From Lift to Release: the Stages of a Launch

In a typical IUS launch, a primary booster (such as a Titan IVB) powers the initial liftoff from the pad. Within a few minutes, the booster's first-stage engine falls away and its second-stage engine kicks in, carrying the system to a low Earth or "parking" orbit. Soon thereafter, the IUS separates from the booster's second stage and assumes control for the remainder of the powered ascent. About an hour later, the first-stage IUS solid-fueled rocket begins firing. The second solid-fueled rocket motor ignites toward the end of the ascent, followed by a coast phase, and finally, separation of the payload. The IUS can hoist a 2268-kilogram satellite into a geosynchronous orbit or heave a 3628-kilogram spacecraft out into the solar system.

The IUS measures about 5.18 meters long by 2.9 meters in diameter and has an overall mass of nearly 14,800 kilograms. Two slightly different versions are available, one for use with the Titan and one for the shuttle launcher. The IUS first-stage solid-rocket motor holds about 9700 kilograms of propellant and generates more than 188,000 newtons of thrust. The second-stage solid-rocket motor holds more than 2700 kilograms of propellant and generates more than 80,000 newtons of thrust.

The first-stage motor can maintain continuous thrust for as long as 150 seconds—longer than any other solid-fueled upper-stage rocket developed for space applications. Actual duration can be tailored to suit mission requirements.