The X-15 is a famous and significant part of aviation history. Its purpose was to fly high and fast, testing the machine and subjecting pilots to conditions that future astronauts would face. It made the first manned flights to the edges of space and was the world’s first piloted aircraft to reach hypersonic speeds, or more than five times the speed of sound. The X-15 was an important tool for developing spaceflight in the 1960s, and pilots flying above 50 miles altitude in the X-15 earned astronaut wings.
Three X-15s were built, and they made 199 flights between 1959-1968. The program was a joint U.S. Air Force/Navy/NASA project.
Like other rocket planes, the X-15 was launched in midair from a B-52 “mothership” at about 45,000 feet. Once its powerful rocket ignited, the X-15 streaked upward to the limits of the atmosphere, then glided unpowered to land on a dry lake bed.
Typical flights lasted about 10 minutes.
Among the pilots who flew this one of a kind research aircraft, there was William “Pete” Knight, who experienced the most notable event of all the X-15 drivers. In fact, as reported by John Anderson and Richard Passman in their book X-15 The World’s Fastest Rocket Plane and the Pilots who Ushered in the Space Age, on Oct. 3, 1967 he achieved the maximum Mach number for X-15, an outcome reached flying the X-15A-2 coated with a white ablative heat shield and equipped with extended fuel tanks as well as with extra external fuel tanks that allowed more full thrust time, bringing it from 90 to 141 seconds.
A dummy model of NASA’s high-speed research engine (HRE), which was part of a research program to develop a scramjet (a supersonic combustion ramjet engine) was attached underneath the aircraft for the sortie.
After about an hour under the wing of the B-52, finally Knight took off at 1:20 p.m., heading for Mud Lake where he was dropped after two launch attempts. Pete recalled the first launch attempt in Dennis R. Jenkins and William H. Dana book X-15: Extending the Frontiers of Flight: “Reached up and hit the launch switch and immediately took my hand off to [go] back to the throttle and found that I had not gone anywhere. It did not launch.” Two minutes later Knight tried a second attempt that resulted in a fine release. At this he accelerated and climbed at an angle of attack of 12 degrees (which was the angle between the wing chord and the free stream airflow direction) at high lift until he reached a climb angle (the angle between the horizontal and the flight path) of 32 degrees. He leveled off at 102,100 feet and reached a speed of 6,600 feet per second (Mach 6.7), a speed that remains today the fastest for a manned powered airplane.
But while Pete was performing some rudder pulses to get data with the yaw damper off after the burn out, at a speed of Mach 5.5, the Hot Peroxide warning light switched on.
The warning was caused by the shock wave from the engine cowling impinged on the bottom surface of the X-15: in fact, the intense aerodynamic heating in the impingement region burned through the attachment pylon, separating the dummy scramjet from the airplane. Moreover the shock waves also impinged on the vertical tail, with some melting and skin rollback severely damaging the airplane and forcing Knight to jettison the remaining peroxide to avoid its explosion.
This unpleasant excitement distracted Pete from the energy management of the X-15 who arrived at the so called “high key” (actually the plan to reach an altitude of about 35,000 feet at a speed of 290 to 350 miles per hour at the highest approach to the runway at Edwards Air Force Base) at supersonic speed rather than the pre planned subsonic speed. To reduce the kinetic energy of its X-15, Knight tried to jettison the scramjet, but nothing seemed to happen and Pete had to dissipate the excess kinetic energy by flying past the landing site, allowing aerodynamic drag to slow the airplane and then landed at the proper speed.
This flight was the last one for the X-15A-2, which is today on display in the Air Force Museum at Wright Patterson Air Force Base in Ohio.
But what really happened to the dummy scramjet? Without any doubt Knight was lucky enough to live through its record flight. In fact the dummy scramjet didn’t release immediately once jettisoned and it was later found on the lakebed: if it remained attached any longer to the airplane, the shock wave would have burned a hole into the primary structure of the fuselage, destroying the X-15 in flight.
Photo credit: U.S. Air Force and NASA
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