X-15A-2 Credit - NASA

The crash-damaged X-15 number 2 was rebuilt to attain even higher speeds. The body frame was stretched, and two drop tanks were added, increasing propellant load by 75%. An ablative heat shield was applied to protect the spaceplane during re-entry. The X-15A-2 reached the highest speeds and altitudes of any manned spaceplane until the space shuttle entered service.

The X-15A-2, modified from the number two aircraft and delivered to NASA in February 1964, included among other new features, a 28-in. fuselage extension to carry liquid hydrogen for a supersonic combustion ramjet that was flown (as a dummy) but never tested. It also had external tanks for liquid ammonia and liquid oxygen. These tanks provided roughly 60 seconds of additional engine burn and were used on the aircraft's Mach 6.7 flight. While adding to the speed the X-15 did achieve, the tanks also increased the aircraft's weight to almost 57,000 lb and added significantly to the drag experienced by the aircraft in flight.

700 hours was needed to refurbish the heat shield. The ablative covering had to be completely stripped off, then reapplied. After the record Mach 6.93 flight, the aircraft received thermal structural damage and the covering was severely pitted and charred. Repair was uneconomical and the aircraft was grounded.

Crew Size: 1. Length: 16.00 m (52.00 ft). Span: 6.80 m (22.30 ft). Mass: 25,455 kg (56,118 lb). Main Engine: XLR-99. Main Engine Thrust: 262.445 kN (59,000 lbf). Main Engine Propellants: Lox/Ammonia. Main Engine Propellants: 17,138 kg (37,782 lb). Main Engine Isp: 276 sec. Spacecraft delta v: 2,020 m/s (6,620 ft/sec). Electrical System: Batteries.

• 1964 February 1 - The X-15A-2 delivered to NASA -
  The X-15A-2, modified from the number two aircraft, included among other new features, a 28-in. fuselage extension to carry liquid hydrogen for a supersonic combustion ramjet that was flown (as a dummy) but never tested. It also had external tanks for liquid ammonia and liquid oxygen. These tanks provided roughly 60 seconds of additional engine burn and were used on the aircraft's Mach 6.7 flight.