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IUS top stages

by Jean-Jacques Serra
At first called Interim Upper Stage since it was devoted to fill the gap until the Space Tug shuttle became available, it became the Intertial Upper Stage when the project was stopped.

Boeing developped the IUS from 1976 to 1981. The vehicle was aimed to act as a top stage for STS but also for the Titan military launchers. IUS is a 2-stage solid fuel vehicle featuring UTC/CSD boosters with deployable nozzles. The propellant load (HTPB) can be adjusted depending of the mission (up to -50%).

Note: a 3-stage version designed for NASA was dropped, but some of the launches did required a third stage (probes).

The whole vehicle is 5.17 m high for 2.9 m diameter and weighs 14.76 tons ( Shuttle version) or 13.1 tons ( Titan 34D version). The first stage (SRM 1) is 3.52 m long and 2.34 m diameter. It used the Orbus-21 motor (up to 9.7 tons propellant) providing a mean thrust of 196 kN during 154 seconds (ISp = 295 seconds). This motor is also used for the TOS perigee stages, and those (spinned) of Intelsat 6 series and for the LLV top stage. The second stage (SRM 2) is 2.1 m long and 1.61 m diameter. It features an Orbus motor (up to 2.75 tons of propellant) available in 2 versions: 6 or 6E depending on the presence of an extensible nozzle. It provides 80 kN during 103 seconds (ISp = 289 seconds or 303 seconds respectively for version 6 and 6E).

This 3-axis stabilized vehicle is very reliable. Its launch capacity to GEO is:
- 2.27 tons from STS
- 1.82 tons from Titan 34D
- 2.36 tons or 2.86 tons from Titan 4 (CSD or Hercules boosters)

Titan IUS

IUS was used twice on Titan 34D: in Octiber 1982 (1st IUS launch) and in September 1989 to launch DSCS satellites.
The Titan 4 + IUS configuration is named Titan 402. It was introduced in June 1989 and is used to launch DSP satellites.


IUS succeeded all its missions from the Shuttle except the first one (partial failure). After the Challenger accident the Centaur was banned from STS so IUS became the only available vehicle.
Late 1995 the missions carried out by STS IUS were: TDRS, Magnum, DSCS, DSP and probes such as Magellan, Galileo, Ulysses

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