In article ,
David Given wrote:
There are people here with more technical knowledge than I, but I do know
that the shuttle's main engines aren't restartable. (Do they even have
on-board fuel tanks?)

Not for the main engines. Which, as you note, are not restartable.
(There is no fundamental reason why they couldn't be, but there is no
requirement for it, and so various details of setup for engine start are
handled with the help of ground equipment.)

How about fitting the shuttle out with a lifeboat? Stick it somewhere in
the cargo bay. If a shuttle gets sufficiently damaged that it can't
reenter, you use the capsule to get the crew down.

It's been proposed many times. It presents some problems of physical
layout, its mass puts a considerable dent in the payload capacity... and
note that it wouldn't have saved Columbia's crew, since they didn't know
something was badly wrong until too late. (Nor is there any plausible
scenario where they would have. Suspicions about TPS damage were focused
on the tiles, not the RCC leading edge, and no plausible imaging -- from
the ground or from elsewhere in space -- would have been at all likely to
notice a small dark hole in a black surface.)

Depending on whether the capsule had its own thruster system, you would
get the choice of putting the shuttle onto a reentry trajectory and then
bailing out, or leaving the shuttle on orbit and just returning in the
capsule.

You'd want the capsule to do its own maneuvering, partly so that entering
it and separating wouldn't be time-critical operations, partly to cover
cases like the orbiter being unable to do its own deorbit burn. This
isn't that big a deal; a deorbit burn isn't large.

...You'd also have to outfit the shuttle
with an automated station-keeping facility using the OMS; you wouldn't
want it to accidentally fall on someone.

The orbiter will be dead and uncontrolled within days anyway: when its
fuel cells run out of reactants, it loses power.

(What's the lightest-weight way of getting a single human down from orbit?
Could you build something like an orbital parachute? If so, would that be
more appropriate than a combined capsule?)

There have been various proposals for "orbital bailout" kits. But a shared
capsule is probably better: it keeps the crew together, it can serve as
shelter or boat, it simplifies providing sizable amounts of survival gear
and electronics, it greatly simplifies cases where someone is injured.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |