Author Topic: Starship On-orbit refueling - Options and Discussion  (Read 596753 times)

Offline TheRadicalModerate

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1700 on: 10/02/2022 04:03 am »
I believe that laddering makes sense when extreme C3 outweighs the need for mission simplicity.  However, for both lunar and Mars missions, that's simply not the case.  In the case of an Artemis mission, you'll want to limit the LSS to a single refueling, simply to reduce risk.  That requires limiting the maximum refueling energy to something that the LSS can reach with its launch prop.  In the case of a Mars mission, with crew already on board, a single refueling in VLEO is more than adequate.
E = 1/2*mv2

The other type of high energy mission is sending a lot of mass.  Laddering through a high energy Earth orbit significantly* increase the mass that can be sent to Mars in a single ship.  Fewer ships tied up in transit and fewer ships that need refueling at Mars.

I think we differ in that you think refueling is complex, risky and difficult.  I think refueling will become routine and easy.  Up to now almost all of the rendezvous and docking has been in human space flight.  Not only are human pilots bad at this but the risk to humans has resulted in extreme conservatism and overly complex procedures.  I expect SpaceX to try new procedures, blow up a few robots and end up with something far faster, safer and more reliable.


* By a factor of 13 if you ladder through a trans-moon injection orbit and use aero braking from Mars transfer orbit down to low Martian orbit or the surface.  This allows an absurd mass of almost 8300t of cargo to Mars in a single SS.  This would allow for example a storm shelter with 10m of water shielding.  (Fitting 10m of water in a 9m SS is left as an exercise, perhaps it would be better to use aluminum.)  Laddering through GTO "only" increase the cargo by a factor of 4.75, which is still very useful.

Four objections:

1) Unless you're using magic at Mars, there's a limit to how much mass you can land using aerodynamic EDL.  I'd frankly be a bit surprised if it's as much as 150t, and anything more seems like you'll run into ballistic coefficient problems.

2) Ridiculously high C3's to Mars need to be offset by a lot of propulsive braking before EDL.  I can't remember who explained this to me (apologies for no citation), but the quick-and-dirty version of this is to look at the centripetal acceleration needed to keep the Starship in the atmosphere at a particular altitude, so it can kill its speed efficiently.  If you want to keep the acceleration below 8g at 100km altitude, you can't hit periapse at more than 16.5km/s--even if you had the unobtanium TPS that allowed that kind of thermal load.

3) Finally, you can ladder as much as you want up to just a smidge below Earth escape speed, but then things get weird.  Any additional tankers have to leave in roughly the same orbit as the Starship.  That gives you two unpalatable choices:

a) You can fill them all completely full in C3=-somethingSmall, but to get maximum Oberth effect, the Starship wants to use as much of its prop as possible, which means that the tankers have to use pretty much the same amount (unless they're bigger).  Net result:  Even if you plan to ladder from here, there's very little prop to ladder with.

b) You can reduce the Oberth effect and ladder at any n:m ratio you want.  But when I worked this out, the C3 you lose from the less-aggressive perigee burn is hard to make up even with efficient laddering.  Sometimes as much as 3:2 makes sense, but you need to be really screaming before it does--and it's still a massive amount of prop to LEO.

We slogged through some of this over on the extrasolar missions thread.  It kinda starts about here, but you have to rummage through lots of posts on other topics to figure it out.

4) It's not that I think that refueling in and of itself is hard, although it certainly will have risks that need to be retired before anybody conservative (e.g. NASA) will think about anything other than a single uncrewed refueling of an expensive asset like an LSS.  The real problem is that everything is harder in an eccentric orbit.  Fiddling with arguments of perigee (or perihelion) is expensive.  Minor RAAN errors are hard to correct.  Phasing takes a long time, because orbital periods are longer.  There are the radiation problems.  Missed-burn contingencies become more and more nightmarish.  There's a lot of operational art that doesn't exist, which will need to be developed.

I'm perfectly happy with a depot in a circular VLEO.  I'm somewhat less happy with a depot or tanker in NRHO, because CR3BP orbits are weird and nobody's done rendezvous in them yet, but you have plenty of time to fool around without things getting too weird.  But HEEO or its heliocentric cousin seems like a lot of trouble for not a lot of gain.  (Again, the obvious exception is a mission that can really benefit from super-high C3.  But Mars ain't it.)

I admit to thinking no more than 10 years out on refueling, and there's nothing but cislunar and maybe a few Mars trips in that timeframe.  At some point in the dim distant future, the operational art will exist, and fancier things will have firm business cases attached to them.  But it's science fiction for right now.

