## The Role of the X-15

NOT SINCE THE WRIGHT BROTHERS solved the basic problems of sustained, controlled flight has there been such an assault upon our atmosphere as during the first years of the space age. Man extended and speeded up his travels within the vast ocean of air surrounding the Earth until he achieved flight outside its confines. This remarkable accomplishment was the culmination of a long history of effort to harness the force of that air so that he could explore the three-dimensional ocean of atmosphere in which he lives. That history had shown him that before he could explore his ethereal ocean, he must first explore the more restrictive world of aerodynamic forces.

Knowledge about this world came as man developed theories and experimental techniques that helped him understand the complex reaction of air upon a vehicle moving through it. One of the earliest theories came from Leonardo da Vinci, who sought to explain the flight of birds. It was Sir Isaac Newton who, among his many achievements, first put a possible explanation for aerodynamic forces into mathematical form. Later, crude experiments began to provide measurable answers to supplement the theories of airflow. Sometimes the theories failed to stand up in the light of experimental evidence. Often both theory and experiment gave incomplete answers.

But man learned to apply this knowledge. Whenever enough theory was available to answer some questions and enough experimental evidence was at hand to answer others, he has advanced in flight, often past his full understanding of how he did it. While the Wright Brothers had learned many answers before their first flight, men were still trying to discover all the theories that explained it long after it was history. Every pioneering flight stimulated the building of other airplanes, and further theoretical and experimental studies. From all of these efforts has come the detailed understanding of the aerodynamics of flight so necessary as the firm foundation upon which aviation progress has been built.

Nowhere is the durability of this foundation more evident than in the most advanced airplane in the world, the X-15 rocket airplane. For the mathematical theory that Newton published in 1726 -long discounted because it couldn't be applied to airflow at low speeds- is now used to help understand the aerodynamic forces encountered by the X-15 at speeds of 4000 miles per hour.

The X-15 program is adding to the historic foundation of aerodynamics, sometimes measurably, often intangibly, in ways as yet unrealized. Not only has it doubled the speed of piloted flight; it has prepared the way for non-orbiting flight into space. It has pushed piloted flight to an altitude of 67 miles, above 99.999 percent of the atmospheric ocean. Although the X-15 has provided much new knowledge about this once-feared region, its return journey from there has proved even more fruitful. Reentry compounds the effects of aerodynamic and space flight with a maneuver that is more demanding of both pilot and aircraft than any heretofore encountered. Yet, though severely taxing, reentry flight has been mastered, and many previous unknowns no longer remain.

Today, after 120 flights, and accumulated flying time of two hours at speeds above 3000 mph, the three X-15 airplanes show the effects of having pushed past man's complete understanding. Wrinkles and buckles mar the once-sleek fuselages. Gaps have been cut elsewhere. Scars are visible where the skin of the wings has been hammered back in place. The three X-15's appear old and tired after many pioneering flights. One of them has a vertical tail with a razor-sharp leading edge, a radical departure from the others. None of them has the vertical tail with which it first flew. Other changes are hidden, such as the added structure that stiffens the fuselage and vertical tail, and the electronics that now help operate the controls.

 Three views of the X-15's original configuration, with which it achieved a maximum speed of Mach 6.06 and a maximum altitude of 354 200 ft. Its launch weight was 33 000 lb.; landing weight, 14 700 lb. The lower half of its vertical tail had to be jettisoned before landing, since, as the little head-on view makes clear, it otherwise would have protruded below the landing gear when the latter was extended.

 This cutaway drawing reveals the volume of tankage needed to give the X-15 its dazzling propulsion, its pressurization, and its attitude control in space. Liquid-oxygen capacity, 1003 gal.; anhydrous-ammonia capacity, 1445 gal.

The changes came from broad-scale attacks, carried out in three phases. The first comprised the early flights, which explored the boundaries of the major research areas. The second consisted of methodical flights to fill in necessary details. Most of this is now history. In the third, and current, phase the X-15 airplanes are being used more as research tools than research craft. This new role includes carrying scientific experiments above the atmosphere-shrouded Earth into regions no satellite or rocket can usefully explore. The X-15 also serves as a test bed for new components and subsystems, subjecting them to a hypersonic flight environment.

Although not all the results of the program are in yet, many important questions have been answered, some of the major ones during design and construction. A structure was developed that has withstood repeated flights into a searing airflow that has heated large areas of the structure to a cherry-red 1300° F. Sometimes the structure responded in an unexpected way, because of uneven heating. Hot spots caused irregular expansion, and those wrinkles and buckles. But while these effects were dramatically visible, they were always localized and merely slowed the pace of the flight program, never stopped it. From this has come a clearer picture of the combined effects of stresses from aerodynamic loads and aerodynamic heating.

It has also shown the interplay between airflow, elastic properties of the structure, and thermodynamic properties of air. The X-15 is the first airplane to push from supersonic speeds to hypersonic speeds, where the river of airflow heats leading edges to 1300° F. It provided the first full-scale hypersonic flight data to researchers who had been concerned with hypersonic theory but who had been limited to the cold-flow results of existing ground facilities. Those cold results had produced little agreement among the several theories for predicting heat flow into an aircraft structure.

From the X-15 data, researchers discovered that theories and experimental techniques were considerably in error. This significant result started detailed measurements and analysis of the airflow nearest the external skin, trying to find the reason. Although the complete story of heat flow is known only in a general way, available theories have been modified so as to yield dependable predictions for it at hypersonic speeds.

