Starship On-orbit refueling - Options and Discussion

[TheRadicalModerate]

However, I'm concerned that a vehicle that regularly uses its engines will have to have maintenance. That rules out ever letting a depot fire its engines once it's in orbit, unless we're actually going to have a manned station to service them.

Depots are cheap.  When you exceed the engine life, dispose of the old one and launch a new one.

I do make a distinction between tankers and depots: A tanker can EDL. A depot cannot. A depot can have a lot of extra mass for things like thermal shielding, power generation, etc. A tanker cannot.

Well, now that we've started the taxonomy discussion, our usual six-month cycle of discussing everything on this thread over and over is complete.  (This isn't necessarily a bad thing.)

The thing we keeping bumping into and not quite discussing is that we're dealing with two different kinds of thermal mitigation:

1) EDL TPS (intentionally low reflectivity, very high emissivity, works great at high temperatures)

2) Low-boiloff insulation (super-high reflectivity + some kind of aerodynamically questionable insulation, works better at vacuum temperatures, which are pretty modest)

Depots need low boiloff, and lift tankers (or any EDL-capable vehicle) needs TPS.  The problem is that we'd really like a "transfer" tanker that can change orbits but still either do EDL or at least deep aerobraking.

That's the thermal axis of the problem.  The other axis is the whole gendering issue.  If we assume that an androgynous QD is hard for some reason,¹ then depots have male QDs and tankers have female.  That means that only depots can transfer prop to target Ships.

Never send a Tanker above VLEO. Always send a Depot instead.

One additional rule:  Never leave a depot in HEEO for an extended period.  They're way too vulnerable to MMOD, and they're a debris hazard if something bad happens in HEEO.  Fill 'em up, boost 'em to HEEO, get the prop transferred as quick as possible, and get 'em back to VLEO.

I don't know if the attached chart will help, but it's a summary of my thinking.

__________
¹Seems obvious to me that, if it were easy, SpaceX would have already done it.  That of course doesn't rule out the possibility that it's hard but doable, and they're still working out the bugs before they declare it reliable enough for use.

[Greg Hullender]

The problem is that we'd really like a "transfer" tanker that can change orbits but still either do EDL or at least deep aerobraking.

This is for the purpose of maintaining a depot in NRHO or L1?

[TheRadicalModerate]

The problem is that we'd really like a "transfer" tanker that can change orbits but still either do EDL or at least deep aerobraking.

This is for the purpose of maintaining a depot in NRHO or L1?

Yes.  At the very least, SLT missions, if the LSS is to be reused, need to refuel in some kind of lunar orbit (NRHO until we hear otherwise).  That requires a depot, at least without androgynous QDs.  You could fly the depot there, let it loiter, refuel the LSS when the time is right, and then fly it back to a propulsive or (possibly) aerobraked LEO insertion, but it's probably more prop-efficient to return a transfer tanker to EDL or a deep-aerobraked insertion (i.e., a single pass to get the apogee where you want it).

I probably should have noted that you can live with a plain ol' lift tanker as the transfer tanker, but only if it has the boil-off characteristics--and tankage--you need.  I think that boil-off will be pretty low with the nose pointed at the sun, so the tank walls are only getting tiny amounts of insolation, and there's little albedo or emissive heating from Earth when the tanker is in deep space.

[Greg Hullender]

The problem is that we'd really like a "transfer" tanker that can change orbits but still either do EDL or at least deep aerobraking.

This is for the purpose of maintaining a depot in NRHO or L1?

Yes.  At the very least, SLT missions, if the LSS is to be reused, need to refuel in some kind of lunar orbit (NRHO until we hear otherwise).  That requires a depot, at least without androgynous QDs.  You could fly the depot there, let it loiter, refuel the LSS when the time is right, and then fly it back to a propulsive or (possibly) aerobraked LEO insertion, but it's probably more prop-efficient to return a transfer tanker to EDL or a deep-aerobraked insertion (i.e., a single pass to get the apogee where you want it).

