Shuttle Report Calls for NDT
After months of investigation, the board investigating the Columbia shuttle disaster has pinpointed its cause and released a series of recommendations including the increased use of nondestructive testing (NDT) and the development of new NDT technologies.
The recommendation to increase NDT usage was made by the Columbia Accident Investigation Board (CAIB) after a nearly seven-month investigation that determined a piece of insulating foam broke from the external fuel tank less than two minutes after takeoff and struck the left wing. The collision breached the Thermal Protection System on the leading edge of the left wing, and during re-entry the breach allowed superheated air to enter the wing and melt electrical wiring and then the wing's aluminum spars. This resulted in a weakening of the structure until aerodynamic forces caused loss of control, failure of the wing and breakup of the orbiter.
"In four simple words, ‘the foam did it,'" says G. Scott Hubbard, director of the Ames Research Center and member of the CAIB. "After months of inquiry, after analysis and series of tests, we have concluded that falling foam impacting the leading edge of the wing caused the breach that ultimately led to the destruction of the orbiter and the loss of the crew."
Doomed from the start
Columbia's 28th and final flight began on Jan. 16, 2003, at 10:30 a.m. EST and while its destruction was more than two weeks away on Feb. 1, its fate was sealed shortly after takeoff. At 81.7 seconds after launch, when the Shuttle was at about 65,600 feet and traveling at Mach 2.46 (1,650 mph), a large piece of hand-crafted insulating foam came off an area where the orbiter attaches to the external tank. At 81.9 seconds, it struck the leading edge of Columbia's left wing. While the event was not immediately detected, and discounted by managers even after the impact was noticed the following day, impact tests would later confirm this to be the cause of the shuttle's breakup over the western United States.
An analysis of post-launch photos show that one large piece of foam weighing 1.67 pounds and approximately 21 to 27 inches long and 12 to 18 inches wide, and two smaller pieces, tumbling at up to 840 feet per second, struck panel 8 on the wing's leading edge. A working group, called Interceptor Photo Working Group, examined the video as standard operating procedure and was worried that Columbia had sustained damage not detectable in the limited number of views that land-based cameras captured. They requested that a high-resolution image be taken by the Department of Defense, but this request, the first of three asking for images and one of nine "missed opportunities" identified by the CAIB, was denied by high-ranking managers. The Mission Management Team declared the debris strike a "turnaround" issue, meaning one that could be taken care of during in-between flight maintenance, and did not pursue a request for imagery.
While by no means a certainty that anything could have been done had the damage been known, the fact is that the damage was like a waiting time bomb. While the unaware crew spent 16 days orbiting the earth and performing numerous scientific experiments, the breach had already occurred.
NDT and inspections lacking
The foam loss may have been prevented before it happened if, according to the board, there were more inspections and better nondestructive testing of the shuttle and especially, in retrospect, the external tank and the foam that insulated it.
The external tank is the largest element of the Space Shuttle and is the common element to which the solid rocket boosters and the orbiter are connected. It contains the propellant for the main engines, holding 143,351 gallons of liquid oxygen at -297 F in its forward (upper) tank and 385,265 gallons of liquid hydrogen at - 423 F in its aft (lower) tank.
The external tank is attached to the solid rocket boosters by bolts and fittings on the thrust panels and near the aft end of the liquid hydrogen tank. The orbiter is attached to the external tank by two umbilical fittings at the bottom and by a "bipod" at the top. The bipod is attached to the external tank by fittings at the right and left of the external tank centerline.
The external tank is coated with two materials that serve as the Thermal Protection System: dense composite ablators for dissipating heat, and low-density closed-cell foams for high-insulation efficiency. The external tank Thermal Protection System is designed to maintain an interior temperature that keeps the fuel stable.
While the CAIB found that the adhesion between sprayed-on foam insulation and the external tank's aluminum substrate is sufficiently strong on most of the tank, the insulated region where the bipod struts attach to the external tank is more of a concern because it is structurally, geometrically and materially complex. Because of concerns that the foam applied over the fittings would not provide enough protection from the high heating of exposed surfaces during ascent, the bipod fittings are coated with ablators. Foam is sprayed by hand over the fittings and ablator materials, allowed to dry and manually shaved into a ramp shape. The board added that the foam insulation is visually inspected at the Michoud Assembly Facility and also at the Kennedy Space Center, but no other nondestructive evaluation is performed.
