Inertial Upper Stage
Painting of Ulysses deploying from the Space Shuttle
United Technologies
Country of originUnited States
Used onSpace Shuttle
Titan 34D
Titan IV
General characteristics
Height5.2 m (17 ft)[1]
Diameter2.8 m (9 ft 2 in)
Gross mass14,700 kg (32,400 lb)
Associated stages
Launch history
Total launches24
(stage only)
Lower stage
First flight30 October 1982
Last flight14 February 2004[2]
Stage 1
Height3.15 m (10.3 ft)[3]
Diameter2.34 m (7 ft 8 in)[3]
Gross mass10,400 kg (22,900 lb)[3]
Propellant mass9,700 kg (21,400 lb)[1]
Powered byOrbus-21
Maximum thrust190 kN (43,000 lbf)[1]
Specific impulse295.5 s[3]
Burn timeup to 150 seconds[1]
Stage 2
Height1.98 m (6 ft 6 in)[3]
Diameter1.60 m (5 ft 3 in)[3]
Gross mass3,000 kg (6,600 lb)
Propellant mass2,700 kg (6,000 lb)[1]
Powered byOrbus-6
Maximum thrust80 kN (18,000 lbf)[1]
Specific impulse289.1 s[3]

The Inertial Upper Stage (IUS), originally designated the Interim Upper Stage, was a two-stage, solid-fueled space launch system developed by Boeing for the United States Air Force beginning in 1976[4] for raising payloads from low Earth orbit to higher orbits or interplanetary trajectories following launch aboard a Titan 34D or Titan IV rocket as its upper stage, or from the payload bay of the Space Shuttle as a space tug.


During the development of the Space Shuttle, NASA, with support from the Air Force, wanted an upper stage that could be used on the Shuttle to deliver payloads from low earth orbit to higher energy orbits such as GTO or GEO or to escape velocity for planetary probes. The candidates were the Centaur, propelled by liquid hydrogen and liquid oxygen, the Transtage, propelled by hypergolic storable propellants Aerozine-50 and dinitrogen tetroxide (N2O4), and the Interim Upper Stage, using solid propellant. The DOD reported that Transtage could support all defense needs but could not meet NASA's scientific requirements, the IUS could support most defense needs and some science missions, while the Centaur could meet all needs of both the Air Force and NASA. Development began on both the Centaur and the IUS, and a second stage was added to the IUS design which could be used either as an apogee kick motor for inserting payloads directly into geostationary orbit or to increase the payload mass brought to escape velocity.[5]

Boeing was the primary contractor for the IUS[6] while Chemical Systems Division of United Technologies built the IUS solid rocket motors.[7]

When launched from the Space Shuttle, IUS could deliver up to 2,270 kilograms (5,000 lb) directly to GEO or up to 4,940 kilograms (10,890 lb) to GTO.[3]

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.[8]

Development of the Shuttle-Centaur was halted after the Challenger disaster, and the Interim Upper Stage became the Inertial Upper Stage.


The solid rocket motor on both stages had a steerable nozzle for thrust vectoring. The second stage had hydrazine reaction control jets for attitude control whilst coasting, and for separation from payload.[9] Depending on mission, one, two or three 54 kg (120 lb) tanks of hydrazine could be fitted.[9]


The Galileo spacecraft and its attached Inertial Upper Stage (IUS) booster being deployed after being launched by the Space Shuttle Atlantis on the STS-34 mission

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 (by the IUS Airborne Support Equipment (ASE)) to a 50-52° angle, and released.[9] 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 in the transfer orbit, 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). IUS could not impart as much velocity to its payload as Centaur would have been able to: while Centaur could have launched Galileo directly on a two-year trip to Jupiter, the IUS required a six-year voyage with multiple gravity assists.[10]

The final flight of the IUS occurred in February 2004.[2]


