Starship On-orbit refueling - Options and Discussion

[TheRadicalModerate]

1) Option A and B missions have to be as simple as possible, and the LSS itself can't be refueled more than once per mission, nor can it be refueled with crew on board.

This is a silly constraint.  By the time a crew certified LSS leaves LEO there will be there will be an order of magnitude more experience refueling than launching SLS or reentering Orion or a great many other parts of the planned mission.  If that's not enough refueling experience, you can get a lot more fairly cheaply and quickly.  Since refueling is an essential part of any BLEO use of Starship you have to do that eventually, so the best plan is to expedite testing refueling and not place unneeded constraints on the eventual mission.

It's a fair criticism, and one I expected.  Here's my reasoning:

1) Pez-dispenser Starships are probably unsuitable for refueling RPOD and transfer tests, so SpaceX is limited to however much hardware they decide to take out of Starlink service to perform the development.

2) Things will be more schedule-bound than you think.  They always are.  I'd also expect depot and refueling R&D to lag launch, EDL, and re-use R&D, simply because refueling testing is expensive without reusability being fairly far along.  And NASA is going to be fairly impatient with delays.

3) An LSS will be close to an order of magnitude more expensive than a vanilla Starship, simply because it has to be crew-certified.  It's an asset for which you want to aggressively minimize the risks you can.  Limiting LSS to a single refueling is a simple way to minimize risks.

4) NASA made a fairly big deal about the refueling complexity in the source selection statement, and the Blue Origin lawsuit picked up on that, forcing NASA to defend the conops as having almost all refueling off the critical path.  I'd expect that anything that reduced that complexity has very nice CYA properties for NASA.

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.

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.

[Greg Hullender]

I'm still curious what SpaceX might do with the used LSS's that will accumulate at Gateway. It might make sense to try to refuel them there (e.g. via a second depot at Gateway or just from regular tankers) and fly them down to the moon and back without crew just to demonstrate they could be reused.

That's assuming, of course, that they can be reused without needing maintenance. And it sure would help if someone built some paved landing pads on the moon!

[TheRadicalModerate]

I'm still curious what SpaceX might do with the used LSS's that will accumulate at Gateway. It might make sense to try to refuel them there (e.g. via a second depot at Gateway or just from regular tankers) and fly them down to the moon and back without crew just to demonstrate they could be reused.

That's assuming, of course, that they can be reused without needing maintenance. And it sure would help if someone built some paved landing pads on the moon!

There should only be two.  After that, you're on to Option B, and they get refueled until they reach end of life.

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 expect Option B (and Appendix P) HLSes to have fairly short lives, due to dust contamination.  It may be that SpaceX would do better using an EDL-capable LSS, if for no other reason that you can completely clean and refurbish it on the ground.  (You can also integrate new heavy cargoes into it, which an Option B LSS can't do.)  But this definitely requires refueling in HEEO or cislunar.

[redneck]

A few days ago I mentioned the concept of tanker and cargo Starship mated for the burn from LEO towards the moon. Mainly I brought it up because I hadn't noticed it in this discussion. It didn't seem to be a popular idea both on the technical side and even whether it would be useful. I concede that the technical end may be more difficult than most would like. Possibly to the point of not being worthwhile.

As for the usefulness if it could be implemented, I didn't make the time to check my assumptions until this afternoon. Did a bit of BOTE to see where I would end up.    The way I see it, if the tanker would detach at ~2,500 m/s and fall back to reenter while the cargo Starship kept thrusting. The Starship would have a mass ratio of about 1.5 remaining to NHRO rendezvous. It seems to me that reaching that rendezvous with an extra 500 tons of propellant on board would be far from useless.

[InterestedEngineer]

A few days ago I mentioned the concept of tanker and cargo Starship mated for the burn from LEO towards the moon. Mainly I brought it up because I hadn't noticed it in this discussion. It didn't seem to be a popular idea both on the technical side and even whether it would be useful. I concede that the technical end may be more difficult than most would like. Possibly to the point of not being worthwhile.

