The X-15 rocket plane flew at Mach 6.72 and at altitudes above 67 miles -- at the threshold of space! The men who flew this amazing plane became the USAF's first astronauts. Powered by an XLR-99 engine capable of producing 70,000 pounds of thrust at peak altitude, the X-15 pushed the envelope for manned aircraft past the post office. Notably, both Apollo commander Neil Armstrong and Space Shuttle commander Joe Engle piloted the X-15. Originally highly...