Author Topic: Starship On-orbit refueling - Options and Discussion  (Read 818685 times)

Offline LMT

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1760 on: 10/08/2022 10:58 pm »
...this maneuver gets you anywhere you want to go in the Solar system, at deltaVs so high the online porkchop calculators can't handle it.

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

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

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

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

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

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

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

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

Refs.

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

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

Yes, with many tankers and SH launches, understood.  Orbit and return maneuvers are vastly harder, but vital for most useful missions. 

Also, your "turn & burn at Jupiter" isn't a remotely realistic Starship maneuver.  Other propulsion is needed.

...for less than 1/10 the cost of the SLS you can refuel a Starship in ~GTO and get anywhere else with Vinf of 15km/sec.  For example it drops Mars down to about 68 days, assuming Starship can actually slow down in the Mars atmosphere...

It's a false assumption.  Your entry speed is north of 30 km/s.  With such immense KE, EDL is not possible.

-- Unless you add a propulsion node to the logistics system.  But where?
« Last Edit: 10/08/2022 11:03 pm by LMT »

Offline InterestedEngineer

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1761 on: 10/09/2022 12:36 am »
Yes, with many tankers and SH launches, understood.  Orbit and return maneuvers are vastly harder, but vital for most useful missions. 

Orbit is achieved by aerobraking anywhere there is atmosphere.   I also wasn't saying to use all 15km/sec of Vinf except for flybys.  The fuel saved for say a Hohmann transfer to Jupiter allows propulsive braking on the far end.  Obviates the need for a several hundred million in yet another "deep space" rocket engine, all of which, as near as I can tell, are custom to each deep space mission.

Quote
Also, your "turn & burn at Jupiter" isn't a remotely realistic Starship maneuver.  Other propulsion is needed.

In what way? Physics?  Logistics?  Cost?  ROI?  Something else?

Compared to billion dollar missions on SLS to get anywhere and billion dollar custom deep space probes, it can't be cost.

I'm not sure we need 150t of cargo leaving the solar system at 60-80km/sec, so ROI may be the best argument against it.

This extreme example of fuel laddering is to show what is possible.

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It's a false assumption.  Your entry speed is north of 30 km/s.  With such immense KE, EDL is not possible.

I'm not sure I get how Vinf 15km/sec leaving Earth SOI gives you 30km/sec at Mars atmosphere.

Traveling to Mars subtracts 1km/sec relative. Dropping from Mars SOI to Mars atmosphere adds 1.5km/sec, so net 15.5km/sec at atmospheric entry.

Not sure if 15.5km/sec is possible in Mars atmosphere, but it's more possible than 30km/sec.

If say only 12.5km/sec is possible, then leave Earth SOI at 13.5km/sec, reserve 1.5km/sec for braking at Mars, and enter Mars atmosphere 12.5km/sec.  Only adds a few days to transit.


Offline LMT

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1762 on: 10/09/2022 03:03 am »
I'm not sure I get how Vinf 15km/sec leaving Earth SOI gives you 30km/sec at Mars atmosphere.

You're taking the relative velocity vector magnitude at Mars, for a trajectory like Jupiter DE-STAR.

Offline TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1763 on: 10/09/2022 06:55 am »
Just to be sure I properly understand you: You're talking about refueling while accelerating, right? That is, you would:

a) launch a tanker and an LSS. (Each with 1500t fuel capacity.)
b) fully fuel both of them in LEO from a (rather large) depot. (Or two depots.)
c) couple the tanker to the LSS for refueling.
d) both of them fire together in formation, with the tanker continuously keeping the LSS topped up.
e) when the tanker is almost dry, it stops firing, disconnects, and reels in the fuel line.
f) at apogee, the tanker fires just a little bit to lower perigee to enable reentry.
g) the LSS goes on firing until it reaches TLI--arriving at the moon with about 500t extra fuel.

