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)
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