(U) Inertial Upper Stage (IUS)

(The effects of any budget/program decisions made since the information was collected during 1997-98 are NOT reflected in the National Security Space Road Map (NSSRM).)

Overview: (U)
(U) The Inertial Upper Stage (IUS) is a two stage, solid fueled, upper stage designed to deliver a satellite to its operational orbit from a LEO parking orbit.  The IUS includes two solid rocket motors, an aft skirt, an interstage, and an equipment support section that includes avionics.  The IUS is used on the Titan IV and the STS and can lift 5,000 pounds from LEO to GEO.  

Description: (U)
(U) The Inertial Upper Stage (IUS) is a two stage, solid rocket propelled, three-axis stabilized vehicle for placing spacecraft in a high-Earth orbit or on an escape trajectory for an interplanetary mission.   The IUS was originally designed as a temporary stand-in for a reusable space tug.  The IUS was then named the Interim Upper Stage. The word "Inertial" (signifying the guidance technique) later replaced "Interim" when it was known that the IUS would be needed through the 1990's.  The IUS is 17 feet (5.18 meters) long and 9.25 feet (2.8 meters) in diameter, with an overall weight of approximately 32,500 pounds (14,742 kilograms). The IUS consists of a first stage comprised of a solid rocket motor (SRM1) containing 21,400 pounds (9,707 kilograms) of propellant generating approximately 42,000 pounds (188,496 newtons) of thrust and an interstage. The second stage consists of a solid rocket motor (SRM2) with 6,000 pounds (2,722 kilograms) of propellant generating approximately 18,000 pounds (80,784 newtons) of thrust, and an equipment support section. 

(U) The large SRM1 motor is the longest thrusting duration solid rocket motor ever developed for space application, with the capability to burn as long as 150 seconds. Mission requirements determine the thrust level and burn duration of the SRM1, which are controlled by tailoring the solid propellant load.

(U) After separation from the launch vehicle, the IUS onboard computers then direct a series of preparatory maneuvers and fire the first stage motor for approximately 140 seconds to propel the IUS and spacecraft toward the desired geosynchronous position. After a coast period of several hours, the second stage motor burns for approximately 100 seconds and injects the IUS into a final circularized orbit. The IUS then separates from the satellite and moves to a position where it will neither collide with or contaminate the satellite.

(U) The equipment support section houses the IUS avionics systems. These systems 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 reliability of better than 98 percent.

User Impact: (U)
(U)  The IUS provides heavy-lift capability to launch surveillance, and exploratory satellites that support DoD and national requirements.

Programmatics: (U)
(U) Operational.

Related Initiatives: (U)
(U) Advanced Upper Stage, Arnold Engineer Dev Cntr (AEDC), DSP, Eastern Range, Shuttle Operations, Titan IVA, Titan IVB

Related Requirements: (U)
(U) None.

Related Categories: (U)
(U) Spacelift, Upper Stages

Road Map Placements: (U)
(U) SPACE FORCES SUPPORT.

Lead Office: (U)
(U) Air Force.

Date of Information: (U)
04 November 1997

(The effects of any budget/program decisions made since the information was collected during 1997-98 are NOT reflected in the National Security Space Road Map (NSSRM).)