There's better version than this, and I think it's probably a winner:

a) Launch a tanker, but it's really not a tanker; it's just a pusher, i.e., it's a first stage that can dock to a "pushee" in VLEO.

b) Dock it to a depot, then leave it docked there until its target (its pushee) is on-orbit, then fill the pusher however full you need it (more in a moment).  This just minimizes boil-off.

c) Launch the EDL-capable lunar Starship, i.e., a vanilla Starship that can land on the Moon, hereinafter "EDLC-LSS".  (I know--catchy, eh?  It sounds like an X.25 standard that never quite caught on.)

d) The EDLC-LSS (with crew), gets filled completely full at a depot.

e) Immediately after the EDLC-LSS undocks, the pusher/tanker does an RPOD, leaving it docked nose-to-tail with the EDLC-LSS.  NOTE!!!!  No prop transfer occurs!!!  It's only a pusher!!!!

f) The pusher adds just enough delta-v to get the EDLC-LSS to go HEEO-LS-EDL on its 1500t of prop.

g) The pusher detaches and the EDLC-LSS immediately burns to TLI.  Note that even though the push technically put it in an HEEO, it's still basically at perigee, just going faster than it was, so there will only be one transit of the VA Belts.

h) EDLC-LSS immediately transits to the Moon by finishing the TLI, does LOI, DOI, PDI, landing, hangs out while astronauts cavort on the surface, then ascent, TEI, and finally EDL, delivering the crew to terra firma.

i) The pusher, separate from the EDLC-LSS, rises to the HEEO apogee, does a minor burn to initiate reentry, and does EDL (long before the EDLC-LSS does).

Below are two conops.  The first one is the HEEO-based refueling from a tanker.  (Yeah, it could be a depot.  Yeah, maybe only depots have refueling equipment.  Yeah, then the depot would have to get back to LEO propulsively.  I don't care.  The result will be pretty much the same.)  The second one is the conops I outlined above.  Several takeaways:

1) In terms of propellant usage, the two conops are, for all intents and purposes, identical.¹

2) If you think about it, this isn't surprising.  You have roughly the same aggregate wet mass and the same delta-v.  The fact that the aggregate "rocket" is flying in two pieces in one case and one two-stage piece in another doesn't really change the mass ratio.

3) The big, big, big deal here is that there's only one refueling for the EDLC-LSS, even though there's a second RPOD as the pusher attaches.  But the crew will spend no time at apogee in the middle of the VA Belts, because the TLI burn, even though it's in two stages, it more-or-less continuous.

4) Obvious problems/risks:
a) If the pusher doesn't detach cleanly, you have yourself a problem.  But this is really well-understood tech.
b) Obviously, a nose-to-tail docking that doesn't mess up the aero and thermal characteristics of the pusher is non-trivial.  But this is a clear winner from a risk-reduction standpoint.
c) If the EDLC-LSS doesn't fire after separate, it'll go to apogee, which makes crew absorb a big bag o' protons, but they can burn at the next perigee.

5) Yeah, you could build yourself a li'l tiny depot that did the pusher's job more efficiently.

6) I'm pretty sure this will work with 1200t EDLC-LSS and tankers, but I haven't cranked it through.

7) I tried this with an LEO-push-HEEO-LS-LEOpropulsive conops (which would eliminate Orion and let D2 bring the crew to/from LEO), and it doesn't work.  You'd still need another refueling in cislunar somewhere.


____________
¹The "refuel in HEEO" conops need contingency prop for loiter, in case the EDLC-LSS gets delayed.  With the pusher version, it can sit in VLEO, hooked up to a depot, where boiloff is minimized until just before RPOD.  This is basically a nit.
« Last Edit: 10/09/2022 06:58 am by TheRadicalModerate »

Offline redneck

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1764 on: 10/09/2022 09:06 am »


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

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

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

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

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

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


Given that, it seems like the original suggestion about transferring the propellant during the boost would work out.  Get the full value of the extra propellant burned during the impulse burn from LEO.

I don't think you were suggesting complete transfer during max boost. Transferring during acceleration does address settling problems as opposed to microgravity transfer. One question is how low can the acceleration be without gravity losses eating too much into the potential gains?  RL10 class with deep throttling?

Offline volker2020

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1765 on: 10/09/2022 12:37 pm »


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

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

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

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

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

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


Given that, it seems like the original suggestion about transferring the propellant during the boost would work out.  Get the full value of the extra propellant burned during the impulse burn from LEO.

I don't think you were suggesting complete transfer during max boost. Transferring during acceleration does address settling problems as opposed to microgravity transfer. One question is how low can the acceleration be without gravity losses eating too much into the potential gains?  RL10 class with deep throttling?

I see some problems with the equation of air formation flying with space operation. In the air, the fuel connection between the planes is flexible, so there is no spinning the other vehicle, when trust gets out of sync, and the air around does stabilize the flight.

