Author Topic: Starship On-orbit refueling - Options and Discussion  (Read 639817 times)

Online TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2100 on: 01/27/2023 04:26 am »
Another factor I didn't consider is plumbing. Apparently cryogenic plumbing is really hard to make work. Every bend in a pipe and every meter of pipe is a potential problem. So proposals that involve running a lot of cryogens through a lot of pipe are generally dead-on-arrival. Nose-to-nose proposals either require really long external pipes or entirely new piping inside the rocket.

The two biggest problems I can see with this for crewed systems, specifically LSS, are:

1) The way the prop transfer docking is arranged interferes with the way the crew-transfer docking is assumed to work.  LSS has no header tanks in the nose, so putting the IDSS-compliant dock in the nose seems like a no-brainer.  Getting that to play nice with the prop transfer seems difficult.

2) Having cryo piping going through the walls of a crew compartment sounds like something hard to get crew-certified.  LSS doesn't need to do this, because it has no header tanks.  A Starship crew-certified for launch and EDL would need to do this, but that just means it's yet another on a long list of things that will be incredibly hard to get launch/EDL certified.  It's certainly not something you'd want to add to your v1.0 refueling architecture, especially since LSS doesn't work without the v1.0 refueling architecture.

Offline mikelepage

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2101 on: 01/27/2023 06:39 am »
One of the biggest issues is that they have to minimize the number of "things no one has ever done before." If we're just talking blue-sky stuff that might be in use in 2100, that's different, but we're talking about what SpaceX might do in the next three or four years. That really rules out the most interesting rotational systems. In fact, there has been so little study of rotating systems in space that merely figuring out what you have to do to make a stable system is a nontrivial task.

Another factor I didn't consider is plumbing. Apparently cryogenic plumbing is really hard to make work. Every bend in a pipe and every meter of pipe is a potential problem. So proposals that involve running a lot of cryogens through a lot of pipe are generally dead-on-arrival. Nose-to-nose proposals either require really long external pipes or entirely new piping inside the rocket.

For the configuration you propose, there's the extra challenge that the depot will have anywhere from 0 to 1500 tons of fuel in it, but the tanker will only have 150, so this thing is going to be awfully unbalanced, and the center of rotation is going to change during fueling. There might be a way to make that work, but, again, it falls into the category of "stuff no one has ever done."

Just considering what happens during the Starship landing swoop, which was practiced with SN8, SN9, SN10, SN11 and SN15, and which SpaceX will have to nail on at least a number of further Starship landings: Cryogenic oxygen has to travel/be pumped the length of Starship from the nose header tank to the engines, whilst the whole structure is rotating through the same spin direction as I've proposed for prop transfer.

Starship's key goal of full reusability has led to a configuration which will be pretty good at "pumping cryogenic propellant through long pipes while rotating". Sure, it hasn't been demonstrated in space, but the principle has been demonstrated in arguably more demanding conditions than what it would experience in space.

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By contrast, ullage burns are well-established technology for settling propellant before an engine burn. For that purpose, they work at amazingly low accelerations--the sort of thing you could do for an hour without burning much fuel. It's a bit of a leap of faith that it will work for refueling, but it seems plausible with a lot less risk. Also, refueling can tolerate more sloshing than an engine burn, since it's not the end of the world if a propellant pump occasionally sucks some gas or a gas pump occasionally sucks some fluid--assuming you designed for that.

So when you've just got a few years to make this work, and you've got a choice between a technology that you're pretty sure will work at an acceptable cost vs one that could take ten or twenty years to work the bugs out, it's obvious why only the former gets serious attention.

I do hear what you're saying, really. But I'd argue this isn't a first-principles approach to the problem, and the difference in current TRL is not going to be that big a deal to SpaceX (*coughs* landing boost stages, steel rockets, mechazilla).

For the sake of argument, I had a go at calculating how much propellant it would take to spin up a 1850 metric ton Depot/tanker combo docked (offset) nose to nose as per the diagram in my post to 0.5rpm (1% of G).

