This is an attitude control motor, or APS (Auxiliary Propulsion System), for the S-IVB (third stage) of the Saturn V launch vehicle which took men to the Moon in Project Apollo. APS motors provided three axis control of roll, pitch and yaw control for the S-IV during Earth orbit and injection into lunar orbit. They produced 150 pounds of thrust.
APS engines were used in all the Apollo Saturn V flights from 1967 to 1972. This particular motor has been fired in combustion tests. It was transferred to the NASM in 1974 by the McDonnell-Douglas Astronautics Co.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.