All that would require that the trust control on both vehicles must be synchronized, so that you get into a feed back loop. Clearly doable, but a lot more complex than air tanking.

The most obvious solution would be a hard dock on the central axis. 

Offline LMT

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1766 on: 10/09/2022 03:48 pm »
...No prop transfer occurs!!!  It's only a pusher!!!!

f) The pusher adds just enough delta-v to get the EDLC-LSS to go HEEO-LS-EDL on its 1500t of prop.

g) The pusher detaches and the EDLC-LSS immediately burns to TLI...

Alternately, a cis-lunar "pusher" or "tug", could run on ASCENT propellant:  [NH3OH] [NO3]

Notably, ASCENT can be manufactured onboard the LEO depot; 96% of its mass can be trawled from the thermosphere, which is almost entirely oxygen and nitrogen.  Lower a trawler's baseline LOX cooler/compressor operating temperature by 13°, to liquefy and store both gases.  Only 4% of ASCENT mass would be launched cargo:  LH2. 

ASCENT is non-toxic and storable in room-temperature inflatables.  Also, it's bimodal.  You can use it as a chemical monopropellant (Isp 250 s) for high-thrust Starship tug maneuvers, or as an electrospray ion engine propellant (Isp 1500 s) on other spacecraft / payloads, for long-duration, low-thrust missions.  Colón and Lightsey 2021.

Manufacture of ionic liquid from cation and anion can be straightforward (video).  How might you produce ASCENT's constituent NH3OH and NO3 from trawled LOX and LN2 plus cargo LH2?

Refs.

Colón, B.J. and Lightsey, E.G., 2021.  Spectre – Design of a Bimodal Propulsion System.
 


« Last Edit: 10/09/2022 07:52 pm by LMT »

Offline OTV Booster

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1767 on: 10/09/2022 05:50 pm »
The fact that SLS and Orion will have less flight experience is irrelevant to the LSS.  Both SpaceX and NASA will do whatever they can to minimize risks, especially if it's cheap or easy to do so.  This is one that's both.
Avoiding a second refueling avoids some risks but it adds others.

It requires a custom extended tank, which adds risk.

I don't think it's a custom extended tank.  There are even more good reasons to go with a 1500t tank for lift tankers than there are for the LSS, not the least of which is that it doesn't require sending two tankers to NRHO for Option B.

Also, remember that "extended" really means "with ring segments, intertank bulkhead, and LCH4 dome rearranged."  That's not nothing, but as modifications go, it's not as bad as a lot of the mods that SpaceX has to make for LSS no matter what.

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

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

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

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

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

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

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

The other factor here is risk tolerance.  In a perfect world, this wouldn't change over time.  However, HLS is a high enough profile project, both for NASA and SpaceX, that reducing tolerance to ensure that both Option A flights go well is a good political move--especially since NASA wrung their hands about operational complexity in the source selection statement, and Blue Origin used the conops to fling FUD around during the protest and appeal process.
Why would HEEO refueling add novelty? What is added that is not demonstrated at LEO? The utility of the operation is another question and I have no opinion. Just trying to follow the arguments.
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Online DanClemmensen

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1768 on: 10/09/2022 05:57 pm »
This thread has drifted far off the topic of how to do refueling. It's now almost exclusively about all the neat missions that are enabled by refueling. Maybe we need two threads?

Offline TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1769 on: 10/09/2022 06:21 pm »
NB:  Even if there turns out to be a lot of experience with tankers fueling depots in VLEO, there will be considerably less experience refueling stuff in HEEO.
Why would HEEO refueling add novelty? What is added that is not demonstrated at LEO? The utility of the operation is another question and I have no opinion. Just trying to follow the arguments.

HEEO rendezvous procedures are going to be different.  Detecting and quantifying insertion errors will be new.  Phasing takes longer.  Thermal characteristics are different.  Avionics failure rates based on VA Belt exposure will be less well-quantified.  Insertion errors cascade forward into departure window uncertainty, which leads to bigger TLI insertion errors.  Contingency planning for docking difficulty or engine burn aborts is more complicated.

None of this is insurmountable.  But if you're a mission assurance engineer plugging values into your failure tree, you'll be assigning larger uncertainties to all of this stuff, and as a result you'll get more uncertainty when the model spits out loss-of-mission probabilities than it would if you were just going round 'n' round in VLEO.

