Ultrasonic inspection of seat-back ejection rocket motors

Ultrasonic methods are described for detecting unbond regions between the steel casing and solid propellant in seat-back ejection rocket motors. Feasibility tests on motors filled with a propellant simulant have demonstrated that at least two techniques are viable: (1) a resonance of the steel casing at a specific ultrasonic frequency, and (2) monitoring the acoustic energy losses produced by the air gaps in regions of steel-propellant unbond. The resonance technique holds more promise since it does not depend on the internal rocket propellant geometry, whereas the second technique does. Recommendations based on results obtained to date are given for developing a nondestructive testing technique suitable for the 100% assembly line inspection of these rocket motors. The envisioned systems, besides being inexpensive, would be independent of operator judgment errors.