Starship On-orbit refueling - Options and Discussion

[Twark_Main]

The fact that SLS and Orion will have less flight experience is irrelevant to the LSS.  Both SpaceX and NASA will do whatever they can to minimize risks, especially if it's cheap or easy to do so.  This is one that's both.

Avoiding a second refueling avoids some risks but it adds others.

It requires a custom extended tank, which adds risk.

I don't think it's a custom extended tank.  There are even more good reasons to go with a 1500t tank for lift tankers than there are for the LSS, not the least of which is that it doesn't require sending two tankers to NRHO for Option B.

Also, remember that "extended" really means "with ring segments, intertank bulkhead, and LCH4 dome rearranged."  That's not nothing, but as modifications go, it's not as bad as a lot of the mods that SpaceX has to make for LSS no matter what.

This also moves the center-of-gravity higher, increasing landing risk.

Landing a tall top-heavy rocket on the Moon is, as you put it, "a procedure with which we have little operational experience," so we should (in your words) "assign high risk to the procedure."

[Twark_Main]

I don't think they need a depot.  In effect, they are depots, or at least spacecraft with good passive resistance to boil-off.  So you can refuel them long before the crew shows up, using plain ol' tankers that go out to NRHO via BLT and straight back to EDL.

I know. I'm just liking the model that says Starships mate with depots, but not with each other, since it makes all the plumbing very unambiguous.

Is "ambiguous plumbing" somehow a real risk? How, exactly?

It also lets the refueling be completely asynchronous. In this vision, a) a steady stream of tankers keeps the LEO depot full, b) occasionally a tanker fills up from the depot, flies to the moon, and pumps ~500t of prop into that depot (which really does achieve ZBO), so c) whenever an LSS needs to refuel, there's always fuel for it either in LEO or at Gateway.

You don't need non-androgynous refilling to "let" that idea work (it works just fine either way), so that's not really an argument in favor.

[TheRadicalModerate]

If you are only 'permitted' one filling, then the math says that VLEO is the only sensible choice.

So essentially your argument is "if we assume no laddering, then there's no laddering." Imagine that... 

In all my laddering calculations I assume the vehicle is fully fueled in VLEO first (ie the sensible approach), so apparently we were talking/calculating past each-other the entire time.

I don't think the "single refueling only" thing is a long-term constraint.  But I'll bet it is for Option A.  Having Artemis III fail, even if it didn't jeopardize the crew, would probably do the program in.  NASA's risk tolerance will be lower on that mission than on subsequent missions.

[Twark_Main]

Just to be sure I properly understand you: You're talking about refueling while accelerating, right? That is, you would:

a) launch a tanker and an LSS. (Each with 1500t fuel capacity.)
b) fully fuel both of them in LEO from a (rather large) depot. (Or two depots.)
c) couple the tanker to the LSS for refueling.
d) both of them fire together in formation, with the tanker continuously keeping the LSS topped up.
e) when the tanker is almost dry, it stops firing, disconnects, and reels in the fuel line.
f) at apogee, the tanker fires just a little bit to lower perigee to enable reentry.
g) the LSS goes on firing until it reaches TLI--arriving at the moon with about 500t extra fuel.

There's better version than this, and I think it's probably a winner:

a) Launch a tanker, but it's really not a tanker; it's just a pusher

Ahh yes, the return of the StarPusher!!

https://forum.nasaspaceflight.com/index.php?topic=49167.msg2006787#msg2006787

https://forum.nasaspaceflight.com/index.php?topic=52033.msg2140950#msg2140950

[TheRadicalModerate]

This also moves the center-of-gravity higher, increasing landing risk.

Landing a tall top-heavy rocket on the Moon is, as you put it, "a procedure with which we have little operational experience," so we should (in your words) "assign high risk to the procedure."

That's a good point.

I have an old spreadsheet (many assumptions and guesses) that looks at CoG for various tank sizes.  Looks like it would move CoG up by 10% (from 17.2m for 1200t to 18.9m for 1500t) to go from 1200t to 1500t. 

That's not nothing.  And landing risk goes towards loss-of-crew, while refueling risk just goes towards loss-of-mission.

