FLYING THE HIGHEST-PERFORMANCE aircraft ever built required intense practice, and still there were surprises. The research program moved at a fast pace, and pilots had few opportunities to share impressions that were not related to the research goals of their flights. Milt Thompson was startled by the violence with which the X-15 detached itself from the B-52 mothership, and at finding, on his first flight, that when acceleration from the huge engine pinned him against his seat, he could no longer scan the instruments in the way he had developed while slouched comfortably in the simulator. Other pilots must have experienced the same thing, but no one had warned him.
“There’s some professional pride there,” Bill Dana comments. “You don’t want to help the other guy do too good a job on his program. I never worried about that—I was never in it for the reputation. But a lot of people did.”
In the chase for records during the X-plane era, some test pilots clearly focused on their own achievements. But Joe Engle remembers the experience differently. “The real thing I’m grateful for is to have gotten to be part of the X-15 program,” he says. “The X-15 program had—I don’t want to sound gooey about it—almost a family attitude about it. Everybody there was family, you weren’t holding back from anybody.... Among the pilots there wasn’t any competition, everybody had the ultimate design limits in mind, and to be part of that climbing-the-mountain process made everybody part of the same team. I would love to do it again.”
One thing most pilots remembered in the same way: the hard physical work of flying the airplane.
“You’re talking about the change in the acceleration as you continue to accelerate faster,” says Bob White. “Going from Mach 2 to Mach 3 took so many seconds, 3 to 4 took less, 4 to 5, you’re cutting down, and the pressure on your chest, you get up to the point where you’ve got 4 Gs and it’s difficult to breathe. And so as Milt said it was the only airplane he ever flew where he was glad when the engine quit.”
As fuel was consumed and the airplane grew lighter, acceleration increased, and all the X-15 pilots experienced a peculiar illusion: the sensation that although they were holding a steady pitch attitude—a 30- to 40-degree climb—the airplane was actually continuing to climb until it was rotating over onto its back. White himself once failed to make his planned altitude because the illusion of over-rotating was so compelling that he had to push the nose down momentarily in order to glimpse the horizon.
Flights were extremely short—usually 10 or 11 minutes from B-52 to lake bed.
“The time went by like a flash,” says Joe Engle. “I remember counting down the last minute of countdown. There are certain things you do and check-list and hitting the release button [to detach from the B-52] and then from then on, right after the flight I would have been hard-pressed to go into a lot of detail, between that and the time when you finally slid to a stop and cracked the canopy.
“The other thing I do recall is that the cockpit and the suit were pressurized with liquid nitrogen that could be released through a valve. Cooling was the same way: You just opened up the valve to cool it down. Some of it I’m sure is because the skin would heat up in flight, but I recall turning it up, because everybody said turn it up all the way before you launch, and being almost cold, you know, and then again, this flight going by like the snap of a finger, and sliding out on the lake bed and cracking the canopy, wanting to get it open because I was just drenched in sweat.”
Ships 1 and 2 had conventional controls, plus a three-axis stability augmentation system, which would weakly counteract any unintended motions in pitch, yaw, or roll. Ship 3 had the fly-by-wire adaptive flight control system that Armstrong helped design. The system had two purposes: to make the airplane handle similarly in all flight regimes, and to seamlessly integrate the thrusters, used during the weightless coast above the atmosphere, with the aerodynamic controls. It was thought that the airplane might not be controllable during reentry without artificial stability augmentation until Pete Knight experienced a total electrical failure.