GRAHAM WARWICK / WASHINGTON DC
As the painstaking reconstruction of the Space Shuttle Columbia continues, investigators are slowly starting to see the bigger picture
Unfolding like a detective story, investigation of the Space Shuttle Columbia accident will have as deep an impact on NASA as the inquiry into the Challenger disaster 17 years earlier. As investigators close in on what caused the orbiter to break up during re-entry, questions are being raised about how NASA inspected its ageing shuttles; how it handled debris impacts on the orbiter; and why it did not request on-orbit imagery of Columbia to check for thermal-protection system damage.
Already the independent Columbia accident investigation board has recommended that NASA require on-orbit imaging of every Shuttle flight, and that the space agency use advanced non-destructive inspection technology to ensure the structural integrity of the orbiters' reinforced carbon-carbon leading edges. These are the easy recommendations, and NASA has already acted on the first, signing an agreement with the US National Imagery and Mapping Agency covering satellite imaging of all future flights.
If images of Columbia on orbit were available, investigators would have been able to confirm their suspicion that the orbiter's left-wing leading-edge thermal protection was damaged by insulation foam shed by the external tank soon after lift-off on 16 January. Lacking suitable imagery, investigators have embarked on a multi-faceted forensic effort combining analysis of telemetry, video, pictures, radar tracks, debris and many other tests.
The effort began immediately after Columbia broke up during re-entry on 1 February, killing all seven astronauts on board. The earliest clues were provided by telemetry downlinked continuously by the orbiter as it headed towards a landing at Kennedy Space Center, Florida - until it disintegrated over Texas.
Entry interface - the start of re-entry - was at 13.44.09 GMT. Virtually non-existent at first, the atmosphere began to thicken and temperatures and pressures on the orbiter soared. Columbia entered the region of peak heating 404s after entry interface (EI+404s), at 243,000ft (74,000m) and Mach 24.1. The first telemetry indication that the re-entry was not nominal - an unusual rise in brake-line temperature on the left main landing gear - came at EI+488s, while the orbiter was still over the Pacific, 480km (260nm) west of the California coastline. But Columbia was already in trouble.
A trail of clues
Quickly convinced that the left wing had been breached, investigators used the pattern of sensor readings, and failures as wires burned through, to "follow the heat". At first it looked like the wheel well had been the source of the breach. Possibilities included impact damage to the tiles covering the landing-gear door, uncommanded deployment of the left main gear, and a heat-induced tyre burst that fatally damaged the wing. Further analysis of the sensor readings, windtunnel tests and recovered debris eventually disproved these theories.
But there is evidence that a plume of superheated air entered the wheel well, possibly through a cutout in the front face, then flowed out at the forward corners of the door, depositing vapourised aluminium on the underside of the wing. A left gear-door uplock roller recovered from the debris is splattered with aluminium and partially melted. The uplock is titanium, which melts at around 1,650°C (3,000°F).
Guided by analysis of video showing external-tank insulation foam hitting the wing during launch, and a blurred photograph of Columbia during re-entry taken by US Air Force researchers at Kirtland, New Mexico, investigators focused their search for the breach on the left wing leading-edge. By now, aerodynamic analysis of the telemetry data had determined that Columbia was experiencing off-nominal yaw forces at EI+476s, and possibly earlier, suggesting some change to the vehicle's external shape.
Debris sightings
What had started as a timeline of telemetry data was expanded to include reports of debris leaving the orbiter. The first of these came at about EI+575s, while Columbia was over northern California. Despite 15 reports of debris leaving the orbiter over California, Nevada, Utah and Arizona, nothing has yet been found on the ground in these states. And despite increasing off-nominal aerodynamic forces, between EI+741s and EI+766s - while crossing the Arizona/New Mexico border at 219,000ft and M20.9 - Columbia's flight control system executed an otherwise normal roll reversal to bleed off speed.
A breakthrough in the search for the breach came with the discovery of the orbiter experiments (OEX) data recorder near Hemphill in east Texas. The reel-to-reel tape recorder, mounted below the floor in the crew cabin, was located intact after an area of the debris field that had already been searched was combed again based on knowledge of where the avionics-bay contents had fallen.
