In article om,
says...

On 14-07-27 22:22, Greg (Strider) Moore wrote:

The problem with this (and NASA was offered this after Endeavour) is you now
have a fleet of 4 different vehicles, which gets costly.



However, in a realistic scenario where the vehicle is still in
development (as opposed to decloaring the beta "Shuttle" in production),
you want to evolve the vehicle until you get something you are generally
happy with.

If Shuttle #4 has significant improvements over Shuttle #1 which result
in Shuttle #4 costing far less to operate, it may compensate for the
added costs of not having commonality within all 4 vehicles.

Not evolving the shuttle and building them all the same is fine of paper
when you have a vehicle ready for production. But doesn't work well for
a vehicle that is still in development.

Consider airraft, the roughly 5 prototypes built for the test flights
have significant differences in them, and as the test flights progress,
they are retrofitted with improvements, and once they are happy with the
design, they can then build production aircraft with all the
enhancements they learned from during the test phase.

Shiuttle should have worked the same way.

Columbia was essentially the orbital prototype, which is why its
structure massed more than later orbiters. The big visible differences
between it and later orbiters were the "scars" for ejection seats, the
"pod" on top of the vertical stabilizer (contained cameras for test
flights), and some TPS differences.

Also, for quite some time, Columbia also contained lots of extra wiring
which was used on the initial flights to collect data. Later orbiters
lacked that as well.

When Challenger was lost, there was a proposal to build two new orbiters
with updated structures and other systems. NASA turned the proposal
down for the reasons Greg mentions.

Jeff
--
"the perennial claim that hypersonic airbreathing propulsion would
magically make space launch cheaper is nonsense -- LOX is much cheaper
than advanced airbreathing engines, and so are the tanks to put it in
and the extra thrust to carry it." - Henry Spencer