[Twark_Main]

Prop consumption for boosting two ships to an HEEO in a coupled system is the same as prop consumption for boosting the two ships separately.¹  If your objection here is, "But what about cross-feed?" the only thing cross-feed would get you is higher thrust

This is wrong. Crossfeed will change the effective mass ratio of the stages (since they'll have different burnout times), which directly increases delta-v, just like staging.

Mind you, I still think the overall idea is insane. 

[TheRadicalModerate]

Prop consumption for boosting two ships to an HEEO in a coupled system is the same as prop consumption for boosting the two ships separately.¹  If your objection here is, "But what about cross-feed?" the only thing cross-feed would get you is higher thrust

This is wrong. Crossfeed will change the effective mass ratio of the stages (since they'll have different burnout times), which directly increases delta-v, just like staging.

Mind you, I still think the overall idea is insane. 

You don't care about the effective mass ratio; it's the same based on where you decide to put the energy of your HEEO.  If you're trying to be as prop-efficient as possible, that energy should be just high enough that the fully fueled target can complete the mission.

All three of these are equivalent in terms of total prop to LEO:

1) A target and a tanker independently boost to LEO+x HEEO, then the tanker completely fills the target, so it has just enough delta-v available to finish the mission.

2) The fully fueled target and a linked, cross-fed tank or tanker boost to LEO+x, then the tank or tanker is jettisoned, leaving the target with a full tank, giving it just enough delta-v to finish the mission.

3) A target is topped off, then a pusher boosts it to LEO+x and is jettisoned, leaving the target with just enough delta-v to finish the mission.

The only difference between them is that there will be slightly different amounts of non-impulsive gravity losses, depending on how much total thrust is generated by the two vehicles.  I'd be surprised if they varied by more than 20m/s.

Cross-feed for launch is actually the same, but the reduction in gravity loss can be much more substantial, because you have more high-thrust prop after the cross-feeding boosters are jettisoned.  Still, I'd be surprised if the difference was more than a couple hundred m/s.

[Twark_Main]

Prop consumption for boosting two ships to an HEEO in a coupled system is the same as prop consumption for boosting the two ships separately.¹  If your objection here is, "But what about cross-feed?" the only thing cross-feed would get you is higher thrust

This is wrong. Crossfeed will change the effective mass ratio of the stages (since they'll have different burnout times), which directly increases delta-v, just like staging.

Mind you, I still think the overall idea is insane. 

You don't care about the effective mass ratio

...

Cross-feed for launch is actually the same

This is obviously wrong.

If we imagine "perfect" crossfeed, the feeding stage would burn 100% of the consumed propellant before dropping off. At that point the math "degenerates" to conventional staging.

You're not suggesting that conventional staging has no delta-v benefits (vs a monolithic stage w the same mass ratio), are you? 

[Barley]

Prop consumption for boosting two ships to an HEEO in a coupled system is the same as prop consumption for boosting the two ships separately.¹  If your objection here is, "But what about cross-feed?" the only thing cross-feed would get you is higher thrust

This is wrong. Crossfeed will change the effective mass ratio of the stages (since they'll have different burnout times), which directly increases delta-v, just like staging.

Mind you, I still think the overall idea is insane. 

You don't care about the effective mass ratio

...

Cross-feed for launch is actually the same

This is obviously wrong.

If we imagine "perfect" crossfeed, the feeding stage would burn 100% of the consumed propellant before dropping off. At that point the math "degenerates" to conventional staging.

You're not suggesting that conventional staging has no delta-v benefits (vs a monolithic stage w the same mass ratio), are you? 

He was explicit about what he is comparing.  You are mistaken.

Compare your continuous cross feed system with one that transfers the fuel instantaneously at the end of the first burn.  It should be obvious that it does not matter when the transfer occurs.  Refueling works out the same in "flat" space or if you use a HEEO so that all burns have the same Oberth effect. 

[TheRadicalModerate]

If we imagine "perfect" crossfeed, the feeding stage would burn 100% of the consumed propellant before dropping off. At that point the math "degenerates" to conventional staging.

