Starship On-orbit refueling - Options and Discussion

[Twark_Main]

[tethers]
So, a flexible dumbell is stable?

No. With a flexible tether, the end-weights can twist back-and-forth on the end of the tether. You need something other than a cable to prevent twist. And the longer the tether, the worse it is and the harder to counter.

Additionally, any AG tether system is likely going to have very similar rotational-inertia in both the short axes. That risks intermediate-axis-instability.

Essentially, tethered systems are likely to be inherently unstable around the long axis.

A tether can be converted into a truss by using pressurized hoses instead of wires. It does not need to be very strong in compression to eliminate twisting.

Is there a physically-possible way to conserve the angular momentum so this system can be spun up and down without much use of thrusters? That would be a really serious reaction wheel, inside the Depot or counterweight. Maybe not feasible.

Still haven't had a chance to catch up with this thread after the holidays, but it looks like we somehow got to tethers.

The shorter the radius, the lower the angular momentum, and the propellant. So either way you do your spin-up and spin-down, you want the shortest workable radius, not the largest.

Given that, and the fact that you need some sort of deployment and docking mechanism anyway, it seems like the "nothing left to take away" implementation would just have two ships hard-dock at their nose. This eliminates the entire tether system, along with any associated instabilities.

[TheRadicalModerate]

Medium-Earth Orbit/High-Earth Orbit/Final Tanking Orbit. Missions beyond LEO will also require space station operations in medium-Earth orbit (“MEO”) to high-Earth orbit (“HEO”). For example, crewed lunar missions will include a secondary propellant transfer in MEO/HEO, the Final Tanking Orbit (“FTO”). Operations in MEO/HEO will occur in an elliptical orbit of 281 km x 34,534 km and an altitude tolerance of +116,000/-24,000 km apogee and +/- 100 km perigee, with inclination between 28 and 33 degrees (+/- 2 degrees).

It looks like we can confirm HEO tanking for Luna.  Interesting!  I didn't think it would be needed.

Now I see why they think they might need 20 launches.  10 for two full starships that go up to FTO, combine fuel, and one boosts for Luna at perigee and one EDLs.

That's an awfully high FTO, generally I've found a half tank for both craft gets you to about 15,000km.  Guess I'd better go check the numbers.  That is basically GTO btw (minus about 1000km)

I'm also surprised.  This is unnecessary with a v3-based LSS, but maybe they've decided that they have to lock down the HLS design earlier than v3 will be reliably available, or that the v3 tilt margins are too narrow.  Or maybe they felt they had to file something, and they'll fix it later when they nail down the LSS details.

But note that the range of final tanking orbits ranges everywhere from 381km x 10,534km to 181 x 150,534km.  A 200 x 10,540 orbit has a delta-v of about 1560m/s above a 200 x 200 LEO.  For a v2-based HLS, that should be pretty close to what the LSS could achieve using only the launch prop.  So maybe they're planning on getting an extra 1560m/s of delta-v simply by going straight to a mildly eccentric orbit.  That would be more than enough to make the margins comfy for an HLS LEO-BLT-NRHO-loiter-RPOD-LLO-LS-LLO-NRHO-RPOD-idle mission.

One of my big problems with the HEEO refueling ops is that you have to keep rotating the apse line every time you add a contingency orbit to clear up some checkout problem.  200x10,540 has a period of 3˝ hours, during which time the Moon will move 1.93ş.  Using the apse line rotation formula:

tan(η) = -v_⟂r_p/(e*μ)*Δv

Perigee speed will be 9349m/s, eccentricity is 0.4408, perigee r=6,571,000m, μ=3.986004418E14mł/s˛, and tan(1.93)=0.034. So Δv=97m/s per extra orbit.  However, that impulse is radially inward, while the TLI delta-v is tangential.  C3=-1.0 (in the ballpark for a BLT) is Δv=3160m/s.  The vector sum of these together has a magnitude of 3161m/s, which is only 1m/s more than starting the TLI/BLT at the proper time.  It obviously gets non-linearly worse as the number of missed orbits piles up, but it's not too terrible for quite a while.