Offline Greg Hullender

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1701 on: 10/02/2022 12:25 pm »
Exactly how much mass does need to be sent to Mars? It seems like we've worked through the numbers for the lunar case pretty thoroughly, but I haven't seen anything comparable for Mars.

For the moon, I think the conclusion is that you need a depot in LEO that gets filled with ~1500 metric tons of propellant via multiple flights by tanker Starships. Then you stretch the lunar Starship enough to hold 1500 metric tons of propellant. Even though it can't launch with that much, you can fill it up once it gets to the depot. From there, it can do the full mission to NRHO, to the moon, and back to NRHO. We haven't worked out how you might reuse a lunar Starship, but NASA clearly doesn't plan on that.

So what's the equivalent process for a Mars Starship? Surely 1500 mt of prop in LEO isn't enough to take you to Mars! Is it? It seems like you'd want another depot much higher up to give the Mars Starship a second refill--maybe in NRHO--and do something like a gravitational assist off the moon to do a deep dive close to the Earth to maximize the Oberth burn. (I'm just making this up as I go--I'm hoping someone has a better-thought-out plan.)

Apologies if this has already been thoroughly debated elsewhere.


Offline Robotbeat

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1702 on: 10/02/2022 12:34 pm »
You need less propellant to go to Mars than land on the Moon, let alone do the ~7-8km/s mission that Starship HLS needs. So no 1200t depot required. 3.7-3.9km/s or so (depending on opportunity… 3.5km/s for TMI, 200-400m/s for landing) is about the minimum amount. 500t would even be enough.

Also, quit taking TheRadicalModerate’s figures for the Moon as gospel.
« Last Edit: 10/02/2022 01:23 pm by Robotbeat »
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Offline Robotbeat

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1703 on: 10/02/2022 12:47 pm »
Pretty annoying when people who post long pages about what “MUST” be required for lunar missions are taken as gospel because they post a lot, while making unqualified assumptions about dry mass that drive their conclusions.

…yes, yes, glass houses and stones, but you shouldn’t take my prognostications as gospel either!
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Offline Greg Hullender

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1704 on: 10/02/2022 01:37 pm »
You need less propellant to go to Mars than land on the Moon, let alone do the ~7-8km/s mission that Starship HLS needs. So no 1200t depot required. 3.7-3.9km/s or so (depending on opportunity… 3.5km/s for TMI, 200-400m/s for landing) is about the minimum amount. 500t would even be enough.

Ah, so the key things that make this possible (correct me if I'm wrong) are a) The Mars trip is one-way b) using the Mars atmosphere for braking saves you a lot of delta-v c) Although the trip to Mars takes much longer, the delta-v to go from LEO to a minimum energy TMI isn't really very much more than what's required for TLI. Is that a fair summary?

Also, quit taking TheRadicalModerate’s figures for the Moon as gospel.

Pretty annoying when people who post long pages about what “MUST” be required for lunar missions are taken as gospel because they post a lot, while making unqualified assumptions about dry mass that drive their conclusions.

…yes, yes, glass houses and stones, but you shouldn’t take my prognostications as gospel either!

Actually, my perception was that a) his logic and his math seem reasonable to me, given his assumptions (I looked over his spreadsheet at length) b) other people's disagreements were all complaining his assumptions were too conservative (e.g. his dry-mass estimate could be too high) but c) even with those assumptions, he sees a fairly simple way to make the system work.

So it seems plausible to me that the system SpaceX will end up using (particularly in light of the Marshall paper) will be something close to what he describes. Doubtless there will be differences, and it may be able to carry more payload than he thinks and/or need fewer refueling flights, but the basic idea seems sound to me.

Do you genuinely feel that this is grossly wrong? Or is your concern just that, despite the precise numbers in his spreadsheet, no one should think that all the details are correct?



Offline Robotbeat

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1705 on: 10/02/2022 02:23 pm »
If you use overly conservative assumptions, your conclusions about what is optimal to do is going to be wrong. This is partly why NASA assumed a 3 stage lunar lander but SpaceX is using one stage. Conservative dry mass leaves you with a whole bunch of stages.
« Last Edit: 10/02/2022 02:34 pm by Robotbeat »
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Offline Greg Hullender

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1706 on: 10/02/2022 04:07 pm »
If you use overly conservative assumptions, your conclusions about what is optimal to do is going to be wrong. This is partly why NASA assumed a 3 stage lunar lander but SpaceX is using one stage. Conservative dry mass leaves you with a whole bunch of stages.
So do you have a simpler approach?