As for the usefulness if it could be implemented, I didn't make the time to check my assumptions until this afternoon. Did a bit of BOTE to see where I would end up.    The way I see it, if the tanker would detach at ~2,500 m/s and fall back to reenter while the cargo Starship kept thrusting. The Starship would have a mass ratio of about 1.5 remaining to NHRO rendezvous. It seems to me that reaching that rendezvous with an extra 500 tons of propellant on board would be far from useless.

Why couple things mechanically (added mass, very bad) when you can couple them with software?  (mass free except for a proximity radar which Starship probably already has).

Two spacecraft can boost simultaneously and adjust to stay relatively close to each other.  It's not rocket science

(Seriously the software wouldn't be very complex).

That, essentially, is what HEEO refuel option is.    Whether they boost at the same time or rendezvous later is an implementation detail (though at the same time is probably simpler)

A full load of fuel with an additional energy of 2.5km/sec beyond LEO orbital speeds is worth a lot, and effectively the same thing you are talking about.

As RadicalModerate points out, this won't be a working solution until LSS is crew-certified and there's been so many refuelings that it's as routine as refueling a B-52 bomber was in ~1960.

[InterestedEngineer]

The HEEO refuel only costs one additional fully refueled standard Starship (said Starship reusable).   You use up so little fuel in the Oberth burn you have enough left over to complete the entire rest of the Moon mission (full round trip).

You use exactly the same amount of delta-v, irrespective of the energy of the HEEO.  If you're trying to get to LEO+3200 and you stop in an LEO+2500 HEEO, then that HEEO to LEO+3200 costs 700m/s.  It's simple addition, nothing more--as long as you always burn at perigee. 

You need to do C3 calculations, not deltaV calculations.

In the end, you end up with far less errors if you always using conservation of momentum and conservation of energy when performing conic section calculations.

Spacecraft have a delta-v budget, not a C3 budget.  And vis-viva is conservation of energy.

v_peri˛ = μ(2/r_peri - 1/a)

Let's set μ=1 and start in a circular orbit, i.e., r=a.  All maneuvers will be performed at r_periapse=1, which will therefore stay the same (i.e., set to 1 in our normalized units), as r_apoapse increases.

If I prove that Δv_a=1→a=2 = Δv_a=1→a=1.5 + Δv_a=1.5→a=2, will you concede?

Here goes:

v_a=1˛ = (2 - 1) = 1, so v_peri,a=1 = 1
v_a=2˛ = (2 - 1/2) = 1.5, so v_peri,a=2 = 1.22
v_a=1.5˛ = (2 - 1/1.5) = 1.33, so v_peri,a=1.5 = 1.15

Δv_a=1→a=2 = v_peri,a=2 - v_peri,a=1 = 1.22 - 1 = 0.22.
Δv_a=1→a=1.5 = v_peri,a=1.5 - v_peri,a=1 = 1.15 - 1 = 0.15
Δv_a=1.5→Δv_a=2 = v_peri,a=2 = v_peri,a=1.5 = 1.22 - 1.15 = 0.07

Δv_a=1→a=1.5 + Δv_a=1.5→Δv_a=2 = 0.15 + 0.07 = 0.22 = Δv_a=1→a=2
QED

Again, all maneuvers are performed at periapse, where you'd get the maximum Oberth effects.

Things are different if you transition from an elliptical orbit to a hyperbolic one, but we're not doing that.

You are correct for elliptical orbits. I plead being sleepy and tired and having seen naive deltaV calcs fail for hyperbolic orbits in previous work.

The real gain comes from getting a full load of fuel that has +2.5km/sec velocity beyond LEO.   1/2mv^2 with v being higher and m being higher.  That's what enables full lunar round trips without remote depots, and exploration of the entire solar system and slightly beyond  (hyperbolic orbits) , making a lot of non-chemical methods technological dead ends.

But won't happen until LSS is EDL crew-certified and refuelings are as routine as they were on B-52s circa 1960.

[Barley]

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.

It also reduces margin.   The extra refueling lets you throw mass at other risks.  It also reduces schedule risk, and in particular a risk of SS underperforming; if it gets to orbit it can complete the mission with two refuelings.  There will be no delay trying to make weight by making the windows so thin they are at risk from a finger tap.

Also a second refueling never risks the crew.  This cannot be said of everything that might be done to avoid a second refueling.

