Build simple boosters with a similar philosophy to the way they built Liberty Ships. Not elegant, but they'll work. You're throwing them away anyway. Since you're building them big (to make it possible to carry their own non-elegant weight) just make them big enough to loft a useful payload (60,000 plus) into orbit, without an elegant (and finicky) upper stage.
Big Honking Single Stage To Orbit!
That was my thought exactly. My only problem is will private industry take the chance of spending the developmental cost, although negligible with today’s tech, without a built in or at least perceived market place beyond the government.
The problem with SSTO is lambda-P. If you don't know what lambda-P is, then you are unqualified to make such statements. A Big Dumb Booster--by definition--has a LOUSY lambda-P. It therefore cannot perform an SSTO mission and achieve orbit with a positive (non-zero) payload. End of discussion. If you don't know what you are talking about it is best to keep silent.
--Boris