Author Topic: SpaceX Falcon Heavy Discussion (Thread 2)  (Read 275115 times)

Offline hypermecanix

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #120 on: 09/15/2014 04:37 pm »
Terrestar-1 was a commercial payload (7 mT) single launched on A5 in 2009.
There is also the Alphabus platform, designed up to 8 mT.
Capacity exists but doesn't seem to attract operators. Whether it's because of the cost, or the lack of backup launch provider is the right question.

Offline pogo661

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #121 on: 09/15/2014 05:27 pm »
Do we know what the Crossfeed plumbing will look like?  I assumed that the center core would have the 3 starboard engines sucking fuel and oxider from the starboard booster.  Ditto for port.  And the center 3 engines were the only ones that could pull from the center core tanks.

Also that fuel and oxidizer hookups to the side boosters would be fairly big, high flow pipes,  without separate valves (beyond the individual engine valves) so that cross feed cores and boosters couldn't be easily repurposed for non-crossfeed service.

Are there other proposals out there?  Could crossed be tank to tank with a separate pump,  so that the engine plumbing for crossfeed was the same as non crossfeed?

Offline oldAtlas_Eguy

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #122 on: 09/15/2014 06:02 pm »
Do we know what the Crossfeed plumbing will look like?  I assumed that the center core would have the 3 starboard engines sucking fuel and oxider from the starboard booster.  Ditto for port.  And the center 3 engines were the only ones that could pull from the center core tanks.

Also that fuel and oxidizer hookups to the side boosters would be fairly big, high flow pipes,  without separate valves (beyond the individual engine valves) so that cross feed cores and boosters couldn't be easily repurposed for non-crossfeed service.

Are there other proposals out there?  Could crossed be tank to tank with a separate pump,  so that the engine plumbing for crossfeed was the same as non crossfeed?

A seperate pump would requires something like a larger size GG and turbo pump to pump the amounts we are talking about on each side.  So I do not think that is is a viable solution.

Offline Jim

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #123 on: 09/15/2014 07:57 pm »
  Could crossed be tank to tank with a separate pump,  so that the engine plumbing for crossfeed was the same as non crossfeed?

There are no additional pumps involved with cross feed, it is just additional feed lines and valves.

Offline IslandPlaya

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #124 on: 09/15/2014 08:56 pm »
  Could crossed be tank to tank with a separate pump,  so that the engine plumbing for crossfeed was the same as non crossfeed?

There are no additional pumps involved with cross feed, it is just additional feed lines and valves.
The existing pumps need more flow-rate than they do in normal operation though.
Are all Merlin turbopumps designed with this extra capacity?
I would think not, but I don't know enough to say for sure.

Offline DanielW

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #125 on: 09/15/2014 09:01 pm »
  Could crossed be tank to tank with a separate pump,  so that the engine plumbing for crossfeed was the same as non crossfeed?

There are no additional pumps involved with cross feed, it is "just" additional feed lines and valves.
The existing pumps need more flow-rate than they do in normal operation though.
Are all Merlin turbopumps designed with this extra capacity?
I would think not, but I don't know enough to say for sure.

I don't think this is correct. The pumps in question are on the center core. They don't know that they are pulling from the side tanks. As Jim said it is just more plumbing and valves. (the word just implying no changes to the turbopumps not a triviality of execution)
« Last Edit: 09/15/2014 09:05 pm by DanielW »

Offline Herb Schaltegger

Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #126 on: 09/15/2014 09:06 pm »
  Could crossed be tank to tank with a separate pump,  so that the engine plumbing for crossfeed was the same as non crossfeed?

There are no additional pumps involved with cross feed, it is just additional feed lines and valves.
The existing pumps need more flow-rate than they do in normal operation though.
Are all Merlin turbopumps designed with this extra capacity?
I would think not, but I don't know enough to say for sure.