Avoiding a refueling is not a no brainer, you have to evaluate all the risks being added against the risk of a refueling, unless you just assign each refueling infinite risk by fiat.

[TheRadicalModerate]

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.

It also reduces margin.   The extra refueling lets you throw mass at other risks.  It also reduces schedule risk, and in particular a risk of SS underperforming; if it gets to orbit it can complete the mission with two refuelings.

Your argument about margin is well-taken--if you're talking about a 1200t LSS.  But even there, when I added in FPR and fixed prop losses, things worked out OK in a low HEEO.  I wouldn't want to use that method with a crew, but it's got pretty good margin for an uncrewed pre-positioning of the LSS in NRHO.

However, it's a lot more janky than just going with 1500t tanks.  Even after adding FPR, fixed losses, and boil-off allowances, the 1500t LSS gets back from the surface to NRHO with several tonnes of usable prop.

There will be no delay trying to make weight by making the windows so thin they are at risk from a finger tap.

Well, I've got other people on the thread complaining that my dry mass and crew module numbers are too conservative.  And again, 1500t of prop hides a multitude of sins.  There's a lot of mass margin here.

Avoiding a refueling is not a no brainer, you have to evaluate all the risks being added against the risk of a refueling, unless you just assign each refueling infinite risk by fiat.

That's fair, but assigning high risk to a procedure with which you have little operational experience is appropriate.  NB:  Even if there turns out to be a lot of experience with tankers fueling depots in VLEO, there will be considerably less experience refueling stuff in HEEO.  Could SpaceX load up on dummy refueling missions to work this out?  Sure.  But why bother if there's a better way to manage the risk?

The other factor here is risk tolerance.  In a perfect world, this wouldn't change over time.  However, HLS is a high enough profile project, both for NASA and SpaceX, that reducing tolerance to ensure that both Option A flights go well is a good political move--especially since NASA wrung their hands about operational complexity in the source selection statement, and Blue Origin used the conops to fling FUD around during the protest and appeal process.

[redneck]

A few days ago I mentioned the concept of tanker and cargo Starship mated for the burn from LEO towards the moon. Mainly I brought it up because I hadn't noticed it in this discussion. It didn't seem to be a popular idea both on the technical side and even whether it would be useful. I concede that the technical end may be more difficult than most would like. Possibly to the point of not being worthwhile.

As for the usefulness if it could be implemented, I didn't make the time to check my assumptions until this afternoon. Did a bit of BOTE to see where I would end up.    The way I see it, if the tanker would detach at ~2,500 m/s and fall back to reenter while the cargo Starship kept thrusting. The Starship would have a mass ratio of about 1.5 remaining to NHRO rendezvous. It seems to me that reaching that rendezvous with an extra 500 tons of propellant on board would be far from useless.

Why couple things mechanically (added mass, very bad) when you can couple them with software?  (mass free except for a proximity radar which Starship probably already has).

Two spacecraft can boost simultaneously and adjust to stay relatively close to each other.  It's not rocket science

(Seriously the software wouldn't be very complex).

That, essentially, is what HEEO refuel option is.    Whether they boost at the same time or rendezvous later is an implementation detail (though at the same time is probably simpler)

A full load of fuel with an additional energy of 2.5km/sec beyond LEO orbital speeds is worth a lot, and effectively the same thing you are talking about.

As RadicalModerate points out, this won't be a working solution until LSS is crew-certified and there's been so many refuelings that it's as routine as refueling a B-52 bomber was in ~1960.

You don't get exactly the same results with software though. With the HEEO refueling, you still have docking, and propellant transfer before resuming boost. At a minimum, this gets another couple of trips through the Van Allens. It may be a better option to do it as you say, but it is not a 1 to 1 match. 

[Greg Hullender]

A few days ago I mentioned the concept of tanker and cargo Starship mated for the burn from LEO towards the moon. Mainly I brought it up because I hadn't noticed it in this discussion. It didn't seem to be a popular idea both on the technical side and even whether it would be useful. I concede that the technical end may be more difficult than most would like. Possibly to the point of not being worthwhile.