NDT inspection would have been important because of the nature of the foam. The foam does not have the same properties in all directions, and there is also variability in the foam itself. Because it consists of small hollow cells, it does not have the same composition at every point. This combination of properties and composition make foam extremely difficult to model analytically or to characterize physically. The great variability in its properties makes for difficulty in predicting its response in even relatively static conditions, much less during the launch and ascent of the Shuttle. The CAIB said, "Too little effort went into understanding the origins of this variability and its failure modes. "
The board was concerned that the way the foam was produced and applied, particularly in the bipod region, also contributed to its variability. Foam consists of two chemical components that must be mixed in an exact ratio and is then sprayed according to strict specifications. Foam is applied to the bipod fitting by hand to make the foam ramp, and this process may be the primary source of foam variability.
While the CAIB said it couldn't say with 100% certainty why the foam broke loose, and that it may never be known, it did discover some possibilities. By dissecting the foam ramps, tests revealed defects including voids, pockets and debris that are likely due to a lack of control in spray-by-hand applications and the complexity of the underlying hardware configuration. These defects often occur along "knit lines," the boundaries between each layer that are formed by the repeated application of thin layers.
Subsurface defects can be detected only through destructive means and the CAIB says that nondestructive evaluation techniques for determining external tank foam strength have not been perfected or qualified. "Therefore, it has been impossible to determine the quality of foam bipod ramps on any external tank. Furthermore, multiple defects in some cases can combine to weaken the foam along a line or plane," the report states.
NDT on the wings
NDT testing on the wings was also found lacking by the CAIB. Each wing's leading edge consists of 22 reinforced carbon-carbon (RCC) panels, and because the shape of the wing changes from inboard to outboard, each panel is unique.
The report says that during the 20 years of Space Shuttle operations, RCC has performed well in the harsh environment it is exposed to during a mission. Within the last several years, a few instances of damage to RCC material have resulted in a re-examination of the current visual inspection process. Concerns about potential oxidation between the silicon carbide layer and the substrate and within the substrate have resulted in further efforts to develop improved nondestructive evaluation methods and a better understanding of sub-surface oxidation.
Oxidation is a particularly difficult problem because it is ongoing and variable. According to the report, "the rate of oxidation is the most important variable in determining the mission life of RCC components. Oxidation of the carbon substrate results when oxygen penetrates the microscopic pores or fissures of the silicon carbide protective coating. The oxidation rate is a function of temperature, pressure, time and the type of heating. Repeated exposure to the orbiter's normal flight environment degrades the protective coating system and accelerates the loss of mass, which weakens components and reduces mission life capability."
U.S. Navy Rear Admiral Stephen Turcotte says that NASA's quality control program went through a series of downsizings and reduced the number of inspection points and left the remaining inspection points "pretty stagnant." "This is an aging orbiter and problems of corrosion are ongoing," he says. "The problem is that as the airframes changes, the inspection points will change and that is an industry standard that we found lacking (in the shuttle program)."
NDT is the only way too truly catch the corrosion problems on the shuttle and its myriad components, he adds. "For example, in the capsule there are some points that short of taking the orbiter apart, we will never be able to get to look at," the Turcotte says. "NASA has to figure out ways to get in their, look at those and find out the true age of the orbiter."
High-ranking officials blamed
The call for increased NDT and more quality assurance vigilance was only one charge made by the board. Turcotte chastised the space agency for falling behind in test and inspection technology saying that its test equipment was 22-years-old and "frozen in time."
This may have been a factor of cost-cutting actions that were felt throughout the agency as it tightened its internal belt and turned over more duties to outside contractors. The board did not find mismanagement by the outside contractor but worried about NASA's loss of control.
Dr. John Logsdon, director of the Space Policy Institute, George Washington University and member of the CAIB, says, "We looked at this as an accident rooted in the history of NASA and history of the space shuttle program."
Some of that history included budget and workforce pressures. "In order to fund other parts of the NASA program," Logsdon says, "the shuttle program was squeezed during the ‘90s. Its budget was cut by 40% and its workforce was cut by 40% and that left too little margin for robust operation of the system in our judgment."
For instance, because the shuttle was considered a mature and reliable system, NASA turned over the oversight to single contractor and turned safety and mission assurance over to the same contractor. "There needs to be the strongest technical oversight of the program by government employees," Logsdon says.
The report hammers NASA's leadership and its culture saying that the agency and its administrator, Sean O'Keefe, pushed scheduling beyond safe limits in its drive to finish building the initial phase of the international space station by February 2004. The report also says the agency "does not provide effective checks and balances, does not have an independent safety program and has not demonstrated the characteristics of a learning organization."
The agency has, what Air Force Major General John Barry calls, a broken safety culture and a silent safety program that echoed the Challenger disaster, which occurred 15 years earlier. "NASA had conflicting goals of costs, scheduling and safety and, unfortunately, safety lost out in a lot of areas."
While NASA's leadership was blamed, QA was praised. "In our time on the shop floor," says Turcotte, "we found that the people down there are working their hearts out. They have the right idea, the right mindset and they are doing the best they can." NDT