6.1 A HISTORY OF FOAM ANOMALIES

¶The shedding of External Tank foam – the physical cause of the Columbia accident – had a long history. Damage caused by debris has occurred on every Space Shuttle flight, and most missions have had insulating foam shed during ascent. This raises an obvious question: Why did NASA continue

¶ion Making at NASA flying the Shuttle with a known problem that violated design requirements? It would seem that the longer the Shuttle Program allowed debris to continue striking the Orbiters, the more opportunity existed to detect the serious threat it posed. But this is not what happened. Although engineers have made numerous changes in foam design and application in the 25 years that the External Tank has been in production, the problem of foam-shedding has not been solved, nor has the Orbiterʼs ability to tolerate impacts from foam or other debris been significantly improved.

¶The Need for Foam Insulation

¶The External Tank contains liquid oxygen and hydrogen propellants stored at minus 297 and minus 423 degrees Fahrenheit. Were the super-cold External Tank not sufficiently insulated from the warm air, its liquid propellants would boil, and atmospheric nitrogen and water vapor would condense and form thick layers of ice on its surface. Upon launch, the ice could break off and damage the Orbiter. (See Chapter 3.)

¶To prevent this from happening, large areas of the External Tank are machine-sprayed with one or two inches of foam, while specific fixtures, such as the bipod ramps, are hand-sculpted with thicker coats. Most of these insulating materials fall into a general category of "foam," and are outwardly similar to hardware store-sprayable foam insulation. The problem is that foam does not always stay where the External Tank manufacturer Lockheed Martin installs it. During flight, popcorn- to briefcase-size chunks detach from the External Tank.

¶Original Design Requirements

¶Early in the Space Shuttle Program, foam loss was considered a dangerous problem. Design engineers were extremely concerned about potential damage to the Orbiter and its fragile Thermal Protection System, parts of which are so vulnerable to impacts that lightly pressing a thumbnail into them leaves a mark. Because of these concerns, the baseline design requirements in the Shuttleʼs "Flight and Ground System Specification-Book 1, Requirements," precluded foam-shedding by the External Tank. Specifically:

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¶3.2.1.2.14 Debris Prevention: The Space Shuttle System, including the ground systems, shall be designed to preclude the shedding of ice and/or other debris from the Shuttle elements during prelaunch and flight operations that would jeopardize the flight crew, vehicle, mission success, or would adversely impact turnaround operations.1

¶3.2.1.1.17 External Tank Debris Limits: No debris shall emanate from the critical zone of the External

¶Tank on the launch pad or during ascent except for such material which may result from normal thermal protection system recession due to ascent heating.2

¶The assumption that only tiny pieces of debris would strike the Orbiter was also built into original design requirements, which specified that the Thermal Protection System (the tiles and Reinforced Carbon-Carbon, or RCC, panels) would be built to withstand impacts with a kinetic energy less than 0.006 foot-pounds. Such a small tolerance leaves the Orbiter vulnerable to strikes from birds, ice, launch pad debris, and pieces of foam.

¶Despite the design requirement that the External Tank shed no debris, and that the Orbiter not be subjected to any significant debris hits, Columbia sustained damage from debris strikes on its inaugural 1981 flight. More than 300 tiles had to be replaced.3 Engineers stated that had they known in ad- vance that the External Tank "was going to produce the debris shower that occurred" during launch, "they would have had a difficult time clearing Columbia for flight."4

¶Discussion of Foam Strikes Prior to the Rogers Commission

¶Foam strikes were a topic of management concern at the time of the Challenger accident. In fact, during the Rogers Commission accident investigation, Shuttle Program Manager Arnold Aldrich cited a contractorʼs concerns about foam shedding to illustrate how well the Shuttle Program manages risk:

¶On a series of four or five external tanks, the thermal insulation around the inner tank … had large divots of insulation coming off and impacting the Orbiter.

¶We found significant amount of damage to one Orbiter after a flight and … on the subsequent flight we had a camera in the equivalent of the wheel well, which took a picture of the tank after separation, and we determined that this was in fact the cause of the damage. At that time, we wanted to be able to proceed with the launch program if it was acceptable … so we undertook discussions of what would be acceptable in terms of potential field repairs, and during those discussions, Rockwell was very conservative because, rightly, damage to the

¶Orbiter TPS [Thermal Protection System] is damage to the Orbiter system, and it has a very stringent environ-

¶Aldrich described the pieces of foam as "… half a foot square or a foot by half a foot, and some of them much smaller and localized to a specific area, but fairly high up on the tank. So they had a good shot at the Orbiter underbelly, and this is where we had the damage."5

¶Continuing Foam Loss

¶Despite the high level of concern after STS-1 and through the Challenger accident, foam continued to separate from the External Tank. Photographic evidence of foam shedding exists for 65 of the 79 missions for which imagery is available. Of the 34 missions for which there are no imagery,⁸⁸ The only actual flight tests conducted of the Orbiter were a series of Printing Office, 1998) for an account of the aftermath of the accident. Approach and Landing Tests where Enterprise (OV-101) was dropped Much of the account in this section is drawn from this source. from its Boeing 747 Shuttle Carrier Aircraft while flying at 25,000 feet. 19 Logsdon, "Return to Flight," p. 348. These tests – with crews aboard – demonstrated the low-speed handling 20 Presidential Commission on the Space Shuttle Challenger Accident capabilities of the Orbiter and allowed an evaluation of the vehicleʼs (Washington: Government Printing Office, June 6, 1986). landing characteristics. See Jenkins, Space Shuttle, pp. 205-212 for more information. 26 Report Volume I August 2003Show full note missions where foam loss is not seen in the imagery, and 6 missions where imagery is inconclusive, foam loss can be inferred from the number of divots on the Orbiterʼs lower surfaces. Over the life of the Space Shuttle Program, Orbiters have returned with an average of 143 divots in the upper and lower surfaces of the Thermal Protection System tiles, with 31 divots averaging over an inch in one dimension.6 (The Orbitersʼ lower surfaces have an average of 101 hits, 23 of which are larger than an inch in diameter.) Though the Orbiter is also struck by ice and pieces of launch-pad hardware during launch, by micrometeoroids and orbital debris in space, and by runway debris during landing, the Board concludes that foam is likely responsible for most debris hits.

¶With each successful landing, it appears that NASA engineers and managers increasingly regarded the foam-shedding as inevitable, and as either unlikely to jeopardize safety or simply an acceptable risk. The distinction between foam loss and debris events also appears to have become blurred. NASA and contractor personnel came to view foam strikes not as a safety of flight issue, but rather a simple maintenance, or "turnaround" issue. In Flight Readiness Review documentation, Mission Management Team minutes, In- Flight Anomaly disposition reports, and elsewhere, what was originally considered a serious threat to the Orbiter