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| Nike computer -
Installed in class room, not BC van.
From Rolf Goerigk
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This information is grouped into the following sections:
1) Mission of the Nike Computer
The Nike Computer had 4 main missions:
a) The Predicted Intercept Point is the point where the missile would intercept the aircraft if:
b) The Predicted Intercept Azimuth is the direction from the launcher to the current Predicted Intercept Point. This direction (azimuth) was sent to the gyro in the selected missile before launch, and was used to provide the missile a sense of "down". This missile gyro and the missile control system kept the belly of the missile "down" and provided the computer and missile a common sense of "down" and left/right.
c) During missile flight, the computer sends Steering Commands to the missile
(via the missile tracking radar) to guide the missile to the continuously
updated Predicted Intercept Point.
d) The Missile Burst Command is generated by the computer since:
e) In the 1950's, digital computers had tens of thousands of vacuum tubes, and because the vacuum tubes had a mean time to failure of only a few thousand hours, the computers had a mean time to failure of only a few hours. 90 percent "up" time was considered outstanding, and required a round the clock staff of real experts. Digital input and output devices were similarly failure prone. (In the 1970s, when reliable transistors and integrated circuits, and cheaper high speed memory became available, some Nike analog computers were replaced by digital computers.)
The only alternative that was reliable enough and accurate enough was the electronic analog computer, which could be implemented for the Nike with fewer than 500 vacuum tubes. There were failures, but the technology was maturing, the tubes were run in a different (not ON/OFF) manner, and the uptime exceeded 99% at most sites. (One failure a month was considered really poor.)
(In the late 1970s, the analog computer was replaced by a digital computer, a PDP-11 W (running the RT-11 operating system. Two of the 4 analog computer cabinets became closets and storage space.)
2) What is an Electronic Analog Computer?
The Nike analog computer was single purpose, a fixed program with a few different modes set by relays to simplify and automate the operation.
An interesting Analog Computer Web site
Schematics and Operation Manual for the Heathkit EC-1 here
From "The Analog Computer Museum and History Center"
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Analog computer
- A computer that performs mathematical operations in a PARALLEL
manner on CONTINUOUS variables. The components of the computer are interconnected to
permit the computer to perform as a model, or in a manner ANALOGOUS to some physical
system.
Electronic analog computer - An analog computer with input, output and program operations that are usually expressed in terms of direct current voltages. Analog computers may seem to be "simple" or "like a toy computer", in fact they are powerful tools that were used during the 1950s and 1960s to design and test systems like ICBMs, supersonic aircraft and spacecraft. But the analog computer can be used to model any physical system that can be described by mathematical formulas, even more mundane ones from modeling the effects of pollution on the fish population in a river to fine tuning the suspension on a new car design. Analog computers will not only test a fixed design but also allows variables to be quickly changed to test "what if' conditions. By scaling time as an independent variable, physical processes that happen quickly can be stretched out, and processes that happen over a long period can be shortened to make the process easier to study. And it is very easy to study variables at any point in the program while it is running to find faults in the program design. Although the analog machine is correctly termed a computer, it does not perform its computations by numerical calculations as does the calculator or the digital computer. The analog computer performs mathematical operations on CONTINUOUS variables instead of counting with digits. Positive numbers are represented by positive voltages and negative numbers are represented by negative voltages, all scaled to the computer's working range, usually -100 volts to +100 volts (vacuum tube) or -10 volts to +10 volts (transistorized), Thus the analog computer does not subtract 20 inches from 45 inches to obtain 25 inches but, rather, it subtracts 4 volts from 9 volts to obtain 5 volts. This 5 volts the operator reads as 25 inches in accordance with his arbitrarily specified "scale factor' of 1 volt equals (or is ANALOGOUS to) 5 inches. The analog computer is basically a set of building blocks, each able to perform specific mathematical operations on direct current voltages and capable of being easily interconnected one to another. Some of the basic operations include addition, subtraction, multiplication, division, inversion, and integration. By interconnecting these building blocks, mathematical equations are modeled. BUT an analog computer is a true PARALLEL computer that can solve one or one thousand equations at the same time. In fact, similar analog computers can be easily connected together to increase their computing power. When you think about the result of many equations being solved simultaneously and becoming the input to other equations, and sometimes these solutions are then fed back or looped back into the original equations with all of the variables changing CONTINUOUSLY with time, then you can get a brief glance into the incredible power of these computers. Output is usually a voltmeter, oscilloscope, or plotter. Many universities today like Massachusetts Institute of Technology, University of Illinois, University of Notre Dame, and Purdue University offer classes or do research using analog. computers, because they realize that the last chapter of the history of analog computers has not being written. It's an ANALOG universe and analog computers are a natural way to study and understand it. Prepared by:
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From the deployment of Nike in 1954 to the late 1970's, the Nike Computer was an analog computer. That means that distances, times, and other values were not digital bit representing numbers (like 12.3) but were (in this case) voltage values (like 12.3 volts).
