane to meet with the Ship to be refuelled. This is much more efficient than having the Tankers rise up to the refuelling orbit because the Tanker is much heavier relative to the fuel mass than is the "Depot". That's because the "Depot" carries a lot more fuel and because the Tanker carries all the hardware needed for EDL.

Thank you for your explanation. So what happens to the propellant accumulator/transport after it has done it's job? Is it discarded? Also, I envisioned my Moon missions proposals to be based from a 500km circular LEO, and hadn't really considered your scenario. I chose 500km because it is still well below the Van Alan belts and above most space debris and satellites orbiting Earth, while requiring slightly less energy for TLI. So I take it that your propellant accumulator/transport. would work from about 200km to 500km? Or did you have larger differences in orbits in mind?

Your 500 km orbit is in some particular orbital plane. The tankers will fill the "Depot" in a lower orbit in that same plane and then rise to your proposed refuellng orbit. You might want to see if you should be using the highest orbit in the plane that is still consistent with radiation safety, since "Depot" can move the propellant more efficiently than your mission spacecraft can. In the extreme, "depot" can accompany the mission spacecraft as far as needed. the Economics is complicated.

The "depot" can retain enough fuel to gradually and efficiently move to a different plane for the next mission. Once that mission is imminent, "Depot" will descend to meet its tankers, before ascending to meet its next customer.

[DanClemmensen]

Dan introduced me to the concept of soft docking. Not really sure how that would be carried out, but sounds promising. The other solution is with mechanical arm(s) that would grab the spaceship at some distance and absorb any sloshing effects as it drew them together for docking.

Take a look at the IDSS spec. There is a video of a soft docking followed by hard capture at:
     https://en.wikipedia.org/wiki/International_Docking_System_Standard

[crandles57]

Thank you for your explanation. So what happens to the propellant accumulator/transport after it has done it's job? Is it discarded?

Stays in orbit with little to no fuel until the next launch to same/similar plane needs refuelling. Don't want it to get so low that it needs boosting to stay in orbit so perhaps slowly lowers orbit to the refuelling level over the time available?

>"That's because the "Depot" carries a lot more fuel and because the Tanker carries all the hardware needed for EDL."

The "depot" on the other hand had all the equipment for boil off minimisation. I would guess this is less than the heat shield mass on tankers for EDL but even if it isn't this is not greatly relevant: the depot has dry mass of one ship whereas using tankers to get to higher orbit would be rising dry mass of several tankers to do several small amounts of fuel to do the same work.

The 'insulation' for cryo boil off could include 'sun shades' in the form of solar panels for ion drive propulsion over the potentially long periods between refuelling campaigns? If this makes the dry mass of the depot heavier than the tankers it doesn't seem like a big issue?

Potentially results in fewer depots needed in LEO because you can change the plane more between refuelling campaign uses?

[DanClemmensen]

>"That's because the "Depot" carries a lot more fuel and because the Tanker carries all the hardware needed for EDL."
The "depot" on the other hand had all the equipment for boil off minimisation. I would guess this is less than the heat shield mass on tankers for EDL but even if it isn't this is not greatly relevant: the depot has dry mass of one ship whereas using tankers to get to higher orbit would be rising dry mass of several tankers to do several small amounts of fuel to do the same work.

EDL mass is a lot more than just heat shield. It includes the flaps and the header tanks, and the Ship must be built to support a catch. I'm not sure but it may be possible to decrease the mass of the "depot" by using lighter-weight construction throughout, since it does not need to withstand the forces of reentry.

[rsdavis9]

One other point I have probably missed is the issue of a permanent depot. I don't see how this concept would work as I can't imagine an orbit suitable for all missions, Moon, Mars, asteroids, GEO. Am I wrong? I think it is only practical to transfer fuel directly to mission specific rockets (in their most optimum orbit) and not have an intermediate host.

In a LEO the plane will rotate around 360 degs in 25 days. So you get to all possible planes with week(s) of waiting for free.

satrot = radian/sqrt((ere+400km)^3/(G*em))
-3/2 * ere^2/(ere+400km)^2 * 1.08262E-3 * satrot * cos(23deg)  where ere is earth radius equator, 23deg is inclination, 400km is orbital height above surface, em is earth mass

EDIT: inclination is hard to change and so is argument of perihelion. So LEO circular. Lower the better for faster rotation of plane.

EDIT2: I just got -50 days for 23incl and 400km orbit