Optical pressure measurement: using fiber optic transducers in hypersonic flight vehicles

The supersonic combustor of a hypersonic engine is a harsh environment in which to apply close-coupled sensors. While fiber optic sensors have some inherent advantages over conventional electronic sensors, the first challenge was to design a fiber optic connector that could support the application of fiber optic sensors. We have designed a high-temperature fiber optic connector that will allow the placement of a fiber optic sensor into the wall of a high-temperature pressure vessel such as the DCR combustor. This connector has proven versatile enough to support monitoring of the ignition status of the combustor via looking at the spectral content of the flame. Additional information about the combustion process can be obtained by using multiple fibers and multiple detectors with different wavelength sensitivities or by connecting the fiber to a spectrometer. We have also shown that two different fiber optic transducer manufacturers, using two different fiber optic sensing technologies, can produce fiber optic pressure transducers that can be installed inside of the high-temperature fiber optic connector. These pressure transducers were closely coupled to the combustor and have successfully monitored the combustor pressure during short ground tests. The transducers used for these tests were modifications of standard designs by Fiso Technologies and Optrand Inc. The small size of the fiber optic pressure transducers and their high-temperature capability allowed them to be applied in locations where the application of conventional strain gauge transducers is not possible. Such close coupling of the transducers allowed observation of phenomena not seen with prior existing instrumentation. Both of the transducers furnished useful data; however, optimization of the transducers was not possible under the scope of this program. It should be noted that the hypersonic DCR tests are short ground tests using heat sink model hardware. The optical sensors have not yet been proven in a flight environment, where the run times will be longer and the combustor temperatures will be higher. We must perform additional work to make them ready for flight tests.