Of course if Interim HOTOL had actually been advanced then StratoLaunch would have had something to point at they could carryInterim HOTOL (the one using the An-225) was not Alan Bond team but BAe, after the two went their own, separate ways (REL was funded in 1989).Thanks. RanulfC, you are correct as far as you go. However, SSTO projects used to be all the rage and they carry far more dead weight into orbit. Any kind of two-stage system is a great advance on them in the terms you describe. The original B.Ae concept when migrating HOTOL to a more realistic two-stage scenario was to use a subsonic mothership for the low-altitude, low-speed bit, where the demands on engine intake geometry vary dramatically from those at high Mach numbers. That really did relieve the Reaction Engines team of some notoriously intractable intake optimisation problems. That they now think it is worth grappling with those problems for the sake of lower orbiter mass is the bit I find interesting.