My other objection is that RPOD in an eccentric orbit is a bit of an unknown.  Maybe they think that the data they gather with the Orion and ICPS doing prox ops maneuvers during Arty 2 will give them what they need.  (Of course, that assumes that there still is an Arty 2...)

I still don't understand why they're not just going for v3 from the git-go.

[Twark_Main]

You'd have to build special sumps into the LOX side of the common dome (and they're not straightforward, since they'd have to be built into the convex side)

There's a single point where all the propellant will pool, so the "sump" is already integrated with the existing geometry. You could just put your LOX pickup point at the appropriate location. There's literally zero extra work here.

The entire weld where the common dome is joined to the ring segment has the same, and largest, centrifugal force being applied to it.  That's not a single point.

We're picturing different things.

However, there's an easier way. You apply a small impulse before spin-up to settle the LOX at the bottom of the tank. This only adds a couple minutes of thrusting to the procedure, so it's definitely worth it.

Nope.  Even when all of the LCH4 is in the bottom of the tanks, it's still "above" (farther forward) the CoM.  When you rotate the system, it'll all fly up to the upper dome.  The same thing will happen to some of the LOX in the LOX tank if its fairly full.

I know all that. The intent was only to settle the LOX tank of the refilling tanker, which has a low enough quantity that this works.

and the LCH4 dome (which will be surrounded by all kinds of stuff, where it can make use of the empty space around the dome).

There's plenty of available space around the top dome. This is not a resource that's in short supply.

Probably true for a tanker or a depot, but not true for a cargo Starship or LSS.

Since you're only filling those (not emptying), you don't need the sump or extra pipes.

The LOX tank only needs a small-diameter pipe (depending on the transfer rate) that's routed to the opposite corner, ~9 meters away.

Is the axis of rotation the x-axis, or the y- or z-axes?

What is your coordinate system convention? The same as Falcon 9?

you need to pull all of the prop that's above the CoM off of the "roof" of the tank

Off the CH4 tank, yes. I presume you're going to reuse the existing header tank pipes for this purpose.

That's a lot farther than 9m.

Indeed, hence why that extra QD alternative is so tempting!

It also has really strange hydrostatics.

It's really just a different equation for how potential changes with height. The hydrostatic change is still just (inlet height potential - outlet height potential)

The QD connectors themselves can get to a very high temperature.  If the pipes behind the connectors are in ullage space, then the heat will conduct along the pipe for some distance, before the cold gas puts everything in equilibrium.  1m is a guess on how far that is.  If the pipes are immersed in liquid prop, then there's no chill-down required at all.

So given these assumptions, you must be on team "put an extra LHC4 QD on top of the tank." 

My objection to the "pull stuff off the roof" scheme is that you either have to run piping along the outside, in which case the whole run as at a hot equilibrium, or do really complex stuff to keep it inside the tanks.

The "really complex stuff" is just T-ing into existing header tank plumbing. The R&D is already done.

If you use only the QD plumbing, most stuff stays cold.

Running pipes inside the tank has the same advantage, by your own admission:

the cold gas puts everything in equilibrium.  1m is a guess on how far that is.

[Greg Hullender]

Please examine the following idea for a different way to get cheap, stable spin settling with ventral/ventral or bidorsal docking.

It involves “wasting” one Ship that is tethered to nose of Depot by maybe a mile of cable. The array is slowly spun. I imagine docking Tanker or Ship-taking-fuel with Depot will not be more complex than other RPOD schemes.

The two advantages I see here are
1) sustained settling acceleration for cheap
2) acceleration direction aligns with launch configuration (on both active systems) for which fuel-refuel plumbing is optimized.
3 of 2) Higher settling accelerations are available, expediting transfer ops.

One way to potentially make this work that would even be stable is if your sidereal spin rate is the same as your sidereal period of revolution. That is, from the perspective of the Earth, the depot is always down and the counterweight is always up. (Or vice versa.) The cable might need to be a hundred kilometers long or so (I keep meaning to estimate this, and I keep forgetting to), but that's not a big deal. Your settling acceleration comes from tidal forces, so you don't need to worry about the usual problems of getting a stable rotating system.

