Starship On-orbit refueling - Options and Discussion

[Twark_Main]

The other point no one has worked out (as far as I know) is how much ∆v you can get if the tanker reserves some fuel to do an apogee burn to allow it to aerobrake.

My spreadsheet does let you reserve a quantity of fuel for deorbit and landing. Because the ∆v is so small, to avoid circular dependencies it's treated as a constant mass that doesn't change based on apogee height.


Using vis-viva, it's relatively easy to show that

        ∆v² = 2 GM [ 1 / (r_a + r_reentry) - 1 / (r_a + r_p) ]

where r_p is the perigee, r_a is the apogee, and r_reentry is the perigee radius for reentry (or aerobraking).

For a 200 km perigee and reentry at 80 km, the graph of ∆v (m/s) vs apogee (km) looks like this.

EDIT: this equation and graph are wrong; see this post for the correct equation.


I think it's safe to say that it's not a big concern. 

[Paul451]

Re: Tanker vs Depot.

I see three classes:

Launch tanker:
EDL capable Starship (with some allowance for early missions if EDL is not solved but SpaceX needs to launch prop for Artemis demo + III.) Extra rings on tank sections, fewer on nose-section. Probably no pumping ability. Visually, it'll look like any other Starship: nose, heatshield, flapperons.

Normal depot:
Non-EDL-capable. Specialised variant of Starship. Has the extra bits required for propellant accumulation and storage. Perhaps solar arrays, sun-shield, pumps, coolers/radiators, different thrusters, etc. I'm not picking a list now that defines "depot", just "whatever turns out to be required", it will be on this vehicle, with as little as possible on the tanker. On the flip-side, it strips out the equipment necessary for EDL, so it will look quite distinct. Less "shuttle", more "skylab".

Orbital propellant transfer depot/tanker:
Moves propellant from one orbit to significantly different one. Can be any combination of properties of the pure-tanker and the pure-depot. It has to be able to dock with and transfer prop to a mission ship, but other that that it might be very close to a pure depot, or it might be EDL capable and closer to a tanker, or anywhere in between. It might be accumulation-fuelled by tankers, or it might grab a single load from a pre-filled regular depot. It might be called a "depot" or a "tanker", depending on the specifics of its mission, and the naming is likely to be pretty arbitrary.
Using these orbital prop transfer vehicles to try to define what is a depot or tanker is therefore an exercise in futility.

[Greg Hullender]

Also you are all obsessed with gender.  Gender swapping adaptors are ubiquitous and small.  A tanker with a gender adaptor that accumulates from several launches in a week before transferring to the mission ship is still a tanker.   It's role may be depot, but if every ship in the fleet can perform that role with minimal work it's a tanker.

That's a fair point, although when it comes to cryogenic fluids, you shouldn't be too quick to assume that something is trivial. But it's about more than just gender; the real issues, as I see it are, pumps, insulation, and ullage burns.

A depot has to be capable of pumping propellants into/out of other vehicles, and it's going to need something to power those pumps. Even if that's just a modest array of solar panels, it adds weight.

As NASA and SpaceX have said, the first depot will only have passive measures to prevent boiloff, but that likely means a coating of something like Solar White (meaning no reenty tiles) and probably a multilevel insulator like the Webb telescope has.

Finally, it's going to need some engine other than a Raptor that's capable of doing very long, very low-thrust burns for the duration of every fueling operation.

Add all of that together, and a depot ends up sporting quite a bit of hardware that you really don't want on any other Starship variant.

[InterestedEngineer]

Also you are all obsessed with gender.  Gender swapping adaptors are ubiquitous and small.  A tanker with a gender adaptor that accumulates from several launches in a week before transferring to the mission ship is still a tanker.   It's role may be depot, but if every ship in the fleet can perform that role with minimal work it's a tanker.

That's a fair point, although when it comes to cryogenic fluids, you shouldn't be too quick to assume that something is trivial. But it's about more than just gender; the real issues, as I see it are, pumps, insulation, and ullage burns.

A depot has to be capable of pumping propellants into/out of other vehicles, and it's going to need something to power those pumps. Even if that's just a modest array of solar panels, it adds weight.

Okay, how much weight, and how does it affect various scenarios?  I don't recall seeing any rocket equation solutions.

