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The ever present need of the field artillery is a means to solve the gunnery problem with greater accuracy and speed. Tactical requirements are changing constantly at a mounting tempo, with increasing demands being placed upon field artillery in its support of the field army. This is a continuous challenge in the area of fire control where new techniques are being devised, the newest advances in science are being adapted and applied, qualified personnel are being trained for new jobs, and where field artillery is finding the means for meeting the demands for its increased support.

The development and standardization of the Field Artillery Fire Control System M35 was a significant step in the right direction. This system utilized an electromechanical computer and opened a new era in gunnery techniques. The Fire Control System M35 was an improvement over graphical means in both speed and accuracy, but experience with it also pointed to needs for improvement in the over-all fire control problem. The analog system used by this device had several disadvantages. Its accuracy was adequate for the shorter range weapons such as the 105-mm and 155-mm howitzers, but was not adequate for guns and free rockets. It was clear that a system of computing was needed which would be more flexible than was possible with an analog system. Ordnance and CONARC agencies cooperated to establish the actual requirements and definitions of problems, and from these Ordnance was able to specify and develop the needed equipment.

Frankford Arsenal studied the basic ballistic problem for about two years and then the Univac Division of Remington Rand studied the problem along with its associated mathematics. The most desirable approach appeared to be simulation of the flight of the projectile from the tube (launcher) to impact, which is referred to as solving the differential equations of motion of the projectile. After reaching an acceptable mathematical solution, the next major problem was the incorporation of this solution into a device compatible with the size, weight, power maintenance requirements, and operation training imposed by the field artillery.

In November 1956 a conference was held at Frankford Arsenal to present the solution and concept of mechanization. This concept was called the Field Artillery Digital Automatic Computer (FADAC). Military Characteristics were drafted and approved. The design criteria for a machine were submitted to industry and a contract placed with Autonetics, a division of North American Aviation, Inc., on 20 June 1958 for the design, development, and manufacture of FADAC. Target schedules called for acceptance tests by Frankford Arsenal in September 1959 of the first prototype FADAC.

The basic considerations and order of priority in arriving at the design of the hardware for FADAC were established as:

1. Accuracy of solution and reliability of operation.

2. Ruggedness (ability to withstand adverse climate and field conditions).

3. Minimal operation (computing time).

4. Ease of operation and operator training.

5. Ease of maintenance and maintenance personnel training.

6. Minimum physical size, weight, and power consumption (not to exceed 200 lb in weight and 500 w in power.)

7. Cost.

The FADAC is a solid-state electronic digital computer. It is compact, portable, and rugged. It is approximately 24 x 14 x 34 inches in size and weighs about 200 pounds. It is designed to operate under severe field conditions and storage, and under extreme temperatures of heat and cold. For use by the fire direction center, FADAC requires only the addition of 3-phase 400-cycle power to the fire direction center facilities.

Transistors are used throughout FADAC circuits. Crystal diodes are used for logical gating. The machine is a stored-program, solid-state (no vacuum tubes), electronic digital computer to be used primarily for automatic computing and visual displaying of firing data (gun orders) for Field Artillery weapons, from inputs defining target and weapon locations together with nonstandard conditions of materiel and weather.

It will provide firing data for a battery of weapons. On a one-battery- at-a-time basis, it can provide firing data for mortars, howitzers, guns, and free rockets, firing any ammunition these weapons will use. In emergencies, it can provide data for five similar type batteries, one at a time. By using the memory loading unit, authorized field personnel can make program changes to permit solution of gunnery problems for other weapons in a few minutes time.

Parts, sub-assemblies, and support equipment are interchangeable with any other FADAC, regardless of weapon or the application to which each may be assigned.

The flexibility of FADAC is demonstrated in the interchangeable control sections, each designed for a particular use. The use of FADAC with different weapons and for different applications is facilitated by changing control sections. For example, a gunnery problem could be solved by FADAC using the control section designed for gunnery, and then the control section could be changed for a counter-battery problem. A removable plug in the control section simplifies such changes. This design is not only a flexible operating feature, but also one that minimizes operator training.

FADAC was designed for high operational dependability and for maintenance to be required only at infrequent intervals. It should be capable of operating under field conditions, without major overhaul, for at least 2,500 hours. The solid-state components are expected to operate for at least 10,000 hours. Errors due to internal malfunctions during a computation will be minimized by internal automatic monitors which aid in detecting such errors.

FADAC was designed to be compatible for transmission purposes with the fieldata family of equipment under development by the Signal Corps as part of the Automatic Data Processing System (ADPS) program. The basic difference between the fieldata code and the teletype system is that the former requires more pulses to transmit a greater amount of information, i.e., an 8-level instead of a 5-level system. Of the 8 levels, 6 are used for information or intelligence, 1 for parity or to check the transmission of the data, and 1 bit for a control-type function. The FADAC can transmit and receive this 8-level fieldata code.

Input consists of a manual keyboard and various arrangements of paper tape or another FADAC. When all the data, such as target location, powder temperature, gun location, and meteorological data, are entered, depression of a button initiates computation. Gun orders, comprising deflection, quadrant elevation, fuze time, and charge are displayed in decimal form.

Output consists of visual display (called Nixie), another FADAC, battery display, printer, magnetic tape, fieldata equipment, and teletype equipment.

The programming and numerical system of FADAC is straight binary for internal operations, with automatic conversions to other codes for input- output.

In the arithmetic unit the execution time for each instruction is 7.8 microseconds. Its arithmetic mode is parallel by function and serial by bit. Timing is synchronous.

The storage consists of a main magnetic disc of 4,096 word capacity and a high-speed magnetic disc of 32 word capacity. There are 32 channels of 128 words each, of which 24 channels are designated as permanent storage and 8 channels as working storage.

The extremely high-speed operation of this machine is made possible by a combination of new techniques incorporated in the logic design. While FADAC is basically a serial computer, it performs some functions in parallel, that is, several operations simultaneously. Instruction search, instruction interpretation, number search, and number read are performed at the same time. This overlapping feature, together with minimum access coding, rapid access loops, and multiplication using two bits at a time, results in a machine capable of performing 12,800 additions or subtractions per second, approximately 750 multiplications per second, and 375 divisions per second, including access time for both instructions and numbers.

There are several additional applications of FADAC. One of these is as an ideal replacement for JUKEBOX as a computer for the Redstone Missile System. Frankford Arsenal had developed JUKEBOX before the development of FADAC had started, but, while an excellent computer, it was designed for vehicle mounting and operation where size, weight, and power were not of prime importance. FADAC can meet all of the requirements and have the advantage of smaller size and weight.

Other missile systems could also use FADAC. These are: Pershing, Sergeant, Lacrosse, and NIKE-Hercules. In addition to the missile systems FADAC can also be employed in fire planning, survey computations, counter- battery computation, reduction of metro data, and as universal automatic check-out equipment.

A universal computer for solving all gunnery problems has always seemed to lie in the future. However, continuous study at Frankford Arsenal on increasing the utility and application of FADAC has yielded results which make this computer a candidate for the title "Universal Field Artillery Computer".

(The material for this chapter was extracted from Frankford Arsenal Technical Memorandum Report M59-5-1, "FADAC Status Report," by R. Brochman, dated 29 December 1958.)

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