However, I couldn't figure out how to cope with the problems that occur when you add/remove propellant to/from the depot. Unless the counterweight is very heavy, the center of mass is going to move a lot and the thing is going to swing. The oscillations will damp out over some period of time (but I haven't worked that out either). Still, the sideways thrust to stabilize are probably a lot less than what's required for an hours-long ullage burn.

If we assume 0.1 mm/s² and rotating once every 90 minutes, that's a radius of ~3 km.

https://futureboy.us/fsp/frink.fsp?fromVal=%280.1+mm%2Fs%2Fs%29+%2F+%281%2F%2890+min%29%29%5E2&toVal=#calc

I think that's way off because it's only computing centrifugal force--it doesn't include the effect of gravity.

The idea behind "tidal ullage settling" is that the depot orbits the Earth at the same angular velocity as the center of mass of the system, which means (if it's closer to the Earth), it feels more gravity and less centrifugal force, for a net acceleration towards the Earth. The counterweight experiences the reverse and feels a net force away from the Earth. The big value over other rotating systems is that this one is very stable.

 I figure the net acceleration of the depot towards the Earth to be

a = μ/(R-r)˛ - (R-r)ω˛

Where R is the distance from the COM of the system to the center of the Earth, r is the distance of the depot from the COM, μ is the standard gravitational parameter of the Earth, and ω is the angular velocity of the system. (Actually, I should use center of gravity instead of center of mass, but for short cable lengths, the difference is very tiny and the extra complication is substantial.)

This equation is cubic in (R-r). To get a = 1 mm/sec˛ for a system at 281 km altitude, I get r = 247 m. If we assume the counterweight has mass equal to or greater than that of the depot, that gives a cable less then 1/2 km long.

If the depot is 150 m tall, and connected at the nose, you'd actually get a = 1.6 mm/sec˛ at the bottom. With no cable at all, you'd get 0.6 mm/sec˛ at the bottom, but I don't think the methane tanks would get enough acceleration to be useful.

However, whenever a vehicle docks with/undocks from the depot, the COM will change, and that will have to be corrected for or else the system will swing back and forth. That may still be a lot less propulsion than doing an ullage burn for a couple of hours though.

[TheRadicalModerate]

We're picturing different things.

Then what are you picturing?

Nope.  Even when all of the LCH4 is in the bottom of the tanks, it's still "above" (farther forward) the CoM.  When you rotate the system, it'll all fly up to the upper dome.  The same thing will happen to some of the LOX in the LOX tank if its fairly full.

I know all that. The intent was only to settle the LOX tank of the refilling tanker, which has a low enough quantity that this works.

You need a solution that allows both kinds of prop to be transferred.

Probably true for a tanker or a depot, but not true for a cargo Starship or LSS.

Since you're only filling those (not emptying), you don't need the sump or extra pipes.

But now you have two completely different sets of domes.  That's a needless manufacturing problem.

What is your coordinate system convention? The same as Falcon 9?

Yes:  x-axis is the long axis.

Off the CH4 tank, yes. I presume you're going to reuse the existing header tank pipes for this purpose.

There aren't going to be any header tanks for the depot or LSS.

It also has really strange hydrostatics.

It's really just a different equation for how potential changes with height. The hydrostatic change is still just (inlet height potential - outlet height potential)

There's a point in the pipe with zero hydrostatic pressure, with pressure on either side of it.  That may be fine, but it's definitely weird.

The QD connectors themselves can get to a very high temperature.  If the pipes behind the connectors are in ullage space, then the heat will conduct along the pipe for some distance, before the cold gas puts everything in equilibrium.  1m is a guess on how far that is.  If the pipes are immersed in liquid prop, then there's no chill-down required at all.

So given these assumptions, you must be on team "put an extra LHC4 QD on top of the tank."

That's insanity.  QDs are major points of failures.  And you don't need it:  the LCH4 fill/drain pipe is the main downcomer.  All you need is a way to make it drain.  I agree that, if you have header tank downcomers, those can be made to work if you tee into them at the right point, and surround them by check valves.  But in many cases, you don't have the lines.  And even when you do, you have a whole bunch of new valves.

[Twark_Main]

We're picturing different things.

Then what are you picturing?

Obviously not that.    Read on...

