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SHUTTLE DEPLOYS CHANDRA AFTER LAUNCH PROBLEMS

"Columbia, Houston. You are go for deploy." With these words, the NASA flight control room clears the crew of Space Shuttle Columbia at the morning of 23 July to deploy the $1,5 billion Chandra X-ray telescope. At that point, which was supposed to be mission STS-93's highlight, the crew had already past the most dramatic moments of the flight. Two technical problems right after lift-off almost led to an early in-flight abort of the mission - or worse.

Five seconds after Columbia lifted off the launch pad, a short in the Shuttle's electric kicked the main computers of two of the three main engines (SSME) off the line. Even though the respective back-up systems took over flawless, this event left the orbiter with a considerably reduced redundancy against a double engine failure. Would such a failure happen right after lift-off, the Shuttle and its crew would be lost.

A second, independent malfunction also occurred during the launch phase of STS-93. A leak developed in three of the numerous cooling tubes through which liquid hydrogen is ducted to cool the nozzle. This fault stayed undetected during the entire 8 minute and 33 ascent.

Only after main engine cut-off did it become evident that something was wrong. The orbiter had used approximately 1800 kg more oxygen than planned. The main engines cut off one second early leaving the orbiter 50 km/h short of the planned acceleration and leaving the Shuttle in a 12 km lower orbit than planned.

According to NASA there was no risk for a catastrophic failure. However, the leakage changed the mixture ratio of liquid oxygen and liquid hydrogen, such generating higher than usual temperatures in the pre-combustion combustor. Had the situation deteriorated, the engine would have overheated and subsequently been automatically shut off, resulting in an in-flight abort of the mission.

The exact abort procedure would have much depended on the position of the Shuttle in its ascent. Had a single engine failed in the first 3 minutes 20 seconds after lift-off, the crew would have tried to fly the orbiter back to the launch site and land it.

This so called Return to Launch Site (RTLS) abort manoeuvre is extremely risky and has not been done yet. After initiating the abort, the Shuttle would first continue in flight direction to burn off the fuel. The goal is to leave just enough fuel in the tank to turn the vehicle around and, after the tank is dropped, glide back to the Kennedy Space Center. During the peak of the manoeuvre the Shuttle reaches a speed of approximately 10000 km/h and an altitude of 400000 ft (120 km).

After a certain point of the ascent the crew also has the option of landing at an emergency landing field on the other side of the Atlantic ocean. NASA has a total of four landing fields available, two in Spain and two in Africa. for STS-93, Ben Guerir in Marocco, which is the primary emergency field, was activated. Ben Guerir is a former US Strategic Air Command base and has a 4260 m long and 60 m wide runway.

Other abort options include the selection of an alternate orbit. The so called Abort to Orbit (ATO) is selected if the main engines cut off too early, but the orbiter can be manoeuvred into a lower but usable orbit with the help of the Shuttle's orbital manoeuvring system. The fourth abort option is the Abort once around (AOA) which would be selected if a major system failure (pressure leak) would require an immediate return of the Orbiter and its crew. During the AOA, the orbital manoeuvring system is used to position the Shuttle for re-entry.

NASA began researching the source of the malfunctions right after landing of the Shuttle on 27 July. STS-93 mission commander Eileen Collins, who was also the first woman to command a Shuttle crew, said after return: "We don't want this to happen again."

Meanwhile the engineers have located the causes for the two defects. The nozzle tubes were damaged by a gold-plated nail-size Inconel pin. Engine manufacturer Boeing Rocketdyne uses these pins routinely to plug up suspect oxygen injectors. One of the two pins in Columbia's right engine came loose. The upcoming Shuttle flights will have newer engines without plugs.

The short in the electric was caused by a damaged wire in the Shuttle's payload bay. 300 miles of cables are running on both sides of the payload bay to connect the front and the rear area of the orbiter. The wire isolation was damaged on a rough screw head.

Following this finding NASA ordered a fleet-wide cable inspection of all Shuttles. There was another damaged wire found on the other side of Columbia's payload bay. Damage was also found in Atlantis. Furthermore, the radar payload for the SRTM mission (STS-99) was unloaded again from Shuttle Endeavours payload bay to be able to inspect this orbiter also. The Shuttle Radar Topography Mission is now rescheduled from its mid-September launch to the beginning of October. Possibly the urgent repair mission to the Hubble space telescope will be launched first.

Hubble's new colleague, the Chandra X-ray observatory, which was launched by STS-93, has reached its final orbit of 10000 by 140000 km. Chandra has already transmitted first images of an superb quality. The American X-ray telescope will not be alone for long. The European X-ray satellite XMM will be launched on board the first commercial Ariane-5 flight on 15 December, 1999.

From page46 of FLUG REVUE 10/99