Offline OTV Booster

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1770 on: 10/09/2022 06:21 pm »
A few days ago I mentioned the concept of tanker and cargo Starship mated for the burn from LEO towards the moon. Mainly I brought it up because I hadn't noticed it in this discussion. It didn't seem to be a popular idea both on the technical side and even whether it would be useful. I concede that the technical end may be more difficult than most would like. Possibly to the point of not being worthwhile.

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

a) launch a tanker and an LSS. (Each with 1500t fuel capacity.)
b) fully fuel both of them in LEO from a (rather large) depot. (Or two depots.)
c) couple the tanker to the LSS for refueling.
d) both of them fire together in formation, with the tanker continuously keeping the LSS topped up.
e) when the tanker is almost dry, it stops firing, disconnects, and reels in the fuel line.
f) at apogee, the tanker fires just a little bit to lower perigee to enable reentry.
g) the LSS goes on firing until it reaches TLI--arriving at the moon with about 500t extra fuel.

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

Anyway, if NASA is worried about the risks of refueling in LEO, I think the risks of refueling while accelerating will really send them into orbit. (So to speak.) :-)
Well, it does do away with ullage thrust.  ;D


IMO, refueling will never be done unless the ships are physically hooked up. IOW, no hose hose line in a gas station. I'll give reasons if you want but ISTM to be a no brainer.


One problem with refueling while under significant thrust is the changing CoM. Between the necessarily constantly changing thrust vector and/or differential throttling, there will be small differences in thrust, maybe constantly changing, between the two rockets. Nothing big. At minimum, the digital engine controls have incremental steps over a range of 256, 512, 1024... steps. This absolutely can be compensated for and almost completely cancelled out but in the large scale non quantum world, nothing is exact.


The net result would be connection points between the ships experiencing small positive and negative shear stress. Adding robustness will make this a minor issue. The question is, is it worth the effort? It's all in the trades and ISTM the branching possibilities have spread so far it's too premature to do more than put this idea into the toolbox to be pulled out and dusted off if things move in this direction.
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Online DanClemmensen

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1771 on: 10/09/2022 06:24 pm »
On further reflection, I am even more convinced that all of the specialized refuelling hardware should be in the depot and the SSs should be at most minimally modified: the best part is no part.

All SS already have a mating connection for thrust, namely the big nine-meter ring that connects to the SH. The depot should implement the SH side of this connection beneath a disposable fairing at the nose. After the fairing is discarded the depot is a cylinder whose top surface looks like the top surface of an SH. The depot will dock nose-to-tail with the SS. The fuel transfer will be via an extendable QD mechanism that is stored beneath the mating ring and is also covered by the disposable fairing. It extends out, up, and around to mate with the SS QD connection. Fuel transfer is done while under a small amount of thrust provided by the depot. Exactly how docking and locking will be accomplished will require actual engineering instead of hand-waving and will depend on the current design of the existing interface. Worst case: use a nine-meter version of the "soft docking" hardware from the IDSS that provides six degrees of freedom through a limited range of motion to complete the soft docking. I strongly suspect this is overkill and real engineers will come up with a simpler system.

Offline OTV Booster

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1772 on: 10/09/2022 07:04 pm »
I don't think they need a depot.  In effect, they are depots, or at least spacecraft with good passive resistance to boil-off.  So you can refuel them long before the crew shows up, using plain ol' tankers that go out to NRHO via BLT and straight back to EDL.
I know. I'm just liking the model that says Starships mate with depots, but not with each other, since it makes all the plumbing very unambiguous. It also lets the refueling be completely asynchronous. In this vision, a) a steady stream of tankers keeps the LEO depot full, b) occasionally a tanker fills up from the depot, flies to the moon, and pumps ~500t of prop into that depot (which really does achieve ZBO), so c) whenever an LSS needs to refuel, there's always fuel for it either in LEO or at Gateway.

I expect Option B (and Appendix P) HLSes to have fairly short lives, due to dust contamination.  It may be that SpaceX would do better using an EDL-capable LSS, if for no other reason that you can completely clean and refurbish it on the ground.  (You can also integrate new heavy cargoes into it, which an Option B LSS can't do.)  But this definitely requires refueling in HEEO or cislunar.
This leads me to wonder whether it's possible to do that at Gateway. Or, conversely, what is the minimum it would take to do the necessary maintenance and checkout at Gateway, rather than returning the LSS to Earth? (Assuming you can skip the static fire.) :-)
It would be great to be able to do repair at gateway and as much as I'd like it, it doesn't seem practical.