Nope.  Even when all of the LCH4 is in the bottom of the tanks, it's still "above" (farther forward) the CoM.  When you rotate the system, it'll all fly up to the upper dome.  The same thing will happen to some of the LOX in the LOX tank if its fairly full.

I know all that. The intent was only to settle the LOX tank of the refilling tanker, which has a low enough quantity that this works.

You need a solution that allows both kinds of prop to be transferred.

  Come on TRM, you're better than this.

As mentioned, the goal here is to minimize additional plumbing. This does so.

Probably true for a tanker or a depot, but not true for a cargo Starship or LSS.

Since you're only filling those (not emptying), you don't need the sump or extra pipes.

But now you have two completely different sets of domes.  That's a needless manufacturing problem.

Not really a "problem," just a cost. There's already plenty of variation between versions, and this is hardly a show-stopper.

What is your coordinate system convention? The same as Falcon 9?

Yes:  x-axis is the long axis.

In that case, I'm proposing rotation about the Y axis, using dorsal-to-dorsal docking.

The dorsal-to-dorsal docking means the fuselages are offset from the spin axis, so the propellant won't settle all along around common dome weld. And besides that, the entire consideration is eliminated by briefly setting the LOX first.

Off the CH4 tank, yes. I presume you're going to reuse the existing header tank pipes for this purpose.

There aren't going to be any header tanks for the depot or LSS.

I'm aware. For some vehicle types you're just re-use the existing engineering and R&D for the pipes. This is still a substantial savings (eliminating your "really complex stuff" worries).

I said from the beginning that you add some pipes where necessary, so I'm not sure how this is a surprise.

It also has really strange hydrostatics.

It's really just a different equation for how potential changes with height. The hydrostatic change is still just (inlet height potential - outlet height potential)

There's a point in the pipe with zero hydrostatic pressure, with pressure on either side of it.  That may be fine, but it's definitely weird.

No more weird than a U-shaped (or inverted U-shaped) pipe run.

The QD connectors themselves can get to a very high temperature.  If the pipes behind the connectors are in ullage space, then the heat will conduct along the pipe for some distance, before the cold gas puts everything in equilibrium.  1m is a guess on how far that is.  If the pipes are immersed in liquid prop, then there's no chill-down required at all.

So given these assumptions, you must be on team "put an extra LHC4 QD on top of the tank."

That's insanity.  QDs are major points of failures.  And you don't need it:  the LCH4 fill/drain pipe is the main downcomer.  All you need is a way to make it drain.  I agree that, if you have header tank downcomers, those can be made to work if you tee into them at the right point, and surround them by check valves.

Nice! Glad to hear.

But in many cases, you don't have the lines.  And even when you do, you have a whole bunch of new valves.

Good trade! A few pipes and valves in exchange for 10-20% more tanker propellant per launch. 

Again, from the start I've considered this to be merely the minimally-clever, rather obvious baseline design. It's probably possible to make an improved AG design by employing some actual cleverness. That's the part I'm really interested in.

[OTV Booster]

[tethers]
So, a flexible dumbell is stable?

No. With a flexible tether, the end-weights can twist back-and-forth on the end of the tether. You need something other than a cable to prevent twist. And the longer the tether, the worse it is and the harder to counter.

Additionally, any AG tether system is likely going to have very similar rotational-inertia in both the short axes. That risks intermediate-axis-instability.

Essentially, tethered systems are likely to be inherently unstable around the long axis.

"... the end-weights can twist back and forth..."

Twist around the axis of the cable or flop to and fro around the connection point?

AIUI, a stiff dumbbell will eventually tumble. It has pseudo stability with one end pointing at earth. In the real world the gravity gradient is not smooth. When a masscon comes over the horizon the bottom weight is slightly closer and is drawn ahead to eventually point at the masscon. As the masscon passes underneath the bottom weight still follows and continues rocking rearward as the masscon passes beyond the receding horizion.

The upshot is the dumbbell will almost always be rocking and given enough time will go ass over teacups into a full tumble.

I'm unsure how to translate this into a tether. One effect I'd expect would be mascon attraction of the bottom ship causing a low frequency wave up the tether that doesn't happen with a solid dumbell.

