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Measuring Nozzle Erosion in a Rocket Motor Using Ultrasound

In this research, nozzle erosion in a small scale hybrid rocket motor is measured using ultrasonic methods. Nozzle throat erosion can lead to large performance losses in long burning rockets, and ultrasonic methods could provide real time information on the nozzle throat geometry in a flight weight package. A challenge arising in the usage of ultrasonic methods is the thermal problem. The speed of sound in the nozzle material can change by a factor of two due to the intense heating experienced during a motor firing. Furthermore, thermal properties of the nozzle material change drastically over the range of temperatures experienced at the throat region, which results in nonlinear thermal behavior in the nozzle material. To account for these changes, it is required to have accurate heat transfer models, thermal properties, and temperature measurements within the nozzle material. An inverse heat transfer algorithm has been developed to compute temperature profiles in the nozzle as well as heat flux experienced at the inner nozzle throat surface from experimental temperature measurements. The temperature profiles are then used in conjunction with ultrasound data to give an accurate time-varying estimate of the nozzle throat diameter.

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