The AN/APS-6 Aircraft Radar
THE AN/APS-6 AIRCRAFT RADAR
The AN/APS-6 Navy Aircraft Radar was designed for carrier-based night fighter planes. With it the fighter pilot located and attacked enemy planes in complete darkness. It also guided the pilot back to his base on a homing radar beacon. Contracts were given the Radio Division to build 3,600 units worth 134,057,331.
In January, 1943, the Radio Division was brought into this Navy carrier based night fighter radar program. At that time, the Radiation Laboratory of the Massachusetts Institute of Technology requested Westinghouse to design and build twenty RF heads to be used in an aircraft radar set then being developed by the Laboratory for the Navy. The RF Head is a composite unit containing the transmitter and the front end of the receiver and appropriate switching mechanisms to permit them to operate from a single antenna. The new equipment was expected to displace AIA equipment then being manufactured by the Sperry Gyroscope Company, and which had been Jointly designed by Sperry and the Radiation Laboratory.
Westinghouse accepted this commission and divided the responsibility between the Research Laboratories and the Baltimore Works. The Research Laboratories were to design the electrical components and Baltimore was to do the mechanical engineering and manufacturing. Work began immediately and early in April, 1943, a semi-operative model was taken to the Radiation Laboratory for discussion. Comment on the unit was very favorable.
At the meeting held April 19 at the Bureau of Ships, it was proposed by the Navy that Westinghouse accept a production contract to include not only the RF Head but the complete radar system. It was stipulated that Westinghouse would purchase the Modulator and the Antenna, which were already under development on contracts previously issued to Stromberg-Carlson and Dalmo-Victor respectively. The proposition was accepted by Westinghouse and contracts were issued eventually for four models and 50 equipments. Delivery of the models was expected in the fall and it was estimated that the production units would start about the beginning of 1944.
By September, it was apparent that new features continually being added to the RF Head would prevent production beginning near the time scheduled. The Navy therefore proposed the production of an AN/APS-6A to be as near as possible like the AN/APS-6 except
that it would use & modified RF Head from the ASD, then in production by Philco, until the Westinghouse RF Head could be put into production. Orders for the equipment had meanwhile been increased, and it was decided that the first 150 equipments would be of the 6A type. Deliveries started in about March, 1944. A rapid increase in production rate requirements and a continual development of AN/APS-6 design forced the original quantity to be extended to a total of 741 6A's equipments before the 6 went into production September, 1944.
Undoubtedly, the introduction of the 6A created some delay in the eventual production of the 6 since many new problems cropped up which interfered with the scheduled plans. However, the final tactical application of the equipments in the field was such that time loss appears to have been worth while because of the very satisfactory performance of the hybrid 6A's and their earlier first appearance in the field.
Requirements for the 6 varied with fluctuations in the fortunes of war and new orders were placed at intervals, usually too late to make uninterrupted production easy. Complete terminations were received with the defeat of Japan. At this time, about 2,100 equipments had been delivered and some 1,400 more were terminated.
Final testing of the AN/APS-6 and 6A and the manufacturing of the Receiver-Amplifier unit and the Westinghouse RF Head was done at Plant #1 of the Baltimore Works. The indicator unit and the Control Boxes were made by the Philadelphia W A- R Shop. The Antenna unit, the Modulator, and the Philco RF Head were bought as complete units from outside suppliers.
From the very beginning, production was plagued with the fact that the engineering and design of the equipment was never completed. Changes, either to meet the existing specification or new requirements of the Navy, were continuously being received. Many of the components were unique and these brought on manufacturing problems.
It was originally expected that the design of the AN/AF&-6, with the exception of the RF Head, would be largely done by the MIT Radiation Laboratory. At the outset, however, it was decided to change to miniature tubes wherever possible and to raise the intermediate frequency to 60 megacycles. This called for new design
work in which Westinghouse engineers participated. Later, it was found necessary to redesign the power supply and to make many other changes in design to meet the specifications.
This equipment was designed to enable carrier-based night fighters to find and attack enemy planes. Initially it was planned for installation in the 74U. Before production was under way, the F6F, the Hellcat, became the outstanding Navy plane for night duty, and the AN/APS-6 was installed in this instead. Plans were also made for installation in the F7F and 200 installations of this type were made. The F7F is a fast twin-engine fighter made by Grumman originally designed as a night fighter but eventually turned over to the Marines for land-based service.
Since each type of plane installation required a different set of cables and connectors, the allocations from plane to plane and from 6A to 6 caused a considerable amount of confusion. Difficulty was experienced in obtaining the sizes of cable lengths from the Navy and complete information was not available even at the end of production.
The AN/APS-6 was a radar equipment capable of determining the direction and range of a target and of presenting this information visually to the pilot. To accomplish this, a pulse generating modulator would pulse an X-band magnetron at repetition rates of 500, 1000, and 2000 cycles per second. The short pulses of microwave energy were radiated from a rapidly rotating paraboloidal antenna. The beam from the antenna described a spiral pattern with a maximum conical angle of 120 degrees. Compared to the speed of light, (186,000 miles a second), the motion of the antenna was so slow that echoes from a transmitted pulse would be received by the antenna before it had moved more than a fraction of a beam width. Thus, the instantaneous direction in which the antenna pointed was the direction to the target.