The OEX was expected to have recorded 721 sensor outputs during ascent and re-entry, covering the wing, fuselage andtail: 182 pressure, 53 temperature and 447 loads, dynamics and stress measurements. Investigators were particularly interested in data from the left wing. By mid-April, NASA had readouts from 622 of the 721 channels. The data indicates Columbia was in trouble as soon as it entered the atmosphere, which suggests the damage that caused the thermal breach had happened before re-entry.
The first unusual reading was from a strain gauge mounted behind the left wing spar, in the vicinity of reinforced carbon-carbon (RCC) leading-edge panel number 9. The sensor started to increase and go off-nominal at EI+270s - 206s earlier than the first off-nominal event on the telemetry timeline, the increase in yaw at EI+476s. Two temperature sensors, one just behind RCC panel 9 and one at the panel 9/10 interface, showed a substantial heat spike about 500s after the orbiter entered the atmosphere, then failed "off-scale low", suggesting the wires had been cut or burned through. Also from the OEX data, NASA believes it has detected a warming trend in the left wing - compared with previous re-entries at similar inclinations - that began just 80s after entry interface.
Other channels monitored by the OEX have provided valuable clues indirectly. Temperatures measured by a sensor on the left orbital manoeuvring system pod started go below nominal at EI+344s then, at around EI+540s, rapidly rose to above nominal, peaking at 650°C just before Columbia disintegrated. This suggests a change in the flow field around the orbiter. NASA has conducted windtunnel tests with individual RCC panels removed, and the pattern of heating on the fuselage with panel 9 missing appears to correlate with the sensor readings.
The new data has spurred efforts to find early debris, and to identify an object that separated from Columbia on its second day in orbit. Ground search of the primary debris area is scheduled to be completed at the end of April. Each day, up to 5,700 people have been involved in searching the 400km-long, 8km-wide debris field, which stretches from Fort Worth, Texas, east into Louisiana. By mid-April, about 70,000 pieces of debris, weighing some 35,500kg (78,200lb) - or around 36% of the orbiter's dry weight - had been recovered.
Reconstruction effort
Debris is being transported to Kennedy Space Center, Florida, where a two-dimensional reconstruction of the orbiter's structure and thermal-protection system is taking shape on a hangar floor. The layout includes three left and right wing planforms, for the lower-surface tiles, lower-surface structure and upper-surface structure. Any pieces of the left wing found by searchers are "fast-tracked" to Kennedy as investigators narrow their search for the breach.
The reconstruction is far from easy - of the 45,000 pieces received by the end of March, only 1,400 had been located accurately on the grid. Most of the parts are small and lack identification. Several have already migrated from the left wing leading-edge to the right, and vice versa, as engineers work to identify the pieces and place them in the correct location on the hangar floor. But already the pattern emerging is providing further valuable clues. One is the lack of left wing structure that has been recovered, compared with the right wing.
The location where debris has been discovered, and its condition, is also providing clues. The westernmost piece of debris so far recovered, found at Littlefield, Texas, near the border with New Mexico, was believed to be black underwing tile, but is now thought to be a white tile from the upper surface of the leading edge, behind RCC panels 8 and 9, which turned black as a result of some form of disturbance of the flow over the wing.
Leading-edge debris, meanwhile, forms a distinct pattern, with pieces of left-wing RCC falling to the west of right-wing RCC along a line stretching from Corsicana, Texas, to east of Nacogdoches. Some of the westernmost fragments of left-wing RCC so far recovered show substantial erosion, suggesting they were exposed to extreme heat for a long time. Pieces from the interface between panels 8 and 9 show "significant and unusual" damage, with the reinforced carbon-carbon eroded to a knife edge. As a result, investigators have asked teams to look west of Corsicana, outside the primary debris field, in search of crucial pieces of RCC.
