Motor case-to-insulation unbond repair accept/reject criterion

Abstract

As the 1986 failure of a Titan 34D launch vehicle was linked directly to an insulation unbond of a solid rocket motor, a comprehensive nondestructive evaluation of bondline integrity was implemented by the motor manufacturer. Unbonds at the insulation-case interface were discovered in several production units of the Titan 34D solid rocker motors along the forward edge of the motor segment. These unbonds were repaired by machining a J-shaped groove in the insulation edge area. In order to assure its flightworthiness, the J-repairs were subjected to acceptance proof tests. Detailed finite element stress analyses were conducted to develop an accept/ reject criterion for use in J-repair proof tests. The proof-test criterion, established in terms of required prooftest loads and test-tool displacements, accounts for the effects of temperature, time-to-load, and J-repair size. The analyses indicate that J-repairs up to 0.25 in. deep can be accepted by the insulation bondline proof test. For a larger repair, however, an additional proof test is required after the J-repair is filled with filler material.