In article ,
Brian Gaff wrote:
But the actual thrust abilities of the smes was increased over the life of
the Shuttle, I'm sure I read that.

Correct -- originally 100% was to be tops (surprise, surprise), but later
the engines were qualified for 104%, and at the time of Challenger there
were plans to qualify them for routine operation at 109%, and possibly
more. Those plans got scaled back in the post-Challenger safety rethink.

There is nothing particularly unusual about this; most rocket engines grow
in thrust as experience builds up and small improvements are made. It
attracted attention on the shuttle only because of how it was expressed:
numbers above 100% sound vaguely alarming to the ignorant. The RS-27A
first-stage engine on modern Delta IIs runs at 153% of its original thrust
rating. The H-1 first-stage engines on the Saturn IBs that launched ASTP
were running at 124% of the thrust of the first H-1s, and even those were
110% upgrades of the S-3D Thor/Jupiter engines, which were themselves
substantially more powerful than still-earlier versions. Had there been a
second production batch of Saturn Vs, almost certainly the first-stage
engines would have been F-1As, running at 120% of the original F-1 thrust.

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.
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