This is the X248-A2 solid fuel rocket motor, also called Altair, that powered the third stage of the Vanguard III launch vehicle, launched in 1959. Altair was also used as the third stage of the Thor-Able-4 vehicle that launched the Pioneer V space probe and Tiros 1, the first weather satellite, and third stage for the Pioneer I, Explorer VI, and other launch vehicles.
The motor, built by the Allegheny Ballistic Laboratory of Hercules Inc., produced 3,070 pounds of thrust and was made with a filament-wound fiber-reinforced epoxy resin. This very strong construction, plus the motor's high performance for its small weight, made it a pioneering development in solid fuel rocket technology. This object was donated to the Smithsonian in 1968 by the Alleghany Ballistics Lab.