Description: This program addresses the need to monitor surface recession, temperature, and through-thickness properties of thermal protection system (TPS) materials. These TPS materials have unique properties for shedding the heat generated under reentry conditions to protect the integrity of the spacecraft. However, ever-increasing mission demands require improved performance and a better understanding for modern heat shield structures. This can be accomplished with a noninvasive, nondestructive method that uses projected sensing fields through the thickness of the TPS material. Novel eddy current methods are proposed that incorporate innovative sensor array constructs, physics-based models, and multivariate inverse methods to nondestructively assess carbon-based TPS materials such as Phenolic Impregnated Carbon Ablator (PICA). The sensors can be mounted behind the TPS material or embedded within the TPS with sensing magnetic fields that are projected through the material. JENTEK�s physics-based methods for diagnostics of layered media using MWM-Array technologies have been demonstrated as a nondestructive evaluation (NDE) method for flexible and rigid ablative TPS materials for condition, orientation, and thickness assessment. These methods are commonly used for NDE, such as coating characterization, and have been extended to surface mounted sensing applications such as torque, fatigue, and heat treatment condition monitoring. This proposed Phase I will demonstrate the feasibility of these methods in an embedded sensor configuration for representative material layer configurations and a heating transient as well as investigate the adaptations required for full-scale testing and operation. JENTEK delivered the MWM-Array solution used by NASA KSC on the Space Shuttle leading edge to detect damage of the reinforced carbon-carbon (RCC) thermal protection tiles; thus JENTEK is well-positioned to deliver a novel method for health monitoring of TPS materials.

Benefits: If the program is successful, it will demonstrate the capability of the MWM-Array technology to monitor condition (e.g., electrical conductivity) thickness and temperature of materials that are exposed to high temperature conditions. Initial implementation is anticipated to support specific spacecraft applications, such as monitoring during ground-based tests of thermal protection system materials. Later versions with dedicated instrumentation designed for reduced size and weight have the potential for implementation on spacecraft. In addition to basic heat shields for ablative TPS materials, it is anticipated that NASA may have a need for this type of projected field high temperature monitoring solution in other structures, such as the carbon-carbon rocket nozzles, Multi-Purpose Crew Vehicles, and exhaust nozzles. Other NASA customers which will have a need for this technology include the Commercial Orbital Transportation Services (COTS) spacecraft manufacturers and other interplanetary programs such as science exploration mission vehicles and human crew vehicles.

There are numerous applications that could use an in-situ health monitoring capability for materials exposed to high temperature conditions. These embedded sensors could be incorporated into high performance analytical instruments to improve observability for high value and complex infrastructure, vehicles, machinery and processes. This type of projected field sensing provides capability for through-thickness monitoring for damage, surface recession, and temperature with sensors placed in more accessible and lower temperature regions of structures. An example is high temperature process monitoring such as heat treatment or annealing processes that are commonly performed on metals. In-situ monitoring of the electrical conductivity and/or magnetic permeability would offer a real-time assessment and process control capability. This could also be applied to composite materials where the electrical conductivity and temperature of the carbon-based material is monitored during impregnation and curing of resins.