Discovery's fuel tank cleared for launch after foam debate
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 18, 2009
After a lengthy technical discussion, NASA managers decided Tuesday the shuttle Discovery's external tank can be safely flown as is, without the need for additional time-consuming inspections of its foam insulation, clearing the way for launch next week on a space station resupply mission.
But late Tuesday, agency officials speaking through Twitter said "the tank discussion is over and everyone had the chance to offer a viewpoint. At this point, the FRR decided we're OK to fly as is."
An official launch date is expected to be set Wednesday, but the internal processing target is Aug. 25 at 1:36 a.m. EDT, one day later than the previous target because of unplanned work at the launch pad.
Discovery's crew - commander Frederick Sturckow, pilot Kevin Ford, flight engineer Jose Hernandez, Patrick Forrester, John "Danny" Olivas, European Space Agency astronaut Christer Fuglesang and space station flight engineer Nicole Stott - plans to fly to the Kennedy Space Center Wednesday evening for the start of Discovery's countdown at 11 p.m. Friday.
Along with ferrying Stott to the lab complex and bringing Timothy Kopra home in her place, the Discovery astronauts plan to deliver critical supplies and equipment, carrying out three spacewalks before landing back at the Florida spaceport around 8:40 p.m. on Sept. 6.
A major topic in Tuesday's review was the integrity of the foam insulation on Discovery's external tank, ET-132, following an unusual amount of foam shedding during the shuttle Endeavour's launching July 15. While some engineers said they would prefer additional data, sources said the community was unanimous in agreeing to proceed.
Most of the foam lost from Endeavour's tank fell away from the central "intertank" area separating the hydrogen and oxygen sections of external tank No. 131. A small amount of foam fell from the base of the left-side bi-pod strut that helps hold the shuttle's nose to the tank and another piece of debris fell from one of the so-called ice-frost ramps on the side of the liquid oxygen section.
Engineers carried out more than 170 so-called "plug-pull" tests on the intertank foam of Discovery's tank earlier to test the adhesion of the insulation and no problems were found. The bi-pod foam loss is believed to be an understood condition and not a major threat to damage the shuttle's heat shield.
But Endeavour's launch was the second in a row to experience foam loss from the same ice-frost ramp on the liquid-oxygen section of the tank. The ice-frost ramps are aerodynamically shaped areas of foam covering fittings that attach pressurization lines to the oxygen section of the tank.
The foam loss during Endeavour's launch presumably occurred because of undetected voids in the insulation. Atmospheric friction during ascent can cause trapped air to expand, popping off overlying foam. Impact-related heat shield damage depends on the size and timing of a release, which can be difficult to model.
The ramp in question on Discovery's tank was subjected to non-destructive terahertz inspections before the shuttle was moved to the launch pad and no significant voids were seen. But three other ramps, which have no history of foam loss, were not inspected.
At a shuttle program review last week, some engineers recommended hauling Discovery back to the Vehicle Assembly Building for terahertz inspections of the other ramps, a move that would delay launch to around Oct. 17.
Instead, managers ordered additional plug-pull tests and terahertz inspections of the ice-frost ramps on the next tank in the sequence, ET-133. The additional pull tests found no problems. The ice-frost ramp scans of ET-133 detected 10 very small voids, none of which would be expected to result in damaging foam losses.
But those inspections shed no direct light on the condition of the ramps on Discovery's tank, which cannot be scanned at the launch pad.
The flight readiness review will continue Wednesday, with presentations by the orbiter project and other elements of the shuttle program. But with the external tank cleared for flight and no other major problems under discussion, NASA managers are expected to set Aug. 25 as the official launch date.