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They could balance one thruster on one vehicle with a similar one on the other vehicle, giving no net thrust during tail to tail fuel transfer.
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The fuel transfer works by unbalanced thrust, Elderflower. The unbalanced thrust creates an acceleration (low as it is) that is indistinguishable from low gravity, for a weak "gravity" feed from the tanker to the ITS. You cannot balance it during the transfer, or there is no "gravity" to power the feed. You can only thrust the other way after the fact to correct your orbit back to what it was. If the transfer takes something on the order of an hour (tens of tons being involved, after all!) then this orbit change is a very significant item.
My idea of a low-pressure-driven transfer works a lot better if the tanks are fitted with expulsion bladders. You pressurize between the bladder and shell to squeeze the propellant out of the bladder. In hindsight, I rather doubt the tanks they are proposing are fitted that way.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Difficult to find bladder material that would withstand LOX. Perhaps some grade of PTFE bellows might survive. Not much else! Transfer could be done with a pump provided you can provide sufficient head for the suction. Locking the two ships together and spinning them about a transverse axis could do that if the transfer pumps are at the forward end of the tanker.
Otherwise the tanker tank can be pressurised simply by raising its temperature so increasing its vapour pressure, but then you have to deal with flashing, mixed flow and recondensing. I think that would be rather difficult.
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Difficult to find bladder material that would withstand LOX.
PCTFE. It doesn't react with oxygen. Service temperature down to -240°C, below that it embrittles. LOX boils/liquefies at -182.96°C at 1 atmosphere pressure. There's also Teflon FEP, also service temperature down to -240°C, also non-reactive with oxygen. PCTFE is more impermeable to oxygen, so better for long-term storage or a greenhouse. PCTFE is also far more immune to UV. Teflon FEP is less expensive, so usable as a bladder for a launch vehicle.
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I don't think these could be made into a bladder which would work effectively without cracking. A bellows would be a better bet but you leave a larger residual quantity of LOX behind. This might not be a drawback as you need it to land your tanker. The bellows would need to be replaced frequently.
Tank pressurisation could be done with helium but you still need to feed liquid to a pump, if you use one, rather than vapour.
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Well, as near as I can tell, the problem is "ullage". There must be some way to move propellant to the suction of the tank. In zero-gee, it wants to form numerous globules floating around inside the tank. That's the action of surface tension in the absence of other forces. There's nothing but vapor for a pump, or for a pressure difference, to act on, at the suction. If you spin a tail-to-tail docked cluster, the artificial gravity pushes the propellant into a pool at the wrong end of the tank.
There are only two conceptual ways that I know of to solve the ullage problem. One is the application of thrust in the forward direction (of the donor vehicle) to accelerate it a bit, causing propellants to pool under artificial gravity at the aft ends of the tanks where the suction pipe really is.
The other is the tank bladder, which literally pressurizes between shell and bladder to squeeze the propellant like squeezing a frosting tube to decorate a cake. That kind of bladder must repeatedly survive massive deformations. What might work for a tank liner (to stop-up porosity in composites) is actually fairly unlikely to work as a bladder with cryogenics. That is because the squeezing motions it must survive are of the same dimension as the tank itself. And it's a crumpling effect as the tank empties, massively-compounding the level of strain that must be survived.
All that being said, there may (or may not) really be any fully-suitable materials for a cryogenic bladder you can crumple down to a small knot. Which lack forces you to the "ullage thruster" solution instead. That may be exactly why Spacex says it will fire a nose thruster on the ITS to get the tanker propellant to pool by artificial gravity (cluster acceleration) at the suction pipe inlet, and to flow from there to the ITS by that same gravity effect.
This will be a slow process, and the orbit-change delta-vee will be significant. But there may not be any other way around this dilemma. Which seems likely why they proposed what they did in the latest concept presentation.
Myself, if I had to do it this way, I'd thrust across the orbit as a plane change delta-vee, which would be a very minor plane change. You alternate sides to minimize the overall effect, and you launch tankers sequentially, taking this tiny plane change into account for the rendezvous.
GW
Last edited by GW Johnson (2017-11-05 10:24:02)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Apollo SM thruster quads used a rubber bladder for their tanks. But they used storable propellants, MMH/N2O4, not cryogens. Those Apollo rubber bladders appeared to collapse along the length of the tank, forming a flat shape that's still the full length. You could do that with a fluoropolymer. I have 8.5"x11" samples of film: PCTFE, FEP, and Tefzel. All clear, colourless, in 1 mil, 2 mil, and 5 mil thicknesses. I intended to create a material exposure rack to test them at MDRS. I should still do that. All samples of film are supple, definitely able to move as a plastic bag.
By the way, Apollo SM main engine used Aerozine 50/N2O4, and its tank didn't have a bladder. Aft-pointing thrusters were used as ullage thrusters to move propellant in the main tanks to the aft end before the main engine could ignite. This meant they needed at least 2 thrusters to work, the aft-pointing thruster of quads on opposite sides of the SM, in order for the main engine to ignite. During Skylab missions the main engine was used to deorbit.
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The ullage motor is a very common solution. The Saturns used it. The plant where I once worked made them as little solid propellant cartridges. That makes sense on one-shot items with cryogenic tanks. Using hydrazine-NTO thrusters as ullage motors makes real sense on multi-use items like Apollo. Only the small room-temperature storables of the thruster system need the collapsible bladders.
The way the bladder collapses has to match the shape of the tank. For a spherical tank. you use a spherical bladder that is pinned at the tank equator, so that one hemisphere everts into the other hemisphere. It wrinkles a bit on the way, but those wrinkles have smoothed-out by the time the propellant is exhausted. That way, the percentage of unusable propellant can be quite low.
For a long cylindrical tank, collapsing from one side to the other (across the tank) is a way to achieve high expulsion percentage, but the vehicle must have a low thrust-to-weight ratio, or else there is a large effective-gravity pressure gradient down the length of the tank. That kind of high pressure at the suction end puts a very large tensile stress on the bladder membrane.
You can evert from one end to the other, similar to the spherical tank, but this has not been as reliable as one could desire. The distance to be everted is quite large as tank L/D gets significant. Any little hang-up for any little reason, and the rig fails: a huge unusable propellant fraction.
The simplest cylindrical tank bladder approach is the collapse radially inward, all along the length. This puts the most crumpling action on the bladder material, and has the highest unusable propellant fraction, but it is utterly reliable, as long as the material doesn't crack from the strain.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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