I don't think this is correct. The pumps in question are on the center core. They don't know that they are pulling from the side tanks. As Jim said it is just more plumbing and valves. (the word just implying no changes to the turbopumps not a triviality of execution)

What they "know" is that there is working fluid with certain characteristics (specifically inlet temperature and pressure) that fall within an expected range. The question is whether pumping that fluid through additional plumbing and fixtures from the boosters into the central core results in significantly-different than expected conditions as compared to pumbing that fluid from tanks in the central core itself. Or to put it another way, will the turbopumps have to be modified or not. This is still an open question as far as I know.
« Last Edit: 09/15/2014 09:31 pm by Herb Schaltegger »
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Offline TrevorMonty

Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #127 on: 09/15/2014 09:16 pm »
Crossfeed seems simple until you start getting into the details. I hadn't thought about the fuel pump requirements.

Offline Nindalf

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #128 on: 09/15/2014 09:18 pm »
  Could crossed be tank to tank with a separate pump,  so that the engine plumbing for crossfeed was the same as non crossfeed?

There are no additional pumps involved with cross feed, it is just additional feed lines and valves.
The existing pumps need more flow-rate than they do in normal operation though.
Are all Merlin turbopumps designed with this extra capacity?
I would think not, but I don't know enough to say for sure.
I think you've got a misconception here.

There are essentially two stages of propellant feed, we could call them the push step and the pull step.  The push step is pressure/gravity feeding.  The tanks are pressurized with helium, and there is additional pressure because you're drawing from the bottom of each tank with all the weight of fuel or oxygen above, so if you simply hooked up a hose from the tank to the open air, it would spray out quite forcefully.  The pull step is the turbopump, which is part of each individual engine.  The main function of the turbopump is to provide high pressure for the combustion chamber.

With crossfeed, you basically have more outlets from the side booster tanks to some of the center core's engines.  The turbopumps don't have to work harder, but you do have to put helium into the side booster tanks faster, since they're emptying faster, so the pressurization system needs to be able to do that.  They could add another pump in the middle but it's an unnecessary complication, so they're probably not doing that.
« Last Edit: 09/15/2014 09:22 pm by Nindalf »

Offline Karloss12

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #129 on: 09/15/2014 09:25 pm »
With the F9 v1.0 tick-tack-toe arrangement, Elon indicated the centre core would fuel the 3 centre engines and the side cores would fuel 3 engines each (3-3-3).

Going to the Octaweb configuration would a 4-1-4 configuration be more optimum or would the side cores run out of fuel to early?

Offline 411rocket

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #130 on: 09/15/2014 09:53 pm »
  Could crossed be tank to tank with a separate pump,  so that the engine plumbing for crossfeed was the same as non crossfeed?

There are no additional pumps involved with cross feed, it is "just" additional feed lines and valves.
The existing pumps need more flow-rate than they do in normal operation though.
Are all Merlin turbopumps designed with this extra capacity?
I would think not, but I don't know enough to say for sure.

I don't think this is correct. The pumps in question are on the center core. They don't know that they are pulling from the side tanks. As Jim said it is just more plumbing and valves. (the word just implying no changes to the turbopumps not a triviality of execution)

It is my understanding, that the turbopumps are part of each engine. The side boosters may require, larger piping for feeding the center core engines, as well as their own engines. This would be in addition, to valves & quick disconnects, interconnecting the side boosters & core similar to the launch pad connections for fuel & oxidizer.

Of course I could be wrong, but I'm thinking of it, as a remote fuel & oxidizer source, instead of initially using the internal tanks.

My analogy for it, would be running a Military AC power generator off a 45 Gallon drum of fuel Vs the built in fuel tank (which can be a pain to refill, if trailer mounted). Fuel source, is controlled by valve selection, remote drum or built in tank.

Offline Nindalf

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #131 on: 09/15/2014 09:54 pm »
With the F9 v1.0 tick-tack-toe arrangement, Elon indicated the centre core would fuel the 3 centre engines and the side cores would fuel 3 engines each (3-3-3).