[Greg Hullender]

Much more severe is what happens when a tanker docks with the depot, changing the center of mass and moment of inertia. Worst is when a depot fully fuels a Starship and that Starship detaches.

You can minimize this by making the counterweight heavy, but that's really going to be expensive.

[butterwaffle]

Much more severe is what happens when a tanker docks with the depot, changing the center of mass and moment of inertia. Worst is when a depot fully fuels a Starship and that Starship detaches.
You can minimize this by making the counterweight heavy, but that's really going to be expensive.

With a pair of launch towers, you can eliminate the need for a counterweight, tether or no tether. The depot is two starships attached nose-to-nose. Launch a pair of starships with fuel to transfer to the depot and they attach tail-to-tail at both ends of the depot, spin up to transfer O2, CH4, and ullage gas. This would require power for pumps to transfer mass toward the center of rotation. Repeat as needed. Then launch a pair of starships to draw from the depot. This would not need much pump energy since the mass is flowing "downhill."

With extra connections between the starship-pair forming the depot, you could balance mass to keep the center of rotation stable.

[Paul451]

[tethers]
So, a flexible dumbell is stable?

No. With a flexible tether, the end-weights can twist back-and-forth on the end of the tether.

Twist around the axis of the cable or flop to and fro around the connection point?

The former. (Although the latter is always an option as well if you start getting waves in the tether.) It means that the system doesn't have a (stable) primary axis, at any moment it might be rotating around the (unstable) intermediate axis.

(As Dan said, there are ways of preventing twisting, but when talking about multi-km tethers (even multi-100m), a pressurised tube becomes as flexible as a cable anyway.)


We should let this die. While I find the topic of AG depots interesting, it's not at all relevant to the thread. And with more details of the actual depot architecture released in the FAA filing, we should probably be discussing that.

(Or else spin-off a narrower "Starship Depot Updates and Discussion" thread, and keep this one as a more general garbage collector.)

[Twark_Main]

[tethers]

We should let this die.

I actually agree. I can see literally zero advantage for these complex tether AG systems over the simpler/cheaper and more performant "naive" implementation.

While I find the topic of AG depots interesting, it's not at all relevant to the thread.

"Re: Starship On-orbit refueling - Options and Discussion"

It's about Starship. It's about on-orbit refueling. It's an option (put aside your "End of History Iteration" bias). It's a discussion.

No need for spurious thread splitting.

And with more details of the actual depot architecture released in the FAA filing, we should probably be discussing that.

Nobody's stopping you.

[Nevyn72]

"Re: Starship On-orbit refueling - Options and Discussion"

It's about Starship. It's about on-orbit refueling. It's an option (put aside your "End of History Iteration" bias). It's a discussion.

It certainly is an evolving discussion, and a very different place to the thread I started almost 5 years ago!

As the design evolves so do the potential mechanisms to achieve the stated goal, remember the proposed mechanism for refueling 5 years ago was tail to tail...

[Greg Hullender]

It certainly is an evolving discussion, and a very different place to the thread I started almost 5 years ago!

As the design evolves so do the potential mechanisms to achieve the stated goal, remember the proposed mechanism for refueling 5 years ago was tail to tail...

Yes, but the data Narnianknight shared with us a week ago definitely narrows down the things SpaceX is going to try first. Here is one thing that puzzled me:

Some of these tanker variants will remain in LEO as “depots,” and will be filled with propellant by subsequent tanker launches.

There won't be a special "depot" version of Starship? How will two tankers plug into each other--doesn't the "depot" need a GSE "spigot" that connects to the QD "socket?" How will a depot be optimized for keeping the fuel cold yet still be EDL-capable?

[OTV Booster]

It certainly is an evolving discussion, and a very different place to the thread I started almost 5 years ago!

As the design evolves so do the potential mechanisms to achieve the stated goal, remember the proposed mechanism for refueling 5 years ago was tail to tail...

Yes, but the data Narnianknight shared with us a week ago definitely narrows down the things SpaceX is going to try first. Here is one thing that puzzled me:

Some of these tanker variants will remain in LEO as “depots,” and will be filled with propellant by subsequent tanker launches.

