Elon Musk tweeted that SpaceX will make a mini-Big Falcon Spaceship upper stage for the Falcon 9.
They are aiming for an orbital flight by June 2019.
The MVac will have too high TWR and have flow separation at sea level. It will practice and testthe reentry regime. Elon said it won’t land propulsively for those reasons.
Mini-BFS will have an ultra-light heat shield and high Mach control surfaces. Those are things that SpaceX cannot test well without orbital entry. SpaceX has confidence in propulsive landings.
SpaceX is building a BFR development ship to do supersonic through landing tests in Boca Chica, Texas.
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