Offline TheRadicalModerate

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1707 on: 10/02/2022 06:20 pm »
If you use overly conservative assumptions, your conclusions about what is optimal to do is going to be wrong. This is partly why NASA assumed a 3 stage lunar lander but SpaceX is using one stage. Conservative dry mass leaves you with a whole bunch of stages.

I never did get around to cranking the very low inert mass and 1200t tanks through the model--an oversight.  If the combined dry + crew module mass is 95t or less, 1200t will work straight from VLEO.  You could take that up to 100t if you were willing to forgo any non-crew payload, which sounds like a mistake.

This isn't a completely unreasonable number, especially if you're willing to skimp on the crew module.  If they manage to get the vanilla Starship down to 100t, then the LSS version might get down to 75t, and you'd be in business.  But there's basically zero design margin, and you're left with having to do something exotic¹ if you ever want to stage the LSS out of VLEO instead of NRHO.

Model added to the others.

______________
¹Update:  The "something exotic" statement is unwarranted.  If you limit total non-crew cargo to 5t, then you could have a 105t dry+crew configuration that would work fine.  I was thinking (without computing) that the requirement to do fast transit instead of a BLT would do you in, but that's more than offset by the fact that you can do LEO-LLO-LS-NRHO when staging out of LEO, eliminating the extra delta-v to get from NRHO down to LLO.

105t is only about 5t lighter than my current "conservative" estimate for dry+crew.  That might have enough margin to be viable.

Note that staging from LEO still requires a refueling, post-lunar-ascent, before returning to LEO propulsively.  The only way you can avoid refueling in cislunar is if you refuel in some kind of fairly energetic HEEO.  I maintain that would be exotic, because you'd wind up subjecting the crew to VA Belt radiation.
« Last Edit: 10/02/2022 08:31 pm by TheRadicalModerate »

Offline redneck

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1708 on: 10/03/2022 01:15 am »
If two fully fueled ships leave LEO docked with crossover plumbing, the attached tanker can empty in a very few minutes. Tanker with 1,500 tons of fuels and LSS with 1,200 tons of fuels at ignition. Tanker empty and detached at about 2,500 m/s while fully fueled LSS continues on to the Lunar mission. Tanker falls back from apogee reenters to base.   

Skimmed through a lot of this discussion without seeing this possibility suggested.

Offline TomH

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1709 on: 10/03/2022 02:07 am »
You need less propellant to go to Mars than land on the Moon, let alone do the ~7-8km/s mission that Starship HLS needs. So no 1200t depot required. 3.7-3.9km/s or so (depending on opportunity… 3.5km/s for TMI, 200-400m/s for landing) is about the minimum amount. 500t would even be enough.

Ah, so the key things that make this possible (correct me if I'm wrong) are a) The Mars trip is one-way b) using the Mars atmosphere for braking saves you a lot of delta-v c) Although the trip to Mars takes much longer, the delta-v to go from LEO to a minimum energy TMI isn't really very much more than what's required for TLI. Is that a fair summary?

Regarding a), some Mars trips will be one way, while others are round trip. This does differ from Luna, however, in that propellant for the return trip, both fuel and oxidizer, will be manufactured on Mars. Lunar trips will offer no aerobraking and also will require return prop to be carried to the surface and re-launched from the surface. (I am a supporter of leaving some prop in LLO and repropping there prior to TEI.) So even though the moon is closer, it has two significant disadvantages when contrasted with Mars.
« Last Edit: 10/03/2022 03:00 am by TomH »

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1710 on: 10/03/2022 03:07 am »
As an aside but still topical for this in-orbit refueling discussion: We are focusing here on good strategies for orbits, ΔV, proximity operations, and fuel settling, all of which is fine. But the risk that worries me is the risk of chopsticks landing accidents. Fireballs have bad optics. Falcon 9 boosters land superbly, but soon SH/SS refueling needs even higher precision landings.
I'd rather be here now

Offline TheRadicalModerate

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1711 on: 10/03/2022 05:12 am »
If two fully fueled ships leave LEO docked with crossover plumbing, the attached tanker can empty in a very few minutes. Tanker with 1,500 tons of fuels and LSS with 1,200 tons of fuels at ignition. Tanker empty and detached at about 2,500 m/s while fully fueled LSS continues on to the Lunar mission. Tanker falls back from apogee reenters to base.   

Skimmed through a lot of this discussion without seeing this possibility suggested.

This architecture does get discussed from time to time.  The biggest problems are that it's very odd structurally, which makes it very odd from a control standpoint (i.e., keeping both ships pointed in the same direction with no significant torques), and the cross-feed plumbing, which has to pump 6.2t/s of mass flow, isn't straightforward.  Note that you'd also either have to double the tanker's amount of prop diverted to autogenous pressurization, or engineer a path to get pressurant from the cross-fed ship back to the tanker supplying the prop.

