Starship On-orbit refueling - Options and Discussion

[OTV Booster]

Possibly a dumb question, but can't there be some kind of solar powered active cooling system? That radiates the excess heat on the side not pointing towards the sun, to reliquidify the boil-off?

Not dumb, but there are a lot of challenges to overcome to make this work. Since SpaceX already announced they aren't planning to do this, it hasn't attracted a lot of interest.

Has SpaceX said Starship will never have actively cooled depots? Or have they only said they intend to first try passive cooling and/or frequent replenishment of boil-off? I imagine replenishment gets more costly when the depot is located beyond LEO. Also any BLEO depot likely needs to hold propellant longer. So that's where it seems like active cooling will eventually be implemented.

LOL. Does it matter? We are talking about SpaceX here. IIRC they said passive but until they put a ship on orbit and see how well models actually characterize boiloff, they can't know for sure. They're probably XX% sure it'll be passive. Plug in any numbers that feel right.


I can loan them a dime if they need to pivot.

[OTV Booster]

Possibly a dumb question, but can't there be some kind of solar powered active cooling system? That radiates the excess heat on the side not pointing towards the sun, to reliquidify the boil-off?

Not dumb, but there are a lot of challenges to overcome to make this work. Since SpaceX already announced they aren't planning to do this, it hasn't attracted a lot of interest.

Has SpaceX said Starship will never have actively cooled depots? Or have they only said they intend to first try passive cooling and/or frequent replenishment of boil-off? I imagine replenishment gets more costly when the depot is located beyond LEO. Also any BLEO depot likely needs to hold propellant longer. So that's where it seems like active cooling will eventually be implemented.

Has a SOFI-MLI combo ever flown? Doesn't MLI depend on she open space between layers?


Unless this is a real thing maybe active cooling and radiators are the best trade for extended ops in the warm clime of VLEO.


The best part is no part but sometimes you need the part.

[Twark_Main]

Possibly a dumb question, but can't there be some kind of solar powered active cooling system? That radiates the excess heat on the side not pointing towards the sun, to reliquidify the boil-off?

Not dumb, but there are a lot of challenges to overcome to make this work. Since SpaceX already announced they aren't planning to do this, it hasn't attracted a lot of interest.

Has SpaceX said Starship will never have actively cooled depots? Or have they only said they intend to first try passive cooling and/or frequent replenishment of boil-off? I imagine replenishment gets more costly when the depot is located beyond LEO. Also any BLEO depot likely needs to hold propellant longer. So that's where it seems like active cooling will eventually be implemented.

Has a SOFI-MLI combo ever flown?

Not AFAIK, but of course neither have the (re)deployable "cone" shields.

Doesn't MLI depend on she open space between layers?

The MLI "quilt" (including an overlayer of protective aerocover fabric) gets glued (RTV) on top of the foam. You don't put foam between the layers.

Unless this is a real thing maybe active cooling and radiators are the best trade for extended ops in the warm clime of VLEO.

You still want to minimize heat ingress to minimize the mass of any active system.

[Greg Hullender]

I'm tying to picture a shield big enough to block ~40% of the sky (Earth), mobile enough to keep the ship shielded as the nose targets the sun and the sun-earth angle changes, not get in the way of a mating ship, do it all repeatedly, and still be reasonably light. Almost forgot. It has to withstand cold thruster blasts. I'm coming up blank but hope it's a lack of imagination.

The complication that troubles me the most is the interaction between the Earth-blocking shield and the sun. When the sun is behind it, it seems there's no way to avoid it reflecting at least some of the sun's back onto the depot. Unless you put the active cooling on the shield, not the depot!

[OTV Booster]

I'm tying to picture a shield big enough to block ~40% of the sky (Earth), mobile enough to keep the ship shielded as the nose targets the sun and the sun-earth angle changes, not get in the way of a mating ship, do it all repeatedly, and still be reasonably light. Almost forgot. It has to withstand cold thruster blasts. I'm coming up blank but hope it's a lack of imagination.

The complication that troubles me the most is the interaction between the Earth-blocking shield and the sun. When the sun is behind it, it seems there's no way to avoid it reflecting at least some of the sun's back onto the depot. Unless you put the active cooling on the shield, not the depot!

Good point.


The conical cloak like shade I suggested 2-3 days ago would avoid this problem. I think it can always find an angle that avoids heat up the kazoo, the only unprotected area.


A blister would run the length of the depot on what would otherwise be the heatshield, and protect the shade during launch. The shade would deploy through an arm waving exercise. That's the hard part.

