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From: (GCHudson)
Subject: Re: Name the Only REUSABLE Rocket Engine --- SSME
Date: Aug 11 1995

The only two reasons that the J-2S  (note I said "S", which was the
Shuttle engine version) was not used in the Shuttle are:

1) when the USAF 65K payload requirement emerged, the engine didn't
produce enough thrust in the available boat-tail space of the orbiter,
thus higher pressure engines seemed warranted; and

2) the engine mafia needed to keep a lot of people employed both at NASA
Marshall and the contractor.

The J-2S had a delivered 455 Isp, BTW, but was meant as an altitude start
engine for a two-stage shuttle system.

Both the J-2 and J-2S demonstrated significantly longer
time-between-overhaul and overall life than the SSME.  In fact, the J-2
had ten times the amount of test time on it that the SSME had at the time
of FMOF (First Shuttle flight in 1981).  Single engines had been operated
for 40000 seconds; this is the smae amount of time the SSME had at first
flight on ALL test engines, let alone a production engine.

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