You're not suggesting that conventional staging has no delta-v benefits (vs a monolithic stage w the same mass ratio), are you?

With the same mass ratio?  No, that definitely wouldn't have any benefits.  But the whole idea behind staging is that the average mass ratio is higher.

But this is all a red herring for the problem at hand, which is to minimize prop to orbit.  I assume that you agree that minimal prop will be achieved by:

1) Having the target Starship fully fueled on the first (and only) rung of the ladder.
2) Having the energy of that first rung only as high as needed so that the target Starship uses up its entire delta-v budget accomplishing the mission.

That means that the aggregate mass ratio needed to achieve the first rung is a constant, irrespective of the staging architecture:  independent tanker/target, cross-fed tank(er)/target, or target pushed by a stage.  They all require exactly the same amount of propellant, mod insignificant differences in non-impulsive gravity loss.

[InterestedEngineer]

But this is all a red herring for the problem at hand, which is to minimize prop to orbit.  I assume that you agree that minimal prop will be achieved by:

Why is that a major goal?  At $100/kg it's still not worth optimizing something that is 1/10 to 1/100 the development cost of something that will be made 3-4 times.

Airlines only optimize fuel because there's nothing else left to optimize.

[sdsds]

Requiring the LSS to have multiple refuelings clearly isn't a complete deal-breaker.  But I suspect that minimization of conops complexity will be high on the list of things that NASA wants to see.

I assert a conops with refilling of HLS Starship in the lunar vicinity reduces mission risk because it increases HLS tolerance of Orion schedule uncertainty.

Put differently, no one knows in which year (or decade) Orion will arrive to rendezvous with HLS. And although some Starships may have low or zero propellant boil-off, HLS likely won't. Having a low boil-off depot Starship in the cis-lunar vicinity allows propellant accumulation potentially years before the propellant is needed. And HLS Starship can be there years early too, patiently waiting for Orion to roll out to the pad. Days before the Orion launch window opens HLS fills from the depot and maneuvers to the rendezvous orbit.

How could NASA not like that plan?

[DanClemmensen]

Requiring the LSS to have multiple refuelings clearly isn't a complete deal-breaker.  But I suspect that minimization of conops complexity will be high on the list of things that NASA wants to see.

I assert a conops with refilling of HLS Starship in the lunar vicinity reduces mission risk because it increases HLS tolerance of Orion schedule uncertainty.

Put differently, no one knows in which year (or decade) Orion will arrive to rendezvous with HLS. And although some Starships may have low or zero propellant boil-off, HLS likely won't. Having a low boil-off depot Starship in the cis-lunar vicinity allows propellant accumulation potentially years before the propellant is needed. And HLS Starship can be there years early too, patiently waiting for Orion to roll out to the pad. Days before the Orion launch window opens HLS fills from the depot and maneuvers to the rendezvous orbit.

How could NASA not like that plan?

Who pays for this? I guess you mean a modification to the milestone payments of the HLS contract? I'd think SpaceX would also want additional compensation for the fairly major extension to the agreed-to loiter time.

[TheRadicalModerate]

But this is all a red herring for the problem at hand, which is to minimize prop to orbit.  I assume that you agree that minimal prop will be achieved by:

Why is that a major goal?  At $100/kg it's still not worth optimizing something that is 1/10 to 1/100 the development cost of something that will be made 3-4 times.

Airlines only optimize fuel because there's nothing else left to optimize.

For the time being, the figure of merit is not $/kg; it's launches/mission.  Eventually, you'll be able to model the process as one or more depots that provides a continuous process, but it's gonna be discrete until cadence is such that the depot doesn't boil dry between missions, which may be tens of months apart.

[sdsds]

Who pays for this? I guess you mean a modification to the milestone payments of the HLS contract? I'd think SpaceX would also want additional compensation for the fairly major extension to the agreed-to loiter time.

Short answer: yes. I believe funding will be available, both NASA-provided and SpaceX-internal, for a lunar vicinity Starship depot.

Caveats: (1) The puzzle has a lot of moving pieces. (2) My crystal ball is sometimes hazy.