Assuming 4x 300s hot gas thrusters (455N each), and calculating a total moment of inertia of 1,248,750,000 kg.m2 for the whole system. I got a total spin-up time of 735s (12.2 minutes), and a total propellant mass use of ~460kg per spin-up / spin-down. It should actually be somewhat better than this in some situations, because the offset centre of mass would give one set of thrusters a longer lever arm and better torque, but let's say 1 ton of prop used per prop transfer.

Using the Radical Moderate's spreadsheet calculator, I found that you have to use hot gas thrusters with half hour transfer times and <1.5mm/s settling acceleration before linear acceleration beats this.

In the TRM's "worst case" post above with a depot in a 3 hour (300x8000km) HEEO - I can't see where I'm doing the math wrong, because I can only get 15 ton of payload of prop to the depot in that orbit (assuming isp=380s, 100 ton tanker dry weight, 7.7km/s LEO, then 1.3km/s transfer burn) - Whatever the case, propellant only gets more precious the further away from LEO you are, and it seems a little absurd to throw it away if you don't need to.

Offline mikelepage

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2102 on: 01/27/2023 06:47 am »
Another factor I didn't consider is plumbing. Apparently cryogenic plumbing is really hard to make work. Every bend in a pipe and every meter of pipe is a potential problem. So proposals that involve running a lot of cryogens through a lot of pipe are generally dead-on-arrival. Nose-to-nose proposals either require really long external pipes or entirely new piping inside the rocket.

The two biggest problems I can see with this for crewed systems, specifically LSS, are:

1) The way the prop transfer docking is arranged interferes with the way the crew-transfer docking is assumed to work.  LSS has no header tanks in the nose, so putting the IDSS-compliant dock in the nose seems like a no-brainer.  Getting that to play nice with the prop transfer seems difficult.

2) Having cryo piping going through the walls of a crew compartment sounds like something hard to get crew-certified.  LSS doesn't need to do this, because it has no header tanks.  A Starship crew-certified for launch and EDL would need to do this, but that just means it's yet another on a long list of things that will be incredibly hard to get launch/EDL certified.  It's certainly not something you'd want to add to your v1.0 refueling architecture, especially since LSS doesn't work without the v1.0 refueling architecture.

Admittedly I've never put too much stock in the claim they'll remove the nose header tank in order to place an IDSS compliant dock in the nose, because it's never been clear to me how that would work with the heat-shield for EDL. Any re-entry will still need a skydiver maneuver, ergo the dock has to be offset, ergo you might as well leave the header tank where it is.

LSS is never intended to return, so I can see them going down a path where they remove the tank and put in the nose dock as you say, but then do they have a hinged, heat-shielded, nose-cone arrangement on every other Starship that needs to EDL so they can keep a common heritage across their vehicles?

As for putting cryo through the crew comparment... well there is no shortage of vacuum in space. 
« Last Edit: 01/27/2023 06:52 am by mikelepage »

Online edzieba

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2103 on: 01/27/2023 09:44 am »
Complicated:  Yes, you can do this only once at the beginning, but in a worst-case transfer (e.g. 1200t from depot to target Starship), and with the assumption that you have 5% ullage space in a full tank and you don't vent down to below half a bar, you wind up with 10.5bar in the receiving tank.  So you're venting multiple times, and you're pressurizing your sending tank to a pretty high value.
Only if you try and one-shot every transfer. Multiple steps with venting in between is not a high complexity operation, and requires bang-bang control of a single vent valve and a single main liquid valve to perform. It is hard to get more simple than that. The 'complexity' of ullage gas regeneration is one that is mandatory to solve anyway for Starship, in order to allow coasts to destinations that require more than the header tank capacity alone at the destination (e.g. HLS) as all autogenous ullage generated during any initial burns would otherwise condense.
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Slow:  In a blow-down tank (which is essentially what you've created here), and assuming that all of your pressure difference can be converted to dynamic pressure, then the volumetric flow rate from one tank to the other is proportional to the square root of the pressure difference.  (Δp = ½⍴v², and v = volumetricFlowRate / (π*rPipe²).  This doesn't include static drops due to viscosity, but they're tiny.)  So as the difference drops, so does your flow rate.  I'm too lazy and stupid to integrate this, but the upshot is you need to do, if not closed-loop control, then lots of vents to keep the pressures reasonable (see "complicated" above).
See also: "not complicated", above for multiple transfer steps.
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Wasteful:  Unless you're willing to vent all of the receiver ullage into a propulsion system on both ships that delivers the ullage thrust (which coincidentally would require exactly the same plumbing you'd need for equalized ullage pressure, but with more valves), then you're dumping a few tonnes of otherwise perfectly good prop into space.
If you utilise a vent routed locally downwards, every vent is some extra free settling thrust. The small (e.g. an entire methane tank at 1 Bar CH4 vented to vacuum is less than 400kg) overall mass loss is not a major concern and is greatly overstated.
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Error-prone:  Uncover the sender's side of the transfer pipe even for a moment, and you've equalized pressures.  Then you get to start all over, with more complexity and waste.  And of course you have to mitigate whatever consequences result from a high-speed gas flow.  (I hereby coin the term "depot fart" to describe this phenomenon.)
An issue shared with pump transfer. If the inlet is uncovered, you also risk having to pick bits of pump out of your teeth. A pressure-based transfer is a single valve closure away from complete system shutdown. A pump-based transfer requires graceful pump shutdown to avoid a catastrophic system failure - resulting in anywhere from loss of prop transfer capability (loss of mission capability) to outright loss of both vehicles.