For example, suppose in the scenario where we want to boost 2400t of fuel to a 3km/sec HEEO, and the dry mass is nominally 130t, and we add 10t of solar panels and cooler to it, what does that do?

The plane jane version leaves you with 989t of fuel remaining.  The solar panel/cooler version leaves you with 983t of fuel remaining.  0.6% difference.

6t is rounding error at these large depot size setups.   I bet the 7 day orbit involved will have enough boil off that solar panels w/ cooler would want to be standard equipment anyways.

The conops scenario that needs to be mass efficient is the launching of fuel from the surface of earth.  Any depot/fueler operating at higher orbits doesn't need to be mass efficient because the mass of fuel is so high it's rounding error.

So there will be eventually two fuelers:   a first for earth-LEO and a second for conops from LEO to higher orbits or injection trajectories.  I think eventually that the second design may have heat shield so if HEEO becomes a standard procedure for getting stuff to the moon, high speed runs to Mars, or high deltaV destinations beyond.

Finally, it's going to need some engine other than a Raptor that's capable of doing very long, very low-thrust burns for the duration of every fueling operation.

That's an assumption.  We haven't seen how propellant transfer works yet.  We have seen videos of how zeroG fluid transfers work with a type of surface tension siphon.  No burns required.

[Greg Hullender]

Using vis-viva, it's relatively easy to show that

        ∆v² = 2 GM [ 1 / (r_a + r_reentry) - 1 / (r_a + r_p) ]

where r_p is the perigee, r_a is the apogee, and r_reentry is the perigee radius for reentry (or aerobraking).

Ah, but I'm trying to solve for the case where you don't know r_a a priori. You know the total impulse of the two burns (i.e. a fixed amount of fuel), and you know r_p and r_reentry, but that's all. In particular, you don't know how big the initial burn is going to be--you have to compute it.

This is still solvable (also using via viva) but not quite so straightforward.

However, as you say, the second burn is so tiny that my enthusiasm for the problem has cooled somewhat. :-)

[Twark_Main]

Ah, but I'm trying to solve for the case where you don't know r_a a priori. You know the total impulse of the two burns (i.e. a fixed amount of fuel), and you know r_p and r_reentry, but that's all. In particular, you don't know how big the initial burn is going to be--you have to compute it.

This is still solvable (also using via viva) but not quite so straightforward.

However, as you say, the second burn is so tiny that my enthusiasm for the problem has cooled somewhat. :-)

Good luck!

Sadly, it turns out my earlier equation was a bit too simplistic to be correct. So far, the simplest correct equation I can work out for calculating the deorbit ∆v is:

        ∆v = sqrt(2 GM/r_a) (sqrt(r_p/(r_a + r_p)) - sqrt(r_reentry/(r_a + r_reentry)))

...which for a perigee of 200 km and 80 km reentry looks like this.

Apologies for the error. If anyone wants to build off this result, it would help if it's correct...   



If you want to analytically find r_a a priori, you "simply" need to solve the following equation for r_a:

        ∆v_total = ∆v_deorbit + ∆v_injection = sqrt(2 GM/r_a) (sqrt(r_p/(r_a + r_p)) - sqrt(r_reentry/(r_a + r_reentry))) + sqrt(GM/r_p) (sqrt(2 r_a / (r_a + r_p)) - 1)

As I said, good luck!    Honestly, it's probably a lot easier to just use numerical methods here.

[OTV Booster]

This is a reasonable definition of a depot:

a: a place for storing goods or motor vehicles
b: store, cache

https://www.merriam-webster.com/dictionary/depot

A depot is for storage. A tanker is a means of transport.

[OTV Booster]

Non political thoughts on gender identity.

The ground based QD is male. Tankers and  other variants, other than depot, are female to allow mating. To mate with other ships the depot must be male. How to do this?

Simplest solution is to build the depot with male QD and put a female-female adapter on the ground based QD. Put it on and take it off as necessary. Extra mass stays on the ground. Depot launches should be rare enough that this will not be a serious impediment to ops.

The ground QD retraction mechanism and protective dog house may need a redesign to work with and without the adapter, but for SX that should be a minor issue.