Drawing a not exact parallel with aircraft and ground vehicles, servicing consists of changing or checking levels of fluids, changing out filters, tensioning belts, and checking critical play. On gas engines it would include plugs and ignition wires. Except maybe for filters and checking for play, none of this would apply. Anything beyond this, including fixing excess play, would be repair.


Can you imagine changing out your cars water pump in a space suit in zero g? They do EVA stuff like this on the ISS only after modeling and scripting every move on the ground and still run into problems.


One optimization we've never seen on a rocket is on orbit maintenance and repair. The day is coming but probably not for another generation or two of rockets. I'd guess that there will be minor tweaks in this direction but the big change will be when they move to on orbit assembly.
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Offline Greg Hullender

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1773 on: 10/09/2022 07:22 pm »
On further reflection, I am even more convinced that all of the specialized refuelling hardware should be in the depot and the SSs should be at most minimally modified: the best part is no part.

All SS already have a mating connection for thrust, namely the big nine-meter ring that connects to the SH. The depot should implement the SH side of this connection beneath a disposable fairing at the nose. After the fairing is discarded the depot is a cylinder whose top surface looks like the top surface of an SH. The depot will dock nose-to-tail with the SS. The fuel transfer will be via an extendable QD mechanism that is stored beneath the mating ring and is also covered by the disposable fairing. It extends out, up, and around to mate with the SS QD connection. Fuel transfer is done while under a small amount of thrust provided by the depot. Exactly how docking and locking will be accomplished will require actual engineering instead of hand-waving and will depend on the current design of the existing interface. Worst case: use a nine-meter version of the "soft docking" hardware from the IDSS that provides six degrees of freedom through a limited range of motion to complete the soft docking. I strongly suspect this is overkill and real engineers will come up with a simpler system.
Doesn't this result in a lot more plumbing, though? (At least, on the depot.) I've been assuming that the depot is little more than a Starship with extended tanks, meaning it fills/drains through a QD port at the bottom. That port would get replaced with a gender-swapped hose of some kind (I'm fuzzy on exactly how this would work, but something a lot like the hoses in the existing ground support equipment that fills them up on the pad) but the interior of the depot would be almost identical to that of a Starship. That means they have to be side-by-side to refuel OR the hose needs to be really long, which (I'm told) introduces a lot of problems.

I do kind of like the idea of the depot securely attaching to the vehicles it refuels. I'm just wondering how the plumbing would work.

Offline TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1774 on: 10/09/2022 07:35 pm »
On further reflection, I am even more convinced that all of the specialized refuelling hardware should be in the depot and the SSs should be at most minimally modified: the best part is no part.

All SS already have a mating connection for thrust, namely the big nine-meter ring that connects to the SH. The depot should implement the SH side of this connection beneath a disposable fairing at the nose. After the fairing is discarded the depot is a cylinder whose top surface looks like the top surface of an SH. The depot will dock nose-to-tail with the SS. The fuel transfer will be via an extendable QD mechanism that is stored beneath the mating ring and is also covered by the disposable fairing. It extends out, up, and around to mate with the SS QD connection. Fuel transfer is done while under a small amount of thrust provided by the depot. Exactly how docking and locking will be accomplished will require actual engineering instead of hand-waving and will depend on the current design of the existing interface. Worst case: use a nine-meter version of the "soft docking" hardware from the IDSS that provides six degrees of freedom through a limited range of motion to complete the soft docking. I strongly suspect this is overkill and real engineers will come up with a simpler system.

Yes, we discussed this in the original pusher/tanker stuff.  If you do this, you lose the ability to go straight back to EDL, but 800m/s or so for a near-empty depot isn't exactly a massive propellant requirement.  If you go with 1200t tanks instead of 1500t, it's a bit more expensive (about 50t of prop), but still reasonable.

I don't think there's a prayer of scaling up an IDSS-like soft-capture ring to 9m.  You'd have soft-capture petals covering up the engines on the tail.  The alignment, translational, and rotational velocity errors would also have to be much, much smaller than those supported by the existing IDSS.  And getting all the hard-capture latches to engage would be harder, too.

I thought that three IDSS-like soft capture rings, spaced 120º apart around the tail, might work pretty well.  The sensors and software to get them to work together to damp out errors is a lot more complicated, but... hey--software. 