Serial number[11] 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.
1 1983-04-04 Space Shuttle
Challenger (STS-6)
TDRS-A (TDRS-1) The second stage tumbled due to a thruster motor problem, resulting in an incorrect orbit. The Boeing staff that was monitoring the flight was able to separate the tumbling IUS from the satellite so it could be maneuvered into its final orbit.
11 1985-01-24 Space Shuttle
Discovery (STS-51-C)
USA-8 (Magnum) Classified DoD payload[12]
12 1985-10-03 Space Shuttle
Atlantis (STS-51-J)
USA-11/12 (DSCS) DoD payload. Declassified in 1998.[13]
3 1986-01-28 Space Shuttle
Challenger (STS-51-L)
TDRS-B Destroyed during launch[14]
7 1988-09-29 Space Shuttle
Discovery (STS-26)
9 1989-03-13 Space Shuttle
Discovery (STS-29)
18 1989-05-04 Space Shuttle
Atlantis (STS-30)
Magellan Probe to Venus. Only one tank of hydrazine.[9]
8 1989-06-14 Titan IV (402) A USA-39 (DSP)
19 1989-10-18 Space Shuttle
Atlantis (STS-34)
Galileo Probe to Jupiter
5 1989-11-23 Space Shuttle
Discovery (STS-33)
USA-48 (Magnum) Classified DoD payload[12]
17 1990-10-06 Space Shuttle
Discovery (STS-41)
Ulysses on PAM-S Probe to the polar regions of the Sun
6 1990-11-13 Titan IV (402) A USA-65 (DSP)
15 1991-08-02 Space Shuttle
Atlantis (STS-43)
14 1991-11-24 Space Shuttle
Atlantis (STS-44)
USA-75 (DSP)
13 1993-01-13 Space Shuttle
Endeavour (STS-54)
20 1994-12-22 Titan IV (402) A USA-107 (DSP)
26 1995-07-13 Space Shuttle
Discovery (STS-70)
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
27 1999-07-23 Space Shuttle
Columbia (STS-93)
Chandra X-ray Observatory Last launch of a payload using IUS on a Space Shuttle.
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)



  1. ^ a b c d e f "Inertial Upper Stage". Archived from the original on March 11, 2016. Retrieved July 13, 2014.
  2. ^ a b "Inertial Upper Stage". Boeing. Archived from the original on 16 July 2012. Retrieved 21 July 2012.
  3. ^ a b c d e f g h "Inertial Upper Stage". Retrieved 21 July 2012.
  4. ^ "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...
  5. ^ Virginia Dawson; Mark Bowles. "Taming liquid hydrogen : the Centaur upper stage rocket" (PDF). p. 172. Retrieved July 24, 2014. They argued that the IUS, which was designed by the Air Force, was a potentially better rocket. The first stage of the two-stage rocket was capable of launching medium-sized payloads at most. This limitation would be overcome by means of the addition of a second stage for larger payloads with destinations into deeper space. Specifically, the Air Force asked NASA to develop an additional stage that could be used for planetary missions such as a proposed probe to Jupiter called Galileo.
  6. ^ "Titan IV Inertial Upper Stage (IUS)". Retrieved 2 February 2019.
  7. ^ "SPACE TRANSPORTATION SYSTEM PAYLOADS". Archived from the original on 21 December 2016. Retrieved 2 February 2019.
  8. ^ "The Cape, Chapter 2, Section 6, TITAN 34D Military Space Operations and". Retrieved 2 February 2019.
  9. ^ a b c d "STS-30 PRESS KIT". April 1989. The IUS is 17 feet long and 9.25 ft. in diameter. It consists of an aft skirt; an aft stage solid rocket motor (SRM) containing approximately 21,400 lb. of propellant and generating approximately 42,000 lb. of thrust; an interstage; a forward stage SRM with 6,000 lb. of propellant generating approximately 18,000 lb. of thrust; and an equipment support section. - The equipment support section contains the avionics, which provide guidance, navigation, control, telemetry, command and data management, reaction control and electrical power. All mission-critical components of the avionics system, along with thrust vector actuators, reaction control thrusters, motor igniter and pyrotechnic stage separation equipment are redundant to assure better than 98 percent reliability. - The IUS two-stage vehicle uses both a large and small SRM. These motors employ movable nozzles for thrust vector control. The nozzles provide up to 4 degrees of steering on the large motor and 7 degrees on the small motor. The large motor is the longest thrusting duration SRM ever developed for space, with the capability to thrust as long as 150 seconds. Mission requirements and constraints (such as weight) can be met by tailoring the amount of propellant carried.
  10. ^ Virginia Dawson; Mark Bowles. "Taming liquid hydrogen : the Centaur upper stage rocket" (PDF). p. 211. Retrieved July 24, 2014.
  11. ^ Krebs, Gunter. "IUS". Gunter's Space Page. Retrieved 21 July 2012.
  12. ^ a b Krebs, Gunter D. "Orion 1, 2 (Magnum 1, 2)". Gunter's Space Page. Retrieved December 5, 2022.
  13. ^ Mars, Kelli (2020-10-02). "35 Years Ago: STS-51J – First Flight of Space Shuttle Atlantis". NASA. Retrieved 2022-06-27.
  14. ^ "Tracking and Data Relay Satellite System (TDRSS)". NASA Space Communications. Archived from the original on 2009-03-20. Retrieved 2009-06-25.