Computer Box, AN/APS-15A Radar System, Bomb Through Overcast, US Navy

Here is a very interesting artifact and the only one we've come across in our many years of collecting. This is a Computer Box for the AN/APS-15A Radar System, which began appearing in bombing aircraft of the US Army Air Force, such as the B-17 and B-29, and US Navy, such as the PBM Mariner, in 1944.  The APS-15A was similar to the Bomb-Through-Overcast radar system AN/APS-13 and APS-15, but with some advanced modifications.  (See description of the Computer Box from the US Army Air Force manual Radar Observers Bombardment Information File dated July 1945 in the photo gallery)

The Computer has 3 setting knobs, one each for Altitude, Ground Speed in Knots, and Sweep Timing.  It also has a compartment to install one Ground Speed Disk for the intended mission, and a leather storage compartment on top to hold the 12 Ground Speed Disks.

The Alt. In Feet, i.e., altitude or slant range, is displayed in the upper left, and is set by rotating the Altitude knob from 0,000 to 90,000 ft. (Our example requires calibration as it travels from 6,000  to 96,000 ft). This varies the position of the Bomb Release marker on the radar scopes.

Each plastic color-coded Ground Speed Disk covers a range of ordnance types and the flight attributes of Indicated Air Speed (IAS) and Altitude.  There are 3 groups of disks, each group is labeled for a primary bomb type (AN-M34, M38A2, and AN-M43) with a list of other types included under the primary type. Each disk is then labeled with one of 4 IAS: 130, 155, 170, and 190 MPH.  Each disk is also labeled for a specific range of altitude, e.g., 5-40M feet, associated with the characteristics of the labeled bomb type at the labeled IAS. Once loaded onto the locating pins in the controller's compartment, it rotates when the Ground Speed in Knots knob is turned to display the Ground Speed through the viewing window on the hatch of the compartment.  The knob automatically adjusts the Bomb Release marker on the scope to agree with the proper bomb-release point for a given ground speed and type of bomb.

The Primary Types and Related Types on each Ground Speed Disk are as follows:

AN-M34:  MkIMII(600lb), M61, AN-M44, M59, M33, AN-MkI, AN-M56

M38A2:  M47AI, Mk4, M70, M31, AN-Mk-17

AN-M43: MkIMII(300lb), AN-M58, M32, M62, M52, MkIIIMI

The third knob, labeled Sweep Timing, is used to adjust the beginning of the radar's sweep.  By turning counterclockwise, the operator can introduce as much as six miles of open center on the scope.  By turning clockwise, the operator can introduce as much as six miles of altitude delay.

The unit has 4 lights across the top to illuminate the altitude display and ground speed dial, and has 2 spares located below the altitude display. Brightness can be adjusted by the rheostat on the right side of the unit. 

The controller measures 7.5 inches tall, 6.5 inches wide, and 4 inches deep, and weighs ~8.5 lbs.