In sci.space.policy Richard Schumacher wrote:
Maybe two. After that much time passes commercial vehicles will be
available. So NASA should try to get it right and go out on a high
note: start with a LOX-kerosene flyback booster for Shuttle (one booster
replacing both SRBs), and then a replacement for the Shuttle orbiter (a

This involves way too much redesign.
Swapping the SRBs for liquids (something thought of after Challenger)
would involve little structural redesign, the changes being almost entirely
operational.
(some abort modes become a whole lot easier, you need to change the fuelling,
and they may even be able to fly back under power.)

I find a paper in JBIS, indicating that it would add almost a third to the
payload.

However, doing this for the few shuttles remaining is almost pointless.

vertical lander which also serves as space tug and CRV, with a
7-passenger module which swaps out for a cargo pallet).

For a space tug, you almost certainly don't want to do it that way, you
want the mass as low as possible.

Or they can just keep doing what they're doing, and that will fill
enough time for the senior people to retire before it all falls apart.
(Anyone remember the United States Railroad Administration? They were
big in their day...)

--
http://inquisitor.i.am/ | | Ian Stirling.
---------------------------+-------------------------+--------------------------
"The device every conquerer, yes, every altruistic liberator should be required
to wear on his shield... is a little girl and her kitten, at ground zero"
- Sir Dominic Flandry in Poul Andersons 'A Knight of Ghosts and Shadows'