Offline colbourne

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2104 on: 01/27/2023 12:11 pm »
It still seems to me having read this thread that it would be a lot easier to transfer full tanks of fuel (maybe with their own rocket engines) to the Starship, rather than trying to transfer the fuel. There would be some weight penalty , but the simplicity and time saved would probably compensate for this especially if their is venting of gas during the refueling and storage.

Offline Greg Hullender

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2105 on: 01/27/2023 07:30 pm »
It still seems to me having read this thread that it would be a lot easier to transfer full tanks of fuel (maybe with their own rocket engines) to the Starship, rather than trying to transfer the fuel. There would be some weight penalty , but the simplicity and time saved would probably compensate for this especially if their is venting of gas during the refueling and storage.
I like that idea too, but, again, you're building something new that has to get designed, built, and tested. Something with new, smaller tanks, a new rocket engine, a new guidance system, etc. Nothing about it is reused from the other work on Starship. It has to somehow get back into the cargo hold for the trip back down as well.

At least to start with, it just looks a whole lot easier to make relatively small modifications to Starship to make it serve as both tanker and depot. And if that costs you something in fuel, well, fuel is cheap. Once the whole system is working, then it'll make sense to study it and find ways to optimize it. But it really looks like the quickest, safest way to build something that'll actually work is to just make three Starship variants.

Offline Oersted

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2106 on: 01/27/2023 08:51 pm »
The best (only?) way for Starships to dock will be back to back, or dorsal to dorsal if you prefer. The dorsal area is the area not covered in tiles and the area least affected by eventual weak spots or protuberances resulting from the docking ports.

That also means the minimum length of tunnels for crew transfers and pipes for fuel/oxidiser transfers, because the crew compartments and the tanks will all be nestled next to each other.

Will a pitch axis rotation of two ships docked dorsally enable a gravity fuel transfer?   
« Last Edit: 01/27/2023 08:51 pm by Oersted »

Online TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2107 on: 01/27/2023 08:54 pm »
The 'complexity' of ullage gas regeneration is one that is mandatory to solve anyway for Starship, in order to allow coasts to destinations that require more than the header tank capacity alone at the destination (e.g. HLS) as all autogenous ullage generated during any initial burns would otherwise condense.

Yes, I agree completely.  But unless you're planning on running all of the things for which you need gas out of the mains (and you can't get high-pressure gas¹ that way), you're going to have to move liquid into a COPV and heat it.  And you know what you need to do that?  A pump.  Why not use it to transfer prop as well?

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If you utilise a vent routed locally downwards, every vent is some extra free settling thrust. The small (e.g. an entire methane tank at 1 Bar CH4 vented to vacuum is less than 400kg) overall mass loss is not a major concern and is greatly overstated.

It's 1.8t for both tanks, and 1bar is a reasonable number for transfer into the depot.  But a target Starship is going to arrive with tanks close to flight pressure, so at least quadruple that.  Unless you're going to wait for them to undergo condensation collapse before transfer?  Not a great thing to do in HEEO.  Not even a great thing to do in LEO or NRHO if you have a crewed refueling.