As an alternative, if you could swing the nose cone of the tanker away and dock it with the tail of your target Starship, everything works like a regular 2-stage rocket.  Then you can do the following conops:

1) Tanker launches and is completely refueled by a depot.
2) Tanker swings nosecone out of the way, exposing docking interface.
3) Tanker docks nose-to-tail with the target Starship.
4) Tanker pumps as much prop into the Starship as possible.
5) While still docked, tanker starts TLI (or TMI) burn.
6) When tanker is down to about 300m/s of delta-v, it shuts down and separates.
7) Target Starship immediately continues burn, does rest of mission.
8) Tanker coasts to apogee, does short burn to lower perigee to reentry altitude.
9) Tanker closes nose cone and does EDL.

But this requires engineering a nose-to-tail docking mechanism, which isn't straightforward.  I suspect that there were good reasons why SpaceX gave up on tail-to-tail docking, and this would be worse.  However, I also suspect that this would be easier to engineer than side-by-side burns with cross-feed.

Update:  Here's a three-year-old thread with the idea in a slightly different form.  And I'll add that this is nowhere near as simple as stretching the LSS tanks a bit.
« Last Edit: 10/03/2022 05:24 am by TheRadicalModerate »

Offline TheRadicalModerate

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1712 on: 10/03/2022 05:34 am »
As an aside but still topical for this in-orbit refueling discussion: We are focusing here on good strategies for orbits, ΔV, proximity operations, and fuel settling, all of which is fine. But the risk that worries me is the risk of chopsticks landing accidents. Fireballs have bad optics. Falcon 9 boosters land superbly, but soon SH/SS refueling needs even higher precision landings.

A more general way of capturing this is that there's a requirement for some average minimum cadence for the lift tankers over the course of the mission's refueling campaign.  If you blow up a Mechazilla, it goes out of service for repairs.  If it's the only Mechazilla, this is... bad... but if there's a second one, then your cadence degrades a lot more smoothly.  If you have a high risk of blowing up all of the landing systems within the course of a single campaign, then you have a more serious problem.

I think the bottom line on the risk is that SpaceX tried landing legs and didn't like the result.  It is absolutely a requirement that landing be sorta-kinda reliable.  If the chopsticks aren't reliable, Plan B is probably landing legs.  But I suspect that somebody did the math before moving forward with Plan A.

Offline redneck

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1713 on: 10/03/2022 08:25 am »
My thought on the side to side docked and cross feed is along the lines of the heavy versions of Delta and Falcon with two cores instead of three. If it were three, the mass ratio would put it beyond escape and getting the boosters back would be a problem. Plus the extra dry mass of another core. 

The tanker pressurant would be the same in either case to empty the tank of liquid whether it is in x minutes or 1/2x minutes.

It seems the tanker would not necessarily need a burn at apogee to put it on a reentry trajectory as long as the TLI trajectory was creating an HEEO with perigee at reentry altitude.

Offline tbellman

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1714 on: 10/03/2022 03:08 pm »
But this requires engineering a nose-to-tail docking mechanism, which isn't straightforward.  I suspect that there were good reasons why SpaceX gave up on tail-to-tail docking, and this would be worse.  However, I also suspect that this would be easier to engineer than side-by-side burns with cross-feed.

The reason given by Elon/SpaceX for giving up on tail-to-tail refilling, was that they realized that they needed a stabilizing arm at the bottom of the ship when it was standing on the launch pad.  They then realized that they could use that arm to fill the tanks of the ship, instead of having plumbing through the entire length of SuperHeavy, filling the ship through the same piping as the booster.  This probably saved them a couple of tonnes on SuperHeavy, and a bunch of valves and other complexity (moving that to stage zero).  And when they had propellant connectors on the side of the ship for filling it when on the launch pad, then they would of course use that when filling it on orbit as well.

I think Elon said this during Tim Dodd's first interview/tour of Starbase, but I don't have time to listen through it at the moment.  So the above is extracted just from my memory.

EDIT: Yes, in part 2 of the interview at 30:02 Elon talks about orbital refilling not being butt-to-butt, and starting at 34:57 they talk about the quick disconnect-arm being used to stabilize the stack.
« Last Edit: 10/03/2022 09:51 pm by tbellman »

Offline OTV Booster

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1715 on: 10/03/2022 08:55 pm »
If two fully fueled ships leave LEO docked with crossover plumbing, the attached tanker can empty in a very few minutes. Tanker with 1,500 tons of fuels and LSS with 1,200 tons of fuels at ignition. Tanker empty and detached at about 2,500 m/s while fully fueled LSS continues on to the Lunar mission. Tanker falls back from apogee reenters to base.   