I probably should have noted that you can live with a plain ol' lift tanker as the transfer tanker, but only if it has the boil-off characteristics--and tankage--you need.  I think that boil-off will be pretty low with the nose pointed at the sun, so the tank walls are only getting tiny amounts of insolation, and there's little albedo or emissive heating from Earth when the tanker is in deep space.

You know, as long as we're playing the nomenclature game again, it seems to me that a really good distinction might be that the difference between a tanker and a depot is that a depot never changes orbit once it goes into operation. Tankers move fuel around; depots merely accumulate it.

This works well with the other definitions; a depot needs serious thermal protection to keep from losing the fuel it's holding. It never ignites its engines, so they never need service, and it lives in orbits with minimal MMOD risks.

A long-distance tanker continually uses its engines, and it's frequently exposed to MMOD and the Van Allen belts, so it needs TPM in order to EDL for service on Earth. You won't have a tanker that just shuttles between a VLEO depot and an NRHO depot.

However, I think it's going to be a while before there's an NRHO depot, hence any need for long-distance tankers. And, as you say, it's not clear there's any need for a specialized vehicle of that sort even if there were.

[TheRadicalModerate]

You know, as long as we're playing the nomenclature game again, it seems to me that a really good distinction might be that the difference between a tanker and a depot is that a depot never changes orbit once it goes into operation. Tankers move fuel around; depots merely accumulate it.

That doesn't work, because you want depots to boost up to HEEOs.  Depots in permanent HEEOs don't make sense because:

1) They are an MMOD danger to themselves and others.
2) HEEO orbital mechanics kinda suck.

You can have depots that rove all the way to LO if you want, but the prop penalty for getting them propulsively back to VLEO gets larger.  That's not true for transit tankers, because they can do deep aerobraking or direct EDLs.

But a transit tanker can't handle servicing HEEO on its own, because it's misgendered.

In a perfect world, you'd have androgynous QDs.  Then you could indeed use lift or transit tankers to service higher-energy FTOs, and we'd be done.  For some reason, we don't have androgynous QDs.  We'll see if they magically appear.

One other dimension we haven't brought into this:  Lift tankers want tank sizes to be optimized for maximizing prop to orbit.  Depots and transit tankers want tank sizes adequate to provide a full load to a target after worst-case boiloff, which may be further complicated by depots and transit tankers having different boiloff rates (maybe, maybe not).

I suspect that the perfect lift tanker has its domes moved forward by about two rings (i.e., 208t of extra prop over the baseline), and then has all additional payload barrel segments removed.  But depots and transit tankers want to keep the barrel, but have its domes moved all the way up to the beginning of the ogive, so it maximizes its capacity on-orbit, even if it doesn't launch completely full.

[meekGee]

Where did the idea come from that a depot could be in HEEO or FTO? It has been proposed to have a tanker that's filled up from a depot at the same time that a mission starship is fueled and that the two would boost into HEEO/FTO in tandem before a final refueling from the tanker to the mission vehicle.

Trying to rendezvous with a depot previously located in HEEO seems like an unnecessary nightmare. Trying to refuel such a depot seems like a really expensive nightmare. Why do it?

I could imagine a useful depot in NRHO or at LL1, but, short of that, I don't see any advantages to a depot above VLEO.

Other than F9 heavy boosters, have we ever seen rockets boosting in formation?

What if the ship doesn't use its engines and keeps the tanks full?  In that case the last refueling never happens, and what used to be a tanker is just an almost-departure stage.  This keeps all fuel transfers in LEO, between ships and depots.

[TheRadicalModerate]

What if the ship doesn't use its engines and keeps the tanks full?  In that case the last refueling never happens, and what used to be a tanker is just an almost-departure stage.  This keeps all fuel transfers in LEO, between ships and depots.

Not sure what you mean.  Are you assuming StarPusher?  Then yes, that works.  But StarPusher is unlikely.

As for refueling in VLEO only, Block 2 can't do VLEO-BLT-NRHO-LS-NRHO with just VLEO refueling.  Block 3 can, though.  (Certain boiloff restrictions apply.)

