In article ,
Kyle Boatright wrote:
That said, the SSMEs are cranky, marginal engines, and taking *them* up to
120% (as was once intended) is much more iffy than doing the same for
robust engines like the H-1 or F-1.

What makes the SS engines "cranky and marginal" vs the H-1 and/or F-1?

Mostly, they were too much of a leap into the technological unknown at the
time: NASA tried to pioneer bold new technology on what was supposed to
be a long-lived reusable engine, and unsurprisingly, this didn't work too
well. The H-1 and F-1 were much more conservative designs -- notably,
although the F-1 was a lot bigger than anything previously built, the
project tried hard to *avoid* pioneering in any other way -- and although
they did hit surprises and ended up breaking some new ground, they had
much more continuity with previous experience.

(For a while there was some feeling that the SSME's staged-combustion
cycle was just *inherently* troublesome, but that idea sort of collapsed
when it became clear that every major Russian rocket engine since about
1960 had been a staged-combustion design, typically including features
like oxidizer-rich preburners, which even NASA had deemed impractically
difficult...)

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.
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