In addition, the forces that support, slow down, and stabilize the X-15 can be reliably calculated. The X-15 data have also shown that small-scale wind-tunnel tests accurately forecast full-scale aerodynamic forces, with but minor exception. This increased researchers' confidence in these experimental tools.

### Significant Help From Flight Simulator

One important contribution of the X-15 program is the development of a pilot-controlled flight-simulation device that has greatly aided research. This device combines aerodynamics with an electronic computer so as to simulate any flight condition likely to be encountered by the X-15. With it, many of the unknowns of controlling the X-15 were explored long before the first flight. The results were somewhat surprising. The region of early concern, control in space, was found to contain no serious problems. Yet the time-honored criteria used to predict aircraft stability had failed to uncover a major pitfall. The result: without some aid from electronic control, the X-15 would be uncontrollable over a large part of the anticipated flight envelope.

This major obstacle was overcome, but not without changes to the airplane's tail surfaces and control system as well as to its stability criteria. Analysis techniques were developed that helped explain the phenomena. Significantly, automatic control came to be looked upon not as a replacement for the pilot but as a useful, helpful, even necessary aid, without which the full potential of the X-15 would not have been achieved.

In addition to contributing to high-speed flight, the X-15 program lowered a barrier at the low-speed end of the flight, for the subsequent landing. This landing was expected to be critical, since it would require such precise judgment and control by the pilot that he would have no margin for error. But techniques were developed that gave back to the pilot enough margin so that the landing is now a routine maneuver. Pilots and aerodynamicists now plan with confidence the landing of future airplanes that will have even more extreme landing characteristics.

The X-15 pilots removed one earlier barrier, a psychological one. When some scientists looked spaceward, they became concerned that man himself would be the limiting factor. Indeed, in the missile dawn of the early 1950's, a large segment of the aeronautical industry began to speculate that man might soon be relegated to pushing buttons. No one working on the X-15 project agreed with this view, least of all the pilots. They viewed hypersonic and space flight as a demanding expansion of previous flight experience, not a radical departure. Now, 120 flights have shown us that this traditional concept for piloted flight research, while needing some modification, is also applicable to the space era. Many now wish hat all the X-15 components would exhibit the same steady, competent reliability that the pilots do.

Perhaps the X-15's most significant role has been to sustain interest in manned, maneuverable flight in high-speed aircraft during a period when the world's gaze turned to orbital space flight. The existence of this active program stimulated creative thought and focused attention on the future of hypersonic aircraft in the rapidly advancing age of space travel. Now that men have begun long-range planning of the nation's space program, they envision daily shuttle runs to orbital space laboratories and foresee the need for efficient, reusable space ferries to cross the aerodynamic river, scientists now talk of two-stage rocket planes and recoverable boosters.

Also proponents of the two principal means for orbital and super-orbital reentry -ballistic capsule and lifting body- are coming closer together, for the force that brakes a capsule can be utilized for maneuvering, as the X-15 has proved. Although the stubby wings of the X-15 may look rather puny, many space officials believe they point the way to the future. Thus the X-15 and Mercury programs are seen, in retrospect, as having made a valuable two-pronged contribution to future manned space flight.

Many strong building blocks have come from the experience of doing-the-job; from leaming safe operational techniques and flight procedures; from gaining experience with piloted hypersonic flight and non-orbiting space flight as well as with the intricacies of missile-type operations with large rocket engines and a two-stage aerospace-booster configuration. This is knowledge that may someday pay off in unexpected ways.

But if the X-15 program has been the source of much new knowledge, it is because there were many unknowns when this bold program was undertaken. A large measure of the success of the program is due to the individuals of extraordinary vision who had the resolution to push ahead of these unknowns. They were men who were prepared to take giant steps, sometimes falteringly, not always successfully, but eventually yielding results. They were men who knew that the foundations upon which the X-15 would be built were sound, yet knew they couldn't wait for all the answers before going ahead. They knew that to go ahead with incomplete knowledge would invite failure, and that technological barriers can become psychological barriers as well. They had no intention of trying to batter down these barriers. They knew that measurable contributions would come from studying and probing until enough unknowns were removed so that they could ease their way through to the next obstacle. For they had their sights set way ahead of the X-15, to its successor, and another.

 This diagram shows how the X-15 has explored the aerodynamic-flight corridor to a speed of 4000 mph and the space-equivalent region above it to an altitude of 67 miles. The aerodynamic-flight corridor is the pathway that reentry spacecraft of the lifting-body type would use to returm to Earth from orbital space stations.

From their stimulus, the United States acted, and acted fast. Initiated as a matter of national urgency, thee program emerged from behind security restrictions to become intimately associated with national prestige. Today, a successor is still many years away, and the X-15 remains the only aircraft capable of studying phenomena at hypersonic speeds, space-equivalent flight, and reentry flight. And it has gained a new role as a workhorse. Rarely has a research program encompassed so many fields of basic and applied science, and less often still has any been able to contribute for such a long period in a fast-advancing technological age. Yet, just as the Wright Brothers left many questions unanswered, today, long after the X-15 first flew 4000 mph, men are still trying to find a complete explanation for airflow. But as long as Earth's atmosphere exists, whenever men fly that fast, they will be traveling in a region whose secrets the X-15 was first to probe.