ICBMs and the first mass-produced “microcomputers”

by Charlie Vono, special for Aerotech News
Because fighting the Cold War involved focused dedication, secrets, and emerging technology, much of the story is unknown to the general public.

Often they are not sufficiently aware of the technological history of the Cold War to even wonder about it. Yet the technology of our world today has been profoundly shaped by this war. And maybe there are even some lessons to be learned. That is, old Cold War solutions and how they might apply to today’s problems.

Here’s an example of something many already know and others should already know: We owe our speed in getting personal computers to the nuclear rockets of the Cold War.

It was just an engineer’s dream for years. What if we could actually install a digital computer in an airplane or a missile?

By the 1950s, computers were filling rooms and engineers were struggling with reliability and performance. In the 1960s, performance improved, but computers were still room-sized. It would take the mass-produced integrated circuit revolution to change that. And this happened because of intercontinental ballistic missiles. You see, something was happening to push our engineers in the 1960s to imagine the modern microcomputer.

Analog computers were an option, of course. And analog computers aren’t old technology – unless I’m old. When I studied control theory at the US Air Force Academy from 1972 to 1976, we used analog computers. There were still many in use and most control systems were still “designed in the s-plane” and made with discrete circuitry. (Remember this sentence, you will see it later.) This is as opposed to today’s digital control systems using on-board computers.

I had my “spirule” and learned the root locus method for designing stable control systems. See image at top of story.

Of course, we also had digital computers at the Academy.

Thanks to Arnold Reinhold for this photo of a FORTRAN punched card from a game (creative commons)

Well, a digital computer. My programming classes and astronautical engineering classes used the Burroughs 6700. I spent many nights in the computer room programming it with a FORTRAN-like language…called ALGOL…using decks of cards. Above is an example card from a computer card game. Look at the top left to see the line of code: “Z(I) = Y + W(I)”. Many lines form a computer program. And so a card game.

In the 1970s, the dream came true in mass-produced hardware, and digital computers appeared on airplanes and missiles. Mass production began with the Minuteman II ICBM and its D37C computer. It was roughly shaped like a loaf of bread, had a volume of about half a cubic foot, and weighed about 26 pounds. It was “micro” in the 1970s. It had a capacity of about 7,000 words of 11 or 24 bits.

By Paul E. Ceruzzi, MIT Press, 1989

Why was the D-37C the first mass-produced small computer? There is a book here on my library to answer this question. If you’re interested in the story of placing computers in missiles and planes, you can still get an old copy on Amazon for around $40. See image above for details. Go to page 93…

“…a shift in demands…made demands…that discrete circuitry could not meet…a shift in US strategic policy from massive retaliation to flexible response. This meant retargeting and therefore reprogramming the missile up to the instant before launch.

Each Minuteman II guidance system required a few thousand Texas Instruments integrated circuits, and initial production included hundreds of missiles. So ICBMs gave us the first mass-produced computer. This lowered IT production costs and paved the way for the ubiquitous home computing industry.

My first posting out of the Academy in 1976 was as a pilot. This is also when Zilog introduced the Z80 chip to the world. On this mission, my fellow pilot, Captain Steve Brown, and I helped introduce Strategic Air Command to small cockpit computers. See my July 1982 article on Combat Crew “We have the technology”, at https://archive.org/details/combatcrewstrate3271offu/page/16/mode/2up.

Thanks to Appaloosa, CC BY-SA 3.0 creativecommons, via Wikimedia Commons and GNU

In 1982 I was assigned to Los Angeles Air Force Base and the Air Force Space Division and was responsible for the flight software used on the IUS. The IUS was a kind of short Minuteman missile (both are Boeing products) that could fit on the back of the Space Shuttle and spy satellites in high orbits. During both periods, I was a computer enthusiast at home. For example, I programmed my TRS80 computer (1977 to 1981) in Z80 assembler and machine language.

Well, you had to program in machine language since the original TRS-80 only had 4KB of memory. I clearly remember the day I went to 16KB. I really felt like someone that day. But that didn’t leave me much more elbow room. Especially since the storage unit was a portable tape recorder. Interestingly, the tape recorder had to be cheap with few filters since the digital data was recorded in the noise bandwidth.

The D37C, on the other hand, kept its data on a spinning disk. However, the read and write heads were “pinned”, so the programmer had to know when the data was close to the read or write head. The programming happened using “word times”.

Assembly language was a convenience, a step up from machine language which programs the Z80 chip directly into its own internal code. But the TRS80 was so small that I needed to resort to machine language to achieve certain efficiencies. This hobby has allowed me to speak more fluently. I got familiar with using holding registers, i.e. where to put data and where to put instructions as an example. How to do binary calculations using “two’s complement”.

Using the Z80 instruction set, I could load data and computer programs into TRS-80 memory. On-chip registers and wiring helped this process.

It wasn’t a big stretch. My degree at the Academy was in Astronautical Engineering. But to calculate these orbits and their perturbations, we had to be in the computer room most of the time. In fact, the department was “The Department of Astronautics and Computer Science”. I took a lot of extra lessons from the computer teachers because I was interested in this field. Prior to being appointed to the Air Force Academy, I was enrolled at CalPoly San Luis Obispo in a brand new field called “Computer Science”. At CalPoly, it was in their math department. OKAY. I may be old.

When I left the Air Force on active duty and started working at TRW in 1985, I was told to “become an expert” on the Minuteman II D37C. And so, I gathered all the technical manuals I could and started studying.

NAND Gate Logic (graphic by author)

At one end were 33 boards, mostly filled with NAND gates, as shown above I learned that we practically owned the Texas Instrument chip production line in the 1960s when they were built. At the other end was a spinning disk with fixed read and write heads. As mentioned earlier, the software had to operate in “word time” to ensure that read and write operations occurred when the heads were aligned with the data.

My “ah!” It was then that I realized that all those machine language instructions, the Z80 instruction set that I had learned for the tiny Z80 chip, had their counterparts in those dozens and dozens of NAND gates spread across most of of the 33 modules. For example, Boeing’s book, “D37C Logical Description”, provides hundreds of pages of the NAND gate connectivity needed to do, for example, additional math. And don’t get me started on their wiring diagrams!! I’ll provide a shock absorber on any raucous part.

I learned that bullets that didn’t go into Minuteman II ended up in the Apollo Moon Shot. Thus, ICBM engineers often referred to “Apollo-era electronics” when discussing our Minuteman electronics, even when discussing the more modern Minuteman III.

The Minuteman III ICBM sported the new and improved D37D computer, which was an upgrade from the D37C, but essentially the same. Minuteman II retired in the 1990s; the last silo imploded in 1997. All of the Minuteman III’s guidance electronics, including the D37D computers, were retrofitted (i.e., replaced) in 1999. A design challenge was to ensure that the new computer might be just as “tough” to nuclear bomb radiation as the older version.

Now you know a bit more about the history of the Cold War and the rise of the personal/office/home computer.

Just remember, EX DE, HL swaps H with D and L with E.

Editor’s note: Charlie Vono is a retired United States Air Force Colonel and retired defense contractor born and raised in Wasco, California, and currently living in Ogden, Utah. See charlesvono.com for his videos on various aerospace topics.