Inertial Upper Stage
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The Inertial Upper Stage or IUS is a two-stage solid-fueled booster rocket developed by NASA and the U.S. Air Force for the launching of large payloads from either a Titan III (later Titan IV) rocket or from the payload bay of the Space Shuttle.
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[edit] Development of the IUS
During the development phase of the Space Shuttle (1969-1974), NASA, with reluctant support from the Air Force, wanted an upper stage that can be used on the Space Shuttle, but at the same time, can be switched over to the Titan III rocket (then the most powerful unmanned rocket in the U.S. fleet, since the Saturn INT-21, a derivative of the Saturn V rocket, was only used once for the launch of Skylab in 1973), in the case the Shuttle ran into lengthy delays either from devleopment or testing. Although NASA wanted to adopt a version of the Centaur upper stage for its planetary missions, the Air Force wanted to use the "Transtage," a hypergolic upper stage used on most Titan III launches in which the Centaur was not needed, and unlike the Centaur, used the same fuel and oxidizer used on the Shuttle OMS and RCS systems.
The solid-fueled IUS was created as a compromise between the Transtage, which was not powerful enough for most NASA payloads, and the Centaur, which was not needed for all military and intelligence payloads. IUS was powerful enough to deliver two large DoD or NSA satellites into proper orbits over the former Soviet Union, or a single NASA payload (most noteably the TDRS (Tracking and Data Relay Satellite) constellation into a geostationary orbit on either the Shuttle or the Titan III.
IUS's first launch was in 1982 on a Titan 34D [1] rocket from the Cape Canaveral Air Force Station shortly before the STS-6 mission. Boeing was the primary contractor for the IUS[2]. Chemical Systems Division of United Technologies built the IUS solid rocket motors [3].
[edit] IUS Function
The IUS is a two-stage rocket motor which operates in two phases. On Titan launches, the Titan III or IV booster would launch the payload into a low earth orbit, upon which the stage and its payload is separated. On Shuttle launches, the IUS and payload is raised to a 50° angle, upon which it is then released and then the Shuttle separates from the payload to a safe distance, much like that employed on the spin-stabilized commercial satellites and their attached Payload Assist Module (PAM) "kick motors." The IUS first stage then ignites and, on most missions, will place the payload in an egg-shaped "transfer orbit", after which the first stage and interstage units are jettisoned. The second stage then fires to circularlize the orbit, after which it too is jettisoned, upon which the satellite will then start its mission.
[edit] Payloads That Used The IUS
As of 2006, the following payloads have used the IUS rocket, most of them from the Space Shuttle, especially after the Shuttle version of the Centaur upper stage was banned due to an after effect of the Challenger Disaster in 1986.
- All Shuttle-launched TDRS satellites
- Any Shuttle-launched DoD and NSA payloads that required its use
- Magellan probe to Venus
- Ulysses polar-orbiting solar probe
- Galileo orbiter and probe to Jupiter
- Chandra X-ray Observatory
[edit] Current Status
Currently, because of the use of the more efficient Centaur upper stage on the Atlas rockets, including the new Atlas V, the IUS has been in effect placed into "semi-retired" status, although it may be used in the future to augment the Delta IV rocket or even the planned Shuttle Derived Launch Vehicle (the Ares I and Ares V rockets).
