IUS malfunction

When Challenger made its debut on 4 April 1983 as STS-6, it carried TDRS 1 for NASA's Tracking and Data

The launch of STS-1.

The deployment of the HS-376 satellite Anik C2.

Relay System. As this 2.2-tonne satellite was too heavy for a PAM, it used the more powerful IUS, and this 18-tonne stack was carried lengthwise, filling the bay. The first step in bringing the stack to life was to elevate the cradle to 30 degrees. The two-stage IUS was heavily instrumented and the control system of each of its stages was verified. By using a camera installed at the rear of the bay, the astronauts confirmed that the first stage's nozzle could gimbal. Once TDRS 1 was verified by the Air Force's satellite operations facility in Sunnyvale in California, the power umbilical was disconnected, and the elevation increased to 60 degrees. At the appointed time, the ring-clamp holding the IUS in place was unlocked to enable the spring to eject it. Ten minutes later (by which time Challenger had withdrawn) the IUS was activated. After a further 30 minutes, it performed a series of star sightings to update its inertial platform. On crossing the equator it ignited its first-stage motor, which delivered 18,500 kilograms of thrust for 150 seconds for insertion into geosynchronous transfer orbit. The first stage then used its small thrusters to orient the stack and initiate a slow roll to even out thermal stresses during the 6-hour climb. A few minutes before reaching apogee, the first stage re-oriented the stack for the circularisation burn, and separated. The second stage was to have delivered 2,750 kilograms of thrust for 103 seconds but, towards the end of its burn, the oil-filled seal of the gimbal deflated, the nozzle slewed, and the offset thrust induced an end-overend tumble at a rate of 30 revolutions per minute. As soon as the flight controllers saw this in the telemetry they commanded the release of the satellite, which readily stabilised itself. The 21,700 by 35,550-kilometre orbit had little to recommend it as a communications relay. It was not only non-synchronous, but the fact that the IUS had not yet fully cancelled the initial 28.5-degree inclination left it nodding 3 degrees each side of the equator. The satellite had its own propulsion system, but this was only to enable it to adjust its station, not to make major manoeuvres. Nevertheless, it was able to assume its operating station. The IUS had redundant systems to overcome many faults, but it had no means of recovering from a mechanical failure in the manifold. All Shuttle missions that required the IUS were postponed pending the resolution of the problem, but it soon became evident that this would be a lengthy process.12 This was bad news for the Air Force, as its most modern satellites

An IUS carrying a TDRS satellite elevated on its cradle.

had been designed to exploit the Shuttle's cavernous payload bay and were too big to be off-loaded onto the rapidly depleting stock of Titan 34Ds. When the IUS flew again as STS-51C in January 1985 the first-stage underperformed, but the second stage made up the 16-metre-per-second velocity shortfall and delivered its Magnum electronic intelligence-gathering satellite as planned.

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