[OTV Booster]

Possibly a dumb question, but can't there be some kind of solar powered active cooling system? That radiates the excess heat on the side not pointing towards the sun, to reliquidify the boil-off?

Not dumb, but there are a lot of challenges to overcome to make this work. Since SpaceX already announced they aren't planning to do this, it hasn't attracted a lot of interest.

Has SpaceX said Starship will never have actively cooled depots? Or have they only said they intend to first try passive cooling and/or frequent replenishment of boil-off? I imagine replenishment gets more costly when the depot is located beyond LEO. Also any BLEO depot likely needs to hold propellant longer. So that's where it seems like active cooling will eventually be implemented.

Has a SOFI-MLI combo ever flown?

Not AFAIK, but of course neither have the (re)deployable "cone" shields.

Doesn't MLI depend on she open space between layers?

The MLI "quilt" (including an overlayer of protective aerocover fabric) gets glued (RTV) on top of the foam. You don't put foam between the layers.

Unless this is a real thing maybe active cooling and radiators are the best trade for extended ops in the warm clime of VLEO.

You still want to minimize heat ingress to minimize the mass of any active system.

There seems to be a misunderstanding. Everybody knows the MLI cells are pumped full of vacuum before use. 



How would the MLI handle MaxQ? I've never seen it directly exposed to aero forces, and if I understand the principals by which it operates, the flimsier it is, the better it does its job.


True that: a shade even during active cooling. But, a simpler and less perfect (and hopefully more robust) shade becomes a more viable option. Shading requirements go from 'stop as much heat as we can' to 'what are the trades between cooler size and shade complexity".

[Twark_Main]

Possibly a dumb question, but can't there be some kind of solar powered active cooling system? That radiates the excess heat on the side not pointing towards the sun, to reliquidify the boil-off?

Not dumb, but there are a lot of challenges to overcome to make this work. Since SpaceX already announced they aren't planning to do this, it hasn't attracted a lot of interest.

Has SpaceX said Starship will never have actively cooled depots? Or have they only said they intend to first try passive cooling and/or frequent replenishment of boil-off? I imagine replenishment gets more costly when the depot is located beyond LEO. Also any BLEO depot likely needs to hold propellant longer. So that's where it seems like active cooling will eventually be implemented.

Has a SOFI-MLI combo ever flown?

Not AFAIK, but of course neither have the (re)deployable "cone" shields.

Doesn't MLI depend on she open space between layers?

The MLI "quilt" (including an overlayer of protective aerocover fabric) gets glued (RTV) on top of the foam. You don't put foam between the layers.

Unless this is a real thing maybe active cooling and radiators are the best trade for extended ops in the warm clime of VLEO.

You still want to minimize heat ingress to minimize the mass of any active system.

There seems to be a misunderstanding. Everybody knows the MLI cells are pumped full of vacuum before use. 

Very funny. 

MLI is typically perforated to let air escape as the rocket ascends.

How would the MLI handle MaxQ? I've never seen it directly exposed to aero forces, and if I understand the principals by which it operates, the flimsier it is, the better it does its job.

As I said,

MLI "quilt" (including an overlayer of protective aerocover fabric)

This is similar in construction to the thermal blankets used on the exterior of the Shuttle, which were also directly exposed to aerodynamic loads.

True that: a shade even during active cooling. But, a simpler and less perfect (and hopefully more robust) shade becomes a more viable option. Shading requirements go from 'stop as much heat as we can' to 'what are the trades between cooler size and shade complexity".

It's always a tradeoff, even in the first scenario. I just assumed we all understand this, but it's good to confirm we're on the same page.

[StraumliBlight]

SpaceNews: DIU studying applications of SpaceX Starship in-space refueling [Feb 6]

Speaking at the Smallsat Symposium here Feb. 6, Gary Henry, a senior adviser at the Defense Innovation Unit (DIU) and a former SpaceX executive, said the agency was working with SpaceX to examine how Starship’s in-space refueling capabilities could support a broader range of users.

[...]

Another Starship application that DIU is exploring is what he called “novel responsive space delivery,” which he defined as delivering unique payloads using rockets. The Air Force Research Lab is already studying “rocket cargo” concepts, like Starship, through a Vanguard program for point-to-point transportation

The DIU approach involves using Starship for delivering cargo from orbit to the ground. “You’ve got payloads on orbit and you want to do something useful with them, and then you want to reenter them and bring them back and exploit them in some way,” he said.

[Asteroza]

18:23 Musk reveals that Starship prop transfer delayed until 2026

Well that means 2025 ain't happening then...

[Eka]

I can't understand why people are bending over backwards to modify a Starship tanker into an orbital depot.

