Inertial Upper Stage
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Painting of Ulysses deploying from the Space Shuttle
|Country of origin||United States|
|Used on||Space Shuttle
|Maiden flight||30 October 1982|
|Last flight||14 February 2004|
|Stage 1 - Orbus-21|
|Stage 2 - Orbus-6E|
The Inertial Upper Stage (IUS), originally designated the Interim Upper Stage, was a two-stage solid-fueled rocket upper stage developed by Boeing for the United States Air Force beginning in 1976 for raising payloads from low Earth orbit to higher orbits following launch aboard a Titan 34D or Titan IV rocket, or from the payload bay of the Space Shuttle. The IUS and similar generation intermediate boosters were able to raise a payload into geosynchronous orbit in approximately 8 hours after obtaining LEO (low Earth orbit.) The IUS spacecraft had on board the same inertial guidance systems used on numerous military and civilian aircraft. This manifested a huge advantage over earlier geosynchronous insertion methodologies that required anywhere between two to three weeks of ground-commanded maneuvering and meticulous tracking to achieve the same result. The higher cost of an IUS booster was offset by the saving of thousands of combined overtime hours incurred by civilian flight controllers during previous types of geostationary insertions.
During the development phase of the Space Shuttle (1969–1974), NASA, with reluctant support from the Air Force, wanted an upper stage that could be used on the Shuttle, but at the same time, could be switched over to the Titan III rocket (then the most powerful unmanned rocket in the U.S. fleet) 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's Orbital Maneuvering System (OMS) and Reaction Control System (RCS).
The solid-fueled IUS, with liquid-fueled 'trim burners', 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. The IUS upper-stage booster rocket design was based on the earlier Burner II upper-stage booster.
The IUS was powerful enough to deliver two large DoD or NSA satellites into orbital inclinations favorable for observing the 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 or Titan rockets.
The first launch of the IUS was in 1982 on a Titan 34D rocket from the Cape Canaveral Air Force Station shortly before the STS-6 Space Shuttle mission.  Boeing was the primary contractor for the IUS  while Chemical Systems Division of United Technologies built the IUS solid rocket motors .
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 a higher altitude. On Shuttle launches, the orbiter's payload 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 a "transfer orbit". Upon reaching apogee, the first stage and interstage structure were jettisoned. The second stage then fired to circularize 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 spacecraft towards planetary trajectories. For these missions, the second IUS stage was separated and ignited immediately after first stage burnout. Igniting the second stage at low altitude (and thus, high orbital speed) provided the extra velocity the spacecraft needed to escape from Earth orbit (see Oberth effect).
The final flight of the IUS occurred in February 2004.
The following missions 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 safety concerns brought on by the Space Shuttle Challenger disaster in 1986.
|S/N||Launch Date||Launch Vehicle||Payload||Remarks||Image|
|2||1982-10-30||Titan 34D||DSCS II F-16/III A-1||Mission successful despite telemetry loss for most of the flight.|
|TDRS-1 (TDRS-A)||Second stage tumbled due to a control system failure, resulting in an incorrect orbit. The spacecraft maneuvered itself into its final orbit.|
|USA-8 (Magnum)||Classified DoD payload|
|USA-11/12 (DSCS)||Classified DoD payload|
|TDRS-B||Destroyed during launch|
|Magellan||Probe to Venus|
|8||1989-06-14||Titan IV (402) A||USA-39 (DSP)|
|Galileo||Probe to Jupiter|
|USA-48 (Magnum)||Classified DoD payload|
|Ulysses||Probe to the polar regions of the Sun|
|6||1990-11-13||Titan IV (402) A||USA-65 (DSP)|
|20||1994-12-22||Titan IV (402) A||USA-107 (DSP)|
|4||1997-02-23||Titan IV (402) B||USA-130 (DSP)|
|21||1999-04-09||Titan IV (402) B||USA-142 (DSP)||IUS first and second stages failed to separate, payload placed into useless orbit|
|Chandra X-ray Observatory|
|22||2000-05-08||Titan IV (402) B||USA-149 (DSP)|
|16||2001-08-06||Titan IV (402) B||USA-159 (DSP)|
|10||2004-02-14||Titan IV (402) B||USA-176 (DSP)|
- "Inertial Upper Stage". Boeing. Archived from the original on 21 July 2012. Retrieved 21 July 2012.
- "Boeing launches two satellites". The Bulletin. UPI. 1 November 1982. p. 3. Retrieved 23 February 2014. "Boeing won the contract to develop the IUS in 1976..."
- Krebs, Gunter. "IUS". Gunter's Space Page. Retrieved 21 July 2012.
- "Tracking and Data Relay Satellite System (TDRSS)". NASA Space Communications. Retrieved 2009-06-25.
- Evolution of the Inertial Upper Stage Crosslink Winter 2003 (published by The Aerospace Corporation)
- Inertial Upper Stage at Federation of American Scientists