The exclamation point that provided closure to the Columbia accident investigation was independent of the analysis of Columbia‘s debris and its data recorder.
Two days after the February 1, 2003 accident, the NASA Accident Investigation Team contacted the Southwest Research Institute (SwRI) near San Antonio for assistance in the accident investigation. SwRI had conducted previous studies for NASA on the effects of impacts of much smaller pieces of foam, cork insulation, and ice on shuttle tiles. NASA wanted incontrovertible proof that foam from the external tank was capable of inflicting mortal damage on the shuttle’s thermal protection system.
That foam could damage the wing seemed counterintuitive on many levels. How could a piece of lightweight insulation— about the density of Styrofoam and weighing less than two pounds—fall off the tank and cause that kind of damage? And wasn’t it traveling about the same speed as the shuttle?
In fact, analysis showed there was a significant velocity difference between the shuttle and the foam at the time of impact. NASA estimated that the shuttle was traveling faster than 1,500 mph—and accelerating—when the foam fell off the tank. After falling off, the foam immediately and rapidly decelerated due to air resistance. The block slowed to about 1,000 mph in the 0.2 seconds between when it came off the tank and when the shuttle’s wing impacted the foam. The relative difference in speeds between the shuttle and foam was therefore more than 500 mph.
The piece of foam that struck Columbia was four hundred times larger than the pieces tested previously by SwRI. Using a special compressed air cannon, SwRI planned to simulate the collision by firing foam blocks at more than 500 mph into samples of shuttle tiles and wing leading edge panels. High-speed cameras photographed the test firings and impacts, and over two hundred sensors measured the effects of the collisions.
By the time the equipment and procedures were ready for the first test on the landing gear door, the investigation had already narrowed its focus to the wing’s leading edge as the impact area. SwRI ran its test anyway using a landing gear door—one borrowed from Enterprise and subsequently covered with silica tiles—to check out the test equipment and processes. As expected, a grazing impact of foam, akin to what would have occurred in flight had the foam hit the underside of the wing, caused only minor damage to the tiles on the landing gear door.
Space shuttle wing leading edge panels are large, expensive, and made to order. The reinforced carbon-carbon (RCC) material also wears and becomes more brittle over time, so SwRI could not use newly manufactured panels to get an accurate assessment of potential damage in its impact tests. The test panels would have to come from the wings of Discovery and Atlantis, the two orbiters that had flown about as many times as Columbia.
NASA decided to check out the test process first using Fiberglas leading edge wing panels from Enterprise, which was not designed to fly in space. Several test shots at Enterprise’s Fiberglas panels—which were stronger than the RCC panels on the flightworthy shuttles—produced scuff marks from the foam blocks, but no breakage. After getting its process and equipment calibrated, SwRI was now ready to try the tests with the space-flown RCC panels.
First, a foam block was fired at panel 6 from Discovery. The impact created a crack nearly six inches long in a rib supporting the leading edge, and it moved the panel enough to create a small gap in the T-seal between panels 6 and 7. This test proved that foam could damage the RCC. However, the damage incurred in this test would not have been severe enough to create the burn-through seen on Columbia. NASA estimated a hole of at least ten inches in diameter would have been needed for the wing to ingest a plasma stream large enough to create the damage shown in Columbia’s debris.
The next test target was panel 8, which had flown twenty-six times on Atlantis. Evidence from the reconstructed debris and the OEX recorder indicated that panel 8 was the probable site of the impact on Columbia’s wing.
At the test on Monday, July 7, 2003 the impact from the foam block blew a hole through the panel about sixteen inches by sixteen inches across, created several other cracks, and caused the T-seal to fail between panels 8 and 9. This was entirely consistent with the type of damage that caused Columbia’s demise.
Witnesses were incredulous, but the evidence was incontrovertible. NASA now had the smoking gun matching the fatal wound on Columbia. The test silenced lingering doubts that a foam strike alone was sufficient to damage the wing and doom the ship.
This text in this section is excerpted from the book “Bringing Columbia Home,” (c) 2017 by Mike Leinbach and Jonathan H. Ward. Video material is from NASA.