[OTV Booster]

Current plans for Artemus III is for passive depot cooling. While I fully expect rapid turnaround eventually, expecting it for Artemus III would be (opinion) a loosing hand. The ships may actually turn around faster than the pad but even that might not be all that fast early on.


IIRC SX has been able to turn an F9 pad around at the Cape in four days and that only recently. Using one pad at BC and one at the cape, maybe two launches a week? That may call for more than two tankers if tanker turnaround is slow. We have to wait to find out. So does SX.


The upshot of this is the depot may have to loiter quite a while waiting for tankers and boiloff may be more than a nominal problem. However long the depot and tankers are stretched, it may need more tanker flights than expected and it'll  probably need one last final topoff just before LSS shows up. Not a deal killer but a kink to keep in mind.

[TheRadicalModerate]

Re: Tanker vs Depot.

I see three classes:

Launch tanker:
EDL capable Starship (with some allowance for early missions if EDL is not solved but SpaceX needs to launch prop for Artemis demo + III.) Extra rings on tank sections, fewer on nose-section. Probably no pumping ability. Visually, it'll look like any other Starship: nose, heatshield, flapperons.

Normal depot:
Non-EDL-capable. Specialised variant of Starship. Has the extra bits required for propellant accumulation and storage. Perhaps solar arrays, sun-shield, pumps, coolers/radiators, different thrusters, etc. I'm not picking a list now that defines "depot", just "whatever turns out to be required", it will be on this vehicle, with as little as possible on the tanker. On the flip-side, it strips out the equipment necessary for EDL, so it will look quite distinct. Less "shuttle", more "skylab".

Orbital propellant transfer depot/tanker:
Moves propellant from one orbit to significantly different one. Can be any combination of properties of the pure-tanker and the pure-depot. It has to be able to dock with and transfer prop to a mission ship, but other that that it might be very close to a pure depot, or it might be EDL capable and closer to a tanker, or anywhere in between. It might be accumulation-fuelled by tankers, or it might grab a single load from a pre-filled regular depot. It might be called a "depot" or a "tanker", depending on the specifics of its mission, and the naming is likely to be pretty arbitrary.
Using these orbital prop transfer vehicles to try to define what is a depot or tanker is therefore an exercise in futility.

Apologies for the tardy response; I kinda lost track of this thread.

I think a hybrid tanker/depot only makes sense if it's EDL-capable.  Otherwise, just use a depot.  And if it's EDL-capable, then the depot-ish parts of it have to be capable of surviving EDL.  That means they have to stow themselves or be positioned on spots on the dorsal side of the vehicle, where they don't mess up the aerodynamics too much.  If all you need is a gender-bender to connect the two QDs and a near-CoM stabilizing dock / grapple / tether / whatever, that might be OK.

If you're going to do HEEO refuelings, then depots need to be capable of >1000m/s orbital maneuvers, for two main reasons:

1) Accumulating prop in a high energy orbit is extremely expensive.  It'll always make more sense to return the depot to VLEO for accumulation, even if the return is propulsive, than it will to have all of the tankers waste a substantial amount of prop getting up to the depot.

2) While it may be OK to deploy a depot operationally to an HEEO, MMOD risk makes it unwise to store it in that orbit.  Note that the depot is simultaneously at risk from MMOD and of becoming MMOD.

If you have a depot in something like a 200 x 9800km operational orbit (200 x 200 + 1500m/s), an explosion or debris-generating MMOD strike is going to strew debris all over a bunch of extremely important orbits.  That's a tolerable risk for the short amount of time needed to do a Starship refueling operation, but then you either need to lower the orbit to VLEO (something like 400 x 400, 1440m/s) or raise the perigee to something like 1000 x 9800 (~150m/s), to get it out of the way of the bulk of MMOD. 

One of these is obviously cheaper than the other, but it doesn't solve the problem of wanting the depot in VLEO for accumulation.  Also, in the event of an explosion, a 1000km perigee means debris forever, while one in VLEO will clear most of itself out in a few years.  But the price for maintaining a depot permanently in VLEO is a non-trivial amount of prop dedicated to stationkeeping.

I continue to think that HEEO refueling is a conops of last resort.  It's a lot easier to make the LSS hold 1500-1600t of prop than to manage shifting depots around in high radiation, with narrow departure windows.