[Greg Hullender]

You know, as long as we're playing the nomenclature game again, it seems to me that a really good distinction might be that the difference between a tanker and a depot is that a depot never changes orbit once it goes into operation. Tankers move fuel around; depots merely accumulate it.

That doesn't work, because you want depots to boost up to HEEOs. 

Ah, but that's exactly what I'm disputing. I think you want to put a stake in the ground, insist that depots never boost into HEEO (or fire their engines at all after reaching final orbit), and bend everything else to make that constraint work.

But a transit tanker can't handle servicing HEEO on its own, because it's misgendered.

If the gender problem is the only reason for boosting depots into HEEO and either discarding them or trying to gently aerobrake them back, then I suggest giving it another thought. Why all the resistance to solving the gender problem? It seems to be a far easier problem.

In a perfect world, you'd have androgynous QDs.  Then you could indeed use lift or transit tankers to service higher-energy FTOs, and we'd be done.  For some reason, we don't have androgynous QDs.  We'll see if they magically appear.

Again, I don't see why it's that hard for a depot to just have two different QDs--e.g. a female dorsal one and a male ventral one. If they need more space in the "attic," perhaps they can remove the three vacuum engines or something. After all, the depot just needs enough thrust to reach VLEO empty. (I'm sure someone else can come up with a better way to fit it in though.)

Regardless of how you do it, you don't need to redesign the connectors if you simply enable two different kinds of connector on the depot.

You do need to have at least some male tankers, since that's how you do the final fueling for the mission ship in HEEO. I think that's fine, though, and you can still use them for regular fueling. It's probably just as well that both sides of the depot see regular use anyway.

One other dimension we haven't brought into this:  Lift tankers want tank sizes to be optimized for maximizing prop to orbit.  Depots and transit tankers want tank sizes adequate to provide a full load to a target after worst-case boiloff, which may be further complicated by depots and transit tankers having different boiloff rates (maybe, maybe not).

I suspect that the perfect lift tanker has its domes moved forward by about two rings (i.e., 208t of extra prop over the baseline), and then has all additional payload barrel segments removed.  But depots and transit tankers want to keep the barrel, but have its domes moved all the way up to the beginning of the ogive, so it maximizes its capacity on-orbit, even if it doesn't launch completely full.

I think the tanker problem is maximizing propellant to orbit per dollar spent. That probably means you want to stuff the thing as full of propellant as possible for each launch.

As far as the depot goes, wouldn't the optimal size be based on estimated boiloff before mission launch? That is, if the mission ship needs (say) 2500 tons of fuel one month after fueling, and if you figure that a load of 3000 tons will dwindle to 2500 in a month, then the depot needs to be big enough to hold 3000 tons full. Figuring out those numbers will be a challenge, of course, and I doubt we have anything like the data needed for more than crude estimates, but it seems straightforward. The fact that they don't think they'll need active cooling suggests they've run the numbers and like the answers they got.

The hardest problem may well be making a transit tanker that can carry fuel for a few days without all of it boiling away, although I'm hopeful that the trick of simply pointing the nose at the sun will be good enough. That or else maybe a quick coat of solar-white paint that's expected to burn off during EDL. :-)

[Twark_Main]

I think you want to put a stake in the ground, insist that depots never boost into HEEO (or fire their engines at all after reaching final orbit), and bend everything else to make that constraint work.

How eager you are to transform optimization goals into straight-jackets.  The children yearn for the mines, apparently. 

[Twark_Main]

So why didn't they build the QDs that way in the first place?

Why didn't they build Raptor 3 in the first place? 

We say "iterative design" a lot, but I think the reality is a lot harder for people to wrap their head around.

Iterative design or not, easy things get done right the first time.  So it's not unreasonable to conclude that an androgynous QD is not easy, for some reason.  It's not like nobody thought about refueling when they designed the interface.  Somebody decided that pushing the problem off was the right thing to do.

Androgynous is probably the optimal solution here, but we don't know why they didn't go that way to begin with.

If we assume that an androgynous QD is hard for some reason,¹

¹Seems obvious to me that, if it were easy, SpaceX would have already done it.  That of course doesn't rule out the possibility that it's hard but doable, and they're still working out the bugs before they declare it reliable enough for use.

