In article ,
David Shannon wrote:
Best reliability calls for a completely reuseable single stage launcher

Alas, SSTO fuel fraction is prohibitive.

Not necessarily. When people have been pushed hard to try to build
expendable stages with that sort of fuel fraction, they have generally
succeeded. And with 1960s technology, too, in some cases.

Reusability is the uncertain part, but that's true of TSTO systems too.

2STO typically uses 1/3 the
propellant for a given payload, although vehicle empty weights are higher.

However, since propellant costs are negligible, and empty mass and
complexity are the expensive parts...

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

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

Note the words "in the atmosphere". The ET separation occurs in vacuum.

The SRB separation may look simple but it isn't; NASA spent a lot of time
and money making sure it would work.

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.

In fact, NASA planned roughly that for the original two-reusable-stage
shuttle.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |