Author Topic: ITS Propulsion The evolution of the SpaceX Raptor engine  (Read 77051 times)

Offline RobLynn

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #220 on: 07/03/2017 06:08 AM »
Isn'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?

lower vapour pressure of subcooled propellants means that pressure on suction sides of pump impellers can go lower before inducing damaging cavitation - so you can make more highly loaded pumps, or spin them faster for higher pressure ratios, or reduce the necessary tank pressure (pump inlet pressure) for tank mass savings.
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Offline envy887

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #221 on: 07/04/2017 10:07 PM »
Isn'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.

Offline rsdavis9

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.
bob

Offline Kaputnik

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #223 on: 07/05/2017 12:29 PM »
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.

Good to know. What about LEO?
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Offline rsdavis9

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.

Good to know. What about LEO?

My comment just has to do with active cooling using standard refrigeration equipment. The difference between LEO and deep space has to do with passive cooling and sun shades. Passive cooling would not work as well in LEO because of the radiation from earth. On the sunside there is not much sky to expose your passive radiators to. On the night side there is half the sky with no thermal radiation. Obviously a active cooling system still needs radiators to get rid of the heat so it will work better depending on the sky it is radiating to.
bob

Offline guckyfan

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #225 on: 07/05/2017 12:42 PM »
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?

Offline envy887

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #226 on: 07/05/2017 12:59 PM »
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?

No propellant is required, just slightly better insulation, cooling and radiators than it takes to keep it from all boiling away in the first place. It depends on the location and cooling strategy. In deep space, passive cooling is likely more than sufficient for ZBO with methalox. On Mars, they will need some active cooling, but there are some heatsinks available so large radiators might not be necessary. The toughest place to do ZBO is in LEO.

Throttling results in a very slight hit to I_sp in vacuum, but the bigger hit by far is the loss of 17% of the propellant mass for the same tank volume at boiling densities. But ITS still has 5,900 m/s of delta-v with a 300 t payload and boiling methalox, which is enough for a fast-ish transit and landing - so they might decide to do boiling non-ZBO in LEO.

Offline Lars-J

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #227 on: 07/05/2017 05:35 PM »
Isn'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.

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.

Offline livingjw

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #228 on: 07/05/2017 09:28 PM »
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

Offline envy887

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #229 on: 07/05/2017 10:38 PM »
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

You better have head pressure whenever the engines are firing whether in space, on Mars, or on Earth - or else you have major problem: you aren't accelerating at all :D

Offline livingjw

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #230 on: 07/06/2017 11:17 AM »
Of course, after they are started. Starting is the problem.

John

Offline envy887

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #231 on: 07/06/2017 12:55 PM »
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.

Offline guckyfan

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #232 on: 07/06/2017 02:58 PM »
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.

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.

Offline envy887

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #233 on: 07/06/2017 03:20 PM »
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.

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?
« Last Edit: 07/06/2017 03:20 PM by envy887 »

Offline guckyfan

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #234 on: 07/06/2017 03:34 PM »
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?

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.
« Last Edit: 07/06/2017 03:38 PM by guckyfan »

Offline MP99

Prop tanks would be full (and so benefit from subcooling) only at Earth launch, TMI and Mars launch.

The two launches can rely on GSE to maintain temps.

Perhaps the refuelling tanker also carries subcooling equipment. Maybe that only the final fuelling delivery uses a special variant with the subcooling hardware, circulating prop until temps are low enough to fit in the full prop load.


There is actually a passive way for the tanker to re-cool ITS's prop load - by freezing its payload prop (in separate tanks from launch prop) before launch, then circulating ITS's prop load through it.

It would make tanker GSE a nightmare, though!

Cheers, Martin

Offline livingjw

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #236 on: 07/06/2017 08:03 PM »
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.

The methane pump is two stage and probably does not need a boost pump, but the LOX pump appears to be a single stage pump and may need one, or they might just hold higher pressure in the small landing tanks and could use these to start without a boost pump.

John

Offline JasonAW3

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #237 on: 07/06/2017 08:54 PM »
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?

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.

Actually, it might not be too much of a problem.  As I understand it, properly "doped", Carbon fiber makes a pretty good insulator.
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Offline cferreir

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......

Online rockets4life97

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Re: ITS Propulsion The evolution of the SpaceX Raptor engine
« Reply #239 on: 07/19/2017 07:54 PM »
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......

It looks like SpaceX's plan is for F9, FH, and smaller-version of the proposed BFR/ITS. BFR looks to be the enabler of the satellite constellation and Mars. I'm not sure that Blue Origin's approach for New Glenn and then New Armstrong is so much different in capability when compared to FH and (the new) BFR.

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