Re: Multiple Engines???

> Best reliability calls for a completely reuseable single stage launcher

  Alas, SSTO fuel fraction is prohibitive. 2STO typically uses 1/3 the
  propellant for a given payload, although vehicle empty weights are higher.

> separating two vehicles at high speed in the atmosphere is not simple.

  Shuttle does it every mission - SRBs from ET, ET from Orbiter.

> Series staging is terrible because you take off without the second stage engines 
> running, and thus without knowing whether they will start and run correctly.

  Agreed

> In any case there's little reason to use multiple different engine designs in one 
> vehicle.

  Servicing a single engine type is cheaper, if all other things are equal.
  4 on the booster and 1 on the Orbiter would support a single engine core,
  with differing bell arrangements.