This is the X-259, or Antares II, the third-stage, solid-fuel rocket motor for the all-solid fuel U.S. Scout launch vehicle. Antares II produced about 23,500 pounds of thrust. Like the other stages of the Scout, Antares was named after a star constellation. (The Scout's first, second, and fourth stages were the Algol, Castor, and Altair, respectively.)
The Scout was conceived in 1958 as a low-cost launch vehicle for light payloads. The first Scout was launched in 1960 and continued as a very successful vehicle. This object was donated to the Smithsonian in 1968 by the Alleghany Ballistic Laboratory.
Gift of Alleghany Ballistics Laboratory