THE X-15 PROGRAM IN RETROSPECT

Piloting Aspects



One of the major initial goals of the program which has been most richly achieved was to explore the capabilities, and the limitations, of the human pilot in an aerospace vehicle. There were those in 1954 who speculated that man had no place in hypersonic or space flight. And there were others who believed that he would prove indispensable. In either event, the space trajectory and reentry maneuver which the X-15 pilot was asked to negotiate were guaranteed to provide a convincing test.

From the outset simulators of all kinds were used to an unprecedented extent in pilot training., flight planning, and also in vehicle design. There was no two-seated version of the X-15 in which pilots could be taught to fly. Twelve pilots trained on the simulators with outstanding success. These experiences paved the way for similar all-out use of simulators in the space program.

It is well known that for greatest effectiveness the use of simulators requires careful correlation with flight testing. In the early stages of X-15 design, of course, flight data were not available, and some of the design features decided upon on the basis of the simulated experiences alone proved to be wrong and had to be altered. One of these was the large ventral tail employed during the first phase of the program. In the original vehicle configuration developed by RACA in 1954 it had been found that this axrangement suffered at high angles of attack at hypersonic speeds from a nearly complete loss of effectiveness of the upper tail, and a large increase in effectiveness of the laver tail, leading to very high and undesirable negative dihedral effect. Thus, our original proposal suggested that only a small ventral tail should be used. The early simulator studies, however, revealed that the large ventral tail was necessary for law angle of-attack controllability to cope with feared thrust misalinement effects of the rocket engine. Furthermore, as shown in figure 3, left side, the simulator studies with the large ventral indicated that the machim could be controlled without dampers at high angles of attack in spite of the negative dihedral. Thus the decision was made to use this symetrical tail configuration.

Figure 3 - three line graph charts showing the handling characteristics of X-15 with dampers inoperative
Figure 3. Handling characteristics of X-15 with dampers inoperative.


This condition of "dampers-off" controllability was an essential design requirement because of the doubtful reliability of the damper system. In the first flights of the program, contrary to the simulator results, the machire was found to be unflyable at angles of attack above about 8 with dampers inoperative (fig. 3,, center). This discrepancy was traced, in part, to the influence of secondary aerodynamic effects (such as trim, for example) on the stability derivatives, effects which were not included in the original simulation. In addition, the pilots naturally felt less secure in flight than in the simulator and were not willing to accept vehicle motions which they had rated "acceptable for emergency" on the early simulator. With flight "calibration" of this kind together with a continuing program of other improvements, the fixed-base simulator eventually achieved satisfactory simulations of instrument flight.

Early in the flight program when the state of affairs shown in figure 3, center graph, had been established there was serious doubt as to vhether the high altitude "space flight" missions of the X-15 could be flown safely. These missions typically required angles of attack in excess of 17 on reentry. One of the major constraints in the problem was eliminated when operational experience with the XLR99 rocket engine revealed that it had no significant thrust misalinement as originally feared. Thus the underlying reason for the large ventral disappeared, the ventral rudder was removed., and the problem was solved by a return to a tail configuration similar to that recommended by NACA in the original 1954 study (fig. 3. right graph). As an added safety measure, a back-up damper system was installed to provide high reliability. With this system the "uncontrollable" region above 20 could be safely penetrated, and reentry trajectories up to 26 were flown.

And so it vas that the absence of flight "calibrations" of the early fixed-base simulator, together with -unfounded worries over thrust misalinement led to a costly excursion in configuration design. A consoling thought in retrospect is that more was learned than if this mistake had somehow been avoided.

The capabilities demonstrated by the pilots in the principal areas of interest are summarized briefly as follows:

Exit phase

The program shows clearly that, given precise displays, the pilot can fly rocket-boosted vehicles into space with great accuracy (refs. 7, 8). He cannot do any better than completely automated systems, however. Perhaps his best role will be as a monitor of automatic systen able to contend with malfunctions or to make trajectory changes as needed.

Attitude control in space

This was considered a major research problem area in 1954. Development of a workable reaction control system was achieved with the aid of a ground-based simula or and flight tests at low dynamic pressure in the X-lB airplane. As a result of this program it became clear that attitude control without aerodynamics and with threshold aerodynamics were skills readily acquired by pilots, and the X-15 high-altitude flights fully confirmed this finding.

Maneuvering reentry

The steep reentries of the X-15 with flight path angles up to -38, Mach numbers approaching 6, and angles of attack up to 26 presented a more difficult piloting problem than the shallow entries of lifting manned vehicles returning from orbital or lunar missions. The prime requisite, of course, is a flyable vehicle, which means in general for hypersonic flight a vehicle incorporating artificial damping systems. When the X-15's damping systems were operative the pilots could perform the reentry maneuver readily (refs- 7, 9). The "self-adaptive" damping system was preferred over the simple rate-responsive dampers. (Footnote: The basic feature of the "self adaptive" system is its automatic gain changer which maintains the desired dynamic response characteristics of the airplane for a wide range of dynamic pressures. Added capabilities of the installation in the X-15-3 airplane were dual redundancy, integra tion of aerodynamic and reaction controls, and automatic stabilization in pitch, roll and yaw. The system was developed under sponsorship of the USAF, Aeronautical Systems Division, and it represents one of the noteworthy advances associated with the X-15 program.) Transitions from reaction to aerodynamic controls were made without difficulty and a control mode in which the two systems were blended was also developed satisfactorily.

Gravity effects on pilot performance

With a few exceptions these proved small - essentially negligible. Weightlessness, which was one of the largest fears of the unknown in the early system studies, produced no difficulties in the few minutes it existed in the high altitude flights. This result, of course, shottly lost its impact after the first Mercury flight in 1962 involving a much longer period of weightlessness.

Pilot plus redundancy

An analysis of the first 44 flights showed that 13 would have failed in the absence of a human pilot together with the various redundant systems provided in the vehicle (refs. 5, 7). Against these'figures in favor of the pilot there were only a few examples where the pilot's error degraded the mission performance, and only one catastrophic accident out of 199 flights. The NASA-USAF board investigating this accident reported that in its judgement, the pilot confused roll and yaw indicators and inadvertently yawed the airplane to 90 or more at the start of the reentry, possibly as a result of display misinterpretation, distraction, or vertigo. This condition apparently lead to complete loss of control and subsequent breakup of the X-15-3 airplane (ref. 10).

The broad positive finding of the program, however, is clear: the capability of the human pilot for sensing, judging, coping with the unexpected, and employing a fantastic variety of acquired skills remains essentially undiminished in all of the key problem areas of aerospace flight. It is equally clear that there are many new areas in aerospace flight in which the pilot's capabilities must be supplemented. The need for artificial damping of hypersonic vehicles is one example.


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