The values as stated above were voltages, and simple circuits (using 76 "operational amplifiers" of 2.5 tubes each) could quickly (1 microsecond):
For images of the motor driven potentiometers see Computer (Servo driven potentiometers) and Computer (Details of Time Potentiometer)
An interesting characteristic of analog computers is that the circuits tend to run in parallel, The relatively slow circuits all working together easily keep up with the anti-aircraft problem in real time.
An analog computer (distances and times were voltages) used
analog (voltage) inputs from the Target Tracking and Missile Tracking Radars.
These values (and derived target and missile velocities) were
used to calculate the remaining flight time and Predicted
Intercept point. The missile was guided to the predicted
intercept point by the computer generated steering commands
sent through the Missile Tracking Radar.
For vacuum tube enthusiasts, here are schematics of a
Another web site
Analog Computer Museum and History Center by
Doug Coward
contains
Definitions.
The Nike analog computer did not use integrators, but did use filtered differentiators for velocity determination as well as motor driven potentiometers for multiplying by a variable (including time to intercept) as mentioned above.
Another system of amplifiers and motor driven switches performed as "Zero Set Amplifiers" to:
A number of preset test cases (switch selectable) helped assure that the computer was performing correctly.
Manual knobs input the offset between the missile tracking and target tracking radars.
(For further information, you can jump ahead to
6) How does the computer know the radar offsets?
In the late 1970's a digital computer was made available for the Nike. These were sent to
"off-shore" locations as all U.S. sites had been de-activated. It is quite possible
that this computer was a Norden PDP-11M. This was a licensed version of the
DEC (Digital Equipment Corporation) PDP-11 that was housed in a tougher case
and made more vibration resistant and in other ways made more suitable for
a military environment.
It is known that the file system format on the floppy disks was RT-11M.
(I have two floppies reported to be from a Nike site, but cannot figure out the data/program.) It had
two 8 inch floppy disk drives, a printer, (and some I/O cards).
As as best I can determine, the tracking antenna potentiometers
were retained - the analog voltages being converted to
digital for digital computing. Maybe the same for the range pot??
Any information about a digital Nike much appreciated.
Here is Rolf Dieter Görigk's
preliminary message - more info to follow:
" On the right hand side of the computer cabinet under the computer power control panel, there was a little monitor about 10 by 6 inches, and a keyboard. There under a printer. The printer printed out sampled data every 100 msec. During a sim track, data were sampled every 100 msec and printed out. For me it was heaven. I already re-calculated a lot of the system capabilities and used the computer during live ECM and T1 ECM exercises. Finally I determined the radar cross section (RCS) of targets and found that the T1 was wrong by/in determining the radar resolution cell LP/SP and therefor misinterpreted the amount of chaff that could be fitted in a resolution cell. Well, that sounds academic but it was the greatest success for me and the reason for many unsuccessful ratings of the crew performances. I found out what the real RCS was for a specific airplane but that was secret. The Airforce told (ordered) me not to talk about it. Just imagine, a live missile firing and data sampled every 100 msec!!!" |
4) Solving the Predicted Intercept problem
To keep flight times down, (and increase the effective range) the missile is aimed ahead of the target at a calculated Predicted Intercept Point. If the aircraft flies a straight line, the missile horizontal flight path will also be straight. (A dog chasing a target runs directly toward the target, involving a longer run if the target runs straight.)
A person can worry that assuming that airplane flying straight might not be a good assumption, but what better can you make? One can wonder about the 10 million year success of dog-like creatures running directly toward the present position of their intended prey. However their prey can change direction and percent speed much faster than an aircraft. Dog chase path succeeds with a slightly different problem.
The computer has built into it average flight times to various ranges
and altitudes. Here is a chart of 
Ajax Constant Time Circles.
This is an
Ajax time of flight chart.
