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.
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.
Mission sensors
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.
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.
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 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.