The received signal, or echo, was amplified and applied as an intensity or brilliance modulation on the oscilloscope indicator where it appeared as a bright dot. Three dimensional data were presented to the pilot in the following manner. A baseline ran across the bottom of the face of the tube. The position of the dot right and left from the center of this line was proportional to the azimuth position of the target. Distance to the target was represented
SCOPE OF THE AN/APS-6 - NAVY AIRCRAFT RADAR
by the elevation of the dot on the indicator above the base line. For long ranges, it was unnecessary to present any elevation data since the angle to a plane could only be small under this condition. At shorter ranges, each signal appeared as two dots side by side. If the target was above or below dead ahead, the right hand dot would be displaced up or down correspondingly.
By means of a control box in the cockpit, the pilot could reduce the range of the equipment as the target was approached, until at about 1/2 mile he would switch to gun-aim position.
In gun-aim position, the antenna no longer scanned in a spiral but rotated in a circular pattern, and the presentation on the indicator became just a dot in the center of the tube face. As long as the target remained dead ahead, in the center of the antenna scanning cone, the dot would remain in the center of the screen. If, however, the target deviated from straight ahead, the indicating dot would make a corresponding motion and provide the pilot instantly with correction information for altering his course to stay on the tail of the target. As firing range was approached, the dot sprouted a pair of wings which grew as the range diminished until at a predetermined firing range, the length of the wings just filled the space between a pair of engraved lines.
The equipment to accomplish all these functions consisted of the following: In the cockpit were the Indicator, the Control Box, and an Auxiliary Control Box. In the fuselage, behind the pilot, were the Modulator, the Rectifier Power unit, which was sort of a junction box for all units named so far; a cable ran through the wing to a Junction Box out in a nacelle near the tip of the right wing. In the nacelle was located the Antenna and the RF Head. The antenna radiated through a plastic radome which formed the front of the nacelle. Behind it was the RF Head which contained the transmitting magnetron, the local oscillators, the mixer, a pre-amplifier, and circuits for automatically tuning the local oscillators. The complete installation weighed about three hundred pounds and was designed to operate on 115 volts, 800 to 2,400 cycles.
In addition to the functions described previously, the equipment was also capable of picking up X-band beacon signals at ranges far in excess of the radar ranges.
One of the problems encountered was the initial use of a brand of resistor which proved to be unstable under exposure to humidity. It was necessary to not only change over in production to a satisfactory brand but to supply retroactive replacements for all sets and spares shipped to the time of the changeover. A long period of time was consumed in finally getting the retroactive shipments made.
Early in production, it was necessary to redesign the grid circuits of the deflection amplifiers when it was found that they became badly unbalanced under high humidity and would drift the picture completely off the screen.
The double-dot method of altitude indication lacked sensitivity at small angles, and an improved circuit giving a deliverately non-linear presentation was developed.
Almost countless other changes were made and several engineers were kept busy continuously assisting the shop and solving minor design problems.
The armed forces have not as yet made any summarized statement as to the benefits derived from the use of the AN/APS-6 and 6A. It is known, however, that even prior to the advent of the Japanese suicide tactics they were having good success and a number of Japs had been destroyed. The large increase in orders which followed the Jap surprise tactics can only lead to the conclusion that the night fighters were an important part of the defense.
In addition to its combat applications, stories have been told of rescues of downed pilots made possible by the use of the AN/APS-6 to locate rescue vessels near which to crash land, for avoiding obstacles in cloudy weather, and as an overhead radar on ships.
There will be little if any commercial peacetime use for this identical equipment. But much of the know-how obtained on this job is directly applicable to the design and manufacture of commercial ship radar, aircraft navigation and beacon radar, and any other equipment operating in X-band, of which there are certain to be an increasing number as time goes on.
A number of new components were developed for the AN/APS-6. Some were applicable only to 6 but others were of far reaching importance. A few of the more significant developments follow:
1) To assist in the testing of the first models of the 6, The Westinghouse Research Laboratories designed and built a portable X-band Spectrum Analyzer, The Navy, interested in the unit, issued contracts for the development of an advanced model and the production of 1365 units. More than 800 analyzers were produced before V-J cancellations. A contract was also issued for a It-band analyzer.
2) To produce a lighter, smaller and more reliable pulse transformer, a coordinated project of the Baltimore Feeder Group and the Research Laboratories resulted in a Fosterite treated transformer of low weight but good performance. The application techniques discovered in the project, made at Baltimore and Sharon, enabled both Sharon and Baltimore to reduce the size and weight of transformers.
3) An improved T-R Box, electronic transmit-receive switch, was developed by the Research Laboratories to improve the performance of the 6. This tube became a standard item, the 1B24, and was Manufactured by the Bloomfield Works. A companion R-T tube waa developed at the same time but was dropped when the Navy decided to use up R-T tubes in stock. The use of the old tube required last minute design changes and a careful machined cavity which was a source of serious trouble in the initial production of the equipment and remained a recurrent problem. In retrospect, it seems that much would have been gained by pushing the use of the new tube.
4) The use of a pressurized RF Head required the development of a wave guide window, to be air tight and yet to transmit microwave energy with little or no attenuation. Such a window was developed at the Research Laboratories and manufactured by the Bloomfield Works.
5) To connect the shook mounted RF Head to the rigidly mounted antenna and yet maintain a tight transmission line between the two required a flexible wave guide. The then existing flexible wave guide did not meet all the requirements for air tightness and low electrical losses. Through the cooperation of Titeflex, Inc., a new rubber covered flexible wave guide was developed which not only was quite satisfactory but found many other applications.
6) The Titeflex Company also assisted in the development of a small light pulse cable. The standard pulse cable, at the beginning of the design period, required special and very bulky connectors. Later, a small and satisfactory cable was evolved by using standard aircraft engine ignition cable and fittings.