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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.