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An Optical Fiber Sensor to Measure Strain in Solid Rocket Motors

Craig Lopatin

A large strain optical measurement method has been developed to determine bore strain
in solid rocket motors. Based on an approach using polymeric multimode optical fibers,
the strain transfer between the solid propellant and sensor is 100% and is independent of
the Young’s modulus of the fiber. While glass fibers would also work in this approach,
polymeric optical fibers were chosen to prevent fiber fracture during installation.
Calibration tests have shown a strain capability of at least 10%, and the method should be
able to accommodate even larger strains. The sensor was used to monitor tangential bore
strain in an inert solid rocket motor during thermal cycling, and the repeatability of the
results was good, indicating little drift over the twenty hour tests. The method may also
find use in other applications where a sensor is needed that not only measures large strain
but retains the advantages of optical fibers.