Every Starship heading to higher orbits or a transfer orbit will be going with a custom orbit and timing. Even batches of Starships to Mars need custom orbits and timings. Even after there is a regular Moon run, they will be launching on custom orbits that change from minute to minute over the Lunar month. Another thing is because acceleration is needed to transfer propellants, the orbit is constantly changing and is thus always a moving target. The additional mass of shades, insulation blankets, cryo coolers, heat radiators, etc requires more propellant for acceleration during transfers.

Even when 1000s of Starships are being sent to Mars and orbits and timings are very similar, one can just leave the last tanker up for Starship N to transfer the dregs to the first tanker for Starship N+1. During the push, it may still be lined up well for the next MTO Starship. When it doesn't line up well, land it. Also many sets of tankers will be doing propellant transfers at the same time. This chaining of tankers works well for fueling cargo Starships being fueled up in parking orbits waiting for the MTO window.

This has the advantage no tanker has spent months in orbit accumulating MMOD damage. If an on orbit tanker is MMOD damaged, it can be replaced by transferring the propellant to the next tanker launched. Maybe an extra tanker needs to be added to make up for the propellants lost. No MMOD shielding is needed. No cooling hardware is needed. Simple orientating the engines toward the sun works for lowering heat gain in the propellants. No sun shades that need deployment and folding back up. No solar arrays need deploying. Just have larger batteries if longer on orbit times are needed. They shouldn't be needed.

This constant cycling of the tankers on orbit allows them to be inspected routinely where it is easy to do it on Earth in a properly fitted out inspection, repair, and storage facility. No need for on orbit inspections. No need for a special orbital depot. This keeps development costs down, and keeps things much more flexible.

A tanker should be able to land with some propellant in it. That propellant can then be detanked, chilled down again, and used later.

Note: Sunshade solar arrays will be a thing for Starships stacked up in parking orbits before Mars transfer windows. One fuels up a cargo Starship and parks it in orbit waiting for the Mars transfer window. The sunshade + solar array keeps down the boil off, and powers the Starship. Near the transfer window a tanker comes by and tops it off to replace any boil off. For MTO, the sunshade solar array is folded up for the boost, then redeployed for the transit. At Mars it can be folded up for landing, and the return journey.

Cleanup: If a tanker is badly damaged in orbit, it does a burn to use up the onboard CH4 before it burns up over Point Nemo. A non responsive tanker can have another tanker attach to it, then does burns to put the dead tanker into an orbit so it burns up over Point Nemo. The "tug" tanker does some additional burns to return home.

[Paul451]

If an on orbit tanker is MMOD damaged, [...] No MMOD shielding is needed.

The risk of MMOD damage to a pressurised tank partially full of liquified-gas is triggering a BLEVE. I believe it has previously caused random satellites and spent stages to kablooey, when their still-pressurised prop tanks were punctured.

[Greg Hullender]

Every Starship heading to higher orbits or a transfer orbit will be going with a custom orbit and timing. Even batches of Starships to Mars need custom orbits and timings. Even after there is a regular Moon run, they will be launching on custom orbits that change from minute to minute over the Lunar month. Another thing is because acceleration is needed to transfer propellants, the orbit is constantly changing and is thus always a moving target. The additional mass of shades, insulation blankets, cryo coolers, heat radiators, etc requires more propellant for acceleration during transfers.

It turns out you can get around this. This paper, Practical Methodologies For Low Delta-V Penalty, On-Time Departures To Arbitrary Interplanetary Destinations From A Medium-Inclination Low-Earth Orbit Depot, describes how you can launch from a depot to an arbitrary orbit for minimal cost.

So there really won't be a need to change the depot's orbit--other than to keep it from burning up.

The argument against using a regular tanker is boiloff. If it takes 20 launches to fill a tanker vs. 10 for a depot, then that's probably a win worth the costs. I think it'll take some experimenting before everyone is sure what the tradeoff space looks like.

[TheRadicalModerate]

I can't understand why people are bending over backwards to modify a Starship tanker into an orbital depot.

Two main reasons:

1) Different thermal management requirements:  Tankers are set up to have high emissivity, because that's what dumps heat most effectively during high-temperature EDL.  Depots need to have high reflectivity and insulation, because they never get to a sufficient temperature for emission to be a good route for heat rejection.

2) Plumbing.  Every Starship variant, irrespective of its on-orbit function, needs to be filled and managed by the GSE quick-disconnect.  Since that's (currently) a gendered connection, the depot needs to replicate the topology of the QD.  That's a fairly significant piece of plumbing that you probably don't want on all tankers, due to mass and aerodynamic constraints.