I agree that it's not the end of the world.  But why would you do this if you didn't have to?
 
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An issue shared with pump transfer. If the inlet is uncovered, you also risk having to pick bits of pump out of your teeth.

You're thinking about this like a rocket engine turbopump.  It's not:  it's an electric pump that has to generate about 0.1bar of pressure.  It draws as much power as a hairdryer or two.  It's not going to overspin, and even if it did, nothing would happen.  And if you're positing a valve control loop that can mitigate a depot fart, you can certainly come up with an electric servo that's responsive enough stop feeding juice into an unprimed pump.

More importantly, you need this pump no matter what, for filling COPVs to produce high-pressure gas.  They don't weigh very much; build in redundancy.  You'll need it for the high-pressure applications anyway, because if you can't generate supercritical gas then the mission fails.

_________________________________
¹Things off the top of my head that need GCH4 or GOX at pressures higher than 4-6bar:

1) Spin-start gas--quite a bit of it, at well more than 6bar.
2) Cold/warm gas thrusters with Isp > 20s.
3) Blow-down for methox or methalox combustion thrusters.  Might not be needed but I'm betting they will.  Hard to get the needed ullage thrust without them.  And of course there are the LSS landing thrusters, if they happen.
4) Ullage pressurization on demand.
5) I'm betting they'll have an APU onboard by Option B or SLT.  It takes surprisingly little methalox to generate a few kW for long durations in the dark.
« Last Edit: 01/27/2023 09:16 pm by TheRadicalModerate »

Online TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2108 on: 01/27/2023 09:13 pm »
The best (only?) way for Starships to dock will be back to back, or dorsal to dorsal if you prefer. The dorsal area is the area not covered in tiles and the area least affected by eventual weak spots or protuberances resulting from the docking ports.

That also means the minimum length of tunnels for crew transfers and pipes for fuel/oxidiser transfers, because the crew compartments and the tanks will all be nestled next to each other.

I think you're right for crew transfer on launch/EDL crew-certified Starships.  But for Option A/B/SLT lunar Starships, I doubt it.  Those don't have thermal tile issues, and I shudder to think what the Orion proximity ops software would make of trying to figure out what to do when approaching what looks like a 9 meter infinite wall.  You also need to think about the limited docking real estate at the Gateway.

Stuff's gonna stay on-axis for a while.

Also, I don't think high-flow cryogenic fluid transfer in close proximity to a crew tunnel is a feature.  My bet is that prop transfer docking and crew docking are two completely separate systems.

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Will a pitch axis rotation of two ships docked dorsally enable a gravity fuel transfer?

For pitch or yaw axis, you'd have to keep the center of mass forward of the tanks to keep them settled at the bottom.  That'll be hard to do.

Roll axis is possible, but you'll have sump slosh problems and the sumps will be on the TPS side, which isn't great.

Offline OTV Booster

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2109 on: 01/28/2023 07:04 pm »
Another factor I didn't consider is plumbing. Apparently cryogenic plumbing is really hard to make work. Every bend in a pipe and every meter of pipe is a potential problem. So proposals that involve running a lot of cryogens through a lot of pipe are generally dead-on-arrival. Nose-to-nose proposals either require really long external pipes or entirely new piping inside the rocket.

The two biggest problems I can see with this for crewed systems, specifically LSS, are:

1) The way the prop transfer docking is arranged interferes with the way the crew-transfer docking is assumed to work.  LSS has no header tanks in the nose, so putting the IDSS-compliant dock in the nose seems like a no-brainer.  Getting that to play nice with the prop transfer seems difficult.

2) Having cryo piping going through the walls of a crew compartment sounds like something hard to get crew-certified.  LSS doesn't need to do this, because it has no header tanks.  A Starship crew-certified for launch and EDL would need to do this, but that just means it's yet another on a long list of things that will be incredibly hard to get launch/EDL certified.  It's certainly not something you'd want to add to your v1.0 refueling architecture, especially since LSS doesn't work without the v1.0 refueling architecture.