I've been thinking about how SpaceX should manage the SLT flights (post-Option B, reused LSS), and it seems as if the best solution is a second depot in NRHO.  You'd still have a VLEO depot, which handled accumulation, but the last tanker to VLEO, instead of moving its prop into the depot, would extract all prop from it.  It would then boost up to NRHO (whether this is a fast NRHO or a BLT depends on how boiloff gets managed in the tanker) and dump almost all prop into the NRHO depot.  The tanker would then do a direct EDL from TEI (or a reverse BLT).

If you really do have an EDL-capable depot/tanker hybrid, then you don't necessarily need the depot in NRHO.  You just launch the hybrid as the last tanker, fill it up from the depot, send it to NRHO, transfer the prop to the LSS, and return to EDL.

[sdsds]

[...] put a female-female adapter on the ground based QD. Put it on and take it off as necessary. Extra mass stays on the ground. Depot launches should be rare enough that this will not be a serious impediment to ops.

Thanks for sharing this thought. If I'm seeing it correctly, any Starship with a male QD interface acts as a depot, or at least a depot prototype that could be used to demonstrate on-orbit propellant transfer. Count me as "extremely eager" to see a Starship fire up its engines with propellant loaded on-orbit!

[Paul451]

Re: Tanker vs Depot.

I see three classes:

Launch tanker:
[...]
Normal depot:
[...]
Orbital propellant transfer depot/tanker:
Moves propellant from one orbit to significantly different one. Can be any combination of properties of the pure-tanker and the pure-depot. [...] It might be called a "depot" or a "tanker", depending on the specifics of its mission, and the naming is likely to be pretty arbitrary.
Using these orbital prop transfer vehicles to try to define what is a depot or tanker is therefore an exercise in futility.

I think a hybrid tanker/depot only makes sense if it's EDL-capable.  Otherwise, just use a depot. [...]

And it might be called a depot. (Perhaps an "orbital transfer depot".) My point was that trying to define "depot" based on the properties of the hybrid cases is futile. Each example will be its own unique combination of properties.

Better to use the "pure" examples to define what is a "tanker" vs "depot". But also accept that there are a bunch of potential use-cases that will be somewhere in between, and whose naming will be somewhat arbitrary.

If you're going to do HEEO refuelings, then depots need to be [...bunch of stuff about HEEO...]

I wasn't referring to HEEO when talking about the third category. Just that their defining trait will be transferring propellant between orbits. It could be LEO to Lunar. It could be interplanetary. Didn't specify. Just that their properties are going to be fuzzier to define than a "pure" depot or tanker.

[Paul451]

The ground based QD is male. Tankers and  other variants, other than depot, are female to allow mating. To mate with other ships the depot must be male. How to do this?
Simplest solution is to build the depot with male QD and put a female-female adapter on the ground based QD. Put it on and take it off as necessary. Extra mass stays on the ground. Depot launches should be rare enough that this will not be a serious impediment to ops.

It's not just the inny and outy bits on the connectors, but the compatibility of all the plumbing and hardware behind those connectors.

[Greg Hullender]

The ground based QD is male. Tankers and  other variants, other than depot, are female to allow mating. To mate with other ships the depot must be male. How to do this?
Simplest solution is to build the depot with male QD and put a female-female adapter on the ground based QD. Put it on and take it off as necessary. Extra mass stays on the ground. Depot launches should be rare enough that this will not be a serious impediment to ops.

It's not just the inny and outy bits on the connectors, but the compatibility of all the plumbing and hardware behind those connectors.

To be more specific, I think the biggest distinction will be pumps. Neither a tanker nor LSS needs to have any pumps to transfer fuel. (Except to the engines, of course.) A depot can pump fuel out of a tanker or into an LSS (or other Starship variant). Nothing else can do this. (Not the way I'd like to define the word, anyway.) In addition, a depot can have big solar panels, active cooling, and whatever else it needs, but it cannot EDL.

If it doesn't have the pumps, there's no point in putting any of that other stuff on a tanker. This makes a tanker as simple as possible; it's just a version of Starship with big tanks and zero cargo space.

