In the last several weeks two companies have landed boosters that have been to space. The boost back RTLS with VTVL is now established as a demonstrated capability. What’s next?
The boost back gets the upper stage(s) to space with some horizontal velocity and returns the booster to the original launch site and has a substantial performance penalty in return for this capability. An expended lower stage of the same size puts considerably more mass in orbit, though at considerably higher cost.
I would suggest that a logical next step would be for the various companies to do a parallel staging of a full first stage booster with RTLS capability along with a derated first stage booster with the upper stage on to of the derated one.
A Falcon Five Nine configuration is an obvious early entrant. The fourteen engines burning at launch is just over half the thrust of the FalconH in development. With cross feed, the Nine will stage at a similar velocity as last Mondays’ launch and RTLS. The Five continues on until upper stage separates and then tries to reach a seaborne recovery. The upper should have a payload of about half that of the FalconH. This is 25-29 tons of payload in the DeltaIV Heavy class.
This would give a capability above that of the expendable Falcon9 for the expenditure of six Merlins instead of ten. The possibility of recovering the Five still exists and a development path similar to that taken with the Nine could be pursued. This concept is to bridge the gap between the light payload of an RTLS Falcon9, and the large payload of the FalconH.
Since capabilities spread through any competitive industry, we could expect to see Vulcan Two One, Antaries Two One, and a Blue Origin entrant within the next several years. Not to mention the other configurations by various other companies.