Long answer:
I predict Artemis II will launch NET 2025, probably NET 3Q2025. Further, there will be considerable change to the way the Artemis program is conducted after the conclusion of that mission. Probably the most important change will be that around 4Q2025 Artemis management will have conducted a formal schedule risk assessment, and will have shared that with NASA leadership and Congress. Expectations will be that Artemis III launches NET 1Q2027, with considerable likelihood it might launch in 2H2027, or even after that.

That will be the assessment in 4Q2025. By that time lots of other puzzle pieces will have moved.
- The PPE/HALO Gateway hardware will be nowhere near its destination orbit.
- Starship propellant transfer in LEO will be routine.
- An uncrewed Starship will have landed on the surface of the Moon.
- A different uncrewed Starship, launched in 2024, will have performed a major propulsive maneuver in close proximity to Mars, having prevented propellant boil-off during the trans-Mars cruise.

Based on that last point, SpaceX will have a proven system for use as a lunar vicinity depot. Combined with the then-acknowledged Artemis III schedule uncertainty and the desire to "buy down risk," the decision to use a lunar vicinity depot for the crewed HLS Starship becomes easy.

[TheRadicalModerate]

Requiring the LSS to have multiple refuelings clearly isn't a complete deal-breaker.  But I suspect that minimization of conops complexity will be high on the list of things that NASA wants to see.

I assert a conops with refilling of HLS Starship in the lunar vicinity reduces mission risk because it increases HLS tolerance of Orion schedule uncertainty.

Put differently, no one knows in which year (or decade) Orion will arrive to rendezvous with HLS. And although some Starships may have low or zero propellant boil-off, HLS likely won't.

According to the source selection statement, LSS (aka HLS Starship) has 100 days of loiter without refueling.  To me, that implies that the prop management tech in LSS is roughly the same as that in a depot.

Having a low boil-off depot Starship in the cis-lunar vicinity allows propellant accumulation potentially years before the propellant is needed. And HLS Starship can be there years early too, patiently waiting for Orion to roll out to the pad. Days before the Orion launch window opens HLS fills from the depot and maneuvers to the rendezvous orbit.

You don't accumulate prop in cislunar; it's insanely expensive.  You accumulate it in LEO, then send it to cislunar shortly before the mission.  Even if the Orion gets massively delayed, topping off the depot in LEO is much, much cheaper than topping it off in cislunar.

However, there are a lot of different conops, only a few of which involve an Orion, or even a transit vehicle that's separate from the lander/ascender.  Before you even thought about a cislunar depot, you'd have to commit to one of the conops that required it.

As it happens, if you ever want to use LSS to stage crews out of LEO, before some form of lunar Starship can be crew-certified for launch and EDL, then you do need to commit to lunar refueling.  That requires at least an EDL-capable lift tanker to go to cislunar.  Whether it transfers prop to a depot or just transfers it straight to the LSS depends a lot on timing, and how boil-off-tolerant your mission is.

[redneck]

You don't accumulate prop in cislunar; it's insanely expensive.  You accumulate it in LEO, then send it to cislunar shortly before the mission.  Even if the Orion gets massively delayed, topping off the depot in LEO is much, much cheaper than topping it off in cislunar.


That depends largely on the operating realities of the company. If tankers are sitting idle, and actual launch costs are as low as some speculate, then keeping them busy with propellant launches may make business sense. Somewhat as Falcon launches Starlinks instead of sitting idle. Or as a cargo truck, plane, or ship is an expense sitting idle at the terminal. 

If more propellant than is needed is accumulating in LEO, and tankers are idle,  then sending some of it to cislunar can make sense. There are many missions enabled by large quantities of propellant in HEO. Any high energy mission would benefit by leaving HEO with full tanks.

[sdsds]

[...] accumulate[d] prop in cislunar [is] insanely expensive.

You and I agree on a lot, and we are in complete agreement on that point.

You accumulate it in LEO, then send it to cislunar ...

Again, complete agreement.

... shortly before the mission.

Sometime between now and 2026 one or the other of us will see this differently than they see it now!

