THE INERTIAL UPPER STAGE (IUS)/PAYLOAD ASSIST MODULE (PAM-S) FOR STS-41 The Inertial Upper Stage (IUS) will be used with a Payload Assist Module (PAM-S) and the Space Shuttle during the STS-41 mission to transport NASA and ESA's Ulysses spacecraft out of Earth's orbital plane to explore the polar regions of the sun. The IUS was originally designed as a temporary stand-in for a reusable space tug and the vehicle was named the Interim Upper Stage. The word "Inertial" (signifying the guidance technique) later replaced "Interim" when it was seen that the IUS would be needed through the mid-1990's. In addition to the Ulysses mission, the IUS has been used to place three Tracking and Data Relay Satellites (TDRS's) into orbit, inject the Magellan spacecraft into its interplanetary trajectory to Venus, and send the Galileo Spacecraft to orbit Jupiter. The IUS, with a third stage PAM-S, is now being used by the agency for the Ulysses solar-polar mission. The IUS was developed jointly with and built under contract to the Air Force Systems Command's Space Systems Division. The Space Systems Division is executive agent for all Department of Defense activities pertaining to the Space Shuttle system and provides the IUS to NASA for Space Shuttle use. After 2-1/2 years of competition, Boeing Aerospace, Seattle, Washington, was selected in August 1976 to begin preliminary design of the IUS. Since then, 23 IUS vehicles have been built and six have been flown on NASA missions. Specifications IUS-17, the IUS to be used on Mission STS-41, is a two stage vehicle weighing approximately 32,500 pounds. The generic IUS is 5.18 meters (17 feet) long and 2.8 meters (9.25 feet) in diameter. It consists of an aft skirt; an aft stage solid rocket motor containing 21,400 pounds of propellant and generating approximately 42,000 pounds of thrust; an interstage; a forward stage solid rocket motor with 6,000 pounds of propellant generating approximately 18,000 pounds 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. The IUS vehicle provides redundant operation for mission-critical components in the avionics system, the thrust vector actuator system, the reaction control thruster system, and the solid rocket motor ignition and pyrotechnic stage separation systems to assure reliability of better than 98 percent. The IUS utilizes a specially developed third stage called the PAM-S to provide the energy required for the Ulysses mission. Payload Assist Module (PAM-S) The Payload Assist Module (PAM-S) is the third stage of the upper stage system used to transfer the Ulysses spacecraft from the Orbiter's low earth orbit into a solar-polar trajectory via Jupiter. The PAM-S utilizes a PAM-D Star-48B solid rocket motor with 4437 pounds of propellant. It is spin stabilized after separation from the IUS using a gas generator powered free spin system. A nutation control system provides coning control of the PAM-S/ULS spacecraft during the spinning operation period following motor burn. The PAM-S utilizes an ordnance sequencing system (OSS) to control its event sequencing which includes an ordnance timer. The timer is activated by IUS command prior to its separation from the IUS. Once separated from the IUS, all commands to the PAM-S are internally generated. Airborne Support Equipment (ASE) The IUS Airborne Support Equipment (ASE) is the mechanical, avionics, and structural equipment located in the Orbiter. The ASE supports the IUS/PAM-S and the Ulysses spacecraft in the Orbiter payload bay and elevates the combination for final checkout and deployment from the Orbiter. The IUS ASE consists of the structure, electromechanical devices, batteries, electronics, and cabling to support the Ulysses/IUS/PAM-S combination. These ASE subsystems enable the deployment of the combined vehicle; provide, distribute and/or control electrical power to the IUS; provide plumbing to cool the radioisotope thermoelectric generator (RTG) aboard Ulysses; and serve as communication paths between the IUS/PAM-S and spacecraft and the Orbiter. IUS/PAM-S Structure The IUS/PAM-S structure physically unites the components of the IUS/PAM-S and attaches them to the spacecraft and the Orbiter. The structure is capable of withstanding all the loads generated during operation including those imposed by the cantilevered spacecraft. In addition, the structure physically supports all the equipment and solid rocket motors within the IUS/PAM-S, and provides the mechanisms for IUS/PAM-S stage separation. The major structural assemblies of the three-stage IUS/PAM-S are the adapters, the IUS equipment support section, and interstage. The adapters attach the IUS to the Orbiter, the IUS to the PAM-S, and the PAM-S to the spacecraft. The Equipment Support Section houses the majority of the avionics of the IUS. The forward end of the equipment support section contains the PAM-S interface mounting adapter and electrical interface connector segment for mating and integrating the PAM-S with the IUS. Thermal protection for the ESS is provided by a multilayer insulation blanket across the interface between the IUS and PAM-S. The interstage joins the two stages of the IUS. IUS Avionics Subsystems The avionics subsystems consist of the telemetry, tracking, and command subsystems; guidance and navigation subsystem; data management; thrust vector control; and electrical power subsystems. These subsystems include all the electronic and electrical hardware used to perform all computations, signal conditioning, data processing, and formatting associated with navigation, guidance, control, data and redundancy management. The IUS avionics subsystems also provide the equipment for communications between the Orbiter and ground stations, as well as electrical power provision, control and distribution. IUS Solid Rocket Motors (SRM's) The IUS/PAM-S three-stage vehicle uses a large IUS solid rocket motor, a small IUS solid rocket motor and a PAM-S solid rocket motor. The IUS motors employ movable nozzles for thrust vector control or steering. These nozzles provide up to four degrees of steering on the large motor and seven degrees on the small motor. The PAM-S solid rocket motor employs no active steering. The IUS-17 first stage motor will carry 21,638 pounds of propellant; the