There won't be a special "depot" version of Starship? How will two tankers plug into each other--doesn't the "depot" need a GSE "spigot" that connects to the QD "socket?" How will a depot be optimized for keeping the fuel cold yet still be EDL-capable?

Assuming a second QD plate isn't planned ISTM the easiest solution is for the depot ship to Have an exactly duplicate of the towers QD plate, then mount a gender bender on the tower QD for depot launch. Keep the mass on the ground.


I'm kinda sorta guessing that the tanker as depot quote was a malformed packet. Maybe meaning the depot would be based on tanker tankage? Or maybe really use a minimally modified tanker for an early trial?  Just good enough for what they want to learn at that point.

[TheRadicalModerate]

I'm kinda sorta guessing that the tanker as depot quote was a malformed packet. Maybe meaning the depot would be based on tanker tankage? Or maybe really use a minimally modified tanker for an early trial?  Just good enough for what they want to learn at that point.

Well, maybe more like a DIX packet running on an 802.3 network.  Nothing a SNAP header can't cure...

There are some good reasons to move the common and LCH4 domes forward on a tanker:

1) When you work out the amount of prop deliverable to LEO on an unmodified tanker, it's a small amount (but non-trivially) less than the maximum payload.  Crank through the mass ratios for an empty Starship vs. one with a max-sized payload and you'll see why.

2) The payload barrel is useless parasitic weight.  You should either get rid of it, shortening the ship and reducing its dry mass, or...

3) ...By using the barrel as tankage, you get rid of the mass ratio problem in issue #1 above, and you may be able to goose the usable prop to LEO above the max payload by a bit.

The obvious issue is that you now have a genuinely different variant, with all of the manufacturing issues associated with that.  However, you also have a variant that's now better suited to become the depot, once you strip off the elonerons, TPS, header tanks, etc.

[OTV Booster]

I'm kinda sorta guessing that the tanker as depot quote was a malformed packet. Maybe meaning the depot would be based on tanker tankage? Or maybe really use a minimally modified tanker for an early trial?  Just good enough for what they want to learn at that point.

Well, maybe more like a DIX packet running on an 802.3 network.  Nothing a SNAP header can't cure...

There are some good reasons to move the common and LCH4 domes forward on a tanker:

1) When you work out the amount of prop deliverable to LEO on an unmodified tanker, it's a small amount (but non-trivially) less than the maximum payload.  Crank through the mass ratios for an empty Starship vs. one with a max-sized payload and you'll see why.

2) The payload barrel is useless parasitic weight.  You should either get rid of it, shortening the ship and reducing its dry mass, or...

3) ...By using the barrel as tankage, you get rid of the mass ratio problem in issue #1 above, and you may be able to goose the usable prop to LEO above the max payload by a bit.

The obvious issue is that you now have a genuinely different variant, with all of the manufacturing issues associated with that.  However, you also have a variant that's now better suited to become the depot, once you strip off the elonerons, TPS, header tanks, etc.

My quote was getting at exactly what you say.

I and most who have spoken up on tanker layout expect what you describe. Stretched tanks and truncated cargo bay. Probably the simplest variant that'll ever be. No cargo handling other than the propellant handling that's already there. That's that tankage I was alluding to.

In my mind this tanker variant is a no brainer. Lengthening the tanks is no big thing. It's just more of the same. Maybe some new lessons in fluid behavior while maneuvering, dealing with a new mass distribution and maybe slightly changed aerodynamics. In this second space age that's all routine.

And the manufacturing issues are there but miniscule. In the automotive world it's a solved problem. You want a pickup? You want that with a standard bed or extended? Crew cab? Dualies? What color? Which interior options? Engine? Transmission? Mix and match any way you want. The process extends from the salesman's desk through purchasing and onto production.

ISTM that a gender bent ship would do for early attempts to hook up and, uh, share fluids. A tanker variant would show up as these trials progress. My guess is as soon as they get two ships to successfully hook up and transfer some 10's of tons of propellant, we'll see some ring arrangements that suggest extended tanks are on the way.

A first try at substantially filling a depot surrogate would probably involve a gender bent tanker as depot and (a) "standard" tanker(s). From this a depot variant will evolve.

