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4 Theory to RealityA maximum zoom at 70-degrees pitch, that Lockheed’s austere studies recommended, would begin at 35,000 feet and Mach 2.2, with a 3.5 g pull up to a pitch angle of 70 degrees, that is, the nose pointed up and maintained throughout much of the climb at that angle above the horizon. The pilot, in a pressure suit could no longer see earth shortly after starting that pull and until completing the climbing zoom. The AARS was the primary flight instrument, and displayed pitch angle, as well as roll and yaw, the latter two held by the pilot at zero, i.e. trim when in the aero region. That constant-pitch climb would result in gradually decreasing speed, a decreasing flight path angle and an increasing angle of attack. All of these (calculated with a rather austere mathematics analysis) should optimize the maximum possible altitude somewhere around 120,000 feet, according to Lockheed Aircraft engineers. The objective was to hold that pitch in the climb until intercepting a new flight path at the point in the climb where alpha increased to 16-degrees, to be held constant until reentry by controlling the attitude through a rapidly changing parabolic path controlled by the pull of gravity. That required the AST pilot to command a constantly varying rate of nose-down pitch, along with maintaining roll and yaw at zero without the airplane’s static stability, which would be lost in all axes in the space-like portion of the flight. It would truly act like and must be controlled like a spacecraft. That phase of the mission would necessitate careful pilot maneuvering solely with the RCS by continuing to maintain zero yaw and roll while changing pitch through 140 degrees of nose down before being in full 70 degree dive recovery, and reentry into enough air for the AST to become an airplane, once again. Not until later, and addition of a modification, were speed brakes permitted during reentry, so the maintenance of 16 degree alpha was necessary to avoid unsafe conditions during the descent. The 140-degree nose down was absolutely critical to be completed in the period between leaving the aero region and before falling back into the atmosphere to avoid loss of control on re-entry into the aerodynamic region. Failure to do so put the AST outside safe and recoverable flight, in any number of uncontrollable maneuvers from tumbling to irrecoverable flat spin. Careful and constant control of alpha was the only way to assure safe flight since failure to constantly react properly could move quickly into a unrecoverable situation, because of RCS power limits and the very rapid changes required in nose-down approaching the apogee of the parabolic path. To achieve that demanded the pilot maintain proper and continually changing pitch in nose down. A nose down pitch rate had to accelerate to be fastest at the apogee and then steadily decelerate to zero upon regaining full aerodynamic flight. That meant continual attention and constant control inputs. We were to find out in actual tests that these pitch changes would also induce motions and necessary corrections in beta adding an unexpected measure of complexity. The pitch axis was clearly the most difficult and critical of the axes, but failure to maintain zero roll and yaw could also prove dangerous, so flying all three with the RCS was imperative. The AARS, attitude indicator, was critical for the zoom. Because the pilot could not see earth after pull up and until most critical controlling was complete, the entire zoom had to depend on instrument flying. Attitude control was primary, but other tasks could not be ignored, during the climb. The afterburner required gradual throttle reduction until over-temperature could no longer be avoided, at about 75,000 feet at which point it had to be shut down. The main jet engine was at full throttle to about 85,000, but requiring similar monitoring and throttle retards, until minimum idle and finally shutdown at temperature limit. After jet shutdown only the Rocket engine was propelling the AST, until it ran out of its oxidizer, hydrogen peroxide (H2O2), which mixed with the JP-4 jet fuel for combustive thrust. It was very important but difficult to maintain the optimum pitch angle without constant attention as the aircraft fought to drop the nose with speed decrease, without retrim of the elevator, which would have added hazard on reentry. And lost pitch angle could not be regained, at least without great loss of energy/altitude. Lacking any indications of energy loss, it was critical to performance and safety to stay on schedule. Later events of a very famous test pilot would demonstrate the risks of that, vividly! Maintenance of yaw and roll were important to safety and performance, but were relatively routine, as in instrument flying, because of aero trim until above 100,000 feet, after which yaw proved to be another thing, altogether. Another very critical aspect surfaced approaching the apogee, as altitude increased and speed decreased. When the normal controls began failing to respond to pilot inputs the AST was changing from airplane to spaceplane and the latter had totally different flying and control requirements. At that point the pilot began a transition from the stick and rudders to the RCS hand-controller on the left side of the instrument panel, combining control for a period. Up to that point the AST was a typical airplane with static and dynamic stability and a normal proportional rate control system. From there on it became a space