Once airborne, the forward bypass doors closed automatically as the undercarriage retracted. At Mach 1.4, the doors began to modulate, again automatically in order to obtain a pre-programed ratio between “dynamic” pressure at the inlet cowl on one side of the “throat” and “static” duct pressure on the other side. Upon reaching 30,000ft, the inlet spike unlocked and at Mach 1.6 began a rearward translation, achieving its fully aft position of 26 inches at Mach 3.2 — the inlet’s most efficient speed. As the spike moved aft, the “capture-air-stream-tube-area” increased by 112 percent, while the “throat” restriction decreased by 46 percent of its former size. A peripheral “shock trap” bleed slot (positioned around the outer circumference of the duct, just forward of the “throat” set at two boundary-layer displacement thicknesses) “shaved” off 7 percent of the stagnant inlet airflow and stabilized the terminal (normal) shock. It was then rammed across the bypass plenum through 32 shock trap tubes spaced at regular intervals around the circumference of the shock trap. As this air was compressed, tertiary air traveled down the secondary bypass passage, firmly closed the suck-in doors, and cooled the exterior of the engine casing before being exhausted through the ejector nozzle.
Potentially turbulent boundary layer air was removed from the surface of the center-body spike at the point of its maximum diameter and then ducted through the spike’s hollow support struts, before being dumped overboard through nacelle exit louvers. The aft bypass doors were opened at mid-Mach to minimize the aerodynamic drag that resulted from dumping air overboard through the forward bypass doors. By carefully dovetailing all the above parameters, the inlet was able to generate internal duct pressures of 18lb per square inch; when this is considered against the ambient air pressure at 82,000ft of just 0.4lb per square inch, it is immediately apparent that this extremely large pressure gradient is capable of producing a similarly large forward thrust vector. In fact, at Mach cruise this accounted for no less than 54 percent of the total thrust being produced; a further 29 percent was produced by the ejector, while the remaining 17 percent was generated by the J58 engine. If, however, airflow disturbances disrupted this delicate pressure-balancing trick, it is equally easy to appreciate the effects that such excursions would have upon the aircraft.
This brings us to yet another of the A-12’s unique idiosyncrasies: the “unstart.” These unstarts, or aerodynamic disruptions (ADs), occurred when the normal shock wave was “belched” forward from the inlet throat, causing an instant drop in the inlet pressure and thrust. With each engine positioned at mid-semi-span, the shock wave departure manifested itself in a vicious yaw in the direction of the “unstarted” engine; sometimes these were so strong that crewmembers would have their helmets knocked against the cockpit canopy framing. Recovery from such an incident required the pilot to re-sequence the inlet in order to get it restarted. This involved the spike being driven forward and opening the forward bypass doors to recapture and reposition the shock wave. The spike was then returned to its correct position, followed by the bypass doors, which reconfigured the inlet to its optimum performance. “Unstarts” were a regular feature of early A-12 flights, but as computer software improved a system known as the Digital Automatic Flight and Inlet Control System (DAFICS) was developed for the SR-71. The DAFICS was able to achieve near-perfect inlet airflow control, which in turn practically rid the jet of its “unstart” problems.The two-seat AT-12T dedicated pilot trainer was powered throughout its life by two Pratt & Whitney J75 engines, which were considerably less powerful than the twin J58s that equipped the single-seat variant. The trainer therefore lacked the ability to cruise at Mach 3+. (Lockheed Martin)
Stability Augmentation System
The A-12’s center of gravity (CG) was automatically moved aft during acceleration to high-Mach flight, to reduce trim drag and improve elevon authority in both the pitch and roll axes. The fuselage chine produced lift forward of the center of gravity, which had the effect of destabilizing the aircraft in the pitch axis and reducing aft CG travel, resulting in low static margins of stability and safety. Additionally, the chine had an adverse aerodynamic effect on the aircraft when performing sideslip maneuvers at cruise angles of attack (approximately 6 degrees of positive alpha). This, coupled with low aerodynamic damping — inherent with flight at high altitudes — conspired to make the A-12 only marginally stable in both pitch and yaw at high Mach.
