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]
Development of the Shuttle-Centaur was halted after the Challenger disaster, and the Interim Upper Stage became the Inertial Upper Stage.
Design
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]
Applications
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]
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.
^"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...
^Virginia Dawson; Mark Bowles. "Taming liquid hydrogen : the Centaur upper stage rocket"(PDF). nasa.gov. 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.
^ abcd"STS-30 PRESS KIT". April 1989. Archived from the original on 2000-08-28. Retrieved 2020-07-25. 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.
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