Date put back to allow extended investigation into foam insulation loss from tank that led to Columbia break-up
NASA has moved the target window for the next Space Shuttle flight to March 2005 to provide more time to understand foam insulation loss from the external tank. Orbiter Discovery has been picked for return-to-flight mission STS 114, with Atlantis to be on standby to launch a rescue mission within 45-90 days.
If Discovery's thermal-protection system is damaged by debris during launch, the seven crew members will be able to use the International Space Station as a safe haven for up to three months. If the flight is successful, Atlantis will fly the second test mission, STS 121, with Discovery on standby.
One reason for delaying the launch, now planned for between 6 March and 18 April next year, is the need to assess the condition of rudder speedbrake actuators on the three surviving orbiters. Actuators are being removed and X-rayed after the Discovery of corrosion and microcracks. Work is progressing fastest on Discovery, which is why it will fly first.
Moving the launch planning window also provides more time for development of a camera/laser boom that will be attached to the Shuttle's remote manipulator arm and used to inspect the vehicle for possible damage while in orbit. More time is also needed to develop potential on-orbit repairs for thermal-protection panels and tiles.
But the need to fully understand the loss of foam from the external tank (ET) is the biggest challenge for the Shuttle return-to-flight effort, says Bill Readdy, NASA associate administrator for spaceflight. Delaying the launch will provide more time to analyse and test a larger area of the ET for potential insulation loss, the agency says.
Readdy says NASA has determined why a large piece of foam was shed during lift-off, damaging Columbia's wing leading-edge and leading to the break up of the orbiter during re-entry. The root cause was cryopumping - air or nitrogen in voids in the insulation near the skin of the cryogenic tank liquified during fuelling, then warmed and expanded explosively during lift-off, blowing off a suitcase-sized chunk of foam. Most debris shed by the ET falls down along the tank, decelerating and not entering the high-velocity airflow between the tank and the orbiter. However, debris shed from the upper surface of the ET can enter that airflow and ram into the orbiter.
It is likely that faulty application of the spray-on foam created air pockets, Readdy says. Gaps around nuts and bolts used in construction of the ET may trap air, and both the bolt configuration and foam application are being reviewed. The aerodynamic foam wedges covering the bi-pod area where struts attach the ET to the orbiter will be replaced with heaters. The largest allowable debris will be 0.02kg (0.04lb), compared with an estimated 0.9kg for the piece of foam that hit Columbia.
TIM FURNISS / LONDON
Source: Flight International