I've been watching these guys for going on seven years now. SpaceX is not only learning how to build spaceships. They are also learning how to structure their learning.

[TheRadicalModerate]

This came up over on the "Replacing SLS/Orion with Starship HLS and D2" thread, and probably ought to be discussed over here:

We've discussed a number of ways to move prop from a depot in VLEO (where optimal prop payload can be recovered from the lift tankers) to some higher orbit.  Three commonly discussed options:

1) Use some kind of tanker that can act as a temporary depot (which would require being able to deploy and stow a QD gender-bender at various times in the conops), and then return to a direct EDL.

2) Boost the depot itself from its VLEO to the higher orbit, fuel the target, and then return propulsively back to the VLEO.

3) Place a second depot in the target orbit, then have a tanker move the prop between them, followed by an EDL.  This probably makes sense for refueling in stable lunar orbits (e.g. NRHO or a frozen LLO), but HEEOs are likely to be defined by specific mission requirements, and will move around, in terms of both altitude/energy and inclination.

But, as Greg Hullender pointed out, you might be able to aerobrake a depot back into VLEO, even if it's not equipped with the full set of EDL gear.

For example, a depot coming back from a lunar distance, even with the high perigee speed, can probably shave off 25m/s of delta-v in an aerobraking pass without doing any structural damage.  If you're trying to brake into a 300x300km VLEO, a 100x385,000 transfer orbit will take about 2 months.  (See attached.)

So, some questions:

1) Do you agree that small delta-v brakings will be fine with a depot?  How large do you think they could get without inflicting any damage or risking aerodynamic instability?

2) Any problems with slosh?  (I'd guess that the acceleration will build up gradually enough that the prop will settle in the bottom windward part of each tank with no problem.) 

3) Any problems with balance, assuming the depot is mostly empty?

4) I've been wondering what happens to solar panels, if the depot deploys them like the LSS, i.e., stuck onto the nose.  If aerobraking fries them, that would be a dela-breaker.  An alternative would be to go to some kind of deployable/stowable solar array, which will probably be needed for Mars, but sounds like it might be some extra work if done soon.  They also need to be pretty reliable to handle 100-200 perigee passes.

5) The depot needs a little bit of prop to lift its perigee at the end of the aerobraking, and it needs some for attitude control and small maneuvers to hit the proper aerobraking entry window each pass.  Does that amount of prop (especially for the window maneuvers) exceed what would be required just to do the propulsive return? 

6) Does the extra heat from the aerobraking increase boil-off enough to make this not worthwhile?

[steveleach]

This came up over on the "Replacing SLS/Orion with Starship HLS and D2" thread, and probably ought to be discussed over here:

We've discussed a number of ways to move prop from a depot in VLEO (where optimal prop payload can be recovered from the lift tankers) to some higher orbit.  Three commonly discussed options:

1) Use some kind of tanker that can act as a temporary depot (which would require being able to deploy and stow a QD gender-bender at various times in the conops), and then return to a direct EDL.

2) Boost the depot itself from its VLEO to the higher orbit, fuel the target, and then return propulsively back to the VLEO.

3) Place a second depot in the target orbit, then have a tanker move the prop between them, followed by an EDL.  This probably makes sense for refueling in stable lunar orbits (e.g. NRHO or a frozen LLO), but HEEOs are likely to be defined by specific mission requirements, and will move around, in terms of both altitude/energy and inclination.

Option 3 feels the most "SpaceXey" to me, with no actual engineering consideration.

Depots would be optimised to stay in roughly the same place for a long time, with tankers visiting them to move propellants around. If they need to change their orbits they will do it entirely with engines. I can imagine challenging mission requirements utilising single-use depots, as the cost of moving them is more than the cost of the depot. Maybe they'll get solar-electric propulsion later on?

Tankers would be optimised for moving propellant to, and between, depots, and for returning to the launch site. They would be able to aerobrake aggressively if returning to a LEO depot is required, as well as EDL to launch site from any orbit.

SpaceX may well be planning a network of depots in a variety of orbits with different energies around different bodies, with tankers constantly moving between them keeping them all supplied.  Ships for cargo and crew would then utilise that depot network, accepting that they won't necessarily have the optimum refuelling orbit as a price to pay for operational efficiency.