As for the usefulness if it could be implemented, I didn't make the time to check my assumptions until this afternoon. Did a bit of BOTE to see where I would end up.    The way I see it, if the tanker would detach at ~2,500 m/s and fall back to reenter while the cargo Starship kept thrusting. The Starship would have a mass ratio of about 1.5 remaining to NHRO rendezvous. It seems to me that reaching that rendezvous with an extra 500 tons of propellant on board would be far from useless.

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.

But I think we previously figured that a fully fueled tanker in LEO could reach the moon with about 500t of fuel still in the tank, so (if two refueling operations were allowed), you'd only be saving a little bit over having both ships accelerate to TLI independently and then have them rendezvous for refueling at any point thereafter. (And have the tanker do a free return or something like it.) Is that right?

Anyway, if NASA is worried about the risks of refueling in LEO, I think the risks of refueling while accelerating will really send them into orbit. (So to speak.) :-)

[Greg Hullender]

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. 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.

I expect Option B (and Appendix P) HLSes to have fairly short lives, due to dust contamination.  It may be that SpaceX would do better using an EDL-capable LSS, if for no other reason that you can completely clean and refurbish it on the ground.  (You can also integrate new heavy cargoes into it, which an Option B LSS can't do.)  But this definitely requires refueling in HEEO or cislunar.

This leads me to wonder whether it's possible to do that at Gateway. Or, conversely, what is the minimum it would take to do the necessary maintenance and checkout at Gateway, rather than returning the LSS to Earth? (Assuming you can skip the static fire.) :-)

[Greg Hullender]

Two concerns I thought of with Highly Elliptical Earth Orbit (HEEO) refueling:

1) These orbits have apogee's further out than the orbit of the moon. That implies they'll eventually have problems with lunar perturbations. Simplest fix is probably to make the period equal to half the lunar period and arrange it so the moon is at 90 degrees to the vector from the depot to the Earth. That makes for a two-week period, though.

2) How long would it take to fill one of these HEEO depots? If it takes 13 tanker trips, assuming you can just do one refueling per perigee, that mean it'd take 3 months to fill the thing. That makes for only four trips a year to the moon. By contrast, a depot in LEO can be refueled in just two weeks, assuming one tanker launch per day.

[InterestedEngineer]

Two concerns I thought of with Highly Elliptical Earth Orbit (HEEO) refueling:

1) These orbits have apogee's further out than the orbit of the moon. That implies they'll eventually have problems with lunar perturbations. Simplest fix is probably to make the period equal to half the lunar period and arrange it so the moon is at 90 degrees to the vector from the depot to the Earth. That makes for a two-week period, though.

2) How long would it take to fill one of these HEEO depots? If it takes 13 tanker trips, assuming you can just do one refueling per perigee, that mean it'd take 3 months to fill the thing. That makes for only four trips a year to the moon. By contrast, a depot in LEO can be refueled in just two weeks, assuming one tanker launch per day.

The optimal approach is for both Starships to use half their fuel so that one full Starship can do the perigee burn.. I calculate that gives an apogee of ~19,000 kilometers, so well inside Lunar orbit and well outside Luna's SOI.  It's almost but not quite  a standard GTO.

The 16 tanker trips is done in LEO, so it doesn't take 3 months.   It's only about 4 times the refueling trips for a min-deltaV Hohmann to Mars.   16 tanker trips (with 2 Starships being refueled) is 8 trips each so about 1.5 days at 4 hours per refuel (that's two orbits per refuel).

Once both Starships are full, they both simultaneously boost to the same HEEO orbit using half their fuel each.  Apogee 19,000 kilometers, Perigee 200-300km.

The final fuel transfer is done at just before Apogee of the HEEO(outside the Van Allen belts) and there's plenty of time to do that out there.  A small adjustment burn is done at apgoee by the full Starship to line up its perigee burn so it'll end up at the Moon (or Mars, or wherever), and the almost empty fuel carrier Starship does a slight apogee burn to initiate re-entry.

The reason to use it, though it may not be as optimal as a 1500t fuel dept in Lunar Orbit is that this maneuver gets you anywhere you want to go in the Solar system, at deltaVs so high the online porkchop calculators can't handle it.

There's only one extra trip through the Van Allens (coming back to perigee to make the Oberth burn).

No special measures for keeping the propellant from boiling are needed.

