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
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:
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.
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
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.