Admittedly I've never put too much stock in the claim they'll remove the nose header tank in order to place an IDSS compliant dock in the nose, because it's never been clear to me how that would work with the heat-shield for EDL. Any re-entry will still need a skydiver maneuver, ergo the dock has to be offset, ergo you might as well leave the header tank where it is.

LSS is never intended to return, so I can see them going down a path where they remove the tank and put in the nose dock as you say, but then do they have a hinged, heat-shielded, nose-cone arrangement on every other Starship that needs to EDL so they can keep a common heritage across their vehicles?

As for putting cryo through the crew comparment... well there is no shortage of vacuum in space.
I think you're over interpreting commonality. SH and SS are the best existing example. Ignoring everything on SS from the cargo space on up and everything having to do with EDL, they have high commonality.


They have the same engines with vacuum optimization where needed. They are made of the same materials assembled with with the same tooling to the same diameter. Same rings with some thickness differences. Same upper dome (I think). Different common and bottom dome/thrust structure due to the transfer tube and different engine count. I haven't checked but expect that the stringers are the same design with differing lengths and placement. They are two variations on one general design. Commonality does not mean exactly the same.


Header tanks are doodads that come and go with need. The automotive industry handles bigger differences on an hourly basis at a production rate orders of magnitude higher than anything SS will ever see. They use build sheets for each vehicle that have tentacles that reach from engineering to marketing to purchasing to production. SX will do the same.


I'm about to post to the engineering thread about human rated ships and header tanks.
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Offline tbellman

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2110 on: 01/28/2023 10:04 pm »
It still seems to me having read this thread that it would be a lot easier to transfer full tanks of fuel (maybe with their own rocket engines) to the Starship, rather than trying to transfer the fuel. There would be some weight penalty , but the simplicity and time saved would probably compensate for this especially if their is venting of gas during the refueling and storage.

Absolutely not.  This is a terrible idea.

Full tanks of Starship is about 260 tonnes of liquid methane, and 940 tonnes of liquid oxygen.  But you need to split that up in multiple smaller tanks that can be lifted to orbit by a tanker Starship.  Just the additional "endcaps" of all those tanks will be several tens of tonnes.  And the dome shape of those endcaps will force the entire ship to be longer (there will be wasted space between the tanks), adding to the mass.

The design of the ship would be entirely different from how it is now.  You would need some kind of backbone truss/beam onto which the tanks are mounted, and each tank also needs "skirts" covering the domes, so the ships profile will still be a cylinder (for aerodynamic purposes), and to carry the TPS tiles.  Those skirts needs to be connected together so there isn't a gap between them letting in hot plasma during EDL.  All this adds to the mass that needs to be lifted by the "tanker" ships (and presumably also brought down back to the surface).

Alternatively, you would design the ship as a large empty tube, with TPS tiles covering one side, and the other side being huge doors through which the smaller tanks can be removed and inserted.  But that will effectively turn the ship into a double-walled ship, which increases its dry mass significantly.

And the mechanics of unmounting empty tanks and mounting new full tanks, and coupling them together, sounds like a nightmare to me.

Offline colbourne

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2111 on: 01/29/2023 01:43 am »
It still seems to me having read this thread that it would be a lot easier to transfer full tanks of fuel (maybe with their own rocket engines) to the Starship, rather than trying to transfer the fuel. There would be some weight penalty , but the simplicity and time saved would probably compensate for this especially if their is venting of gas during the refueling and storage.

Absolutely not.  This is a terrible idea.

Full tanks of Starship is about 260 tonnes of liquid methane, and 940 tonnes of liquid oxygen.  But you need to split that up in multiple smaller tanks that can be lifted to orbit by a tanker Starship.  Just the additional "endcaps" of all those tanks will be several tens of tonnes.  And the dome shape of those endcaps will force the entire ship to be longer (there will be wasted space between the tanks), adding to the mass.

The design of the ship would be entirely different from how it is now.  You would need some kind of backbone truss/beam onto which the tanks are mounted, and each tank also needs "skirts" covering the domes, so the ships profile will still be a cylinder (for aerodynamic purposes), and to carry the TPS tiles.  Those skirts needs to be connected together so there isn't a gap between them letting in hot plasma during EDL.  All this adds to the mass that needs to be lifted by the "tanker" ships (and presumably also brought down back to the surface).

