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My understanding of the Falcon Heavy cross feed system is that the outer cores feed six of the nine engines directly on the center core. Thus, the cross feeds don't pump propellent and oxidizer into the center core tanks but rather feed six of the nine center core engines directly.

This is very similar to how the Space Shuttle worked. The Shuttle's cross feed between the External Tank and the Orbiter was 17" in diameter -- one for propellent, one for oxidizer. A project to reduce the size of the cross feed to 14" was cancelled. A video of the disconnect being tested can be seen here:

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Some questions:

How large are the cross feeds between the Falcon Heavy's outer cores and the inner core?

Are there four (2 for propellent, 2 for oxidizers)?

Has cross-feeding been done on any other vehicles other than the Shuttle and the Falcon Heavy?

Do the pumps reside on the center core?

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  • $\begingroup$ Interesting question, but one that will have to remain without answers for now as there's no public information on the crossfeed system other than some offhand comments by Elon Musk, plus lots of speculation. $\endgroup$
    – Hobbes
    Commented Jan 30, 2015 at 12:02
  • $\begingroup$ Crossfeed on FH has been all but canceled. $\endgroup$
    – marked-down
    Commented Mar 30, 2015 at 18:18
  • $\begingroup$ Where have you heard this @EchoLogic? $\endgroup$
    – Erik
    Commented Mar 30, 2015 at 18:20
  • $\begingroup$ @Erik, I wish I could say more, but I can't. You'll have to take my word for it. From what I hear, CF will only be developed if a customer decides they want to pay for its development. It's no longer in the planning pipeline otherwise. $\endgroup$
    – marked-down
    Commented Mar 30, 2015 at 18:23

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Cross-feeding has been used (sort of) on the early versions of Atlas: these had booster engines that were fed from the first stage tanks. These engines were jettisoned while the first stage was still running. Of course this is a simpler construction than the proposed design for Falcon Heavy.
The Space Shuttle also isn't really comparable: the SSME's are fed only from the ET, there is no switchover to another tank.

This will be the critical point for Falcon Heavy: they need to switch the engines on the center core from one set of tanks to the other, without interrupting the flow of oxygen and propellant. High-speed turbopumps are sensitive to flow issues, and inserting a gas bubble into the liquid flow could be disastrous.

Also, when you close a valve in a high-speed flow of liquid, it's difficult to avoid creating a fluid hammer (a pressure spike traveling upstream from the valve).

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  • $\begingroup$ The comparison to STS was for the question about size. STS' is 17" -- which I'm sure is larger than the FH's. Agreed that STS did not have to worry about closing the flapper while under powered flight. $\endgroup$
    – Erik
    Commented Jan 30, 2015 at 16:21
  • $\begingroup$ I know it's not exactly what you mean, but STS worried a LOT about that flapper closing inadvertently in powered flight. After the 51-L accident, pneumatically operated locks were installed to hold the valves in the open position. $\endgroup$
    – Organic Marble
    Commented Mar 30, 2015 at 19:58
  • $\begingroup$ Do they need to switch tanks, or could they just shut down the cross-fed engines? At the point the boosters fall away, the center stage has burned a lot of its propellant. So you could probably feed one (or maybe even two) engines from each side stage, then boost on 7 (or maybe just 5) engines after staging. It's not as cool as feeding three on a side, but it would help. $\endgroup$
    – ChrisPhoenix
    Commented Jan 17, 2016 at 9:43
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There is an image floating around, I do not recall where I got, called the LOXtopus, which is the manifold at the bottom of the tank showing the plumbing.

LoxToPus Manifolfd

There are a total of ten connections. Nine are for the nine engines hosted locally. The tenth is for draining/filling.

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    $\begingroup$ The tenth is the inlet used to fill the tank. $\endgroup$
    – Hobbes
    Commented Jan 30, 2015 at 14:47
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    $\begingroup$ So the tank is filled from the bottom? $\endgroup$
    – Erik
    Commented Jan 30, 2015 at 16:52
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    $\begingroup$ Upvote for 'LOXtopus' $\endgroup$
    – pericynthion
    Commented Jan 30, 2015 at 19:36
  • $\begingroup$ @Erik: yes. Fuel and LOX filler lines enter at the bottom of the stage. I'll find a reference later. $\endgroup$
    – Hobbes
    Commented Jan 30, 2015 at 21:23
  • $\begingroup$ @Hobbes thx. Interesting as I believe STS fueled ET from above. $\endgroup$
    – Erik
    Commented Jan 30, 2015 at 21:42

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