For some reason, we don't have androgynous QDs.  We'll see if they magically appear.

The thing you're forgetting about iterative design is that it's "long pole first."

Androgynous QD isn't the long pole, so it's not a priority to get it in the "final state" right now. Androgyny is a quick modification, so the bigger priority up until now has been the harder parts of the design.

[TheRadicalModerate]

Again, I don't see why it's that hard for a depot to just have two different QDs...

It doesn't solve the problem.  If you want tankers to be able to connect directly to target Starships, then every tanker--which is the thing you most want to mass-reduce--needs the double QDs.  The plumbing is non-trivial.

I think you want to put a stake in the ground, insist that depots never boost into HEEO (or fire their engines at all after reaching final orbit), and bend everything else to make that constraint work.

How eager you are to transform optimization goals into straight-jackets.

I agree with Twark in this case.  Depots are the cheapest components in the system.  If there are restart or burn time limits, they're probably worth finding early.

The thing you're forgetting about iterative design is that it's "long pole first."

Androgynous QD isn't the long pole, so it's not a priority to get it in the "final state" right now. Androgyny is a quick modification, so the bigger priority up until now has been the harder parts of the design.

Iterative development doesn't absolve you from scheduling intelligently.  It just acknowledges that sometimes, you have to refactor.  Best to refactor things that are cheap over things that are expensive.

Replacing QDs for Boeing and Bechtel would be a $200M ECO with three months of design reviews and approvals.  For SpaceX, it's not exactly a weekend, but it might only be a few weeks.  My point was simply that it's such an obvious thing to do that it would be something you'd make the time for if it were super-easy, because it would save you a lot of money and, more importantly, pad down-time, which is likely the thing that SpaceX would most like to minimize.

SpaceX has been fooling a bit with the QD recently, although it's not a huge change.  If there were something androgynous good to go, that would have been a good place to insert it.  That doesn't prove or disprove your point; it merely says that if there's an androgynous change coming, it's not ready yet.

If it's going to be done, it's better to do it before the beginning of the refueling orbital test campaign, rather than the end.  Accumulating data on QD reliability is going to be a major requirement for making NASA comfy--and for silencing the naysayers.  You want as big a stable sample size as possible.

[Nomadd]

 I haven't kept up very well, and it might have been covered, but has there been any discussion of defueling a Starship in low orbit? Say, if it flunks checkout or something else goes wrong and it needs to return to Earth.

[TheRadicalModerate]

I haven't kept up very well, and it might have been covered, but has there been any discussion of defueling a Starship in low orbit? Say, if it flunks checkout or something else goes wrong and it needs to return to Earth.

It’s a good contingency to understand. But it seems like dumping the prop is probably the easiest/safest thing to do.

Note that there are no cases where an LSS returns to Earth.

[Greg Hullender]

I think you want to put a stake in the ground, insist that depots never boost into HEEO (or fire their engines at all after reaching final orbit), and bend everything else to make that constraint work.

How eager you are to transform optimization goals into straight-jackets.

Absolutely! It's one of the best ways to explore a space where there are a lot of variables. Fix one or two and see what happens to the rest.

I gather you dislike partial derivatives too? :-)

Again, I don't see why it's that hard for a depot to just have two different QDs...

It doesn't solve the problem.  If you want tankers to be able to connect directly to target Starships, then every tanker--which is the thing you most want to mass-reduce--needs the double QDs.  The plumbing is non-trivial.

No, you just need to make some number of male tankers and some number of female tankers. As far as I can tell, the difference amounts to swapping out a plate on the outside of the vehicle. (This may be my fatal misassumption though.) :-)

I agree with Twark in this case.  Depots are the cheapest components in the system. If there are restart or burn time limits, they're probably worth finding early.

Cheaper than a single metal plate? And the best limit is no limit. Never restarting those engines eliminates a whole class of potential problems--and losing a full depot is very expensive indeed. In fact, having two different ways to drain a full depot is a plus.