[Barley]

Two concerns I thought of with Highly Elliptical Earth Orbit (HEEO) refueling:

1) These orbits have apogee's further out than the orbit of the moon. That implies they'll eventually have problems with lunar perturbations. Simplest fix is probably to make the period equal to half the lunar period and arrange it so the moon is at 90 degrees to the vector from the depot to the Earth. That makes for a two-week period, though.

2) How long would it take to fill one of these HEEO depots? If it takes 13 tanker trips, assuming you can just do one refueling per perigee, that mean it'd take 3 months to fill the thing. That makes for only four trips a year to the moon. By contrast, a depot in LEO can be refueled in just two weeks, assuming one tanker launch per day.

I think you are being over specific in what you mean by HEEO.

By some usage a Molniya or Geostationary transfer orbit would qualify as HEEO.  ΔV wise these are a little less than halfway to the Moon's surface, have periods close to 12 hours and are reasonably stable*.  A somewhat higher 400x70000km with a period of about a day would also be a HEEO.   Something like this would address the issues you raise

* You don't need absolute stability.  For example, a depot near L2 in a James Webb style halo orbit would require very little station keeping, as long as you are paying attention, there's probably no reason to do that, but you could.  Many orbits near the boundary of Earth's influence are like this.  They are unstable because small perturbations send them off into the void, but they are potentially usable because slightly less small control inputs bring them back.

[LMT]

...this maneuver gets you anywhere you want to go in the Solar system, at deltaVs so high the online porkchop calculators can't handle it.

Getting to the outer solar system is straightforward.  Getting back isn't.

As with a notional 2-year Starship Jupiter mission, delta-v far exceeds depot capacity, whether in LEO, HEEO, or elsewhere.  Depots must be supplemented, somehow.  In that mission:

- (Directional) laser propulsion was based in LEO and LJO.

- (Omnidirectional) methalox depots were based in LEO and on Callisto.

Together, and only together, they gave thrust vectors for mission success.

Some such network of pre-positioned directional and omnidirectional propulsion solutions would seem necessary for crewed missions to the outer solar system.

In practice, mission planners would optimize use of the network, in the spirit of Ishimatsu et al. 2016.  That is, applying an extended network flow model to the logistics system.

Q:  To support outer solar system Starship missions, where might you base the next directional and omnidirectional propulsion solutions?  What are some further good moves on the logistics chessboard?

Refs.

Ishimatsu, T., de Weck, O.L., Hoffman, J.A., Ohkami, Y. and Shishko, R., 2016. Generalized multicommodity network flow model for the earth–moon–mars logistics system. Journal of Spacecraft and Rockets, 53(1), pp.25-38.

[InterestedEngineer]

...this maneuver gets you anywhere you want to go in the Solar system, at deltaVs so high the online porkchop calculators can't handle it.

Getting to the outer solar system is straightforward.  Getting back isn't.

As with a notional 2-year Starship Jupiter mission, delta-v far exceeds depot capacity, whether in LEO, HEEO, or elsewhere.  Depots must be supplemented, somehow.  In that mission:

- (Directional) laser propulsion was based in LEO and LJO.

- (Omnidirectional) methalox depots were based in LEO and on Callisto.

Together, and only together, they gave thrust vectors for mission success.

Some such network of pre-positioned directional and omnidirectional propulsion solutions would seem necessary for crewed missions to the outer solar system.

In practice, mission planners would optimize use of the network, in the spirit of Ishimatsu et al. 2016.  That is, applying an extended network flow model to the logistics system.

Q:  To support outer solar system Starship missions, where might you base the next directional and omnidirectional propulsion solutions?  What are some further good moves on the logistics chessboard?

Refs.

Ishimatsu, T., de Weck, O.L., Hoffman, J.A., Ohkami, Y. and Shishko, R., 2016. Generalized multicommodity network flow model for the earth–moon–mars logistics system. Journal of Spacecraft and Rockets, 53(1), pp.25-38.

By "anywhere in the solar system" I meant one way flybys.

The only two way trip that is worthwhile, AFAICT, is Mars and the Moon.  Mars is very well covered, and the Moon is the tough one, which is why all the debate.

