Stock Photo - Both the HL_10 and X_15A2, shown here parked beside one another on the NASA ramp in 1966, underwent modifications. The X_15 No. 2 had been damaged in a crash landing in November 1962. Subsequently, the fuselage was lengthened, and it was outfitted with two large drop tanks. These modifications allowed the X_15A_2 to reach the speed of Mach 6.7. On the HL_10, the stability problems that appeared on the first flight at the end of 1966 required a reshaping of the fins´ leading edges to eliminate the separated airflow that was causing the unstable flight. By cambering the leading edges of the fins, the HL_10 team achieved attached flow and stable flight.