The SpaceX Mini-BFR for the Falcon 9 will only be used for testing and will not become a fully reusable second stage for the F9 or FH.
If SpaceX makes the F9 upper stage more reusable. Elon has said they will add more thermal protection to the upper stage. They will see what the least amount of mass necessary to return the upper stage in a condition that is reusable. Elon is confident reusability can be achieved. However, effort and focus is on the BFR.
Mod to SpaceX tech tree build: Falcon 9 second stage will be upgraded to be like a mini-BFR Ship
— Elon Musk (@elonmusk) November 7, 2018
Won’t land propulsively for those reasons. Ultra light heat shield & high Mach control surfaces are what we can’t test well without orbital entry. I think we have a handle on propulsive landings.
— Elon Musk (@elonmusk) November 7, 2018
New SpaceX BFR Simulation from Low Earth Orbit and Return
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