This weeks selection from the Flight International Spaceflight archive

STS-114 "Return to Flight" Space Shuttle Mission

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Space Shuttle Main Engines being installed on the Discovery at the Orbiter Processing Facility at NASA's Kennedy Space Center.

Good guess but not the right answer have another shot..........

 Saturn V

No, jetrobert.

Without the shadow of a doubt SSME, but ...... which mission ?

Not having seen any of the archives I would say Columbia for STS-1 or Discovery for STS-26R.

The very first mission in 1981 or the very first "Return to Flight" in 1988 .....

Maybe.

I found the same image (black and white) at p. 65 of  Aerofax Datagraph 5 -  "Rockwell International Space Shuttle" -  by Dennis Jenkins - 1989 - Aerofax, Inc. ...... with the following (useless) caption: "Each SSME produces 375,000lbs. of thrust at sea level.  The engines are regeneratively cooled by liquid hydrogen flowing through thrust chamber walls".

Well we shall see all will be revealed on Wednesday

Space Shuttle Orbiter Columbia main propulsion system after re-installation in July 1980, months before the April 12, 1981 first flight.