Design and Laboratory Validation of a Capacitive Sensor for Measuring the Recession of a Thin-Layered Ablator

Noffz, Gregory K. and Bowman, Michael P. (1996) Design and Laboratory Validation of a Capacitive Sensor for Measuring the Recession of a Thin-Layered Ablator. Technical Report NASA TM-4777, Research Engineering, NASA Dryden Flight Research Center.

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Abstract

Flight vehicles are typically instrumented with subsurface thermocouples to estimate heat transfer at the surface using inverse analysis procedures. If the vehicle has an ablating heat shield, however, temperature time histories from subsurface thermocouples no longer provide enough information to estimate heat flux at the surface. In this situation, the geometry changes and thermal energy leaves the surface in the form of ablation products. The ablation rate is required to estimate heat transfer to the surface. A new concept for a capacitive sensor has been developed to measure ablator depth using the ablator’s dielectric effect on a capacitor’s fringe region. Relying on the capacitor’s fringe region enables the gage to be flush mounted in the vehicle’s permanent structure and not intrude into the ablative heat shield applied over the gage. This sensor’s design allows nonintrusive measurement of the thickness of dielectric materials, in particular, the recession rates of low-temperature ablators applied in thin (0.020 to 0.060 in. (0.05 to 0.15 mm)) layers. Twenty capacitive gages with 13 different sensing element geometries were designed, fabricated, and tested. A two-dimensional finite-element analysis was performed on several candidate geometries. Calibration procedures using ablator-simulating shims are described. A one-to-one correspondence between system output and dielectric material thickness was observed out to a thickness of 0.055 in. (1.4 mm) for a material with a permittivity about three times that of air or vacuum. A novel method of monitoring the change in sensor capacitance was developed. This technical memorandum suggests further improvements in gage design and fabrication techniques.

EPrint Type:	NASA Technical Memorandum
Keywords:	Ablation, Ablation measurement, Capacitive sensors, Fringe field, Thin film measurement
Subjects:	(31 - 39) Engineering: (34) Fluid Mechanics and Thermodynamics
ID Code:	196
Deposited On:	16 July 2004
Additional Information:	38 pages.