Debris recovery and reconstruction has helped eliminate what was at one time considered the leading candidate for the mysterious object in orbit - and source of the thermal breach - a carrier, or closeout, panel from under the wing leading-edge. Carrier panels bridge the gap between the leading-edge RCC panels and the underwing tiles, and can be removed to provide access to the inside of the leading edge. Each access panel is secured by two bolts and covered with small tiles.
A carrier or closeout panel became the suspect when analysis of the launch video showed the chunk of foam shed by the external tank had struck the wing leading-edge, and not the landing-gear door as originally believed. Calculations of the foam's trajectory suggested the impact area was centred on the underside of RCC panel 6, but encompassed adjacent RCC and closeout panels. After further analysis, however, the impact area was moved outboard to panels 7 and 8, bringing it more into line with other indicators of leading-edge damage.
The mystery object separated from Columbia following a series of manoeuvres on the second day in orbit, but was not discovered until after the disaster, when USAF Space Command sifted through more than 3,000 different observations of the orbiter made by ground radars during the 16-day mission. In a bid to identify the object, 26 different articles underwent UHF radar cross-section testing. A carrier panel emerged as the leading candidate based on reflectivity, but its ballistics did not match the object's observed re-entry characteristics.
By mid-April, investigators were focusing on the leading-edge RCC panels, and the T-seals that bridge the gaps between panels. While healthy panels and seals should have withstood the 700ft/s (215m/s) impact of the 0.9kg chunk of foam, age-related damage has been discovered that could have weakened the RCC. This includes pinholes that allow subsurface oxidation of the load-carrying carbon substrate. The pinholes are thought to be caused by contamination of the leading edge with zinc primer falling from the deteriorating paintwork of the Shuttle launch platform.
T-seal suspected
Investigators are now almost certain the orbital object was a T-seal, however. If all or part of a bow-shaped T-seal was dislodged, it would open up a slot in the leading edge that would allow a jet of superheated air to enter the wing. The jet could have eroded the leading edge and played around inside the wing - "like a fire hose that no-one was holding", one investigator says - vapourising the aluminium substructure.
While one arm of the investigation has been following the heat, another has been following the foam: trying to understand what was shed by the external tank 82s after lift-off, and why. The "what" is believed to be a piece of cryoinsulation foam up to 150 x 355 x 600mm (6 x 14 x 24in) in size and 0.9kg in weight, shed from the left bipod ramp. The "why" could be defects in the manually sprayed and sculpted polyurethane-foam ramps - aerodynamic fairings over the attachments for the struts connecting the nose of the orbiter to the external tank.
Because the foam-covered external tank holds liquid oxygen and liquid nitrogen at cryogenic temperatures, investigators suspect cryopumping - air condensing in voids in the foam and slowly drawing in more air while the Shuttle sits on the pad, until the cavities are filled. Then, as the foam warms during ascent, liquified air in the voids gasifies quickly. If something blocks the escape of the gases - like the duct tape found by investigators during dissection of a bipod ramp - then a chunk of foam could pop off.
Another possibility is cryoingestion - the absorption of cryogenic nitrogen into the layer of super-light ablator that lies under the foam. Nitrogen is used to purge the region between the liquid oxygen and liquid nitrogen tanks during fuelling, and cracks have been discovered in the stringer channels that form the intertank structure, and which run under the forward edges of the bipod ramps.
Knowledge of at least four previous incidents of foam loss from the bipod ramp - three resulting in damage to the undersurface of the orbiter - has raised serious questions about why NASA came to regard the problem as a maintenance turnaround issue and not a safety-of-flight concern. Investigators have already concluded that the computer program used to assess the damage caused to Columbia by the debris strike was inadequate, as it was based on tests using far smaller pieces of foam.
Full-scale tests of foam strikes against orbiter leading-edge structure are just getting under way, and could confirm whether the thermal breach was caused by a large chunk of external-tank foam hitting a T-seal. Whatever the detective work reveals, the ultimate test of the independence of the accident investigation board will come when it tackles the part played by NASA's culture in the decisions that led to the loss of Space Shuttle Columbia and its seven astronauts.
Source: Flight International
