Survey of Nondestructive Methodology for TPS Vehicle Health Monitoring for Reusable Launch Vehicles

For NASA's Ames Research Center, SRI analyzed the potential for application of nondestructive inspection (NDI), testing (NDT), and evaluation (NDE) technologies for maintenance of the thermal protection system (TPS) on the Space Shuttle and future reusable launch vehicles. The inspection and maintenance procedures and requirements of existing and future components and materials were reviewed. Inspection technologies in thirteen general areas, including optical, electromagnetic, electrical, magnetic, acoustic, and chemical, were discussed in detail. Included were descriptions of the physical basis of the technology, current applications, potential applications to space vehicle health monitoring, and outlines of the research and development requirements and opportunities for each technology. Several new concepts were proposed.

Between flights each shuttle vehicle spends about 10 weeks undergoing extended inspection and maintenance. The most significant and time-consuming activity is maintenance of the thermal protection system, consisting of ceramic tiles, silica fabric blankets, carbon composite wing leading edges and nose cone, and other components. The entire surface the shuttle is inspected by the trained eyes of maintenance technicians. After each flight, about 150 defects are identified and documented in detail for subsequent repair. The repairs themselves take many hours each.

Future reusable launch vehicles have a target turnaround time of only 24 hours. This means that future vehicles must a high probability of zero defects and zero maintenance between flights. Thus, the primary purpose of inspection becomes verification of the vehicle's health. What is needed are alarms rather than measurements. Knowing that a problem has occurred or is developing is important for making the decision to take the vehicle out of service. Technologies for health verification need to be fast, but do not need good spatial resolution. Localization and characterization of the fault can be the first phase of the maintenance procedure for a vehicle that fails inspection or is scheduled for periodic maintenance.

Key conclusions of this study are

New approaches are needed for subsurface inspection
The temperature history (maximum temperature reached during reentry) is the best warning indicator of potential future damage
Improved inspection technology is needed for reinforced carbon-carbon components (e.g. nose cone and wing leading edges) that are likely to be used on all future vehicles

Among the recommended technologies were schemes for passive maximum-temperature monitors, such as plastics or alloys that melt at known temperatures, that can be placed below the vehicle surface, between or under thermal-protection ceramic tiles or metal "fish scales." Also recommended were schemes for remote reporting of temperature excursion warnings, such as release of a visible chemical or dye or by wireless communication with an embedded microchip.

Two new approaches were recommended for verification of the integrity of the silicate/silicon-carbide coating (on wing leading edges and nose cone) and vulnerability of the underlying reinforced carbon-carbon composite to oxygen attack. These components already fly repeatedly without repair. Detailed inspection and analysis could be scheduled infrequently and use special-purpose instruments developed for health verification.

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SRI's Inquiry Line, E-mail: <inquiry_line@sri.com>,
Phone: (650) 859-4771, Fax: (650) 859-4111
David L. Huestis, E-mail: <david.huestis@sri.com>

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