Why does the shuttle throttle on ascent?

"Henry Spencer" wrote in message
...
Three specific snags also aggravated this problem on the SSME:

(a) Most of the technology development on staged combustion had been done
by Pratt & Whitney, but oddly, the contract for the staged-combustion SSME
went to Rocketdyne instead. So the experienced people were shut out, and
the guys who were actually doing the work were having to come up to speed
on a technology that was new to them.

(b) The SSME program, like the shuttle in general, was starved for money
and opted to cut corners on subsystem testing in particular. The result
was an unusually long and painful development process, with subsystem
problems often not surfacing until whole-engine testing.

(c) Partly as a result of (a) and (b), it didn't become clear until too
late that the main LOX (I think it was) turbopump really needed one more
pump stage. Since a major redesign was politically and financially
impossible at that point, the result was a pump in which each stage was
pushed to the ragged edge of engineering practicality to meet a very
ambitious spec.

The combination of (b) and (c) was particularly nasty, because all too
often, a LOX-pump failure becomes a LOX-pump fire, which destroys the
evidence of what went wrong. Having this happen repeatedly to whole test
engines was just what an already-stressed development program didn't need.

To be fair, the engine has improved greatly since the first ones were built.

The current Block IIs appear to have incorporated several major changes
improving them, prolonging their cycles between tear-downs and over-all
making them far better than the originals.

(and much of the work was done by Pratt & Whitney.)

(of course I still think some people continue to hold old biases against the
SSME :-)

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