NDE and repair of damaged Minotaur fairing shell

A composite grid-stiffened structural concept was selected for the payload fairing of the Minotaur launch vehicle. Compared to sandwich structures, this concept has the advantages of lower manufacturing costs and lighter weight. Various failure modes were examined for the composite grid-stiffened structure. The controlling criterion for this design was a joint failure in tension between the ribs and skin of the structure. The identification of this failure mechanism and the assessment of bounding strains required to control it required extensive test and analysis effort. Increasing skin thickness to control skin buckling resulted in reduced strains between the skin and ribs. During the final steps of manufacture of this composite structure an industrial accident occurred resulting in extensive damage to one half of the fairing. The failure modes developed during the design of the fairing were used to identify probable regions of damage and effective means of repair for this structure. Conventional ultrasound NDE confirmed that extensive disbonding of ribs and skin had occurred in portions of the damaged fairing half-shell. The resulting repair procedures were applied and the structural testing procedure was modified to insure that the repairs had minimal impact on the test results. The structure has passed structural flight qualification and a second unit is being prepared for launch in 2005.