This is an unflown Uni-Jet D3 model rocket motor. The "D3" signifies that the power of the motor is in the fourth highest class (out of seven) established by the National Association of Rocketry. It is made of pasteboard and contains non-explosive propellant. A motor is used only once to power a model rocket.
G. Harry Stine, a pioneer in the field of flying rocket models, donated it to NASM in 1973.
Gift of G. Harry Stine