Alternatively, you would design the ship as a large empty tube, with TPS tiles covering one side, and the other side being huge doors through which the smaller tanks can be removed and inserted.  But that will effectively turn the ship into a double-walled ship, which increases its dry mass significantly.

And the mechanics of unmounting empty tanks and mounting new full tanks, and coupling them together, sounds like a nightmare to me.
Starship can lift about 100 tonnes to LEO. Each trip will be using a tank with  endcaps , so you have already paid the weight price. For the trip to Mars there will be some penalty, but these empty tanks can be jettisoned (hopefully to be reused or used as valuable scrap on Mars in the future) and do not need to be streamlined (They would have to be streamlined for the trip to LEO).
I would see only a small Mars lander Starship making the whole trip to Mars.

Online TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2112 on: 01/29/2023 03:55 am »
Starship can lift about 100 tonnes to LEO. Each trip will be using a tank with  endcaps , so you have already paid the weight price. For the trip to Mars there will be some penalty, but these empty tanks can be jettisoned (hopefully to be reused or used as valuable scrap on Mars in the future) and do not need to be streamlined (They would have to be streamlined for the trip to LEO).
I would see only a small Mars lander Starship making the whole trip to Mars.

This is certainly a debatable point, but it shouldn't be debated in a Starship refueling thread.  You're not going to refuel a Starship this way.  The architecture won't support it.

Offline Hog

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2113 on: 01/29/2023 07:03 pm »
Why are we pumping fluids, when we can be loading blocks of solid propellant, yes solid propellant, "but it's not what you think." 

Let's solidify that O2 and CH4. The Ship pulls up to a refueler and loads a certain quantity of propellants, the Ship backs away, methane/oxygen "melts" into subcooled props and following a quick ullage kick, away she goes firing the Raptors.

What's a kilogram of methane ice worth?  oxygen ice?
Paul

Online TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2114 on: 01/29/2023 07:42 pm »
Why are we pumping fluids, when we can be loading blocks of solid propellant, yes solid propellant, "but it's not what you think." 

Let's solidify that O2 and CH4. The Ship pulls up to a refueler and loads a certain quantity of propellants, the Ship backs away, methane/oxygen "melts" into subcooled props and following a quick ullage kick, away she goes firing the Raptors.

What's a kilogram of methane ice worth?  oxygen ice?

So... you're going to put doors on the main tanks?  The densities of solid oxygen and methane are a little bit higher than the liquids, but not by a huge amount.  The blocks of prop are going to be big.  How are you planning on making those big doors reliably leak-proof at 6bar?

Nobody has ever frozen CH4 and O2 at high scale, and they certainly haven't done it in a high-temperature vacuum.  Nor have they frozen it on the ground and launched it as a solid.

Cryogenic pumps aren't exactly new technology.  Adapting them to work in microgravity is trivial compared to developing whole new orbital industrial processes.

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2115 on: 01/30/2023 02:00 pm »
Also, I don't think high-flow cryogenic fluid transfer in close proximity to a crew tunnel is a feature.  My bet is that prop transfer docking and crew docking are two completely separate systems.

Not meaning to rehash this excessively, but the main advantage I was aiming to point out was that spin-G settling acceleration means it wouldn’t have to be high flow. You could take 18+ hours to pump it over with at a relatively tiny flow rate, and still be launching tankers daily. I’m picturing the walls of the LOX header downcomer pipe embedded with a set of 4 relatively tiny pipes, then surround the whole structure with a shell pipe that gets vented to vacuum once in space. Also, if you could size your pumping system for those tiny flow rates, how much power would that save? It’s not like on the ground where ambient temperatures will lead to unacceptable levels of off-gassing if you don’t move it fast enough.

Offline OTV Booster

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2116 on: 01/30/2023 09:35 pm »
Pumps will not cavitate as long as the ullage pressure is large enough.

Good point.  So you can still have equalized pressure between the two ullage spaces, as long as the absolute pressure is a few bar, correct?
If you already have high ullage pressures, then as calculated above you can omit the pumps entirely and move fluids by pressure difference along. Can't cavitate pumps if you don't have any pumps.