vehicle with zero stability and a fixed thrust angular-acceleration control system. In fact, it proved to become somewhat unstable due to cross-coupling of yaw and pitch from the gyroscopic effects of the rapidly rotating jet engine, long after the jet engine was shut down. This had not been predicted. In short, thrusters are rotation accelerators, so AST attained a constant rotational velocity about any axis depending on how long the thrust command was applied. So it would take the same duration counter-command to stop it. Since position of AST was the critical issue, it would more than double that effort just to stop it, to then move back to a given position and all of that without the rapid and variable control power available in the proportional control system of an airplane. The difficulties of the zoom climb were significant in the many actions required besides controlling the craft, all of which was exacerbated because the pilot had no visual reference, just sky. With the seat canted back 14 degrees to the aircraft axis, we were on our back at 84 degree pitch, in a full pressure suit with fixed helmet. I didn’t see earth from pull-up until over the top and it took a few flights before I found time to look away from the instruments even for a couple of seconds, but the view at apogee proved to be well worth a glance. Shaped like a pump handle of a yard spray tank, the RCS controller was rotated in the desired roll direction, pushed left and right for corresponding yaw and raised to nose up for increased alpha or down to lower it. Any and all command combinations were possible. Factors that exacerbated the problems for max zoom, were the limited time allowable to complete the 140-degree pitch-over with the fixed power of each thruster at 250 pounds, a total of 500, always fired in a pair. These, adapted from the X-15, proved sufficient, even redundant, when used properly, since the control torque was therefore 500 times the distance to the AST center of gravity , measured in foot pounds. There were other dangerously critical things like oxidizer temperature necessary in flight, but once the rocket was lit and the nose pointed up 70 degrees that was the best you could do to fix a problem, in fact, was the only way to rid that explosive risk. A brief summary might put the difficulties faced by the AST pilot in perspective. Propulsion in space requires rockets to supply their own fuel and oxidizer for combustion and gain thrust from their reaction to the gas products of combustion expelled from the motor nozzle, in opposite direction from the exhaust gases. The jet engine is different only in that its fuel is combined with ambient air as the oxidizer, so it becomes useless as space is approached. Aerodynamic forces are employed to control attitude on aircraft, but they too become useless in space and miniature rockets can serve as attitude thrusters, as they did for the AST. Directional control requires not only thrusting of a rocket, but the addition of aiming, which requires control of the crafts attitude. Control-of-attitude is most complex and demanding. It is totally different between the regions of atmosphere and space. An airplane can be configured by design to be inherently stable in its attitude (stay where put) but spacecraft lack stability, no matter how configured. Only with the advent of supercomputers, not available at the time of AST, has it been possible to incorporate auto-stability in both air and space craft. The attitude control with thrusters was vitally critical ‘over-the-top’ in a maximum zoom of AST because if the pilot varied much he could reenter the aerodynamic region outside the airplane’s stability limits resulting in certain loss of control with the possibility of being unable to recover and land safely. These facts were demonstrated during the AST test program on all three occasions, when the limits were exceeded. Here’s a brief summary of some of the pilot’s challenges in attitude control of AST, explained more technically in the AIRCRAFT section of this website. Airplanes’ have “Proportional Rate” attitude-control systems that allow the pilot to move them about all three axes of motion within a large range of rotational velocities merely by selecting a control position. To stop the motion immediately, the control is returned to null, aided by the natural stability of the craft. Space flight allows only “Fixed Acceleration” for attitude control systems, which, coupled with lack of stability, made the AST very difficult to control safely in space. Difficulties in controlling it were increased by gyroscopic effects from the jet engine’s rotation. Space controls activated in any of the three axes of motion cause a spacecraft to continue accelerating around that axis at the system’s design rate for as long as the control is held on. Because of the space vehicle’s lack of stability it doesn’t stop like the airplane when released but will continue to rotate at the final velocity achieved based on the period of acceleration. That is where the toughest challenge existed. It took just as long to stop the motion as it took to cause it, and if you consider that the intention was merely to change the attitude to a safe position, it is easy to see how difficult to estimate leads and lags in trying to achieve an aim position. Occurring in three orthogonal rotation-axes and disturbed with outside influences and the challenge became daunting. There is no way to equate it to the experiences of aerial flight. |
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