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.
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."
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.
Quote from: Greg Hullender on 01/27/2023 12:40 amAnother 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.
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.
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).
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.
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.)
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.
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.
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.
An issue shared with pump transfer. If the inlet is uncovered, you also risk having to pick bits of pump out of your teeth.
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?
Quote from: TheRadicalModerate on 01/27/2023 04:26 amQuote from: Greg Hullender on 01/27/2023 12:40 amAnother 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.
Quote from: colbourne on 01/27/2023 12:11 pmIt 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.
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?
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.
Quote from: TheRadicalModerate on 01/25/2023 09:45 pmQuote from: edzieba on 01/25/2023 08:12 amQuote from: TheRadicalModerate on 01/25/2023 04:01 amQuote from: eriblo on 01/24/2023 11:18 pmPumps 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.
Quote from: edzieba on 01/25/2023 08:12 amQuote from: TheRadicalModerate on 01/25/2023 04:01 amQuote from: eriblo on 01/24/2023 11:18 pmPumps 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.
Quote from: TheRadicalModerate on 01/25/2023 04:01 amQuote from: eriblo on 01/24/2023 11:18 pmPumps 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.
Quote from: eriblo on 01/24/2023 11:18 pmPumps 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?
Pumps will not cavitate as long as the ullage pressure is large enough.
Quote from: Twark_Main on 01/18/2023 11:27 amQuote from: edzieba on 01/18/2023 11:18 am.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).
Quote from: edzieba on 01/18/2023 11:18 am.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.
.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.
Quote from: Twark_Main on 01/18/2023 11:27 amYou 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.
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.
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.
Quote from: mikelepage on 01/30/2023 02:00 pmAlso, 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.