There's a fundamental difference between high ullage pressures and high ullage pressure differences.  The first will eliminate cavitation.  The second will move propellant.  If you're going to pressure-feed, you need to manage the ullage pressures in both tanks, via venting (on the receiver) and heating (on the sender).

I'm still struggling a bit trying to figure out pump power requirements, so really big numbers there could change my mind.  But unless that happens, pressure-feeding sounds insanely more complicated than equalizing pressures, especially since the QD has pre-press plumbing built into it.
Equalising pressures during pumping requires continuous ullage gas generation for the sender tank, and continuous venting for the receiver tank. This requires continuous closed-loop control of both gas generation and tank venting throughout the entire transfer process.

Pressure transfer requires pressurising the sender tank once at the start of transfer, venting the receiver tank once at the start of transfer, then opening the inter-tank valve(s) to allow fluid to flow. No additional venting or pressurisation is required during transfer. In the event of an extreme fluid volume transfer (e.g. completely full sender to completely empty receiver) then once transfer ceases (pressure equalised) the tanks can be isolated again, the sender repressurised, the receiver vented, and the inter-tank valve opened again to repeat the process. Venting and pressurisation are "run to completion" processes with no direct constraints on pressurisation/venting time or rate.
ISTM that continuous venting of the receiver during pressure powered transfer would simplify the operation. If parasitic losses cause things to stall out before completion the COPV's on the supply side could add more ullage pressure.


That said, has anyone looked at how this would line up with cold or hot gas consumption for settling thrust? The receiver ullage would be lost anyway whether continuous or pulsed. Might as well make use of it if it adds enough thrust to make the added complexity worth while. I've the echo of a memory of this being asked but no memory of a quantitative answer.
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Offline Twark_Main

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2117 on: 01/30/2023 11:06 pm »
.This loses some amount of gaseous prop from venting back down from equilibrium to the target low pressure, but that mass loss is relatively small: e.g. for CH4 assuming a 3 Bar sender tank pressure and the absolute worst case of the entire tank volume to be vented, the maximum mass loss is around 1.5 tonnes.

Except that's not the "absolute worst case," because (as you point out) liquid propellant will be constantly evaporating to replenish the ullage gas.

It's also not the "absolute worst case" because we don't know if it will actually reliably transfer 100% of the propellant. If some is missed that needs to be counted, and even a small amount of liquid could exceed 1.5 tonnes.
Only when in a vacuum state. The vents would not be opened again until the tank has reached pressure equilibrium with the sender tank, and then settles, so venting would not lose liquids (settled so no mixed phase) and pressure would not be allowed to drop before the triple point (so no flash boiling).

"Flash boiling" isn't the only kind of boiling. Lower pressure will always cause an increase in evaporation rate, even before hitting the vapor pressure.

You misunderstood my point about liquid. I wasn't saying that liquid would be vented, but rather that some might be left in the tank.

You also missed the ullage thrusters. If the technique works but takes a lot longer, then that additional ullage prop mass should also be accounted for.
By transferring at a high flow rate and then settling with a sealed tank, you can minimise the time at the maximum settling thrust needed (to keep the inlet covered on the sender tank) and then switch to the minimum thrust needed for the post-transfer sealed tank settling (equivalent to a cost-phase settling thrust). This is opposed to needing to keep sufficient thrust to both keep the sender tank inlet covered and keep the receiving tank from geysering for the entire transfer duration.

Again, you're missing my point. It has yet to be shown that this doesn't take more time vs. using pumps. More time = more ullage propellant used.
« Last Edit: 01/30/2023 11:24 pm by Twark_Main »

Online TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2118 on: 01/31/2023 09:05 pm »
Also, if you could size your pumping system for those tiny flow rates, how much power would that save? It’s not like on the ground where ambient temperatures will lead to unacceptable levels of off-gassing if you don’t move it fast enough.

If you're doing a full depot-to-target transfer (1600t depot transferring 1200t of prop to a target with 0t of prop), a 1kW pump can do the transfer sequentially (i.e., first LOX, then methane) in 4 hours 44 minutes.¹  If the same transfer takes 18 hours, the pump only has to be 200W.  If you're solar-panel-limited, that might be important.  If you're battery-limited, it's less important.