You could also go with grapples, like we discussed in the side-to-side docking.  But then you really do need a robust hard capture in either case.  Quick back-of-napkin:
Fully fueled (1500t) EDLC-LSS (EDL-capable LSS) with 25t payload:  1660t. 
Pusher mass near burnout, with return prop: 190t. 
Total wet mass of docked system: 1850t. 
Thrust of 3 RVacs throttled down to 70%: 4620kN. 
Burnout acceleration: 2.50m/s². 
Load on the docking system at burnout: 1660t * 2.50m/s² = 4150kN (423tf). 

That's a lot of force on a docking system.  So any hard-capture probably has to be able to align the ships to use the launch load points.

In the conops I did just up-thread, I assumed that there was no prop transfer at all, and that both the EDLC-LSS and its pusher had refueled at one or more depots before RPOD.  There is of course nothing¹ that prevents a depot from fulfilling the role of pusher, as long as you're willing to forgo reusing some or all of the QD hardware.  In that case, the EDLC-LSS and the depot would do the nose-to-tail RPOD, the depot would transfer prop to the EDLC-LSS until it was full, and then the depot would use the remainder of its prop to provide the extra delta-v to allow the EDLC-LSS to complete the whole mission without cislunar refueling.  The depot would then have to make its own way back into VLEO, which is a little more prop-intensive, but not much.

In either case, you're looking at a significant mod to the target Starship's configuration to support pushing.  Irrespective of the style of docking system you use, it's a whole bunch of new stuff in the engine compartment, with some interesting plumbing if you want pusher/depot functionality in one vehicle.

But a pusher/depot only adds a single RPOD to an EDLC-LSS crew's mission, and it gives them minimal exposure to VA belt radiation, and almost all of your contingency planning occurs in a stable, circular VLEO.  There are some kinda gnarly post-push contingencies (failure to undock, timely engine start failure on the EDLC-LSS), but I'd think that they'd have pretty low probabilities.

___________
¹Well, maybe one thing:  The depot would have to be able to hold a bit more than 1900t of prop to do a mission with 25t of cargo in addition to the crew module.

Offline Greg Hullender

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1775 on: 10/09/2022 07:37 pm »
It would be great to be able to do repair at gateway and as much as I'd like it, it doesn't seem practical.
I know, but it wasn't that long ago that we all thought having an empty first-stage return to the launch pad wasn't practical either. :-) You're probably right, of course, but I'm just wondering how much thought anyone has given to it lately.

Can you imagine changing out your cars water pump in a space suit in zero g? They do EVA stuff like this on the ISS only after modeling and scripting every move on the ground and still run into problems.
Sure, but I don't think anyone has given thought to how you'd set up an orbital repair station if you thought you were going to have to do the same set of things over and over. E.g. bespoke hardware to let you swap out engines. Or maybe a giant balloon big enough to hold a whole Starship so people could work on it in shirtsleeves (probably wearing oxygen masks). Or (more likely) something I haven't even thought about.

One optimization we've never seen on a rocket is on orbit maintenance and repair. The day is coming but probably not for another generation or two of rockets. I'd guess that there will be minor tweaks in this direction but the big change will be when they move to on orbit assembly.
Again, you're likely right, although I wonder if it wouldn't be easier to attempt repair before you try to do complete assembly.

(The connection this has with the refueling thread is that I don't think you can usefully refuel anything that can't EDL. Not past the initial mission, anyway.)

Offline TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1776 on: 10/09/2022 07:42 pm »
I do kind of like the idea of the depot securely attaching to the vehicles it refuels. I'm just wondering how the plumbing would work.

The depot always has to securely attach to the vehicles it refuels.  The real question is whether it has to transmit main engine thrust loads to them.  If it's a pusher, it does.

Just to reiterate:  There is almost no difference, from a propellant efficiency standpoint, between a depot that refuels something in HEEO and a pusher that provides the delta-v to get to HEEO before detaching, allowing the target Starship to go straight to TLI.  The difference is in the operational details, especially for crewed flights.

The pusher is a more complex piece of hardware, and a pusher/depot is even more complex, with more complex plumbing.  But the conops is quite a bit simpler.

So many trades, so little time...

Offline LMT

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1777 on: 10/09/2022 07:48 pm »
Actual cryogenic depot tech to be aware of:  Eta Space LOXSAT 1, and its successor, Cryo-Dock.