second stage 6,057 pounds; the PAM-S has 4,437 pounds of propellant and expendable inerts. Reaction Control System (RCS) and Nutation Control System (NCS) The IUS reaction control system controls the IUS and payload's attitude during coasting, roll control during SRM thrusting, and a final collision avoidance maneuver after separation from the PAM-S/Ulysses spacecraft. The reaction control system consists of hydrazine fuel tankage and reaction control thrusters. IUS-17 will carry one tank, with 50 pounds of fuel, for the IUS RCS. To minimize spacecraft contamination, the IUS has no forward facing thrusters. The PAM-S has no RCS but uses spin stabilization and nutation suppression to control vehicle pointing during its operation. PAM-S carries propellant (six pounds) for its nutation control system. Following separation of the PAM-S/Ulysses spacecraft from the IUS/PAM-S aft adapter, a gas generator system mounted on the motor support section spins the PAM-S/spacecraft to approximately 70 RPM. After spin-up the motor support section is separated by release of a clamp band assembly. The PAM-S and Ulysses spacecraft continue to spin throughout PAM-S burn. At PAM-S burnout, the PAM-S/Ulysses are despun using the despin assembly. Despin Assembly The despin assembly is used to reduce the spin rate of the PAM-S/SC from approximately 70 RPM down to approximately 7 RPM after SRM burnout and prior to spacecraft separation. The despin assembly consists of two weights (approximately 2 pounds each) attached to a 477-inch flat kevlar cable which dissipate energy to slow the PAM-S during deployment. These weights are released by cable cutters initiated by the ordnance sequencing system and are detached automatically from the PAM-S after the deployment. Spacecraft Separation Separation of the PAM-S from the spacecraft is accomplished with four redundantly actuated pyrotechnic devices at the spacecraft-to-PAM-S interface and two redundantly actuated pyrotechnic disconnects at the spacecraft RTG interface, and two electrical disconnects between PAM-S and the spacecraft. Flight Sequence After the Orbiter payload bay doors are opened in orbit, the Orbiter will maintain a preselected attitude to keep the payload within thermal requirements and constraints. On-orbit predeployment checkout begins, followed by an IUS command link check and spacecraft communications command check. Orbiter trim maneuver(s) are normally performed at this time. Forward payload restraints will be released and the aft frame of the airborne support equipment will tilt the Ulysses/IUS/PAM-S to 29 degrees. This will elevate the spacecraft just outside the Orbiter payload bay, allowing direct communication with the ground during systems checkout. The Orbiter will then be maneuvered to the deployment attitude. If a problem develops within the spacecraft or upper stage, the upper stage and its payload can be restowed from this position. Prior to deployment, the payload electrical power source will be switched from Orbiter power to IUS internal power by the Orbiter flight crew. After verifying that the payload is on IUS internal power and that all Ulysses/IUS/PAM-S predeployment operations have been successfully completed, a GO/NO-GO decision for Ulysses/IUS/PAM-S deployment will be sent to the crew. When the Orbiter flight crew is given a GO decision, they will activate the ordnance that separates the IUS umbilical cables. The crew will then command the electromechanical tilt actuator to raise the tilt table to a 58-degree deployment position. The Orbiter's RCS thrusters will be inhibited and an ordnance separation device initiated to physically separate the Ulysses/IUS/PAM-S combination from the tilt table. Compressed springs provide the force to separate the Ulysses/IUS/PAM-S from the Orbiter payload bay at approximately 0.14 meters (5.7 inches) per second. The deployment is normally performed in the shadow or the Orbiter or in Earth eclipse. Following deployment, the tilt table will then be lowered into the payload bay. A small Orbiter maneuver will be made to back away from the Ulysses/IUS/PAM-S. Approximately 19 minutes after deployment, the Orbiter's engines will be ignited to move the Orbiter farther away from the IUS/spacecraft. At this point, the IUS/PAM-S is controlled by the IUS onboard computers. Approximately 10 minutes after deployment from the Orbiter, the IUS primary onboard computer will send out signals used by the IUS to begin mission sequence events. This signal will also enable the reaction control system and the Ulysses RTG pressure release device. All subsequent operations from transfer orbit injection through PAM-S separation and IUS deactivation will be sequenced by the IUS computer. When the IUS RCS has been activated, the IUS will maneuver to the required thermal attitude and perform any required spacecraft thermal control maneuvers. At approximately 45 minutes after deployment from the Orbiter, the ordnance inhibits for the first solid rocket motor will be removed. The belly of the Orbiter will have already been oriented towards the Ulysses/IUS/PAM-S combination to protect the Orbiter windows from the IUS's plume. The IUS will recompute the first ignition time and maneuvers necessary to attain the proper attitude for the first thrusting period. When the proper transfer orbit opportunity is reached, the IUS will orient to the proper burn attitude and the IUS computer will send the signal to ignite the first-stage motor. This is expected at approximately 65 minutes after deployment. After firing approximately 150 seconds, the IUS first stage will have expended its fuel and then be separated from the second IUS stage. Approximately 125 seconds after burnout of the first stage, the second-stage motor will be ignited, thrusting about 108 seconds. The IUS second stage will then be separated from the PAM-S and perform a final collision/contamination avoidance maneuver before deactivating. The PAM-S will ignite at approximately 83 seconds after IUS SRM-2 burnout. PAM-S burns approximately 88 seconds. Following a 10-minute coast and despin to approximately 7 RPM, the Ulysses spacecraft will separate, leaving the Ulysses spacecraft on its trajectory to Jupiter and its final mission orbiting the poles of the Sun.