GRAHAM WARWICK / WASHINGTON DC & TIM FURNISS / LONDON
Revised external tank fitting to eliminate foam shedding on take-off is a key measure
NASA says returning the Space Shuttle to flight "will be safety-driven not schedule-driven". Although the return to flight (RTF) plan unveiled last week establishes 11 March to 6 April 2004 as the first window for a daylight launch to the International Space Station (ISS), this is expected to slip, with 19 May to 9 June and 18 July to 6 August as the next suitable opportunities.
Achieving even the later dates requires near-term solutions to meeting the Columbia Accident Investigation Board's (CAIB) recommendations, while work continues on long-term answers. The STS-114 mission will demonstrate safety features recommended by the CAIB. The Space Shuttle Atlantis, with a crew of six or seven, will visit the ISS, where repairs are needed. The mission, lasting around 11 days, will not carry an ISS expedition crew, and Russian Soyuz TMA craft will be used for crew transfer.
NASA's near-term RTF plan is to eliminate critical debris shedding from the external tank (ET); improve ground- and vehicle-based imaging for debris damage discovery; survey the vehicle in orbit with cameras on the Shuttle and ISS manipulator arms; and develop a capability to repair thermal-protection system (TPS) tiles and reinforced carbon-carbon (RCC) panels in orbit. "Our first line of defence is: don't have debris come off and strike the Shuttle. The second is to have inspection techniques and repair capabilities so we can come home safely," says Shuttle programme manager Bill Parsons.
NASA is redesigning the ET forward bipodfitting to eliminate the foam ramps that shed debris and caused the Columbia accident, and incorporating redundant heaters to prevent ice formation. Liquid-oxygen feedline bellows will be redesigned to preclude ice and large foam ramps covering cable trays and pressurisation lines will be made smaller. Ideas are being evaluated to eliminate foam shedding from the liquid hydrogen/intertank flange area. The RTF ET is due for delivery in November.
NASA is testing tile repair techniques, using a silicon-based cure-in-place ablator, and plans to begin testing RCC repair concepts this month. Plans call for Shuttle and ISS cameras to be used for inspection, augmented in the short term by crew spacewalks to measure the depth of any tile damage. In the longer term, NASA plans to develop a laser sensor capable of producing three-dimensional maps of the Shuttle's exterior. Initially, TPS repair would be performed at the ISS, but NASA is planning an autonomous inspection and repair capability for non-ISS missions.
Although NASA is studying ways to enable the Shuttle to re-enter with minor RCC damage, it plans a contingency capability to sustain Shuttle crews on the ISS for up to 180 days should the orbiter be unfit for re-entry. This assumes three crew members can return to Earth in the Soyuz attached to the ISS, Shuttle fuel-cell water can be transferred to the station and Russian Progress vessels can bring critical consumables.
Source: Flight International
