TIM FURNISS/ LONDON & GRAHAM WARWICK / WASHINGTON DC
Orbiter experiments recorder reveals anomalous sensor readings while Columbia was still off Californian coast
Investigators are almost certain the Space Shuttle Columbia lost a wing access panel in orbit, but have not determined whether this is the source of the thermal breach that caused the orbiter to break up during re-entry. Radar cross-section testing has eliminated all but a carrier panel from consideration as the object observed separating from Columbia on its second day in orbit.
Carrier panels seal the gap between the leading-edge reinforced carbon-carbon (RCC) panels and the underwing thermal-protection tiles. They carry smaller tiles through which pass bolts that allow the panels to be removed to provide access to the RCC attachments.
Analysis of images from Columbia's launch indicate the piece of insulation foam that separated from the external-tank bipod area 82s after lift-off struck the underside of RCC panel 6 on the left wing leading-edge. The estimated impact footprint covers two carrier panels and the T-seals between neighbouring RCC panels.
Data on the orbiter experiments (OEX) recorder recovered from the Columbia debris points to a breach of the left wing leading-edge, with anomalous sensor readings earlier than first thought. The first to go off-nominal is a strain gauge behind the wing spar near RCC panel 9, outboard of 6. The reading started to rise 270s after re-entry began, 206s before the earliest anomalous event - and increase in yaw - detected in telemetry.
The first temperature anomaly, recorded from a sensor in front of the spar on an outboard attachment point for RCC panel 9, was 290s after entry interface. Another sensor, behind the same spar, went off-nominal 135s later, while Columbia was still off the coast of California, peaking at 218°C (450°F) before going off-line when the wire burned through. This is at least 63s before the first temperature rise detected in telemetry from the left wheel well.
Columbia accident investigation board member Roger Tetrault says the OEX recorded data from 670 sensors, including 15 in the left wing that all went "off-scale low" while Columbia was still off California. Seventeen sensors in the left wing show "a warming trend compared to previous flights" just 80s after entry interface, he adds.
Impact testing has been delayed after analysis of the launch images showed the foam shed by the external tank was larger than previously thought. Trajectory analysis suggests the piece weighed 0.9kg (2lb) and hit at 195m/s (640ft/s), says Tertault.
Another possibility is potential weakening of the RCC panels by pinholes caused by contamination by zinc oxide primer, exposed by deterioration of the paint, falling from the launch pad structure.
Source: Flight International