What decisions can be inferred from the designs?

Quote
LOXSAT 1 will test a suite of CFM technologies previously developed and ground tested. CFM technologies include:

Active and passive thermal control
Cryogenic chill down and transfer
Pressure control
Ground densification
Fluid surface visualization tools
Autogenous vs. helium pressurization
Liquid acquisition devices (LAD)
Zero boil off (ZBO) with pump mixing
High capacity 90K cryocoolers
Ground to flight insulation
Low conductivity supports
Zero-g chill down and transfer
Cryogenic quick disconnects
Ground densification for thermal energy storage
« Last Edit: 10/09/2022 07:54 pm by LMT »

Online DanClemmensen

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1778 on: 10/09/2022 07:56 pm »
On further reflection, I am even more convinced that all of the specialized refuelling hardware should be in the depot and the SSs should be at most minimally modified: the best part is no part.

All SS already have a mating connection for thrust, namely the big nine-meter ring that connects to the SH. The depot should implement the SH side of this connection beneath a disposable fairing at the nose. After the fairing is discarded the depot is a cylinder whose top surface looks like the top surface of an SH. The depot will dock nose-to-tail with the SS. The fuel transfer will be via an extendable QD mechanism that is stored beneath the mating ring and is also covered by the disposable fairing. It extends out, up, and around to mate with the SS QD connection. Fuel transfer is done while under a small amount of thrust provided by the depot. Exactly how docking and locking will be accomplished will require actual engineering instead of hand-waving and will depend on the current design of the existing interface. Worst case: use a nine-meter version of the "soft docking" hardware from the IDSS that provides six degrees of freedom through a limited range of motion to complete the soft docking. I strongly suspect this is overkill and real engineers will come up with a simpler system.
Doesn't this result in a lot more plumbing, though? (At least, on the depot.) I've been assuming that the depot is little more than a Starship with extended tanks, meaning it fills/drains through a QD port at the bottom. That port would get replaced with a gender-swapped hose of some kind (I'm fuzzy on exactly how this would work, but something a lot like the hoses in the existing ground support equipment that fills them up on the pad) but the interior of the depot would be almost identical to that of a Starship. That means they have to be side-by-side to refuel OR the hose needs to be really long, which (I'm told) introduces a lot of problems.

I do kind of like the idea of the depot securely attaching to the vehicles it refuels. I'm just wondering how the plumbing would work.
Yes, lots of complex plumbing, but It's all in the depot. That's the whole idea. Move the entire design problem into the depot so all the other SS variants are minimally affected. Although this configuration can probably be used as a pusher, that's a side effect, not a design goal. Yes, this depot is a whole lot more complicated than a simple tank, because it must implement the entire fuel transfer system with no change to the other SS. The fuel transfer QD is separate from the depot's standard QD, which is still there at the tail. I'm not a rocket engineer, so I don't know if you run the pipes up the outside of the depot or if you run them inside the tanks, but they extend from the "bottom" to the "top" to connect the tanks to the refuelling QD. Since the depot does not EDL, they can probably be on the outside.

Offline OTV Booster

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1779 on: 10/09/2022 08:11 pm »


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

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

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

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

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

If you are worried about formation refueling, then LEO refueling won't work either, and the entire architecture of Starship is doomed.  Somehow I don't think so.
Sometimes atmosphere is as much a friend as an enemy. Air to air refueling is in a viscus environment that both damps and inputs small variations. Drogue systems have inherent stability and flexibility in six degrees. Boom systems have a third control system (boom operator). Impulse from fluid flow is a tiny force that is less than random atmospheric burbling. It's a very different environment. Not impossible, but very different.


The idea of rigidly bonding two aircraft together in flight has been demonstrated but not widely used because it's a ticklish PITA in atmosphere. Bonding two spacecraft together in vacuum is a routine operation. I just can't see snaking two propellant hoses, two ullage gas hoses, maybe a nitrogen hose and maybe a power line between ships except as the back side of an extendable QD plate on the depot. Maybe .5-1m.


Maybe they can do without additional bonding points and I fully expect this to be way they'll first try it. My gut says that between the changing CoM and the unpredictable impulse from fluid flowing into the receiving tanks, its dicy. A slow transfer rate will keep fluid flow more controlled but this might not be viable when moving from proof of concept to operations.
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