If it's going to be done, it's better to do it before the beginning of the refueling orbital test campaign, rather than the end.  Accumulating data on QD reliability is going to be a major requirement for making NASA comfy--and for silencing the naysayers.  You want as big a stable sample size as possible.

Again, the best change is no change. Use the same connectors you have now. Make the depot hermaphroditic, make the tankers mixed sex. I really do think that solves everything we've been talking about--and it's much simpler in operation. The question is how much does it cost?

[Twark_Main]

I think you want to put a stake in the ground, insist that depots never boost into HEEO (or fire their engines at all after reaching final orbit), and bend everything else to make that constraint work.

How eager you are to transform optimization goals into straight-jackets.

Absolutely! It's one of the best ways to explore a space where there are a lot of variables. Fix one or two and see what happens to the rest.

I gather you dislike partial derivatives too? :-)

If it's just a braiinstorming trick and not an actual engineering constraint, then say that

And the best limit is no limit. Never restarting those engines...

1. That's not a phrase.

2. "No limit" would (by definition) mean the limit is infinity, not zero. 

...Never restarting those engines eliminates a whole class of potential problems--and losing a full depot is very expensive indeed. In fact, having two different ways to drain a full depot is a plus.

It also eliminates a whole class of capabilities too.

For instance you can kiss controlled re-entry goodbye, which means the depot probably can't even (legally) launch under the current space debris regulatory environment.

[volker2020]

Maybe I missed 20 pages of analysis, but I don't get why not simply having the depot get 2 connects, one male, one female. The small asymmetry in docking should really be no concern and the mass constrains for the depot is minimal. There is no reason not to make it bigger and heavier. The other ship flying to the depot any way.
I even think it would make sense to extra shield and actively cool down the fuel in there. I think any mass used for that would pay off.

Yes it is not the simplest solution, but I go with Einstein, find the simplest solution that works, but don't make it even simpler.

[meekGee]

What if the ship doesn't use its engines and keeps the tanks full?  In that case the last refueling never happens, and what used to be a tanker is just an almost-departure stage.  This keeps all fuel transfers in LEO, between ships and depots.

Not sure what you mean.  Are you assuming StarPusher?  Then yes, that works.  But StarPusher is unlikely.

As for refueling in VLEO only, Block 2 can't do VLEO-BLT-NRHO-LS-NRHO with just VLEO refueling.  Block 3 can, though.  (Certain boiloff restrictions apply.)

Yeah, I wasn't up to speed on this thread.  So yeah, the idea of a push-off stage - why not?

You've effectively created a an orbital booster stage, it takes the ship to HEO, maybe even help it into the Oberth burn since it can later aerocapture or aerobreak.  If the pusher never re-enters, it can be optimized for the job - no heatshield, no payload bay, etc.  Blunt nose to help with the pushin'.

Requires none of the refueling issues stated above to be solved.  Everyone fuels from depots in LEO.

[Greg Hullender]

...Never restarting those engines eliminates a whole class of potential problems--and losing a full depot is very expensive indeed. In fact, having two different ways to drain a full depot is a plus.

It also eliminates a whole class of capabilities too.

For instance you can kiss controlled re-entry goodbye, which means the depot probably can't even (legally) launch under the current space debris regulatory environment.

Why would you think that? Depots still need to be able to accelerate for station keeping and ullage control. They just don't use their main engines for that. (Exactly how they do this probably deserves some discussion.) Anyway, they should be able to use those thrusters for controlled reentry, when the time comes. But the Raptor engines clearly cannot be used for this.

Did you have any other use case that benefited from mobile depots rather than just using existing tankers?

Personally, I still favor the idea of avoiding ullage burns entirely by having two depots connected nose-to-nose by a cable ~ 1 km long, relying on tidal forces to settle the propellant. That gives you 2 mm/sec at the noses of the two vehicles, assuming equal masses. This configuration is stable, but it can swing like a pendulum with a period of a few minutes. (I'm assuming something like solar panels to the sides--otherwise it can spin on the long axis.) Given the plan is for two depots anyway, it seems a shame not to try to take advantage of this.

[TheRadicalModerate]

Did you have any other use case that benefited from mobile depots rather than just using existing tankers?

