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Quote from: spacenut on 07/03/2017 02:02 amIsn't the only reason for super cooling propellants is to provide more propellant in a given fixed space? Certainly, this is the main point, even if engine operation is affected by propellant density. The question is whenter the TMI, landing and return DeltaV expectations are calculated with assuming the extra propellant, or not?
Isn't the only reason for super cooling propellants is to provide more propellant in a given fixed space?
Quote from: geza on 07/03/2017 03:25 amQuote from: spacenut on 07/03/2017 02:02 amIsn't the only reason for super cooling propellants is to provide more propellant in a given fixed space? Certainly, this is the main point, even if engine operation is affected by propellant density. The question is whenter the TMI, landing and return DeltaV expectations are calculated with assuming the extra propellant, or not?Why wouldn't it be? Otherwise there would be no performance gain.
So lox at 66k vapor pressure is .029 bar. So Mars surface at .005 bar and space at 0 bar will cool lox just fine. All you have to do is collect the gaseous oxygen boil off at .029 bar and compress it in a Linde liquefaction cycle and put it back in the tank. Takes energy but not the special stuff you do at 1 bar. Like vacuum pumps or LN2 baths.
Quote from: rsdavis9 on 07/04/2017 10:49 pmSo lox at 66k vapor pressure is .029 bar. So Mars surface at .005 bar and space at 0 bar will cool lox just fine. All you have to do is collect the gaseous oxygen boil off at .029 bar and compress it in a Linde liquefaction cycle and put it back in the tank. Takes energy but not the special stuff you do at 1 bar. Like vacuum pumps or LN2 baths.Good to know. What about LEO?
My understanding was that they use subcooled propellant to eliminate cavitation in the turbopump. Full power would mainly be needed on earth ascent, both in the first and second stage. That can be provided with subcooled propellant on tanking. Can cavitation also be avoided with some throttling? For TMI full power would not be needed, also on Mars ascent it is not as essential.If subcooled is needed in every phase, can you calculate, how much propellant would be wasted to cool propellant a few degrees below sea level pressure boiling temperature?
Quote from: Lars-J on 07/03/2017 03:42 amQuote from: geza on 07/03/2017 03:25 amQuote from: spacenut on 07/03/2017 02:02 amIsn't the only reason for super cooling propellants is to provide more propellant in a given fixed space? Certainly, this is the main point, even if engine operation is affected by propellant density. The question is whenter the TMI, landing and return DeltaV expectations are calculated with assuming the extra propellant, or not?Why wouldn't it be? Otherwise there would be no performance gain.Because subcooling prop on-orbit or on the Mars surface is somewhat more challenging than on the launch pad, and the leg from staging to Earth orbit is more challenging than LEO to mars surface or Earth return empty.I don't think subcooled props are strictly necessary for TMI or Earth return, but if they can solve long term boiling storage than long term subcooled isn't all that much more difficult, so they might do it. It does help with fast transits and next-synod reuse.
Engine pumps are very sensitive to vapor pressure and tank pressure (head or otherwise). In space you don't have head pressure. On Mars, you obviously have less than on earth, but better than in space. In space, BFS will probably need to use either sub-cooled propellants, higher tank pressures or boost pumps or some combination. I was surprised that Raptor doesn't have them (yet). :^)John
I still don't see the problem. If the propellant is no longer subcooled, it doesn't go away. (conservation of mass and all that) It is still there, just taking up more volume. The engines will certainly be able to handle a bit of temperature range.
Quote from: Lars-J on 07/05/2017 05:35 pmI still don't see the problem. If the propellant is no longer subcooled, it doesn't go away. (conservation of mass and all that) It is still there, just taking up more volume. The engines will certainly be able to handle a bit of temperature range.Assume you have filled the tank in orbit to capacity with subcooled propellant. Then the temperature drifts to the boiling point. Some of the propellant is going to go away unless you keep the vents closed. In that case it will stay until the tanks burst and it all goes away.
It will take 6 tanker launches to fill the tanks. If you can keep it subcooled for that long, what's stopping you from keeping it subcooled until you use it?
Of course.Didn't Elon say they were using multistage pumps on Raptor? The low pressure pump might be designed to handle lower vapor pressure without cavitation.
Quote from: envy887 on 07/06/2017 03:20 pmIt will take 6 tanker launches to fill the tanks. If you can keep it subcooled for that long, what's stopping you from keeping it subcooled until you use it?Time maybe? They can fill it in a week with daily launches. But if it waits for months in LEO for the Mars window to open it will be hard to keep propellants subcooled without any active measures. Less hard while in interplanetary space away from IR emitting earth.Edit: I was mostly repying to the "it does not go away".There are ways to handle it. Fill up to boiling temperature, wait for departure time, with hopefully minimal boiloff. Subcool by opening to vacuum and have a last tanker fill up before departure. One tanker can probably do the topping off for several departing vehicles.
What vehicle will use the Raptor? I know all about the ITS but the Raptor will be done well in advance of ITS and if it's only use is ITS then it seems like the economics of SpaceX won't work. Raptor has got to have more use than that. Is it only me or there is a BIG gap in the SpaceX launch family from F9 to ITS......