Space Control & Dynamics
The AST in its aircraft form was statically
and dynamically stable in all axes, except that
it became uncontrollable due to loss of pitch
stability above its “pitch-up angle of attack”
of 28 degrees. It had a flight control system
with angular velocity proportional to control
displacement, a typical airplane’s system. The
AST in spacecraft environment acted as any space
vehicle, with neutral (zero) stability in all
axes and a reaction control system with a fixed
angular acceleration from rocket thrusters.
Neutral stability meant that only the imposition
of forced rotations (controlled or
environmental) changed the rotational motions
and velocities, therefore the 3 axes were
free-coasting.
To clarify the performance of space type of
control, let’s look at the pitch axis, which
represents the performance of all, but in which
the mission task was the most demanding, by
far. Beginning with a zero pitch rate, push
down on the 3-axis controller for two seconds
and during that time AST pitches nose down,
accelerated by the two 250 pound thrusters
acting on the lever arm from the tip of the nose
to the center of gravity. When the controller
is released the pitch rate remains constant at
that level meaning the AST continues to nose
down, and if in space will continue unendingly.
To stop the rotation it would require another
two seconds of thrusting in the nose up
direction. It could be stopped in short bursts
so long as the total increments added to the two
second disturbance that started it, for example
four one-half second bursts. Operating with a
series of short bursts has some control
advantages and is referred to as a bang-bang
system.
But now comes the stickler. To control
flight, rate alone is not the primary issue but
controlling the vehicle’s position is. It is
very difficult to have to stop on a dime, so to
speak, when you have to judge when to start the
opposite control motion so precisely, because
unlike aerial control you have ‘only one power
setting’ for changing rate. It is made easier
and safer by bang-bang changes, which weren’t
always practical, because of the steep parabola
on which the mission was performed, but there
was serious risk in overreacting also. The AST
airplane pilot could pitch his airplane rapidly
or slowly by the amount of control deflection
and stop by merely discontinuing his control,
but the AST space flyer, could only increase
pitch rate by maintaining a long burst from his
RCS. But when he discontinued firing for
angular velocity he introduced the need to
estimate the position to begin to undo what was
done, using the fixed thruster force. If he
overshot the mark it would start the process
again in the opposite correction. And in fact
it was ideally a constant series of small
corrections and deletions in all three axes.
Simply for clarity, the explanation that follows
presumes the desired position of the nose was
directly along the zoom path when it actually
was up 16 degrees to assure safe reentry angle
of attack, but that offset was built into the
command needle positioning: The pilot had no
way to predict or know when that critical point
to safe entry was actually reached. The only
indication was when normal aerodynamic flight
control began to return. For roll and yaw the
process and reactions were identical but the
task was much easier because there was no
inherent change in the orientation of those two
axes during the zoom, because roll and yaw were
always steered to be true to the flight path,
but the tasks were not trivial since
disturbances changed both in any direction,
requiring corrections.
The process of the pitch rate control for a
maximum zoom from the moment RCS became
necessary was demanding. There was a very
gradually increased nose down stick position as
space flight was approaching, due to
deceleration and gravity forcing commencement of
a decrease from the 70 degree pitch climb. Once
in space mode the nose down rate had to
continually increase at a greater rate with the
loss of jet thrust and then rocket propulsion,
until the exact top of the parabolic track, it’s
apogee, because the slope changed most rapidly
there. This is a good point to compare airplane
and spaceplane controls. The airplane stick
needed only to be gradually pushed further
forward to increase the nose over rate, but as
the space environment took over the pitch
controller had to be pulsed more often and
longer to increase the nose down rate.
At the point the AST was momentarily
horizontal, which was the point of maximum nose
down rate and the pitch angle had decreased by
70 degrees, but another 70 degrees nose down was
needed before reentry of the atmosphere, to
achieve the safe 70 degree diving reentry. The
nose down rotation must continue but at a
decreasing rate until then. Unlike aircraft
control, where the pilot would only need to
gradually reduce the amount of nose down with
his stick, the statically neutral AST would
continue nosing down with that highest rate
unless commanded to decrease the rate.
Therefore at the apex the controller would
suddenly need a great deal of nose up commands
since the decrease was most severe near the top
and continued to reentry at diminished rate.
The flight path was a mirror image climbing and
descending and so were the commands in space
controls. Suddenly, at apogee, the process
reversed and long nose up commands had to begin
reversing that highest nose-down to largest
nose-up, a very big change, very rapidly and a
critical chance for overshoot cycling. As the
descent continued the nose up rate had to slow
down to mirror the upward flight path, until
reentry to aero flight control, maintaining 16
degree alpha throughout. It was never possible
to meet this ideal and commands in both
directions were frequent to remain very
controlled in attitude throughout the path.
Even with very good technique, the necessary
changes were unpredictable and required constant
attention and change due to many factors. One
such condition was significant wind shear,
gusts, to displace AST in every direction,
negating a perfect plan. I was at first
surprised to find that up to 120,000 feet, but
the age of the Space Shuttle shows extensive
disturbances, far above our altitude. Another
major factor was the unexpected gyroscopic
effect from the free-spinning jet engine, which
caused an unwanted yaw rate with each command
for a pitch rate change, and conversely. So
that, when the gyro-induced yaw occurred,
another smaller pitch change resulted from that
coupling. And, as with aerial flight, the
usually benign roll axis had to be considered
due to disturbances.
True space vehicles use thrusters for
attitude control, a la AST, but also employ
rocket thrusts through the center of gravity to
change their orbital paths, but that was not a
consideration in the space portion of AST since
the rocket motor was used to achieve the
altitude needed for the zoom, and was exhausted
about the time the primary portion of the space
control mission occurred. The combining of the
orbital changes with attitude controls made
spacecraft docking extremely complex, but
computers coupled with techniques invented by
Dr. Buzz Aldrin PhD, who with Neil Armstrong and
Mike Collins completed the first lunar landing
mission, have made docking routine in space.
On the other hand AST pilots’ tasks to
control were exacerbated by the time limit on
attitude changes and the critical need for
maintaining alignment in a rapidly changing
reference. Coupled with the total loss of
aerodynamic stability, as the vehicle moved into
a virtual vacuum, the die was cast for a rapid
set of changes. The pilot had to assure that
those changes were completed while the AST
continued to maintain near-perfect alignment
with what had become a precise parabolic path,
over which gravity had full control, in order
that when he returned to the sensible atmosphere
the AST would not suddenly become airplane again
in an uncontrollable and irrecoverable attitude.
At that point it would be too late for
corrections. |