SLP-330 Laser Scanning Probe Laser Design
The loss of the Space Shuttle Columbia because of damage to its thermal protection system (TPS) during launch spurred a search for methods of repairing the TPS in space; specifically, repairing the Reinforced Carbon-Carbon (RCC) material used to protect the nose cap and wing leading edge panels that experience the most extreme heating — more than 3,000° F — during the return to Earth. In testing possible repair methods, it is critical to accurately measure the complex freeform 3D RCC panel shape after the damage, after the repair, and after tests that simulate re-entry. NASA used the SLP-330 laser scanning probe from Laser Design, integrated with Romer portable coordinate measuring machine (CMM) arms for this task.
Each of the orbiter’s wings has 22 RCC panels that are 0.250 to 0.500" thick. RCC is a laminated composite material made from graphitized rayon cloth impregnated with a phenolic resin; a three-stage pyrolysis process converts the phenolic resin to the carbon matrix. The outer layers of the RCC are converted to silicon carbide to provide oxidation resistance for reuse. While the other components of the TPS serve as insulators, the areas protected by the RCC are too hot to insulate, so the RCC is designed to radiate heat from the hot lower surfaces to the cooler upper surfaces and to the internal portion of the wing.
NASA has developed a pre-ceramic polymer sealant impregnated with silicon carbide that is referred to as Non-Oxide Adhesive eXperimental (NOAX). Damage in the protective outer silicon-carbide coating of the RCC may be repaired with NOAX, which converts to silicon carbide during re-entry. NASA also has designed a special space caulking gun that astronauts will use to apply a small amount of repair material to the damage site. NASA has planned several arc jet tests (reentry simulation), as well as thermal-vacuum tests prior to certifying the new material. Astronauts Piers Sellers and Mike Fossum conducted repairs of arc jet samples during a spacewalk on STS-121 in July 2006.
To perform these measurements, NASA selected laser scanning, which projects a line of laser light onto surfaces while cameras continuously triangulate the changing distance and profile of the laser line as it sweeps along, enabling the object to be accurately replicated. The laser probe computer translates the video image of the line into 3D coordinates, providing real-time data renderings that give the operator immediate feedback on areas that might have been missed.
The scanning laser probe scans parts as long as 12 feet without moving the arm, yet can be packed up and moved to another location. The probe captures up to 50,000 points per second and has dual receptors to see the laser line from opposed angles for faster scanning with fewer passes over the part. The probes use Geomagic Studio as their scanning interface. The laser probe offers manual point-and-shoot scanning with a spray-painting-type display.
NASA tests simulated the repair of damage ranging from a crack to a 2"-diameter silicon-carbide coating loss. After the damage was generated, NASA scanned the panels to record the damaged areas. They uploaded the resulting point cloud in Geomagic Studio, which generated a surface model of the panel profile. Then, the crew member applied the paste, which has a consistency of peanut butter, to the damaged panel, being careful to keep the contour of the repair as close as possible to the outer mold line of the panel. An engineer scanned the repaired section and again imported the point cloud into Geomagic. The engineer used Geomagic Qualify software to compare the difference between the original damaged panel and the repaired panel.
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