Skimmed through a lot of this discussion without seeing this possibility suggested.
How fast props can be transferred is an open question. IIUC, you're suggesting the two ships be locked together while thrusting. This is done with FH at launch but my gut says that this is different enough to need some serious testing. Not a deal breaker but it is something new in the mix.


I've been noodling an elliptical orbit with apogee at L1. It only takes a puff to stop and it's literally down hill in either direction.
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Offline OTV Booster

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1716 on: 10/03/2022 08:59 pm »
As an aside but still topical for this in-orbit refueling discussion: We are focusing here on good strategies for orbits, ΔV, proximity operations, and fuel settling, all of which is fine. But the risk that worries me is the risk of chopsticks landing accidents. Fireballs have bad optics. Falcon 9 boosters land superbly, but soon SH/SS refueling needs even higher precision landings.
We need to understand the mission before we can judge what details even need to be considered. All on topic, IMO.
We are on the cusp of revolutionary access to space. One hallmark of a revolution is that there is a disjuncture through which projections do not work. The thread must be picked up anew and the tapestry of history woven with a fresh pattern.

Offline TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1717 on: 10/03/2022 09:36 pm »
My thought on the side to side docked and cross feed is along the lines of the heavy versions of Delta and Falcon with two cores instead of three. If it were three, the mass ratio would put it beyond escape and getting the boosters back would be a problem. Plus the extra dry mass of another core.

Applying lots of shear stress to something that docked, rather than something that was pre-stressed on the ground with heavy equipment, seems pretty challenging.  The only way I can think that a tandem burn would work is if the accelerations of both ships are identical.

Quote
The tanker pressurant would be the same in either case to empty the tank of liquid whether it is in x minutes or 1/2x minutes.

It's the "x minutes vs. 1/2x minutes" that's the problem.  1/2x implies double the pressurant mass flow (rate), and it all has to go in the tanker if you want to preserve the prop in the target Starship.

That pressurant is tapped off from the Raptor pumps, fed through a heat exchanger, and then piped up to the ullage space in each tank.  If you're pumping out prop at twice the rate (for both the tanker and the target Starship), either the engines on the tanker need to handle twice the tap-off flow (a decent-sized design change) or you need to import hot ullage gas from the engines on the target Starship (more complex inter-ship plumbing).

Quote
It seems the tanker would not necessarily need a burn at apogee to put it on a reentry trajectory as long as the TLI trajectory was creating an HEEO with perigee at reentry altitude.

If you do a burn from a circular orbit or from perigee, the TLI can't have a lower perigee; it can only have a higher  apogee.

If you apply 2500m/s of delta-v to a pair of mated spacecraft in a 300x300km orbit, you wind up with the tanker in a 300x40,800km orbit.  If you then want to lower your 300km perigee to, say, 70km for entry, you need to do an apogee burn with a retrograde delta-v of 22m/s.  Not a lot, but not zero.

Offline edzieba

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #1718 on: 10/04/2022 08:20 am »
If two fully fueled ships leave LEO docked with crossover plumbing, the attached tanker can empty in a very few minutes. Tanker with 1,500 tons of fuels and LSS with 1,200 tons of fuels at ignition. Tanker empty and detached at about 2,500 m/s while fully fueled LSS continues on to the Lunar mission. Tanker falls back from apogee reenters to base.   

Skimmed through a lot of this discussion without seeing this possibility suggested.
How fast props can be transferred is an open question. IIUC, you're suggesting the two ships be locked together while thrusting. This is done with FH at launch but my gut says that this is different enough to need some serious testing. Not a deal breaker but it is something new in the mix.
Falcon Heavy does not perform propellant cross-feed. This was dropped as being 'too complex' for insufficient benefit in actual performance.
As far as I am aware, no vehicle has flown with cross-feed between stages both under power, with the closest being feeding from a dedicated tank structure to a dedicated engine structure with the two being in-flight separable (STS, stage-and-a-half Atlas).

Offline redneck

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #1719 on: 10/04/2022 09:06 am »
What I would like is an extremely lightweight drop tank (20 ton tank 1,200 ton prop???) mated to the LSS as it leaves LEO. I don't see that being feasible short term due to the logistics of getting it in orbit. Plus development, attachment and several other issues. Possibly not even long term as it might not make any sense with Lunar ISRU. Trying to not engage in too much handwavium engineering.


Tags: HLS 
 

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