A NASA Fact Sheet on ET Thermal Protection describes the different types of foam used on the ET. This fact sheet is dated April 2005 and describes the key areas where improvements were required after the Columbia accident including the bipod, PAL ramps, and LOX feedline.
Different types of foam are used on different sections of the External Tank. Most of the 'acreage foam' on the large cylindrical surfaces are sprayed on by a robotic machine. Some of the smaller specialized pieces, like the bipod, are manufactured by hand.
CAIB member Scott Hubbard holds a full-size mockup of the left bipod. The dark line indicates the approximate volume of foam which came off of Columbia's left bipod. The actual bipod has a volume of 1.1 cubic feet and weighs roughly 2 pounds.
The External Tank had lost foam on every shuttle mission, some more than others. This graph shows the number of tiles damaged on each mission and cases where the damage was greater than one inch. The marshmallow-size pieces of foam had a fairly small amount of energy, not enough to do serious damage. In several cases far more foam was lost but the Marshall Spaceflight Center didn't consider it to be important. Phrases like "in family" and "acceptable risk" were used to describe the foam damage to the shuttle's tiles.
In about a dozen cases much more foam was lost than usual. On STS-87 (Kalpana Chawla's first mission), 308 tiles were damaged by foam debris from the External Tank. In this case the problem was attributed to a new formula for spraying the foam which mixed too much air in with the foam.
Most of the ET's lost foam was acreage foam. The pieces were relatively small and didn't have enough energy to do serious damage. It was considered a maintenance issue, not a safety concern. However in a handful of cases far larger chunks of foam came off of the bipod. With higher mass they had far greater energy than the acreage foam debris and the bipod foam pieces could cause much more damage if they hit a vulnerable portion of the shuttle.
A misleading version of this graph appears in one of the supplementary volume to the CAIB's report. In the External Tank Working Group's Final Report the graph shows the amount of foam lost on previous missions - but excludes the STS-107 mission where almost twice as much foam was lost as any previous shuttle mission.
The Southwest Research Institute (SWRI) in San Antonio, Texas has a large air-gun used to test the strength of aircraft components. It features a large compressed nitrogen tank (blue tank on the left) and barrel to propel the piece of debris at the target. In this case the target was a simulated leading edge of the space shuttle's wing and the 'bullet' was a suitcase-size block of foam. Early tests were performed with simulated panels made out of fiberglass, later tests used actual reinforced carbon-carbon panels which had flown on as many flights as Columbia.1.67 pound blocks of foam were tossed at the simulated wing. Movies of the early and final foam test show a dramatic difference in the amount of damage. While dramatic the tests were misleading since they didn't show what actually happened, just what could have happened. The hole in the final test is about twice the size of what actually happened to Columbia.
A bipod fitting without the insulating foam. The electrical heater in the foreground replaces the foam insulation which caused the Columbia accident.
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