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Tucked away in a corner outside the main entrance to Building 4205 were three signs with no accompanying engines. This one reads

The J-2 Engine

Marshall's Legacy of Flight

The first liquid-hydrogen-fueled American rocket engine, the J-2 is best-known for powering the second and third stages of the powerful Saturn V rocket for NASA's Apollo moon missions, supporting America's successful mission of discovery to the moon.

The innovative J-2 spawned numerous engine development programs, including the space shuttle main engine. Today engineers at NASA's Marshall Space Flight Center and Pratt & Whitney Rocketdyne Inc. are developing the next generation, the J-2X, to boost the upper stage of the Ares I crew launch vehicle to orbit and help tomorrow's explorers carry on our mission in space.

J-2 Quick Facts

Propellants:	Liquid Oxygen/Liquid Hydrogen (5.5 mix)
Thrust (at sea Level):	200,000 lbs
Height:	11.1 feet
Diameter:	6.8 feet
Dry Weight:	3,480 lbs
First Flight:	1966
Last Flight:	1975
No. of Flights:	87

The fact that the J-2 was "the first liquid-hydrogen-fueled American rocket engine" might come as a surprise to quite a few rocket scientists (not to mention us Space Race nuts). The first production hydrogen engine was the RL-10, which powered both the Centaur upper stage and the S-IV (second) stage of the Saturn I.

And I'm not quite sure to make of the "No. of Flights: 87". If it meant "Total No. of Engines Flown", the number would be 86 (which is at least close to 87): As the engine on the S-IVB (second) stage of the Saturn IB, a single J-2 flew on each of the unmanned AS-201 through AS-204 missions (4) and on Apollo 7 (5). Five J-2s flew as second stage engines on each of the 13 Saturn Vs launched (70) and a single J-2 flew as a third stage engine on the first 12 Saturn Vs (82; the final Saturn V launched Skylab, so there was no third stage engine). After the last of the Saturn Vs, a single J-2 flew as second stage engines on each of the three Saturn IB missions to service Skylab (85), and one final J-2 powered the second stage of the Saturn IB used for the Apollo-Soyuz Test Project (ASTP) (86).

Perhaps the missing engine was the one which was secretly developed and launched before the RL-10? :-)