Yes.  A mobile depot can refuel stuff in HEEO.  As of right now, a tanker can't.

Let's go back to first principles and distinguish between roles and Starship variants.
There are four roles:

1) Lifter (hauls methalox up from Earth)
2) Prop accumulator (VLEO or LO, but not HEEO, because HEEO isn't a single place)
3) Target fueler (feeds prop to missions that need it)
4) Rover (moves methalox to higher orbits, reasonably efficiently)

If you could build a single variant that fulfills all of these roles, you would.  However, right now we have a fundamental conflict:

a) Depots are the only reasonable prop accumulator and target fueler, but they can't EDL, and may not even be able to aerobrake.  That makes them not great as rovers.
b) Tankers can't be target fuelers, due to gendering issues.  That makes them not great, because we really need something that's a rover and a target fueler for HEEO.

If the gendering issue gets fixed, depots are accumulators and target fuelers, and tankers are lifters, rovers, and target fuelers.  All roles covered, and we're good.

If the gendering issue does not get fixed, then tankers can never be target fuelers.  They can be rovers between two target fuelers, but they can't slurp methalox from an accumulator, rove up to HEEO or LO, and fuel a target.¹

The obvious solution is to fix the gendering problem.  This is the single biggest gap we can see, given what we know about tankers and depots as of right now.  I think an androgynous QD is a much better solution than a tanker with two QDs of different gender, but that's an implementation detail at this level.

The second biggest gap is making rovers both prop-efficient and boiloff-proof.  You can do this by making a tanker well-enough insulated, or by making a depot capable of aerobraking (bonus points for making it capable of aerobraking quickly, but that's not an issue for early Artemis).

FWIW, I think a tanker is well-enough insulated to rove to LO using fast transit., and it's certainly well-enough insulated to rove to HEEO.  However, I'm not sure it's well-enough insulated to get to LO via a BLT.  This may not be a trivial distinction if we're using Block 2 propulsion, because a Block 2 tanker can't carry enough prop to NRHO to refuel a Block 2 LSS.²  However, Block 3's work fine.

_________
¹This is problematic for HEEO, because HEEO isn't a single orbit; it's a class of different orbits.  Therefore, you can't just plunk a depot at "HEEO" to be the refueler, and have a rover bring it prop.

²You could send two Block 2 tankers, but that's so inelegant that my delicate sensibilities are offended.

[Twark_Main]

...Never restarting those engines eliminates a whole class of potential problems--and losing a full depot is very expensive indeed. In fact, having two different ways to drain a full depot is a plus.

It also eliminates a whole class of capabilities too.

For instance you can kiss controlled re-entry goodbye, which means the depot probably can't even (legally) launch under the current space debris regulatory environment.

Why would you think that? Depots still need to be able to accelerate for station keeping and ullage control. They just don't use their main engines for that. (Exactly how they do this probably deserves some discussion.) Anyway, they should be able to use those thrusters for controlled reentry, when the time comes. But the Raptor engines clearly cannot be used for this.

"Clearly," lol. Is that meant to suggest some real engineering limitation here, or just that it "clearly" follows from the limit you artificially imposed?


If only there were some way you could have not made this problem for yourself... 

Did you have any other use case that benefited from mobile depots rather than just using existing tankers?

Lunar and deep space depots are of course options (notwithstanding anyone's made-up "rules"), and the requirement for long-duration storage means you can't just use existing tankers.

Personally, I still favor the idea of avoiding ullage burns entirely by having two depots connected nose-to-nose by a cable ~ 1 km long, relying on tidal forces to settle the propellant. That gives you 2 mm/sec at the noses of the two vehicles, assuming equal masses. This configuration is stable, but it can swing like a pendulum with a period of a few minutes. (I'm assuming something like solar panels to the sides--otherwise it can spin on the long axis.) Given the plan is for two depots anyway, it seems a shame not to try to take advantage of this.

Sounds like it will make for a very interesting rendezvous-and-docking. 

Has there ever been a successful rendezvous-and-docking with a non-inertial target before?  Let alone one that's also swinging like a see-saw??