What was the Inertial Upper Stage? A Short Summary
Galileo sits on top of the Inertial Upper stage, on a Space Shuttle. Credit: NASA
History
From 1982 to 2004, the Inertial Upper stage made 21 flights. The start of the IUS goes well before 1982, to when the Space Shuttle was in development. The National Aeronautics and Space Administration (NASA) and the United States Air Force wanted an upper stage that could be attached to the Space Shuttle to bring their payloads to higher orbits that the shuttle couldn’t reach on its own. Three options were available. There was the Transtage, used on earlier Titan rockets. The Transtage used liquid propellants and ran on hypergolic fuels. The other option was the Centaur, which was also liquid fuelled, but it ran on the cryogenic propellants LH2/LOX. Centaur was used on Atlas rockets before. The third and final option was the Interim Upper Stage, a solid fueled upper stage that was new. In the end, works were being done on getting the Interim Upper Stage and the Centaur on the shuttle. On October 30, 1982, the first IUS flew on a Titan 34D rocket, carrying USAF’s DSCS II F-16 satellite.
Available Launch Vehicles
A US Air Force Titan IV rocket launches with the final flight of the Inertial Upper Stage from Cape Canaveral. This photo was taken on February 14, 2004. Credit: United States Air Force
Available to the Inertial Upper Stage were the Space Shuttle, the Titan 34D, and the Titan IV.
The Titan IV was a two stage expendable launch vehicle developed by Martin Marietta in the late 1980s to satisfy the needs of the Air Force. The Titan IV would end up making 39 flights, with 8 of them carrying Inertial Upper Stages.
The Space Shuttle was a two stage partially reusable launch vehicle developed by NASA in the 1970s. The Shuttle would carry 15 Inertial Upper Stages throughout its lifespan, with many of those being both Department of Defense, and NASA payloads. Unfortunately, an IUS was lost during flight carrying NASA’s TDRS-B satellite during the STS-51-L mission.
Anatomy
The Inertial Upper Stage was not just one stage, but two stages. The first stage of the IUS comprised of an Orbus-21 motor that ran on HTPB.
Orbus-21:
ISP: 295s
Thrust (max): 190 kilonewtons
Burn time: 150 seconds
The first stage of the Inertial Upper Stage fires, carrying a TDRS satellite to Geosynchronous Transfer Orbit. This image is a simulation and is not real.
Attached to the top of the first stage was the second stage. The second stage was an Orbus-6E motor that was less powerful & efficient than the first stage.
Orbus-6E:
ISP: 289s
Thrust (max): 80 kilonewtons
Burn time: 103 seconds
The second stage of the Inertial Upper Stage fires over South America. The Orbus-6E motor is putting this satellite into Geosynchronous orbit. This image is not real and is a simulation added for visual reference.
The IUS’ second stage housed 12 Kaiser Marquardt 133 N thrusters that could be used to re-orient the vehicle. The IUS was also special because the nozzle on each motor was able to be moved, meaning they can control themselves and don’t require any sort of stabilization. If the second stage flew without the Extendible Exit Cone (EEC), it would lose ISP and be less efficient.
Notable Payloads on the Inertial Upper Stage
NASA’s TDRS satellites
USAF DSCS satellites
DSP Satellites
Magellan
Ulysses
Chandra X-Ray Telescope
Chandra X-Ray Telescope sits in the payload bay of Space Shuttle Columbia on the STS-93 mission. Credit: NASA
Magellan, Galileo, Ulysses, and Chanda X-Ray Telescope were all launched on the Space Shuttle on STS-30, STS-34, STS-41, and STS-93, respectively. Due to the energy needed for Ulysses’ orbit, the IUS carried a third stage, known as the PAM-S (Payload Assist Module - Shuttle).
Of the 24 IUS flights, only 1 was a complete failure. The flight was launched on a Titan IV, carrying a DSP satellite known as USA-142. Because there was an issue separating the first and second stage of the DSP, the satellite was placed into a useless transfer orbit.
The Inertial Upper Stage’s lesser-known successor
Developed by Martin Marietta in the early 90s, the IUS had a successor that not many know about. Meet the Transfer Orbit Stage, which only had two flights. Unlike the IUS, the TOS only used one stage. Like the IUS, it used an Orbus-21, though a different variant named the -21H. The TOS also had hydrazine control thrusters to orient the vehicle and was also able to be gimballed. It only had two launch vehicle options: the Space Shuttle, and the Commercial Titan III.
Its first flight was on the Commercial Titan III, carrying NASA’s Mars Observer spacecraft. While the TOS did its job well in delivering Mars Observer to interplanetary transfer orbit, the probe exploded before orbital insertion.
Its second and final was on STS-51, carrying NASA’s ACTS satellite.
The Transfer Orbit Stage is seen from Space Shuttle Discovery on the STS-51 mission. September 12, 1993. Credit: NASA
References:
National Museum of the USAF
NTRS
b14643.eu
Federation of American Scientists, archived
Excerpt from Taming liquid hydrogen: the Centaur upper stage rocket
STS-30 Press Kit - NASA
http://www.braeunig.us/space/specs/ius.htm
