centaurbooster

I do believe this is a second stage Centaur buster.

Mock-up of the Centaur-G Prime stage (a.k.a. "Shuttle Centaur") in a mock-up Space Shuttle Payload Bay. Of course, post-Challenger disaster, the Centaur-G would never fly aboard the Shuttle, but would instead be adapted for use on the Titan IV as the Centaur-T. See: arstechnica.com/science/2015/10/dispatches-from-the-death...

These are models of the two variants of Shuttle Centaur. On the right is Centaur G-prime, to be used for civilian (NASA) interplanetary probes launched from KSC, Florida. On the left is Centaur G, to be used for military (USAF) polar spy satellites to be launched from Vandenberg AFB, California, and GEO satellites to be launched from KSC. The Challenger disaster ended the Shuttle Centaur program: Centaur G-prime flight articles were built but never flew, and the launch complex at Vandenberg AFB was built but never used. N.B.: The Centaur G-prime model (right) is shown with its payload, an interplanetary probe (probably Galileo), whereas the Centaur G model (left) does not include a model of its payload, which would have been classified information.

Here's a link to a great article about Shuttle-Centaur: www.cleveland.com/science/index.ssf/2011/12/long-forgotte...