Development of Radar Bombing - Appendix III

APPENDIX III

VISAR CONTINUOUS TRACKING RANGE COMPUTER

1. 1. 1. The Continuous Tracking Range Computer built by the Fifteenth Air Force is one of the several possible H2N type computers to convert the AN/APS-15 or similar type equipment from a fixed angle bomb release system to a continuous range tracking type computing bomb release equipment.
      2. The bombing circle that is presented in the fluorescent face of the PPI is created by a phantastron circuit in the range unit of the Mickey equipment.  The radius of the bombing circle is proportional to the control voltage applied to the phantastron circuit.  This voltage is normally taken from the wiper of a linear potentiometer which is mounted in the Mickey computer unit.  When the computer drum is rotated, the potentiometer wiper is also rotated and the voltage changed, thus changing the radius of the bombing circle.  (In the APS-15 A, the potentiometer is controlled by the H and by the B function but the voltage that it sends to the phantastron is variable in the same manner as in the APS-15).  To produce a synchronous bomb circle, i.e. one with which the target may be continuously tracked to determine the proper release point, it is necessary to use a computing mechanism that produces a voltage which varies in proportion to the slant range between the aircraft and the aiming point.  The most readily available computers in this theater are the Norden M-Series and the Sperry S-Series.  The Norden is used because it is better understood, more available, and the Norden representatives were at hand to construct the devices necessary.
      3. By connecting the bomb circle control potentiometer of the Mickey range unit to the Norden computer through proper linkages, a bomb circle control system in constructed which is continuously tracking.  The control of the tracking, or synchronous bomb circle is then exactly the same as the control of the Norden cross, or range hair and tracking is the same.  The hair is displaced in range with the Norden displacement control (displacement knob).  The displacement knob also changes the diameter of the bombing circle.  The rate of change of range position of the cross-hair is controlled by the Norden rate control (rate knob).  This also controls the rate at which the bombing circle diminishes.
      4. In this manner, the range and range rate are controlled by using either the Norden sight or the Mickey sight.  The computation of the release point is independent of the sight being used, and the bombing team may use whichever data is the best.
      5. The Norden computer solves the bombing problem by maintaining a triangle (ABC – Fig. III – 1) always similar to the space triangle ADT formed by the aircraft A, the target T, and the perpendicular AD from the horizontal plane containing the target to the aircraft.  The length (s) of the hypotenuse AC of this right triangle is proportional to the slant range (S) from the aircraft

to the ground, the vertical side of the triangle (h) is proportional to the altitude (H) of the aircraft

above the target, and the base of the triangle r is proportional to the ground range (R).  The angle 0 between the hypotenuse and the altitude is the sighting angle.  The sighting telescope is coupled to look along the slant range line.  In the sight, the base (r) is composed of a rack.  This rack is driven by a pinion P.  The angular rate of the pinion P is constant at any given altitude, but is inversely proportional to altitude.  The angular displacement of the pinon P while the angle 0 is changed from 70^o to zero degrees is the same regardless of altitude.  The physical dimensions of the sight triangle (0 equal to 70^o) are unchanged as altitude changes.  The rate of the rack r (0 equal to 70^o) is always a function of ground rate and altitude.  The displacement (from the 70^o position) of the rack is proportional to the time that the computer has been in operation.  The displacement of the rack in turning the telescope through 70^o is always the same regardless of the aircraft velocity and altitude.  The rate of the rack r is a factor in determining the release point, or the dropping angle.  The angular rate of the pinion (and the linear rate of the rack) is controlled by the disc speed setting and by the rate control (rate knob).  The angular position of the pinion (and the linear displacement of the rack) is controlled with the displacement control (displacement knob) and with a search control (search knob) which is connected directly to the pinion P.  Indications of the position of the telescope may be conveniently brought out of the Norden computer either from the rack r or from the pinion P.  If connection is made to the rack, it is most conveniently done at the C end.  The pinon P motions are conveniently brought out through the search control shaft.

1. 1. 6. The present VISAR system connects to the rack directly and obtains the phantastron voltage control by using this motion to drive a triangular linkage which controls the linear potentiometer from the APS-15 computer.  Two other systems were considered but not used because of the difficulty of obtaining the needed parts.  A non-linear potentiometer might be driven from the search shaft.  In series with this potentiometer, it would be necessary to connect a resistance which would give an altitude control.  It is possible to design a cam which would be driven by the search shaft to control a linear potentiometer.  The cam would need to be either a two dimensional one with an altitude control in series with it, or a three dimensional cam.
      7. The VISAR range unit employs a triangle (ABC, Fig. III-2) which is congruent to the Norden computer triangle (ABC, Fig. III-1).  Triangle AEX (Fig. III-2) is always similar to the space triangle.  A carriage is centered at X and the potentiometer arm is driven so that as the point X is displaced along the hypotenuse rack AC the electrical displacement of the potentiometer wiper is proportional to the distance AX.  The potential controlled by the wiper is then used to control the bomb circle radius through the phantastron circuit, the bomb circle radius, the electrical potential of the wiper, and the rack displacement AD, all being proportional to the slant range of the aircraft in relation to the aiming point (usually the target).
      8. Appendix V, Fig. V-1, shows the VISAR system as now experimentally installed in the test B-17.  Fig. V-7 shows the right hand operated functions of the Norden and the altitude adjusting control of the H2N type VISAR control mounted on the far side of the sight.  The cable which is attached to the end of the Norden rate rack comes out through the mounting box which supports the right hand end of the H2N Unit, around a small pulley, and down between the rails to the carriage which is seen just behind the mounting block.  The cable and the carriage transmit the rate rack motion to the H2N slant range rack R shown in Fig. V-7.  Mounted upon the slant range rack is a carriage (visible in Fig. V-7) which is restrained from moving vertically by another carriage mounted between the rails of the altitude rack R_h.  The potentiometer is mounted on the slant range rack carriage and thus rotates through the same angle as does the slant range rack.  The potentiometer wiper is driven by a gear which meshes the slant range rack teeth.  The displacement of the wiper is then proportional to the displacement of the wiper gear along the slant range rack.  The slant range rack is returned to the 70^o position by the spring loaded cable and pulley seen in the left hand side of Fig. V-7.
      9. Altitude adjustments are made by turning the control H, Fig. V-6, and comparing the position of the altitude marker mounted to the left of the handle to the scale appearing behind the acx marker.  Zeroing adjustments are made by turning the potentiometer wiper arm in relation to its shaft upon which it is friction mounted.  Calibration is accomplished by comparing the circle which the Nosmo produces with the circle that the Mickey drum computer produces.  A switch is mounted beside the Mickey operator so that he can display the circle desired.  A switch is mounted for the bombardier so that he can turn the bombing circle on and off at will only when the H2N is producing the circle.  Schematics appear in Appendix V.

 

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