Beyond Earth/Moon starting at LEO is IMHO a fairly obsolete technique when for less than 1/10 the cost of the SLS you can refuel a Starship in ~GTO and get anywhere else with Vinf of 15km/sec.  For example it drops Mars down to about 68 days, assuming Starship can actually slow down in the Mars atmosphere on the far end.  (in general if you want to *stop* somewhere the 15km/sec is problematic)

Trying to stay on topic, one can extend the fuel depot idea via "laddering" to e.g. an Oberth/gravity turn & burn at Jupiter, but the benefit is marginal.   That topic was explored on the extra-solar probe thread.

So are there missions beyond Luna/Mars that need a return and would be helped with a fuel depot?  Would it change the requirements that have been detailed in this thread?

[redneck]

A few days ago I mentioned the concept of tanker and cargo Starship mated for the burn from LEO towards the moon. Mainly I brought it up because I hadn't noticed it in this discussion. It didn't seem to be a popular idea both on the technical side and even whether it would be useful. I concede that the technical end may be more difficult than most would like. Possibly to the point of not being worthwhile.

As for the usefulness if it could be implemented, I didn't make the time to check my assumptions until this afternoon. Did a bit of BOTE to see where I would end up.    The way I see it, if the tanker would detach at ~2,500 m/s and fall back to reenter while the cargo Starship kept thrusting. The Starship would have a mass ratio of about 1.5 remaining to NHRO rendezvous. It seems to me that reaching that rendezvous with an extra 500 tons of propellant on board would be far from useless.

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.

But I think we previously figured that a fully fueled tanker in LEO could reach the moon with about 500t of fuel still in the tank, so (if two refueling operations were allowed), you'd only be saving a little bit over having both ships accelerate to TLI independently and then have them rendezvous for refueling at any point thereafter. (And have the tanker do a free return or something like it.) Is that right?

Anyway, if NASA is worried about the risks of refueling in LEO, I think the risks of refueling while accelerating will really send them into orbit. (So to speak.) :-)

I'm say hard couple the ships together to control the variations. Formation flying while refueling would be even riskier. The tanker would crossfeed the cargo vehicle during the boost. If it works out that fuel cannot be transferred between ships at that rate, a lessor acceleration by both ships would reduce the required cross flow.

I think it works out that one ship would reach NHRO at 500 tons less dry mass. This would allow reaching it with 1,000 tons less the same dry mass. Approximately 380 tons net vs 880 tons net.

[redneck]

Two concerns I thought of with Highly Elliptical Earth Orbit (HEEO) refueling:


There's only one extra trip through the Van Allens (coming back to perigee to make the Oberth burn).

No special measures for keeping the propellant from boiling are needed.

I get two extra trips through the Van Allens. Coming back to perigee and then again on the way out.   Not saying it's a showstopper. Depends on the risk, or perceived risk, of transiting them.

Patience please, still learning to use the quotes on this site.

[InterestedEngineer]

I get two extra trips through the Van Allens. Coming back to perigee and then again on the way out.   Not saying it's a showstopper. Depends on the risk, or perceived risk, of transiting them.

Patience please, still learning to use the quotes on this site.

A direct flight from  LEO w/ return is 2 passes through VA belts.

A ~GTO orbit takes one orbit to get back to perigee.  The boost out and the final return is one effective orbit.

At 2 passes through VA  belts per orbit, that's two extra passes for a round trip.

Thanks for the correction.

There's so much more mass and space on Starship to protect crew from VA radiation that I don't think the risk is that great.

You literally wrap the crew area in plastic.

[InterestedEngineer]

I'm say hard couple the ships together to control the variations. Formation flying while refueling would be even riskier.

Formation flying while refueling is what US Air Force has been doing since the late 1940s.

The trick is to not be under relative acceleration.  That's the default for spacecraft not under thrust, whether it's HEEO, LEO, or Luna orbit.

Other than radiation, there's no difference between a refueling procedure in orbit around Luna, HEEO, or LEO, although the refueling has to be not controlled by Earth because of lag problems in the first two cases. 

Since Dragon is docking autonomously I think they'll figure out how to do refueling > 0.01 light second away from Earth.

If you are worried about formation refueling, then LEO refueling won't work either, and the entire architecture of Starship is doomed.  Somehow I don't think so.