A "hybrid" would be something that capable of EDL which also had pumps to let it exchange fuel with a tanker, an LSS, or even another hybrid. You can do some cool things with hybrids, but they seem to add a lot of engineering and operational complexity.

1) Accumulating prop in a high energy orbit is extremely expensive.  It'll always make more sense to return the depot to VLEO for accumulation, even if the return is propulsive, than it will to have all of the tankers waste a substantial amount of prop getting up to the depot.

2) While it may be OK to deploy a depot operationally to an HEEO, MMOD risk makes it unwise to store it in that orbit.  Note that the depot is simultaneously at risk from MMOD and of becoming MMOD.

Oh, I don't think anyone is talking about doing that! (This is the confusion that comes from calling hybrids "depots.") Let's see if I can describe what I think is meant by "HEEO refueling." I'll over-simplify a bit in the interest of brevity.

1) One hybrid and one LSS are launched into the same circular LEO with just a bit of space between them.

2) Tankers make as many trips as necessary to fuel them both up.

3) When they're both full, they make a single burn at the same time that consumes a bit less than half of their fuel. They are now together in HEEO.

4) Before they reach apogee, the hybrid transfers the rest of its fuel to the LSS, saving just enough for EDL. The LSS is now fully fueled and the hybrid is almost empty.

5) At apogee, the hybrid does a very small burn to lower its perigee enough for EDL at perigee.

6) At its own perigee, the LSS does a second burn that lifts it to the moon.

No one would ever try to refuel an actual depot in HEEO. No one would ever try to put one there in the first place. Or, at least, that the conclusion I drew from the (extensive) discussions upthread. (Pretty much for all the reasons you give.)

It's still pretty complicated, though, and it only works if you have hybrids.

By contrast, a depot in NRHO has a lot going for it. It's a lot easier to keep the propellant cool. (For methalox, passive measures alone may suffice.) And a fully-fueled depot should only have to burn 2/3 of its fuel getting from LEO to NRHO. You can top it up by sending two regular tankers, which fill up at an LEO depot, deliver about 1/3 of their load of fuel to NRHO, and then return to Earth for EDL.

In this scenario, we don't introduce any new vehicle types (beyond the four SpaceX has already talked about), and the genders on the connectors even work out.

If only we could convince NASA to build an NRHO depot instead of Gateway. :-)

[BitterJim]

Non political thoughts on gender identity.

The ground based QD is male. Tankers and  other variants, other than depot, are female to allow mating. To mate with other ships the depot must be male. How to do this?

Simplest solution is to build the depot with male QD and put a female-female adapter on the ground based QD. Put it on and take it off as necessary. Extra mass stays on the ground. Depot launches should be rare enough that this will not be a serious impediment to ops.

The ground QD retraction mechanism and protective dog house may need a redesign to work with and without the adapter, but for SX that should be a minor issue.

Who not skip the female-female adapter and just give the depot both male and female versions of the QD?

[TheRadicalModerate]

Better to use the "pure" examples to define what is a "tanker" vs "depot". But also accept that there are a bunch of potential use-cases that will be somewhere in between, and whose naming will be somewhat arbitrary.

I think what you're saying is that there will be an a la carte menu of accessories to be added to a common Starship core.  Something like the attached?  (I've called your hybrid "OTV Tanker" below.)

It's not just the inny and outy bits on the connectors, but the compatibility of all the plumbing and hardware behind those connectors.

I've got this subsumed under "Pumping System" in the attachment, but, other than the actual pumps, I'd guess that the existing plumbing, which should handle both fill and drain under gravity or ullage acceleration, should be just about it.  If you can avoid lots of internal plumbing changes, things get a lot easier.

1) One hybrid and one LSS are launched into the same circular LEO with just a bit of space between them.

2) Tankers make as many trips as necessary to fuel them both up.

3) When they're both full, they make a single burn at the same time that consumes a bit less than half of their fuel. They are now together in HEEO.

4) Before they reach apogee, the hybrid transfers the rest of its fuel to the LSS, saving just enough for EDL. The LSS is now fully fueled and the hybrid is almost empty.

This sounds wildly improbable and terrifying to me.  Formation thrusting, even if it weren't dangerous, is unlikely to result in two ships in the same orbit.  There are always unquantifiable residuals.  Those alone make a first-orbit RPOD unlikely, and certainly not something that you'd count on.

