Elon Musk said that the integration of the Falcon Heavy was "shockingly difficult". I can absolutely believe that. But at the same time, SpaceX is reusing two "flight proven" boosters for the Heavy. Presumably, they could be modified rather easily for the job, they would not be used.

So what explains this apparent contradiction? Is the central booster so much different from the Falcon 9 first stage?

  • $\begingroup$ Keyword integration. Each of these taken apart is not much more difficult than your normal Falcon 9. Making them work together well, that's the tricky part! $\endgroup$
    – SF.
    Commented May 11, 2017 at 10:06

1 Answer 1


In the end, they decided that the existing Falcon 9 cores with minor modifications could be used as the side boosters.

The difficulty came in the center booster. This needs to handle the load of the two side boosters, and carries the heaviest load (Second stage and payload).

Additionally seperation is trickier than any they have tried to date. Currently the use a pusher to shove away the second stage. This is a totally different type of separation event. Have to make sure they fly away, not back into the center core, they have to be pushed away firmly enough, but not with something that can damage the remaining center core.

Issues with handling engine out become much more problematic. On a single core, if an engine has trouble, it gets shut down, the computer decides whether to throttle the others up, or else just run longer to compensate.

Now you have two side boosters each with 9 engines, and one engine shutting down you have to balance the thrust difference across the entire stack. That is probably a lot more complex than you would imagine. Especially with 27 engines across three cores.

  • $\begingroup$ I remember a while ago Space X intended to develop the ability to cross feed fuel in the heavy configuration. This could potentially mean all 3 cores would use fuel from the strap on boosters until they were empty. Then the center core would still have a full full load at separation. The advantages of this are clearly great, but the difficulties also great. Engineering some kind of 3 rocket manifold and managing pressure changes when it switched sounds daunting. I wonder if this is this still in the works? $\endgroup$ Commented May 31, 2017 at 3:30
  • $\begingroup$ @JohnnyRobinson MAke it a standalone question and we can answer that. You even can get points... $\endgroup$
    – geoffc
    Commented Jun 2, 2017 at 2:05

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