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?
Could crossed be tank to tank with a separate pump, so that the engine plumbing for crossfeed was the same as non crossfeed?
Quote from: pogo661 on 09/15/2014 05:27 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.
Quote from: Jim on 09/15/2014 07:57 pmQuote from: pogo661 on 09/15/2014 05:27 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.
Quote from: pogo661 on 09/15/2014 05:27 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.
Quote from: IslandPlaya on 09/15/2014 08:56 pmQuote from: Jim on 09/15/2014 07:57 pmQuote from: pogo661 on 09/15/2014 05:27 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)
Quote from: Jim on 09/15/2014 07:57 pmQuote from: pogo661 on 09/15/2014 05:27 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.
Quote from: IslandPlaya on 09/15/2014 08:56 pmQuote from: Jim on 09/15/2014 07:57 pmQuote from: pogo661 on 09/15/2014 05:27 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)
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?
Quote from: IslandPlaya on 09/15/2014 08:56 pmQuote from: Jim on 09/15/2014 07:57 pmQuote from: pogo661 on 09/15/2014 05:27 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.
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.
The existing pumps need more flow-rate than they do in normal operation though.
Quote from: hypermecanix on 09/15/2014 04:37 pmTerrestar-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.
Quote from: Space Ghost 1962 on 09/15/2014 10:39 pmQuote from: hypermecanix on 09/15/2014 04:37 pmTerrestar-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).
Quote from: Karloss12 on 09/15/2014 09:25 pmWith 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.
Quote from: Nindalf on 09/15/2014 09:54 pmSetting 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
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.