Every Starship heading to higher orbits or a transfer orbit will be going with a custom orbit and timing.

See Greg's comment just up-thread.  If you're in a sufficiently high HEEO, there's a circle of hyperbolic asymptotes that are easily accessible from apogee, and you can fiddle with the angle for a relative modest change in altitude.  Precessing your argument of perigee is a bit harder, but still fairly cheap.

That said, I don't disagree that it may be better to use a gender-bent tanker to shuttle  prop from VLEO (where it's always more energetically favorable to accumulate prop) to the FTO (Final Tanking Orbit).  But that requires making the tanker's EDL capability compatible with the gender-bender.¹

Another thing is because acceleration is needed to transfer propellants, the orbit is constantly changing and is thus always a moving target. The additional mass of shades, insulation blankets, cryo coolers, heat radiators, etc requires more propellant for acceleration during transfers.

We're dealing with accelerations that are less than 1E-03m/s² for settling.  It's also possible that propellant management devices will drive that acceleration even lower, once the bulk of the prop has been herded into contact with the PMDs.

This has the advantage no tanker has spent months in orbit accumulating MMOD damage.

The real requirement here is that full Starships not spend a lot of time in orbits with high MMOD risk.  This is yet another reason why depots should accumulate prop in VLEO, which stays pretty clean due to atmospheric drag.

But the requirement to spend little time in higher orbits can be fulfilled by either a depot or a gender-fluid tanker.  The tanker does indeed have the nice property that minor MMOD damage can be repaired on the ground.  Even nicer is the fact that it doesn't have to propulsively return to VLEO, which improves prop efficiency by a non-trivial amount.

Simple orientating the engines toward the sun works for lowering heat gain in the propellants.

It's never simple in LEO, because there's no attitude that's going to be low-boiloff when a tanker is between the Sun and the day-side Earth.

A tanker should be able to land with some propellant in it. That propellant can then be detanked, chilled down again, and used later.

This is unlikely.  We already know that SpaceX is struggling with getting entry balance right, or they wouldn't have gone to the header tanks in the nose, which are a real pain.  So even fairly modest amounts of prop left in the mains probably aren't going to be a thing.

___________
¹An obvious solution to the whole gender-bender issue is to cut over to using a QD that's androgynous.  However, this is so obvious that I would've thought that SpaceX would have designed the GSE around it if it were easy.  So I conclude that, for some reason, it's not easy.

[InterestedEngineer]

This has the advantage no tanker has spent months in orbit accumulating MMOD damage. If an on orbit tanker is MMOD damaged, it can be replaced by transferring the propellant to the next tanker launched. Maybe an extra tanker needs to be added to make up for the propellants lost. No MMOD shielding is needed. No cooling hardware is needed. Simple orientating the engines toward the sun works for lowering heat gain in the propellants. No sun shades that need deployment and folding back up. No solar arrays need deploying. Just have larger batteries if longer on orbit times are needed. They shouldn't be needed.

An important point is that that it's not sunshine that will have the most heating impact on a starship or depot, it's earth-shine, which is 1/4 the heat flux of sunshine but unlike sunshine comes from half the sky (versus a point source of the sun).   Thus  simple tricks like pointing the nose or tail at the sun won't work.

That being said, I think it's a real toss-up between these two different approaches.  They both of drawbacks and benefits.  "try and find out" is the usual SpaceX approach to that kind of problem, so I suspect we'll see both methods tried.

[TheRadicalModerate]

Something I don't think we've discussed, or at least haven't discussed for a while (everything old is new again on this thread):

We've been mostly working on the assumption that refueling will reuse non-depot Starship's QD, which means that the depot has to have a gender-reversed connector on it.  This is the main reason why we think that all refueling steps either have to be depot-to-x or x-to-depot; x-to-x won't work, nor will depot-to-depot.

That seems like a nice, parsimonious design.  Unfortunately, the depot still needs to be launched.  So its QD interface with the GSE needs to be the normal interface.  So how does the gender-reversed version get put in place?  Some possibilities:

1) What I used to call the "depot kit", which is a deployable payload that grafts on all the stuff needed by the depot after it reaches VLEO.  In this case, a male-to-male QD attachment would glom on to the female QD and latch itself in place.

2) There are two separate QDs on a depot, one male and one female.  The female one is only used for launch, and the docking/berthing hardware lines all depot clients up with the male one.
Update:  Per Dan, below, note that this could allow depot-to-depot fueling.

3) SpaceX decides to replace the existing gendered QD with an androgynous one.  This is a pretty big change, and it's likely to be considerably heavier than the existing female QD on the Starship side.

