Some Results

In mid-1965 the X-15 program was roughly three-quarters of the way through its eventual flight program, and flight surgeons at the AFFTC published a report on their findings to date. At the time, nine different pilots had flown the X-15 (Adams, Dana, and Knight had not yet flown); however, researchers only collected data from the six who had flown a sufficient number of flights to be statistically relevant to the analysis (omitting Armstrong, Petersen, and Thompson). The researchers noted that a "potentially very useful comparison of pilot performance and concurrent physiologic response is not possible because the X-15 flight test program is not structured as a psycho-physiological experiment. The aforementioned variability in flight profiles and unpredictable aircraft malfunctions makes possible only a general, qualitative comparison, rather than a specific, quantitative one."[37]

It is important when considering the physiological data obtained during the X-15 program to keep in mind the conditions under which researchers collected the data. In addition to the normal variability of physiologic responses, no two X-15 flights were the same. Different flight profiles and random aircraft malfunctions varied the physiological and psychic stresses to which the pilots were exposed.[38]

During an altitude mission, immediately after launch, the X-15 rotated to a preplanned climb angle and accelerated at 3-3.5 g. The pilot experienced a front-to-back ("eyes-in") inertial force that increased the apparent weight of the body, particularly the chest area, and resulted in a prompt increase in respiratory rate that continued until the acceleration subsided. After approximately 40 seconds of acceleration, the pilot pushed over to a "zero-normal" acceleration. The pulse rate, which had been increasing up to and throughout the launch operation, tended to decline during this period. At engine burnout, approximately 80 seconds after launch, the longitudinal acceleration dropped abruptly to zero, followed by a variable period in which all accelerations were essentially zero. The pilot was in an essentially weightless state during this period and the respiration rate showed a prompt decline.[39]

Immediately after engine burnout, the pilot invariably immediately experienced an increased heart rate that tended to decrease during the zero-g period. The increased heart rate at this point was probably a psychic response to the abrupt transition from a hypergravic state (in the normal plane) to a hypogravic ("weightless") state. From this point to the landing phase, the pilot was busily engaged in usually complex flight maneuvers, including "accomplishing deliberate aircraft perturbations in roll, pitch, and yaw for the purpose of collecting stability and control information." Heart rates and respiratory rates tended to reflect the difficulties the pilot encountered in managing the flight.*40

After the aircraft passed over the top of its trajectory and was descending at a steep angle, the pilot had to pull out of the dive into level flight. This pull-up generated a positive normal acceleration that the pilot experienced as increased body weight. At this point, the anti-g portion of the David Clark full-pressure suit activated to counteract these forces. Nevertheless, during this maneuver, the blood tended to pool in the lower parts of the body, the carotid arteries experienced decreased pressure, and the cardio-accelerator reflex produced a prompt increase in heart rate. The heart rate lowered as the accelerations decreased. The landing maneuver produced only mild accelerations, and the small increase in both heart rate and respiratory rate during this phase was entirely a psychic response to the task of accomplishing the landing. Since steady-state conditions existed for only a few seconds at a time during the brief 8-10-minute flights, physical and psychic stimuli were usually occurring concurrently and independently. This meant that "only the grossest correlation with heart rate and respiratory rate responses [could] be made."*41

In most professions, including piloting, there is a general trend for heart rates and respiratory rates to decrease as an individual gains experience in performing a task. However, an analysis showed "no statistically significant difference" between early flights and later flights for each of the six pilots analyzed. Researchers believed a number of factors could explain this failure to adhere to the expected trend. The first, and probably most important, was that there were no "easy" X-15 flights. Trying to obtain the maximum amount of data on each flight kept the pilot very busy performing the required maneuvers at the proper time while maintaining the desired flight profile. This required intense concentration, and the pilot also had to monitor aircraft systems during this period.-*42*

In psychological terms, the pilot had a fast-moving, intensive task to perform continuously during his 8-10-minute flight, plus a few minutes on each end. Added to this was the psychic stress of actual or potential system failures, which were not uncommon during the flight program. Another factor was the variation in flight profiles, which meant that the pilots had little or no opportunity to develop a routine for a familiar flight. Furthermore, there was often a considerable interval between flights flown by individual pilots. In the end, the researchers found that it was "not surprising that a rapid reduction in responsiveness" was not seen. The researchers found that, overall, "the spectrum of physiological response of the pilots to X-15 flights, in terms of heart rate, respiratory rate, blood pressure, and pulse pressure, has remained quite stable throughout the X-15 Program regardless of pilot experience level. This pattern of physiological response may be tentatively considered the norm for this type of operation."*431

In the end, the X-15 program was both a contributor to and a recipient of biomedical instrumentation. It was the first program to generate meaningful requirements in airborne biotelemetry and was the impetus for the development of several pieces of instrumentation that later found their way into standard clinical practice. Although Mercury and Gemini gathered better data, the X-15 nevertheless contributed to the physiological database that helped establish baselines for future programs. However, perhaps the most significant contribution of the X-15 program from a biomedical perspective was "the unequivocal, and at times dramatic, demonstration of the capabilities of the human pilot in managing a vehicle and a flight profile from launch to landing, which is a true space flight in miniature.’444*