The Inertial Upper Stage (initially known as the Interim Upper Stage) was a two-stage solid-propellant upper stage combination used for high-energy orbital insertions of satellites launched aboard the Space Shuttle and Titan rocket. The first stage was composed of an Orbus-21 solid rocket motor and a metallic interstage structure; the second stage was built around an Orbus-6E SRM and also contained the avionics and attitude control/orbit trimming thrusters used for the combined assembly.
IUS was initially intended as a temporary stage to support Shuttle-based satellite launches to geosynchronous orbit prior to the development of Shuttle-Centaur. Following the Challenger disaster, Shuttle-Centaur was canceled, and IUS was used for all remaining missions of this type. IUS also flew several times on the Titan-34D and Titan IV expendable launch vehicle, offering performance in between the basic no-upper-stage and Titan-Centaur variants. A few other upper stages were developed or proposed based on IUS components, such as the Transfer Orbit Stage.
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