Note that there are lots of reasons to do faster transfers:

1) Minimizes prop consumption for ullage thrust (the big advantage of a rotating scheme).

2) Gives you more flexibility in terms of orbital dynamics.  The big one here is probably HEEO-based refueling, where you'd like to do the bulk of refueling near apogee, so the Starship is ready to do a departure burn by perigee.  Note also that a GTO-sized HEEO has a period of about 10 hours, so 18+ likely requires 3 orbits, each of which makes two transits through the Van Allen Belts.  (FWIW, I don't think HEEO refueling will be happening any time soon, but that's been extensively litigated up-thread.  Let's just say that it's an option that you wouldn't want to preclude and move on.)

3) I agree that boil-off isn't a huge issue, but it's not a trivial one, especially if you're transferring prop to a target without good boil-off properties, or if you're transferring to a target whose mission plan needs an amount of prop that's close to needing an integral number of tankers, and a little boil-off could force an extra launch.

4) When you finally get to crewed transfers, minimizing the amount of time that humans are literally bouncing off the walls while strapped to a giant bomb might be nice. 

PS:
_____________
¹Assumes a 15cm pipe and 5mm/s² acceleration.
« Last Edit: 01/31/2023 11:19 pm by TheRadicalModerate »

Offline mikelepage

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2119 on: 02/03/2023 07:31 am »
Also, if you could size your pumping system for those tiny flow rates, how much power would that save? It’s not like on the ground where ambient temperatures will lead to unacceptable levels of off-gassing if you don’t move it fast enough.

If you're doing a full depot-to-target transfer (1600t depot transferring 1200t of prop to a target with 0t of prop), a 1kW pump can do the transfer sequentially (i.e., first LOX, then methane) in 4 hours 44 minutes.¹  If the same transfer takes 18 hours, the pump only has to be 200W.  If you're solar-panel-limited, that might be important.  If you're battery-limited, it's less important.

Note that there are lots of reasons to do faster transfers:

1) Minimizes prop consumption for ullage thrust (the big advantage of a rotating scheme).

2) Gives you more flexibility in terms of orbital dynamics.  The big one here is probably HEEO-based refueling, where you'd like to do the bulk of refueling near apogee, so the Starship is ready to do a departure burn by perigee.  Note also that a GTO-sized HEEO has a period of about 10 hours, so 18+ likely requires 3 orbits, each of which makes two transits through the Van Allen Belts.  (FWIW, I don't think HEEO refueling will be happening any time soon, but that's been extensively litigated up-thread.  Let's just say that it's an option that you wouldn't want to preclude and move on.)

3) I agree that boil-off isn't a huge issue, but it's not a trivial one, especially if you're transferring prop to a target without good boil-off properties, or if you're transferring to a target whose mission plan needs an amount of prop that's close to needing an integral number of tankers, and a little boil-off could force an extra launch.

4) When you finally get to crewed transfers, minimizing the amount of time that humans are literally bouncing off the walls while strapped to a giant bomb might be nice. 

PS:
_____________
¹Assumes a 15cm pipe and 5mm/s² acceleration.

Yikes, you weren't kidding about "extensive" - I just read from page 80 up to the present, only having occasionally lurked on this thread up until this month.

Re number 2) HEEO-based refuelling: I think specifically, the conops where you fill up LSS in vLEO, then top it up during a single orbit in vLEO+1600-2000 m/s with a buddy tanker, rendezvousing shortly after the first burn - seems to me the most obvious way to handle that case, and as good a reason as any to set a lower limit on how slowly the system will pump propellant. In that case you would need to transfer 500-600t of prop during a 3-4 hour orbit (~50kg/s). So presumably the pump power can be less than 1kW, but not by very much.

My other thought while reading the thread was around whether the effect of milli-G acceleration - if done in a "spiral in" direction during this single orbit in vLEO+1600-2000m/s - could cause an appreciable reduction of TLI delta V. You could justify significantly higher than 5mm/s² acceleration if it was all adding to your velocity at perigee. You might not be getting the Oberth multiplier, but it wouldn't be "wasted", even so.

« Last Edit: 02/03/2023 07:32 am by mikelepage »

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