Vehicle Upgrades: Tile and Reinforced Carbon-Carbon (RCC) panel repairs
With the many changes made to the External Tank, the dangers of large pieces of foam striking the orbiter have been greatly reduced. Although no harmful debris is expected to strike the orbiter, the on-orbit tile and RCC repair activities are considered a second or third level of redundancy or mitigation. Boeing provided extensive support to NASA and other industry partners on tile and RCC panel repair development. NASA has been leading these repair efforts with inputs from a wide variety of contractors.
The biggest challenge is to develop and qualify materials that can work in space. As the Shuttle docks at the ISS, it rotates and goes from facing the cold of space to the heat of the sun. The temperature, materials, and to be able to put it on in a vacuum in space where temperatures can be as low -150 or -200 degrees in the shade on the leeward side, presents unique challenges to cure the repair materials on orbit. During STS-114, a Detailed Test Objective (DTO) will be conducted to prove that the material can cure both in a vacuum and in this cyclical cold-to-hot environment within a relatively short timeframe.
Repair of damaged tiles or RCC panels can also be very challenging because of problems with filling a hole, swelling, extreme on-orbit and reentry temperatures, application by an astronaut in a bulky space suit and the fact that repairs could even create worse problems with overheating. Repairs must be able to survive temperatures approaching 3,000 degrees for 900 seconds -- a very harsh and unforgiving environment.
RCC repair is different and much more challenging than tile repair in the unique material properties and re-entry conditions that it must sustain. RCC is a thin shell composite material that incorporates both structural and thermal loading during reentry. RCC temperatures at the hottest areas of the wing leading edge can exceed 3,200° F and must sustain significant thermal, aerodynamic, and vibroacoustic loading to survive. The repaired surface has strict surface contour requirements that must be maintained to keep additional aeroheating to a minimum.
Two principle repair techniques have been considered for the RCC panels. Crack repair concepts are the prime repair method in work by NASA and its industry team. These concepts are similar to tile repair in that a putty-like substance is spread over a crack to cover the crack and the surrounding damaged coating. This material then cures and shields the damaged area during entry.
Another RCC solution is called the "plug repair" concept, which is being lead by Johnson Space Center in Houston in combination with ATK Thiokol in Utah. It's essentially a toggle bolt, like the kind you'd use to hang a picture on dry-wall, where the back of the assembly springs open inside the structure and clamps securely as you screw it from the outside.