Update:  From sdsds, below:
4) Depot launches with a female-female adapter, allowing the GSE male QD to fuel it.  Then:
a) It's pulled off during the GSE disconnect process.
b) It's jettisoned after reaching orbit.
Either way, it's now a male QD.

Update: from billh, below:
5) Replace GSE QD with a female version, as needed for launching depots.

Are there any other options?

[sdsds]

Are there any other options?

The reverse of your #1: the depot launches with a depot-gender QD supplemented with a kit that makes it appear ship-gender to the GSE. The kit is jettisoned after launch.

[DanClemmensen]

2) There are two separate QDs on a depot, one male and one female.  The female one is only used for launch, and the docking/berthing hardware lines all depot clients up with the male one.

Not just on launch. depot's normal QD is also used for depot-to-depot.

[Twark_Main]

3) SpaceX decides to replace the existing gendered QD with an androgynous one.  This is a pretty big change, and it's likely to be considerably heavier than the existing female QD on the Starship side.

This is the opposite of how SpaceX prioritizes hardware development and schedule.

SpaceX doesn't ask "why aren't we building the ultimate design now??"  Instead, SpaceX asks "why are we building the ultimate design now, when we could go faster using an easy interim solution?"   

I wouldn't take androgyny off the table just based on that logic.

[billh]

Are there any other options?

The reverse of your #1: the depot launches with a depot-gender QD supplemented with a kit that makes it appear ship-gender to the GSE. The kit is jettisoned after launch.

Wouldn't it make more sense for the tower to have two QD versions instead of the ship?

[OTV Booster]

Something I don't think we've discussed, or at least haven't discussed for a while (everything old is new again on this thread):

We've been mostly working on the assumption that refueling will reuse non-depot Starship's QD, which means that the depot has to have a gender-reversed connector on it.  This is the main reason why we think that all refueling steps either have to be depot-to-x or x-to-depot; x-to-x won't work, nor will depot-to-depot.

That seems like a nice, parsimonious design.  Unfortunately, the depot still needs to be launched.  So its QD interface with the GSE needs to be the normal interface.  So how does the gender-reversed version get put in place?  Some possibilities:

1) What I used to call the "depot kit", which is a deployable payload that grafts on all the stuff needed by the depot after it reaches VLEO.  In this case, a male-to-male QD attachment would glom on to the female QD and latch itself in place.

2) There are two separate QDs on a depot, one male and one female.  The female one is only used for launch, and the docking/berthing hardware lines all depot clients up with the male one.
Update:  Per Dan, below, note that this could allow depot-to-depot fueling.

3) SpaceX decides to replace the existing gendered QD with an androgynous one.  This is a pretty big change, and it's likely to be considerably heavier than the existing female QD on the Starship side.

Update:  From sdsds, below:
4) Depot launches with a female-female adapter, allowing the GSE male QD to fuel it.  Then:
a) It's pulled off during the GSE disconnect process.
b) It's jettisoned after reaching orbit.
Either way, it's now a male QD.

Are there any other options?

Androgynous is potentially the most elegant and maybe it'll eventually show up but cryo coupling is tough. Show me the coupling and it'll push me towards believing.


Then there's the layout. Unless I'm missing something it's either a single vertical line of connectors or a crazy amount of cross connect and valving for fluids and switching for data. Adding one service increases the connector stack height. It's not impossible but doesn't look (to my eye) to be a good solution.


Two installed opposite gender QD plates only works for non-EDL ships (i.e. depots) unless the plates are at different heights. I have no idea what the mass penalty would be but expect it to be non-trivial.


What I've been advocating is your 4a. Depot QD would be exactly the same as GSE QD. GSE would have a gender bender temporarily installed then removed after launch. On the face of it this violates the goal of rapid turnaround but depot launch's will be a tiny portion of the total.


The downside is the depot will always be an intermediary for transfer. No depot to depot, no tanker to tanker, no ship to tanker, no ship to ship. Additional hardware needed for any of these.


Depot to depot has a few use cases. Ship to ship even fewer. Tanker to tanker and ship to tanker have enough use cases to be worth looking at.


IMO, (here it comes. The world is about to crash down around us) yet another build would cover most but not all unmet use cases. Something between a tanker and a depot that is tricked out for some heat rejection but not intended for long term storage. No EDL capability. Maybe two QD plates. Maybe one plate and a deployable gender bender. Maybe truncated tanks to save mass.


It would act as the AAA service truck delivering gas to an idiot driver that ran out. If built light enough the penalties for orbital changes would be reduced. Even if the penalties are high they may be justified at the operational level by worse alternatives.