"John Doe" wrote in message ...
How much more would it cost to replace SRBs with liquid fueled flyback
boosters ? (or whatever technology is deemed best to replace the SRBs) ?


NASA replowed this ground in 2001-02 as part of the so-called Space Launch
Initiative (SLI) effort. The SLI folks spent time and money on studying a
shuttle replacement consisting of a fully-reusable two-stage vehicle with a
large flyback booster. The development cost was an estimated $30-35B (in
today's bucks). Sean O'Keefe had several independent studies made to keep
the SLI folks honest and these came up with essentially the same numbers.
Not surprising. When I worked on the original shuttle Phase A effort
(1969-70) at McDonnell Douglas, this two-stage monster was our baseline for
awhile and our estimated development cost was $37B (in today's money).
Again, not surprising that these numbers agree despite over 30 years of
separation, because the technology remains the same.

How much more would it cost to rework the engine area to make it simple,
fast
and cheap to remove/replace engines ?

Don't know.


Would there be any significant advantage in having the SSMEs attached to
the
ET instead of actual shuttle ? (either as a capsule with its parachutes
for
re-usability, or using disposable engines).

Each SSME costs about $50M to manufacture (current dollars). NASA spends
about $200M per year (current dollars) to maintain the SSME inventory. The
shuttle flies, on average, 5.5 times per year. So 15-18 SSMEs are flown each
year. So NASA is paying about $11M per SSME flown. If you can build an SSME
substitute (e.g. Rocketdyne's RS-68 that powers the Delta IV CBC) for less
than $11M per copy, then you might consider strapping a few of these engines
to the bottom of the ET and dumping them into the Indian Ocean along with
that tank.

BTW, NASA studied a similar configuration in the late 1970s when it became
evident that the shuttle could not meet the USAF requirement to place 32,000
pounds payloads launched out of VAFB into a polar reference orbit (100 nm
altitude circular orbit at 90 deg inclination). NASA and Martin figured out
how to beef up the structure in the bottom of the ET to attach liquid or
solid rocket boosters to give the shuttle more lift for the launches to
polar orbit.


Instead of dreaming of a totally radically new vehicle NASA has no idea
how to
build, wouldn't it be more realistic to just build new and improved
shuttles ?
If you start with the base model and then add each new improvement, at one
point, you will reach the "sorry, we can't afford this" level, at which
point
your new shuttle's designed is finalised with only the improvements you
can
afford ?


What improvements? As I mentioned at the start of this thread, there have
been no "improvements" made in spacecraft and launch vehicle technology
during the last 30 years that will produce large reductions in either
development cost or operations costs. Why? First, because the technology is
difficult ( especially in the high temperature materials area, where I spent
about 15 years of my aerospace career) and we have been in a region of
diminishing returns since the early 1970s. And, second, there just hasn't
been much R&D money spent during the last 30 years on this technology. For
example, there has been only one new high thrust engine developed in the
last 20 years, namely, the RS-68, which, BTW, is a legacy engine that relies
heavily on technology developed in the late 1960s for the excellent
Rocketdyne J-2S engine, which, in turn, was a simplified version of the
venerable J-2S engine that powered the 2nd and 3rd stages of von Braun's
Saturn V ELV.

Yes, I know all the tooling for shuttle is gone. But the tooling for CEV
doesn't exist either. However, there are a lot of things about Shuttle
which
are still manufactured (tiles, engines and I am sure countless other
components that are regularly replaced).