Control in this delicate but critical corner of the flight envelope was achieved by the aircraft’s elevons and rudders, which were worked through an automatic flight control system (AFCS). The AFCS consisted of a redundant three-axis stability augmentation system (SAS), a two-axis autopilot, an air data computer, and a Mach trim system. Other associated equipment included an inertial navigation system (INS), a flight reference system (FRS), hydraulic servos, and a pitch actuator. The AFCS provided pitch, roll, and yaw stabilization via the flight control surfaces. Eight rate-sensing gyros detected divergence from stable flight and together with three lateral accelerometers, also provided motion-sensing signals relative to the rate of change in all three of the aircraft’s axes, thus damping excessive changes in attitude. Because these SAS corrections were applied through a series of servos, they weren’t apparent to the pilot at the control stick or rudder. Control over the AFCS was provided to the pilot via “Pitch SAS,” “Roll SAS,” and “Yaw SAS” switches, located on the right-console panel. The servos could also be activated by direct stick and rudder-pedal inputs.
The two-channel (pitch and roll) autopilot processed INS and FRS inputs, then applied the data through the SAS electronics to transfer valves for control surface positioning. This provided the autopilot with two separate “hold functions.” Pitch control was achieved via the basic attitude hold mode, Knots Equivalent Airspeed “hold,” or Mach “hold.” In roll mode, control was exercised via the basic roll attitude hold mode, heading hold mode, or auto-steering “Auto Nav” mode; this latter mode was programed to obey heading commands from the INS. When the autopilot was engaged, the aircraft was held in the roll attitude established at the time of engagement. With “Auto Nav” selected, the autopilot controlled roll to ensure that the aircraft adhered to the predetermined navigation track that the INS accurately maintained. During operational sorties the aircraft was invariably flown in this mode to ensure that it remained stable and on an accurate track whilst the onboard sensors were activated.Although the A-12 was highly advanced, the cockpit instrumentation and its layout were very unspectacular and straightforward. The hooded view scope can be seen at the top of the picture. (Roadrunners Internationale)
The Mach trim system provided speed stability up to Mach 1.5, while the aircraft was either accelerating or decelerating — a period during which the autopilot could not be engaged. It compensated, via the pitch trim actuator, for the aircraft’s propensity to “tuck” nose-down while accelerating through the Mach and rise nose-up while decelerating.
Inertial Navigation System (INS)
The A-12’s INS was completely self-contained and provided the principal navigation references to the aircraft without recourse to any electromagnetic radiation or other external references. The system provided attitude, true heading, command course, ground speed, distance, and geographic position data for automatic or manual navigation between waypoints on the flight plan. The pilot could, if required, update position information periodically to correct gyro drift by taking fixes with a view scope that provided an optical display of the terrain along the flight path, or by taking sun fixes with an optical device to measure the sun azimuth angles for determination of true heading.
The flight reference system provided magnetic heading information and served as an alternate navigation reference. A gyromagnetic compass provided both slaved gyro and free gyro heading information, while a gyro platform provided pitch and roll information.
A set of integrated flight instruments consisting of an attitude indicator, a bearing-distance-heading indicator and related signal-switching equipment, displayed navigation information to the pilot. The indicators operated in conjunction with the inertial navigation and flight reference system to provide data to the pilot.
