More work needed to eliminate risk of take-off damage.
NASA’s Space Shuttle fleet could be grounded for a year or more, according to agency insiders, following the loss of a 0.4kg (0.9lb) foam chunk from the external tank’s (ET) upper half during Space Shuttle Discovery’s ascent.
Following the identification of foam and thermal protection system (TPS) tile chip debris falling from Discovery during its launch on 26 July, Shuttle managers have grounded the fleet until the causes are known and fixed.
Discovery was filmed and photographed by over 100 cameras, which were located on the ET, on the solid rocket boosters, on the ground, on board chase aircraft, and in the Discovery crew’s own hands. ET-umbilical digital cameras revealed two bipod divots and NASA has concluded that a 15-17cm (6-7in) foam piece had peeled off the redesigned bipod area. An ET liquid oxygen feed line camera showed a TPS tile on the edge of the nose-gear door was chipped in one corner. A nose gear tile corner had broken off during ground handling and was repaired before flight. This may be the same tile. A tile in the left wing-fuselage chine area was also seen to be damaged.
Worst of all, as the ET fell away from Discovery, the crew’s cameras showed that a 0.4kg piece of foam, measuring 60-85cm by 6-20cm had fallen from the ET’s protuberance air load (PAL) ramp. This is believed to be the foam chunk seen to fall in ET camera footage during launch. It did not hit the orbiter.
“If it had come off earlier we think it would have been really bad,” says Wayne Hale, NASA’s deputy Shuttle programme manager and chairman of the mission management team.
Hale says imagery analysis revealed that small pieces of foam from an ET ice frost ramp may have struck the orbiter’s right wing during ascent as well. But this is not considered to be a problem.
In images released on 28 July, damage to a nose cone insulation blanket on the port side is also noticeable.
By 29 July NASA had identified 25 impacts on Discovery’s underside. Of these six are to be examined closely.
One is an area of protruding tile-gap filler. This is a felt-like material that could be cut away during extra vehicular activity. This might be necessary because protruding filler would cause boundary-layer turbulence resulting in higher localised temperatures for the heat shield.
The PAL ramp foam that is causing so much concern is a long, hand-applied dam used to improve the tank’s aerodynamics in the area of a hydrogen fuel line. NASA has encountered this foam-shedding problem before. Space Shuttle Challenger’s mission STS 7 lost PAL foam, but NASA thought it had solved the problem.
Now the agency will only confirm that it has an ET team carrying out a fault tree analysis as the first step towards solving the new ramp problem.
ET manufacturer Lockheed Martin was not available for comment before Flight International went to press. However the PAL ramp foam issue is seen as a straightforward problem.
Hale has suggested replacing the foam, which protects the cable trays, with a metal sheet. But NASA engineers interviewed by Flight International still expect the fleet to be grounded for 12 months at least and expressed greater concern over the bipod divots.
“The two large divots where the bipod ramp used to be…were supposed to have been applied with a new very controlled and very well-understood process. Obviously it was not,” says a senior engineer.
The engineers point out that the foam pieces that came off are smaller than the bipod ramp that damaged Columbia, but they are large enough to have fatally damaged Discovery. The foam process may be improved with ongoing development work.
NASA has said that an enhanced spray process is being developed for future tanks, as well as continued redesigns to eliminate ramps.
Although it technically ceased to exist last month, the Return to Flight Task Group, created to monitor and report on NASA’s compliance with Columbia Accident Investigation Board (CAIB) recommendations, could be reactivated for the new ET work.
In its analysis of NASA’s compliance with CAIB, the group deemed that the agency had failed to meet three of the 15 recommendations, including stopping foam debris shedding.
Discovery is expected to land at Kennedy Space Center at 09:46 Greenwich Mean Time on 7 August.
ROB COPPINGER / LONDON
Source: Flight International
