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Introduction & Configuration - Flight Profile - Max Zoom - Aerodynamics of the AST - Space Control & Dynamics

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.

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