“Pete Knight was the best test pilot that I’ve seen,” says Bill Dana. “He had a flight that launched over Smith’s Ranch and headed for Edwards. At 100,000 feet and Mach 4, both his generators went offline. All the lights came on for a few seconds, and then they all went out. And he never had another electron, that he could see, in the whole flight. He flew the climb using ballistic controls to keep the wings level. He didn’t have an artificial horizon, but he could apparently see out. He kept the wings level over the top and then he wanted to get back to Mud Lake to land there, because it’s a long runway. So he made a 180-degree turn to the left and he used more back stick when he wasn’t developing wing rock or lateral-directional instability, and when the airplane was flying too squirrelly he backed off on the G and he came around, and now he had aerodynamic controls—he had reentered, in other words—and so he flew a dead-stick landing into Mud Lake. To me, that’s the greatest single feat of airmanship that I know of.”
Malfunctions as severe as Knight’s were rare, but if the X-15’s pilots felt anxiety, it probably would have been over that sort of thing—being left helpless out at the edge of the world to be burned up or torn apart by an airplane that had turned savage. Says Bob White: “If you didn’t have a little fear when you stepped into this thing, there was something wrong with you, believe me.”
Milt Thompson was the only pilot to write a book about flying the X-15 (At the Edge of Space, Smithsonian Institution Press, 1992), and because writing about it forced him to turn it over and over in his mind and examine, as writers will, all the feelings that it brought forth, he clothed his memories in metaphor. He described the airplane as a “black bull,” and even imagined a scene in which a young warrior is sent out to explore a mysterious land, clad in armor and mounted on just such a beast. “Beware of the bull,” the tribal elders tell the young man as he is about to set out. “He is awesome in battle. However, if you lose control of him or fall off, he will kill you as quickly as he would kill your enemy.”
Though Bill Dana and Milt Thompson were close friends, “I didn’t understand that part,” Dana says. “I don’t know what he was trying to get across. I didn’t think it was dangerous until right toward the end of the program.”
That was when the long lucky streak ended.
X-15s had never been reliable airplanes. They were complex and novel, and on most flights one system or another would act up. But they had always brought their pilots back alive, no one had ever had to eject, and only one pilot, Jack McKay, had been seriously injured. Even McKay—who was measurably shorter after suffering several crushed vertebrae when a landing skid failed and his airplane flipped over—eventually returned to fly the X-15 again. But Mike Adams didn’t return.
It happened in the Number 3 airplane, the one with the adaptive flight control system, which constantly adjusted the authority or “gain” of the controls in order to make the airplane feel the same regardless of speed and altitude. It was the latter phase of the program, by which point the original research goals had all been met and the X-15s were being used as mules to carry scientific experiments to extreme speeds or altitudes. An electric motor that was part of an experiment carried on the wingtip created a disturbance that interfered with the flight control system as the airplane shot out of the atmosphere. Adams, whose known susceptibility to vertigo had been ignored when he was assigned to the X-15 program, apparently became disoriented. An additional trap lay in wait: A needle on his primary attitude indicator could be selected to display either roll or yaw. Adams got mixed up and tried to correct with yaw for what was actually a roll cue. Controllers on the ground could not tell what was happening, but the airplane was rotating about its vertical axis until, when it reentered the atmosphere, it was flying sideways. A violent and dizzying ride followed. Adams reported that he was in a spin—a situation that had never before been encountered in hypersonic flight, and for which no recovery procedure was known. At first, however, the black bull corrected itself, its rotation slowing as it weathercocked back into alignment with its flight path. For a brief period it was inverted but stable, with sufficient altitude for a recovery.
But then the adaptive flight control system began pitching the aircraft up and down with increasing violence until, somewhere beyond 8 Gs, the airplane broke apart. A switch on the panel could have shut the runaway system off, but no one thought of it until too late.
“I have always associated the end of the program with Mike’s accident,” says Bill Dana. “We were going along with three airplanes, getting lots of data, and had lots of plans. And when Mike was killed, it kind of took the heart out of the program. And I think there were a lot of people that would have liked to quit the program right there. I think the program quit itself. You can imagine the emotion involved there, when Mike got killed.”
But it did not end immediately.