[DanClemmensen]

Depots would be optimised to stay in roughly the same place for a long time, with tankers visiting them to move propellants around. If they need to change their orbits they will do it entirely with engines. I can imagine challenging mission requirements utilising single-use depots, as the cost of moving them is more than the cost of the depot. Maybe they'll get solar-electric propulsion later on?

Tankers would be optimised for moving propellant to, and between, depots, and for returning to the launch site. They would be able to aerobrake aggressively if returning to a LEO depot is required, as well as EDL to launch site from any orbit.

SpaceX may well be planning a network of depots in a variety of orbits with different energies around different bodies, with tankers constantly moving between them keeping them all supplied.  Ships for cargo and crew would then utilise that depot network, accepting that they won't necessarily have the optimum refuelling orbit as a price to pay for operational efficiency.

I think we have been misleading ourselves based on the names "Tanker" and "Depot".  Tanker is optimized for EDL. This capability adds a lot of dry mass.  Depot is optimized to contain as much propellant as is feasible and stay in space for a long time. But when you are in space, you can move from place to place easily and efficiently, while the name "Depot" makes you think of a fixed location. To efficiently move a large quantity of fuel, you should use Tankers to fill Depots in VLEO, and then move the entire Depot to its destination. If needed, use additional Depots to fill the first Depot after it arrives. Using Tankers is less efficient because they must move more dry mass. Except possibly for edge cases, sending a Tanker beyond VLEO is a waste of fuel. You can mentally rename the ships "fuel lifter" and "fuel transporter" to help clarify the situation.

When designing missions, you can pretty much always send a Depot together with the mission Ship. Think of the Depot as a tank extender for the Ship. Instead of thinking about where the ship will rendezvous with Depot, think of where the extended Ship/Depot must go. You can also think of Depot as a third stage. Note that you are still flying the same missions, but you are thinking about them differently. You leave Depot with enough fuel to recover itself, just as you leave fuel in Booster for the boost-back or leave Tanker with enough fuel to EDL.

[Narnianknight]

This came up over on the "Replacing SLS/Orion with Starship HLS and D2" thread, and probably ought to be discussed over here:

We've discussed a number of ways to move prop from a depot in VLEO (where optimal prop payload can be recovered from the lift tankers) to some higher orbit.  Three commonly discussed options:

1) Use some kind of tanker that can act as a temporary depot (which would require being able to deploy and stow a QD gender-bender at various times in the conops), and then return to a direct EDL.

2) Boost the depot itself from its VLEO to the higher orbit, fuel the target, and then return propulsively back to the VLEO.

3) Place a second depot in the target orbit, then have a tanker move the prop between them, followed by an EDL.  This probably makes sense for refueling in stable lunar orbits (e.g. NRHO or a frozen LLO), but HEEOs are likely to be defined by specific mission requirements, and will move around, in terms of both altitude/energy and inclination.

But, as Greg Hullender pointed out, you might be able to aerobrake a depot back into VLEO, even if it's not equipped with the full set of EDL gear.

For example, a depot coming back from a lunar distance, even with the high perigee speed, can probably shave off 25m/s of delta-v in an aerobraking pass without doing any structural damage.  If you're trying to brake into a 300x300km VLEO, a 100x385,000 transfer orbit will take about 2 months.  (See attached.)

For the sake of simplicity, option 2 sits best with me. As to the second question, a returning depot will have very little mass, so propulsion shouldn't be too inefficient. I doubt the Δv benefit is worth the risks and time drain of aerobraking.

However, assuming the depot must transfer more than a full tank to the LSS, it would have to refuel several times between the first transfer and the Final Tanking Orbit. That adds loiter time to the LSS. Some version of option 3 solves this problem by having all necessary propellant in orbit before the LSS launches. Regarding Dan's note of depot maneuvers' greater efficiency, perhaps the VLEO depot fills the FTO depot, returns to VLEO, and gets topped up before the LSS launches.

I'm very curious to see whether SpaceX would prioritize minimizing depot count or loiter time. It's quite likely neither of these options materialize, in favor of another, of course.