This is the S-IVB-D, or third stage of the Dynamic Test version of the Saturn V. This stage of the rocket was not meant to fly but was used for ground tests pior to the actual flight tests. As in the flyable version of the Saturn V, the stage shown here is fitted with one J-2 rocket engine, although it was inoperable. The Dynamic Test version of the rocket was extremely important in Apollo program - it proved the validity of the vehicle's structural design. In its flyable form, the 363-foot long, 33-foot diameter operational version of the Saturn V rocket took a dozen astronauts toward the Moon during 1969-1972 and is considered one of the greatest engineering achievements in history. The third stage was transferred to from NASA to the Smithsonian in 1975.