In article ,
David Shannon wrote:
Consider the design path where the stages are externaly identical
(eg General Dynamics "Triamese"...
This approach has theoretical limitations that are outweighed by
practical advantages. You only have to spend skull-sweat on the orbiter -
the boosters are simplified variants (no OMS, less TPS, etc, etc)

The practical problem with biamese and triamese is that almost anything
you do to simplify the boosters starts you off down the slippery slope of
building two different vehicles. It's very hard to stop that.

Just leaving systems out looks easy, but often it means a lot of extra
engineering to assess what *happens* when you leave those systems out,
and what drives development cost is not materials but engineering effort.
Later on, when weight is excessive or there's a bit of a performance
shortfall, well, we're already building two different configurations, so
we'll just make them a little *more* different...

Biamese or triamese is a win only if the boosters are the *same* as the
orbiter. Same TPS; if it doesn't get as hot, that's nice. Same OMS;
okay, we can leave its tanks empty on the boosters. Same systems, all of
them. Maybe we fill the boosters' cargo bays with tanks, but if so, any
permanent fittings we need to add go in the orbiters too. It takes very
strong engineering leadership to make this work.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |