The Inertial Upper Stage (IUS), originally known as the Interim Upper Stage, is a two-stage solid-fueled booster rocket developed by 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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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 development 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 notably the TDRS (Tracking and Data Relay Satellite) constellation into a geostationary orbit on either the Shuttle (STS), Titan III, or Titan IV.
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].
The IUS was a two-stage rocket launched by either a Titan family booster or as a "cargo" by the Space Shuttle. On Titan launches, the Titan booster would launch the IUS, carrying the payload into low earth orbit where it was separated from the Titan and ignited its first stage, which carried it into an elliptical "transfer" orbit to higher altitude. On Shuttle launches, the cargo bay was opened, the IUS and its payload raised to a 50° angle, and released. After the Shuttle separated from the payload to a safe distance, the IUS first stage ignited and, as on a Titan booster mission, entered "transfer orbit". Upon reaching apogee, the first stage and an interstage structure were jettisoned. The second stage then fired to circularlize the orbit, after which it released the satellite and, using its attitude control jets, began a retrograde maneuver to enter a lower orbit to avoid any possibility of collision with its payload.
In addition to the Communication and Reconnaissance missions described above, which placed the payload into stationary (24 hour) orbit, the IUS was also used to boost Planetary spacecraft. For these missions, the second IUS stage was separated and ignited immediately after first stage burnout. Igniting the second stage at low altitude provided the extra velocity the spacecraft needed to escape from earth orbit.
As of 2007[update], the following missions 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.
| # | Launch Date | Launch Vehicle | Payload | Notes | Image |
| 1 | October 30, 1982 | Titan 34D | DSCS-II F16 DSCS-III A1 |
Mission successful despite telemetry loss for most of the flight. | |
| 2 | April 4, 1983 | STS-6 | TDRS A | Second stage tumbled due to a control system failure. Over the period of several weeks, ground controllers used excess fuel in the TDRS to move it into proper orbit. | |
| 3 | January 24, 1985 | STS-51-C | Magnum 1 | Classified DoD payload. | |
| 4 | October 3, 1985 | STS-51-J | DSCS-III B4 DSCS-III B5 |
Classified DoD payload. | |
| 5 | January 28, 1986 | STS-51-L | TDRS B | Destroyed due to Challenger disaster. | |
| 6 | September 29, 1988 | STS-26 | TDRS C | ||
| 7 | March 13, 1989 | STS-29 | TDRS D | ||
| 8 | May 4, 1989 | STS-30 | Magellan | Science probe to Venus | |
| 9 | June 14, 1989 | Titan IVA | DSP 14 | ||
| 10 | October 18, 1989 | STS-34 | Galileo | Science probe for Jupiter and its moons | |
| 11 | November 22, 1989 | STS-33 | Magnum 2 | Classified DoD payload. | |
| 12 | October 6, 1990 | STS-41 | Ulysses | ||
| 13 | November 13, 1990 | Titan IVA | DSP 15 | ||
| 14 | November 15, 1990 | STS-38 | Magnum 3 or an SDS-2 Exact payload unknown |
Classified DoD payload. | |
| 15 | August 2, 1991 | STS-43 | TDRS E | ||
| 16 | November 24, 1991 | STS-44 | DSP 16 | ||
| 17 | January 13, 1993 | STS-54 | TDRS F | ||
| 18 | December 22, 1994 | Titan IVA | DSP 17 | ||
| 19 | July 13, 1995 | STS-70 | TDRS G | ||
| 20 | February 23, 1997 | Titan IVB | DSP 18 | ||
| 21 | April 9, 1999 | Titan IVB | DSP 19 | Spacecraft failed to separate from the second IUS stage. | |
| 22 | July 23, 1999 | STS-93 | Chandra X-ray Observatory | ||
| 23 | May 8, 2000 | Titan IVB | DSP 20 | ||
| 24 | August 6, 2001 | Titan IVB | DSP 21 | ||
| 25 | February 14, 2004 | Titan IVB | DSP 22 |
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). Although this is highly unlikely, as the final IUS launch used the last IUS vehicle and the Boeing IUS team has been disbanded, making the future production of IUS vehicles quite costly.
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