Going to the Octaweb configuration would a 4-1-4 configuration be more optimum or would the side cores run out of fuel to early?
Setting aside the complex question of the best time to accept a reduction of thrust in order to reduce vehicle mass and drag, I suspect that the cross-fed engines will shut down during separation, leaving only the non-crossfed engines to provide control authority, and at least two are needed for strong roll-control.

Offline IslandPlaya

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #132 on: 09/15/2014 09:57 pm »
  Could crossed be tank to tank with a separate pump,  so that the engine plumbing for crossfeed was the same as non crossfeed?

There are no additional pumps involved with cross feed, it is just additional feed lines and valves.
The existing pumps need more flow-rate than they do in normal operation though.
Are all Merlin turbopumps designed with this extra capacity?
I would think not, but I don't know enough to say for sure.
I think you've got a misconception here.

There are essentially two stages of propellant feed, we could call them the push step and the pull step.  The push step is pressure/gravity feeding.  The tanks are pressurized with helium, and there is additional pressure because you're drawing from the bottom of each tank with all the weight of fuel or oxygen above, so if you simply hooked up a hose from the tank to the open air, it would spray out quite forcefully.  The pull step is the turbopump, which is part of each individual engine.  The main function of the turbopump is to provide high pressure for the combustion chamber.

With crossfeed, you basically have more outlets from the side booster tanks to some of the center core's engines.  The turbopumps don't have to work harder, but you do have to put helium into the side booster tanks faster, since they're emptying faster, so the pressurization system needs to be able to do that.  They could add another pump in the middle but it's an unnecessary complication, so they're probably not doing that.
Thanks. I think I understand more clearly now.
It's not at all trivial though and that's why x-feed will come later than the FH debut, I guess.

Offline Space Ghost 1962

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #133 on: 09/15/2014 10:39 pm »
Terrestar-1 was a commercial payload (7 mT) single launched on A5 in 2009.
Business failure. An example of "what not to do". And of not "getting away with" a big sat as a win.

There is also the Alphabus platform, designed up to 8 mT.
Capacity exists but doesn't seem to attract operators. Whether it's because of the cost, or the lack of backup launch provider is the right question.
It's not the bus that is the big deal, it's the solar and antenna array(s).

The temptation is to have one big sat to do 2-3 smaller sat's service footprint.

One bus. Multiple antenna cluster, feeds, and transponders. Large area solar that supplies enough even after 15 years in space degradation.

Ironically, the primary point of a big sat is to reduce, among other things, launch cost and launch risk. If launch risk stays the same (or worse), better to not have the big guys, because you risk less capacity loss. If your launch risk drops with increased launch cost, then go with big sat. If both launch costs and launch risks both go down, its life cycle costs of the entire constellation that matter, so you look for the "right size" by modelling factors - like small/large/dispersed geo coverage.

Offline Jim

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #134 on: 09/15/2014 11:56 pm »

The existing pumps need more flow-rate than they do in normal operation though.


No, they don't, they are just drawing propellant from a different tank.

Offline Nomadd

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #135 on: 09/16/2014 12:16 am »
Terrestar-1 was a commercial payload (7 mT) single launched on A5 in 2009.
Business failure. An example of "what not to do". And of not "getting away with" a big sat as a win.

Skyterra 1 was of similar size and also a huge failure. It has a 70' antenna with 500 spot beams and as far as I know, it's not doing much besides replacing an old MSAT bird. And it seems like SpaceX making launch prices even cheaper might ironically make the case for giant comms sats even weaker since it would make more frequent replacement with newer technology birds more attractive and hurt the case for satellites that cost $200 million to build and take 20 year lifespans to justify.
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Offline S.Paulissen

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #136 on: 09/16/2014 03:45 am »
Terrestar-1 was a commercial payload (7 mT) single launched on A5 in 2009.
Business failure. An example of "what not to do". And of not "getting away with" a big sat as a win.