This information is compared with the speed and direction of the
target until a valid time of flight to predicted intercept point
is computed. This information is presented to the battery commander
on the About the Nike Plotting Boards
to assist him in making the firing decision.
The computer computes the Predicted Intercept Point by:
How Predicted Intercept Point is computed.
At longer ranges (flight times) the Predicted Intercept Point can vary greatly due to target aircraft maneuvers. The battery commander must make allowances for many possibilities.
The predicted time to intercept is also continuously updated during the missile flight to update the predicted intercept point to assist in making steering command commands.
The missile is launched (essentially) straight up, boosted to about mach 1.7 in 3.4 seconds. It then turns its belly toward the calculated Predicted Intercept (allow 1 second). A sustainer rocket starts to increase the speed to mach 3.5. A full dive command (7 g's) is sent to the missile to dive it from vertical toward horizontal to intercept the flight path of the target. When the missile has reached a vertical angle that will be a good flight path to the intercept point, the full dive command is removed and normal steering begins. For a preview of the "good flight path", jump ahead to End of 7 G dive . (Use your browser's "back" function to return here.)
The missile must not fly directly over the Missile Tracking Radar (MTR) because of limitions inherent in that type of antenna mount and the pointing system. To avoid flying directly over the MTR, special circuits are included in the computer to fly the missile in a path skirting around a flight path over the MTR. This situation is normally avoided by placing the launching area toward the expected direction of enemy aircraft - but the battery can be effective in any case.
The predicted intercept point is constantly being updated by the computer from data from the target tracking radar and the missile tracking radar. Using the missile position from the Missile Tracking Radar, missile velocity and attitude generated in the computer, and the Predicted Intercept Point, the computer generates analog steering commands in gravity units (g's) for the missile. These commands are sent to the Missile Tracking Radar, where the analog commands are converted into radar pulse sequences indicating the command to the missile.
About 0.1 seconds before the missile will be closest to the target, a missile burst command is send by a coded pulse sequence to the missile by the missile tracking radar. This burst command is decoded and the missile warhead exploded. The goal is to explode the missile just before (10 meters) the missile would impact (or be at the closest point with) the aircraft. This way, the expanding blast of fragements goes through the space where the target aircraft would be - giving maximum damage even if a near miss.
Actually the missile fins are 45 degrees from horizontal, and the accelerometers are also 45 degrees from horizontal. The last stage of the computer rotates the command so that a 1.0 g right command will be sent as 0.707 g up/right and 0.707 g down/right which will result in 1.0 g turn right. You do remember your high school trig - don't you? Oh yes, you must be a "rocket scientist".
The missile steering commands are sent in analog form to the Missile Tracking Radar electronics where they are converted to radar pulse pairs which are received by the missile. For a description of the commands as they are sent by the Missile Tracking Radar see Missile Tracking Radar.
About 0.1 second before intercept, a burst command is sent to explode the missile and hopefully disable the target.
Return to beginning of Nike Computer
7) About the Nike Plotting Boards
The horizontal plotting board had lights to identify which pen was plotting what.
For a photo of the plotting boards, go to Battery Commander's View of the World.
8) How does the computer know the radar offsets?
The computer must know where the Missile Tracking Radar (MTR) is relative to the Target Tracking Radar (TTR), and also the relative location of the Target Ranging Radar (TRR) relative to the Target Tracking Radar (TTR). This permits the computer to subtract out the positional differences, so that all three radars are, for computational purposes, located at the same point is space.
The offsets of the MTR and the TRR are dialed into potentiomenters located on the computer. The coordinates are as North/South, East/West, Up/Down from the Target Tracking Radar. Any errors dialed into these potentiometers will cause errors in flying the missile to the exact position of the target.
The positional differences of the various acquisition radars are not important because they are always within 50 yards of the TTR and errors in range and parallax due to this offset do not affect operation (primarily the locking on to the designated target by the TTR operators). It is important that the acquisition radars and the TTR (and any area supervision radars) have the same "North" reference to within about a degree for more exact target designation. (Which target is it?)
The Tracking Radars have the same North very precisely due to their alignment procedures.
9) Gotta have a little fun - Mechanical computers.
I haven't the heart - time for fun.
Tim Robinson makes analog computers for fun - out of Meccano building "blocks".
If you have comments or suggestions, Send e-mail to Ed Thelen
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Updated Sept, 2008