The X-15 was designed to smash existing aerospace records. Jets had reached mach 2, manned rocket planes had reached mach 3, and missiles and rockets (X-7 thru 12) had reached into space. The X-15 was designed to double the mach number and reach 250,000ft (space) – all with a man onboard. Forced to listen to the haunting radio signals of the orbiting Soviet Sputnik 1 satellite, reaching into space was vital to the US. But the challenge was enormous.

In January 1955, 12 aircraft contractors were requested to attend a bidder’s conference at Wright Field for the development of a hypersonic vehicle. Three of them (Martin, Grumman, and Lockheed) presumably thought the challenge too great, and didn’t attend. In the end, only four companies submitted proposals: Bell, Douglas, North American, and Republic. North American won, even though, at $56M, it had submitted the highest bid.

Despite their high bid, North American had woefully underestimated their efforts, the final cost was around $300M. North American put Harrison “Stormy” Storms in charge of the design, ably backed by engineer, pilot, and rocket plane old hand, Scott Crossfield. Four years later they had an airplane.

Over its life, the X-15 was modified many times, its objectives were broadened and new uses were found for its unique capabilities. The X-15 flew 199 flights. A 200^th flight was planned, but technical and weather issues caused it to be delayed several times, until the flight was abandoned and the aircraft decommissioned. A sad end.

An X-15 flight was remarkably brief – about 10 minutes from B-52 drop through to landing. It went up like, well, a rocket, and descended like a brick. The typical flight profile was to launch on the B-52 from Wendover (in the far northwestern corner of Utah) and land at Edwards AFB, California. That means they crossed the length of Nevada in ten minutes.brick

X-15 Mission Profile

Despite the short duration of the flight, pilots trained for 10 to 20 hours in a simulator before each flight. They ran through the flight’s objectives, and the hundreds of failures that might occur. If a fire warning light lit up, was there really a fire? Or had the sensor failed? Or was something else burning, shorting out the wire between the cockpit and the sensor? Even if he couldn’t tell the difference, what was the best course of action? He might not know, he might make the wrong choice, but in the simulator there was always a second chance.

X-15 Simulator

Simulator tests like this are done with all modern aircraft development. Failures are injected blind, meaning that the pilot isn’t advised of what or when a failure occurs. It’s up to him to spot the problem while he’s trying to carry out the planned mission. It’s easy to think that people wouldn’t treat this seriously, and the pilots know “it’s just a simulation,” but its not uncommon for them to emerge from the sim having had a good workout.

This is a video of Pilot Milt Thompson in the X-15 Simulator (and it certainly looks like it was posed for the camera).

However, simulators are no substitute for flight test.

The early flights of the X-15 were unpowered to test the aircraft’s systems. On the first, Scott Crossfield encountered no significant problems until his approach to landing. In the last moments when he extended the flaps, the aircraft pitched up. As he tried to correct, the aircraft started to oscillate. This is a problem called Pilot Induced Oscillation (PIO). In this case, the aircraft’s control surfaces were taking too long to respond to Crossfield’s movement of the joystick. Crossfield would then move the joystick even further, resulting in the aircraft eventually pitching more than he wanted. He’d reverse the motion of the joystick, but again the lag would cause him to overshoot. Fortunately, Crossfield was a highly experienced pilot and PIO wasn’t new to him. He rapidly reduced his movement of the joystick (against what his eyes and senses were telling him was necessary) and brought the aircraft under control just moments before landing. The following sequence of pictures showing the event have been put together on the excellent SierriaFoot X-15 site.

<PIO picture>

X-15 Pilot Induced Oscillation (PIO)

Twelve pilots flew the X-15 over a period of nearly a decade. Scott Crossfield was the first and William Dana the last. Robert White was the first man to pass mach 4, 5 and 6. Joe Walker set the X-15’s altitude record at 354,200ft. Pete Knight flew the fastest flight at 4,520mph. Robert Rushworth flew 34 flights, the most of any pilot.

It was an unblemished record of success until flight 191 took the life of Michael Adams.

Over the next couple of posts we’ll look at some of these people and their experiences, starting with the tragedy of Major Michael J Adams.

Cheers

(Images courtsey of www.sierrafoot.org and NASA).