Inertial Upper Stage

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.{{cite web|author1=Virginia Dawson|author2=Mark Bowles|title=Taming liquid hydrogen : the Centaur upper stage rocket|url=https://history.nasa.gov/SP-4230.pdf|website=nasa.gov|access-date=July 24, 2014|page=172|quote=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.}}

Boeing was the primary contractor for the IUS{{cite web|url=https://www.globalsecurity.org/space/systems/t4-config-2b.htm|title=Titan IV Inertial Upper Stage (IUS)|website=www.globalsecurity.org|access-date=2 February 2019}} while Chemical Systems Division of United Technologies built the IUS solid rocket motors.{{cite web|url=http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/carriers.html|title=SPACE TRANSPORTATION SYSTEM PAYLOADS|website=science.ksc.nasa.gov|access-date=2 February 2019|archive-date=21 December 2016|archive-url=https://web.archive.org/web/20161221070222/http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/carriers.html|url-status=dead}}

When launched from the Space Shuttle, IUS could deliver up to {{Convert|2,270|kg|lb}} directly to GEO or up to {{Convert|4,940|kg|lb}} to GTO.{{cite web|url=http://www.braeunig.us/space/specs/ius.htm|title=Inertial Upper Stage|access-date=21 July 2012}}

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.{{cite web|url=https://www.globalsecurity.org/space/library/report/1994/cape/cape2-6.htm|title=The Cape, Chapter 2, Section 6, TITAN 34D Military Space Operations and|website=www.globalsecurity.org|access-date=2 February 2019}}

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.{{cite web |url=https://science.ksc.nasa.gov/shuttle/missions/sts-30/sts-30-press-kit.txt |title=STS-30 PRESS KIT |date=April 1989 |quote=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. |access-date=2020-07-25 |archive-date=2000-08-28 |archive-url=https://web.archive.org/web/20000828140155/https://science.ksc.nasa.gov/shuttle/missions/sts-30/sts-30-press-kit.txt |url-status=dead }} Depending on mission, one, two or three {{cvt|120|lb|kg|order=flip}} tanks of hydrazine could be fitted.

Image:Galileo Deployment (high res).jpg 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. 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.{{cite web|author1=Virginia Dawson|author2=Mark Bowles|title=Taming liquid hydrogen : the Centaur upper stage rocket|url=https://history.nasa.gov/SP-4230.pdf|website=nasa.gov|access-date=July 24, 2014|page=211}}

The final flight of the IUS occurred in February 2004.

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File:1988 s26 TDRS-C.jpg|TDRS-C in Space Shuttle Discovery's payload bay

File:1988 s26TDRS-C Released.jpg|Release of TDRS-C

File:Ulysses sits atop the PAM-S and IUS combination.jpg|Ulysses used a PAM-S and IUS combination

File:Inertial Upper Stage mockup.jpg|An Inertial Upper Stage at the Museum of Flight in Seattle

{{reflist}}

• [https://web.archive.org/web/20140417071751/http://www.aerospace.org/wp-content/uploads/crosslink/V4N1.pdf Evolution of the Inertial Upper Stage] Crosslink Winter 2003 Vol 4 Num 1 (published by The Aerospace Corporation), page 38
• [http://www.fas.org/spp/military/program/launch/ius.htm Inertial Upper Stage] at Federation of American Scientists

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Category:Expendable space launch systems

Category:Rocket stages

Category:Space Shuttle program