Inside the cockpit a large view scope, located in the center of the front instrument panel, enabled the pilot to select several different functions. Utilizing a Baird-Atomic 6642-1 periscope system, it was possible to view the ground below. The scope had two settings: a wide-angle field of view, about 85 degrees forward of nadir (a point on the earth’s surface directly below the aircraft); and a narrow field of view, which provided coverage of about 47 degrees forward of nadir. An upwards view function enabled sun compass readings to be taken to cross-check the INS. Additionally, a route filmstrip could be selected, providing the pilot with a visual reference of the aircraft’s progress or other pictorials like let-down plates for landing.Cameras
Although three different cameras were developed for the Oxcart program, only the Perkin-Elmer Type I camera was used during operational missions. Equipped with an f/4.0, 18in lens, image frame size was 27.6×6.3in. The unique camera system or “package” utilized two reflecting cube scanners, positioned one behind the other, enabling imagery to be scanned simultaneously onto either the left or right film spool. The forward unit scanned from 21 degrees to the right of vertical, then out to 67 degrees to the left; the aft scanner rotated 21 degrees left from the vertical, then out to 67 degrees to the right — thereby providing 42 degrees of stereo coverage directly below the aircraft, and a total swath 134 degrees wide (which from 80,000ft was 72 miles). The scan cycle time was 4.8 seconds and each frame was timed to produce a 30 percent overlap. Ground resolution was 1ft at nadir (80,000ft vertically below the aircraft) and 3ft, depending upon haze degradation at the outer edges of the image — that’s 36 miles left or right of the Oxcart’s track. Transport of the 5,000ft of film within the camera utilized a concentric supply and take-up system to ensure that film weight remained centralized, thereby minimizing potential changes in the aircraft’s CG as the film advanced.The essence of Project Oxcart was the Type I camera, manufactured by Perkin-Elmer. The inner works of this very complex camera system remain highly sensitive and even after nearly 50 years, no detailed photographs of the unit have yet emerged. (CIA Pilots Manual)
Because in-flight temperatures could vary between -40 and +290 degrees C, an isothermal window was provided as a protective barrier between such severe temperature gradients and the camera’s film. This window was sealed to the Type I camera and a pump was then used to create a vacuum between the camera base and the glass. The entire camera assembly was lowered through a removable hatch into the Q Bay; the camera lens sought out its targets through the high-quality quartz window that measured 22in×23in. Problems encountered when bonding the window to its metal frame were eventually overcome during a three-year, $2 million program, which developed a unique fusing process using high-frequency sound waves.Birdwatcher
Birdwatcher was a monitoring system unique to the A-12. It utilized a multiplexed High Frequency, Single SideBand (HF/SSB) radio system and was designed to telemeter signals concerning the operation/non-operation of various aircraft systems down to a specially equipped ground station. The frequencies selected for any particular mission were briefed and noted by the pilot on hand-cards; they were also annotated on the mission filmstrip, which was displayed to the pilot in the cockpit. The system consisted of two main elements: an air element and a ground element. The air element was a subset of the aircraft’s HF/SSB radio, the antenna consisting of a tube structure within the aircraft’s pitot/static system located at the front of the aircraft. The relatively short antenna was closely matched to the ground plane, furnished by the airframe, and made a fairy efficient antenna. Of its 40 channels, 32 were used to monitor individual aircraft systems; for example, channel 54 covered the starboard engine’s EGT; channel 7, the starboard engine’s fuel flow; while channel 3 covered the aircraft’s altitude. If any pre-set parameters of the systems being monitored were breached, Birdwatcher keyed and modulated the HF transmitter with a coded signal consisting of three consecutive half-second bursts, each separated by a five-second period of silence. During each of the half-second bursts, the aircraft’s identity and the condition of each of the systems being monitored was transmitted. These three bursts could be heard through the pilot’s headphones as three chirps — hence the name Birdwatcher.
The forward and aft scanning heads of the Type I camera provided a level of stereo overlap directly below the aircraft before each then scanned out to the left and right, 67 degrees either side of the Oxcart’s track. (CIA Pilots Manual)
The A-12 pilot had only limited control over the system. In the cockpit there were two switches; these were labeled “A” and “B.” The “A” code was usually used by the pilot to signify to the ground station that the aircraft had reached a pre-designated point in the mission and that it was in a “GO” condition, such as at the end of a successful air refueling, or upon reaching a predetermined distance-to-go point or turn point. The pilot activated the “B” code usually to indicate that the aircraft had experienced some sort of abort condition. The two buttons could be used sequentially to indicate that something unusual had occurred; for example, “B” followed by “A” might be interpreted as meaning that the aircraft had an abort condition, but was not in an emergency situation. Involuntary Birdwatcher codes were transmitted automatically by the system when any one of a number of sensors tripped the encoder. All radio emitters on the A-12 could be inhibited by the pilot by activating a “Mute” switch — except the Birdwatcher. The Mute system was installed on the aircraft to prevent accidental transmissions by a device such as the Tacan, UHF radio, etc. The pilot was usually instructed to operate the Mute switch by the mission filmstrip prior to entering denied territory.
Birdwatcher could also be interrogated by appropriately equipped ground stations. The Command Post (CP) could, if required, cause Birdwatcher to transmit a short burst of information that would include only the coded identification of the Oxcart — if no other sensors had already tripped. To the dismay of a couple of pilots, some of those monitoring the aircraft’s progress from the ground station at Kadena AB occasionally interrogated the aircraft when it was over enemy territory!
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DESIGN AND DEVELOPMENT
A-12 PROJECTS AND VARIANTS
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