Testing would reveal that the initial prototypes did not have adequate fatigue life, with early cracks forming in the fuselage. The five initial prototypes required additional structural reinforcements in order to continue flight tests. The aircraft subsequently underwent a structural redesign, with increased composite material usage, reinforced airframe to meet full life cycle requirements, elongated tail "sting", and slightly greater wingspan; the sixth flyable prototype was the first of the redesigned "second stage" aircraft, with the five previous prototypes considered "first stage" vehicles.