Tests reveal potential danger to Shuttle despite modifications to boost safety

Modifications to the Space Shuttle external tank (ET) to prevent the shedding of foam insulation cannot guarantee that no debris will damage the orbiter during launch, testing has revealed.

The orbiter Columbia broke up during re-entry on 1 February 2003 after tank debris damaged the leading edge of the left wing during launch.

STS 114 Discovery's ET, now at the Kennedy Space Center in preparation for the Shuttle return to flight mission in May, has been modified with the aim of ensuring that no piece of debris heavier than 0.013kg can break away from the upper section during launch.

However, ongoing tests at the Southwest Research Institute (SRI) in San Antonio, Texas have resulted in slight damage to the thermal protection system's (TPS) reinforced carbon-carbon leading edge with pieces of foam weighing 0.01kg.

The object that hit the leading edge of Columbia weighed 0.75kg. This broke away from the raised ramp in the bi-pod region of the upper tank where it is attached to the upper part of the orbiter, adjacent to the liquid oxygen tank inside the ET. This region has been redesigned so there are no raised areas.

The criticality of debris shedding from progressively lower levels of the ET is reduced so that in the region of the liquid hydrogen tank the debris limit is 0.03kg.

Testing at the SRI indicates that there is still the potential for "entry critical" damage to the orbiter from ET debris shedding, although the risk has been lessened because it could cause a small crack rather than a fracture. The TPS has been damaged to some extent on all Shuttle missions.

The STS 114 crew will use a camera on the remote manipulator system and a laser sensor package to inspect the orbiter, particularly the leading edge panels, and will be able to spot even coating damage. Repairs to TPS material will be demonstrated during the mission.


Source: Flight International