A "traveling depot" (one that changes orbits but isn't EDL-capable) is more expensive than an EDL-capable tanker, but not by massive amounts.  Remember that, even if you need 1500-2000m/s of delta-v times two to accomplish the refueling, the second 1500-2000m/s takes a lot less prop than the first one.

Put the depot or tanker in HEEO first and determine its orbital parameters as best as possible.  Then put the target Starship into the HEEO, slightly out of phase, and then phase to do the RPOD.  (I have an HEEO RPOD thread that didn't really catch on somewhere.  I still think HEEO RPOD is hard.)

The other option is to make a "pusher/tanker" depot, which we've discussed elsewhere.  It acts more like a first stage than a formation-flyer, and eliminates all the RPOD scariness.

By contrast, a depot in NRHO has a lot going for it. It's a lot easier to keep the propellant cool. (For methalox, passive measures alone may suffice.) And a fully-fueled depot should only have to burn 2/3 of its fuel getting from LEO to NRHO. You can top it up by sending two regular tankers, which fill up at an LEO depot, deliver about 1/3 of their load of fuel to NRHO, and then return to Earth for EDL.

Best I can tell, a 1625t tanker (which is what I get if the tanker moves the domes and bulkheads to eat all of the 8m of the cylindrical portion of the payload bay) winds up in NRHO with only a tad more prop than is necessary to transfer to the LSS for NRHO-LS-NRHO, plus what's needed to get back to EDL.  If a tanker can refuel the LSS directly, that's fine.  If not, a depot can mediate the transfer.  But when it's almost exactly one tanker per mission, things seem pretty straightforward.

[Barley]

To be more specific, I think the biggest distinction will be pumps. Neither a tanker nor LSS needs to have any pumps to transfer fuel. (Except to the engines, of course.) A depot can pump fuel out of a tanker or into an LSS (or other Starship variant). Nothing else can do this. (Not the way I'd like to define the word, anyway.) In addition, a depot can have big solar panels, active cooling, and whatever else it needs, but it cannot EDL.

I suspect you are overestimating the complexity of pumps.  They'll need significantly less than 100kg of pump/motors to do the transfer in a couple of hours.*  Putting pumps in the tanker to do the offload will significantly simplify the rest of the plumbing and pushing rather than sucking fluids should make ullage a lot easier.

* They might need more than that in solar cells or batteries, but those can be on the depot.

[Greg Hullender]

I suspect you are overestimating the complexity of pumps.

Could be. I'm a software guy, and this is definitely a hardware problem. :-)

They'll need significantly less than 100kg of pump/motors to do the transfer in a couple of hours.*  Putting pumps in the tanker to do the offload will significantly simplify the rest of the plumbing and pushing rather than sucking fluids should make ullage a lot easier.

Are you sure about that? Is pushing that much easier than sucking? (Uh, maybe I should rephrase that . . .) ;-)

They might need more than that in solar cells or batteries, but those can be on the depot.

Ah, but that only works if the other vehicle is actually a depot. If it's some other Starship variant (or even another tanker), then the power has got to come from somewhere. Not a problem if the rule is that tankers (and only tankers) fill depos; depos (and only depos) fill everything else.

[edzieba]

Pumps are also an option rather than a necessity: you have a free vacuum-sink all around you, and your propellants boil off with minimal effort (i.e. you have to actively stop your boiloff mitigation for a bit). That means you have both a sink-to-vacuum and source of pressurised gas without any energy input beyond that needed to open vent valves. Since the tanks need vent valves already, that means pressure-based fluid transfer can be performed with effectively no additional hardware or energy input.

[DanClemmensen]

Pumps are also an option rather than a necessity: you have a free vacuum-sink all around you, and your propellants boil off with minimal effort (i.e. you have to actively stop your boiloff mitigation for a bit). That means you have both a sink-to-vacuum and source of pressurised gas without any energy input beyond that needed to open vent valves. Since the tanks need vent valves already, that means pressure-based fluid transfer can be performed with effectively no additional hardware or energy input.

This also means that all SS types are "equipped" to supply pressure or vacuum to move the fluids in either direction. I still think all transfers will either be to the Depot or from the Depot for other reasons, though.