Skyterra 1 was of similar size and also a huge failure. It has a 70' antenna with 500 spot beams and as far as I know, it's not doing much besides replacing an old MSAT bird. And it seems like SpaceX making launch prices even cheaper might ironically make the case for giant comms sats even weaker since it would make more frequent replacement with newer technology birds more attractive and hurt the case for satellites that cost $200 million to build and take 20 year lifespans to justify.

I don't think lifespan of satellite will be highly effected by launch prices.  I believe that's largely a function of the economics of delayed return on investment (takes longer to turn a true profit) coupled with technology advancement (new technology may make your bird obsolete before ROI has been achieved).   Add in the risk of loss with a single bigsat versus three sats and it becomes risk management to avoid a single launch.
 
Think of the risk statistics three dice rolls versus one.

Further, if launch costs are lowered by SpaceX as planned the cost for big payloads will be decreasing as well in the long run (potentially even more than regular sats if payloads are sufficiently high enough up the cost curve elbow).
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Offline macpacheco

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #137 on: 09/16/2014 04:29 am »
I don't think lifespan of satellite will be highly effected by launch prices.  I believe that's largely a function of the economics of delayed return on investment (takes longer to turn a true profit) coupled with technology advancement (new technology may make your bird obsolete before ROI has been achieved).   Add in the risk of loss with a single bigsat versus three sats and it becomes risk management to avoid a single launch.
 
Think of the risk statistics three dice rolls versus one.

Further, if launch costs are lowered by SpaceX as planned the cost for big payloads will be decreasing as well in the long run (potentially even more than regular sats if payloads are sufficiently high enough up the cost curve elbow).
The best investment a satellite operator can do today to reduce total cost of ownership for a given GEO mission is going full electric. Allows launching a 8ton bird with F9R class launcher, while still leaving an extra decade worth of station keeping xenon. The main drawback is having another decade to use the satellite might not make much sense, birds launched today will be utterly obsolete 30+ years from now.
Looking for companies doing great things for much more than money

Offline MP99

Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #138 on: 09/16/2014 08:24 am »
With the F9 v1.0 tick-tack-toe arrangement, Elon indicated the centre core would fuel the 3 centre engines and the side cores would fuel 3 engines each (3-3-3).

Going to the Octaweb configuration would a 4-1-4 configuration be more optimum or would the side cores run out of fuel to early?
Setting aside the complex question of the best time to accept a reduction of thrust in order to reduce vehicle mass and drag, I suspect that the cross-fed engines will shut down during separation, leaving only the non-crossfed engines to provide control authority, and at least two are needed for strong roll-control.
No, it seems clear that the core will fly on nine engines after booster separation. Otherwise, gravity losses would be too high.

(I know this is past the point of major gravity losses, but this is essentially still a full F9 + F9US + payload, and 3x engines would cut T:W from "something over 1.2" to "something over 0.4".)

Cheers, Martin

Offline guckyfan

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Re: SpaceX Falcon Heavy Discussion (Thread 2)
« Reply #139 on: 09/16/2014 09:30 am »
Setting aside the complex question of the best time to accept a reduction of thrust in order to reduce vehicle mass and drag, I suspect that the cross-fed engines will shut down during separation, leaving only the non-crossfed engines to provide control authority, and at least two are needed for strong roll-control.
No, it seems clear that the core will fly on nine engines after booster separation. Otherwise, gravity losses would be too high.

(I know this is past the point of major gravity losses, but this is essentially still a full F9 + F9US + payload, and 3x engines would cut T:W from "something over 1.2" to "something over 0.4".)

Cheers, Martin

I think the question was for the moment of separation. Would the crossfeed engines shut down for separation and then restart? At least this was my understanding.

I agree with you that the engines will not shot down even temporarily. It would be easier and safer to keep them running IMO.


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