The Altair was a solid-fuel rocket with a fiberglass casing, initially developed for use as the third stage of Vanguard rockets. It was manufactured by Allegany Ballistics Laboratory (ABL) as the X-248. It was also sometimes called the Burner 1.
The X-248 was one of two third-stage designs used during Project Vanguard. Early launches used a stage developed by the Grand Central Rocket Company, but later launches used the X-248 which enabled the Vanguard to launch more massive payloads.
Altairs were used as the third stage of early Delta rockets.
The fourth stage of the Scout rocket also used the "Altair" stage.