Propellant in cis-lunar space is certainly costly. It is also valuable. The question is simply under what circumstances is the value greater than the cost?

[TheRadicalModerate]

That depends largely on the operating realities of the company. If tankers are sitting idle, and actual launch costs are as low as some speculate, then keeping them busy with propellant launches may make business sense. Somewhat as Falcon launches Starlinks instead of sitting idle. Or as a cargo truck, plane, or ship is an expense sitting idle at the terminal. 

It's always cheaper to let something sit idle than it is to send it out for no reason.  Yes, it will always make sense to launch full tankers, even if the mission only requires a fractional tanker.  But you're not just going to launch prop to LEO depots for no reason.

You should also consider that if things are really that cheap, two things will happen:

1) Demand will spike, which will first soak up spare capacity and then increase prices.

2) The market will be hot enough that Starship will get some competition.  That may reduce prices somewhat, but it will also make SpaceX want to be as ops-efficient as possible.

In both cases, nobody's gonna be launching prop just for grins and giggles.

If more propellant than is needed is accumulating in LEO, and tankers are idle,  then sending some of it to cislunar can make sense.

No.  It never makes sense, absent a conops with sufficient cadence to generate the demand.  It's always better to leave excess prop in LEO until you need it.  Even if there's boil-off, the boiled-off prop costs a fraction of what it costs to replace the equivalent amount of prop in cislunar.¹

There are many missions enabled by large quantities of propellant in HEO. Any high energy mission would benefit by leaving HEO with full tanks.

Which missions do you have in mind?  And in what HEEO, with what RAAN and argument of perigee?  Are they crewed missions?  What are their abort contingencies?

One of the nice things about using cislunar depots for high-energy missions is that, unlike an HEEO, which is in a near-constant sidereal reference frame, the cis-lunar ones are in an Earth-Moon rotating reference frame, which means that you have a not-terrible window to any departure asymptote once a month, instead of once a year.

I do think that if refueling in HEEO is really the best way to do lunar missions, then an HEEO depot might make sense.  But even then, you don't move the prop to it until you need it, for exactly the same reason you don't move prop to cislunar depots until you need it:  boil-off costs more at high energy than at low energy.

____________
¹Weasel words:  If all you have is passive boil-off management, then cislunar is colder than LEO.  Whether it's enough colder that boil-off is cheaper would require some figuring.  But if there's enough traffic going to cislunar to make this at all useful, I'm pretty sure that SpaceX will have a zero boil-off depot technology, and LEO will then be just as good as anywhere else to store excess prop.

[redneck]

It's always cheaper to let something sit idle than it is to send it out for no reason.  Yes, it will always make sense to launch full tankers, even if the mission only requires a fractional tanker.  But you're not just going to launch prop to LEO depots for no reason.

Actually it is not cheaper to let equipment and personnel sit idle if there is any revenue, or potential revenue work for them to do. Idle personnel cost as much as busy ones, and building inventory in good locations is potential revenue work. The alternatives are to eat the expense for losses, or lay them off. Neither is good for the long term if something useful can be done.

Which missions do you have in mind?  And in what HEEO, with what RAAN and argument of perigee?  Are they crewed missions?  What are their abort contingencies?

Anyone here can think of missions enabled by more propellant in a convenient location. Dropping down for an Oberth burn at perigee with a 20/1 mass ratio can get 18 km/sec Earth relative at infinity. Most planets and asteroids can be reached without gravity assist. HEO is different from HEEO. The most convenient orbits can be used based on demand.

One of the nice things about using cislunar depots for high-energy missions is that, unlike an HEEO, which is in a near-constant sidereal reference frame, the cis-lunar ones are in an Earth-Moon rotating reference frame, which means that you have a not-terrible window to any departure asymptote once a month, instead of once a year.

I do think that if refueling in HEEO is really the best way to do lunar missions, then an HEEO depot might make sense.  But even then, you don't move the prop to it until you need it, for exactly the same reason you don't move prop to cislunar depots until you need it:  boil-off costs more at high energy than at low energy.

Boil off in HEO can be eliminated with a fairly simple sun shade. In LEO it requires more technology.