Tim Furniss/LONDON

The recent failure of the Boeing Inertial Upper Stage (IUS) has put into doubt the planned launch of the Chandra X-ray Observatory aboard Space Shuttle STS93/Columbia on 9 July.

The IUS, which is due to be used on the Chandra mission, failed to place a Defence Support Programme (DSP) early warning craft into the correct orbit on 10 April (Flight International, 21-27 April).

NASA is monitoring the US Air Force's investigation of the failure. The Chandra launch is unlikely to go ahead until the cause is identified. Although the IUS first stage succeeded in placing the DSP craft into an elliptical transfer orbit, the second stage failed to circularise the orbit to geostationary.

The USAF probe is focusing on the possible failure of the second stage nozzle to extend fully, and on other systems, including the stage's new flight control unit.

The IUS used "a flight controller introduced as part of an effort to update the vehicle's avionics", says Boeing. The new hardware "delivers higher reliability and greater accuracy, but is lighter and uses less power".

The IUS stage has flown on previous Shuttle missions for standard satellite deployments, but its Chandra flight profile is unique and challenging. The spacecraft and its IUS must be deployed quickly from the Columbia's payload bay.

The IUS stages must fire perfectly in quick succession to place the observatory into a 140,000km operating orbit before the spacecraft can deploy its solar arrays.

Source: Flight International