While the 1st prototype suffered hydraulic failure on its second flight and crashed into the woods on final for Calverton 12-30-1970 and the 2nd prototype was written off 05-13-1974 when the aircraft suffered an in flight hydrazine APU fire.  It landed safely but was listed as a write off due to extensive damage.

The 3rd prototype.......BuNo-157982

was used for non-destructive structural testing.

A famous picture of this aircraft in testing