In a recent National Public Radio interview, former NASA space shuttle program director Wayne Hale was asked what the shuttle program’s most important lesson had been. Without hesitation, he replied “safety.” Certainly among the most important safety lessons taught during space shuttle operations is the vulnerability of the orbiter’s thermal protection system (TPS) to debris impacts. Damage from such an impact was determined to be the root cause of orbiter Columbia’s destruction, and the loss of its seven-person crew, during hypersonic atmospheric entry Feb. 1, 2003.
During a typical mission, orbiter TPS is continually at risk of potentially fatal impacts. Immediately following launch, debris threats arise from spray-on foam insulation liberated at high dynamic pressures from the space shuttle’s external tank immediately adjacent to an orbiter’s TPS. The demise of Columbia was traced to a specific foam insulation debris impact on the left wing’s leading edge. Other hazardous TPS impacts are possible throughout orbit operations. These arise from untracked orbiting debris moving at speeds often exceeding 10 kilometers per second with respect to an orbiter. Finally, landing operations are subject to TPS impacts from runway debris that may be from the ambient environment or from the orbiter’s tires.
Debris impacts to orbiter TPS pose relatively little safety risk when incurred on the runway as a mission ends. But damage from these and all other impacts accumulated during that mission requires expensive and time-consuming repair before the orbiter launches again. If the orbiter lands in California and is to be ferried piggyback atop a Boeing 747 to Florida, TPS damage must first be treated with a clear sealant to prevent additional damage from airflow erosion.
Particularly after Columbia’s loss, heroic in-flight measures were implemented to detect and mitigate orbiter TPS impact damage. These measures included impact sensors installed in the orbiter’s wings whose data were recorded during launch and orbit operations, many crew member hours dedicated to close-up inspection using on-board robotic systems, and imagery of the orbiter during terminal approach obtained by the international space station crew. All data relating to TPS integrity were transmitted to the ground for painstaking expert analysis before an orbiter was cleared for entry. These transmissions often monopolized significant air-to-ground communications bandwidth.
With this history in mind, arguably the most compelling story a decommissioned orbiter can relate when displayed to the public is told by TPS damage incurred during its final flight. This damage should be rendered safe for public viewing, likely by a sealant akin to that used for ferry flights, but it should be otherwise faithfully preserved. The only TPS damage subject to repair after a final flight should be any arising from deservicing and other postflight activity required by NASA as part of an orbiter’s decommissioning.
Three institutions have been granted an orbiter with flown space mission history for display: Discovery to the National Air and Space Museum’s exhibit near Washington, Atlantis to the Kennedy Space Center Visitor Complex, and Endeavour to the California Science Center in Los Angeles. These institutions are urged to adopt in-flight TPS damage preservation as a curatorial policy. From the heroic operational measures outlined here, such preservation is the only responsible course to take from the standpoint of exhibiting aerospace history to our progeny with minimal distortion. The message conveyed is that humanity’s first attempts to operate with a reusable TPS to and from low Earth orbit typically incurred this degree of damage during just one flight. Future aerospace technologists will be far better informed by viewing such damage firsthand than by reading secondhand reports.
In addition to avoiding the cost of most TPS repairs, which NASA is not obligated to fund, exhibiting institutions will be wise to preserve in-flight TPS damage because this policy also preserves each orbiter’s monetary value. There is a huge body of anecdotal precedent for this in valuations associated with all manner of human artifacts, from furniture to automobiles, damaged by meteorite impacts. As experts appearing on “Antiques Roadshow” often attest, a damaged historic artifact’s value drops dramatically after repair.
Dan Adamo is an independent astrodynamics consultant with research interests in space mission design throughout our solar system. From 1990 until 2008, he supported 60 space shuttle missions from NASA Mission Control’s Flight Dynamics Officer Console. He welcomes feedback at adamod@earthlink.net.
