ICBMs and the first mass-produced ‘micro’ computers

Figure 1 (Photo thanks to Nathan Zeldes, CC BY-SA 4.0 creativecommons, via Wikimedia Commons)

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

They often are not aware enough of the tech history of the Cold War to even wonder about it. Yet the technology of our world today was profoundly shaped by this war. And perhaps there are even a few lessons to be learned. That is, old Cold War solutions, and how they might apply to today’s problems.

Here is an example of something that many already know and more should: We owe our speed in getting personal computers to Cold War nuclear tipped rockets.

It was just an engineer’s dream for years. What if we could actually fit a digital computer into an aircraft or a missile?

In the 1950s, computers filled rooms and engineers struggled with reliability and performance. In the 1960s, performance improved, but computers were still room-sized. It would take the revolution in mass-produced integrated circuits to change this. And this happened because of Intercontinental Ballistic Missiles. You see, something was happening to push our 1960s engineers to dream up the modern micro-computer.

Analog computers were an option, of course. And analog computers are not an ancient technology — unless I am ancient. When I studied control theory at the U.S. Air Force Academy from 1972 to 1976, we were using analog computers. There were still plenty in service and most control systems were still “designed in the s-plane” and realized with discrete circuits. (Remember that phrase, you’ll see it later.) This is as opposed to today’s digital control systems using embedded computers.

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

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

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

Well, one 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 language much like FORTRAN … called ALGOL … using card decks. Above is an example card from a computer card deck. Look at the top left to see the line of code: “Z(I) = Y + W(I)”. Many lines make a computer program. And, thus, a deck of cards.

In the 1970s, the dream was realized in mass-produced hardware and there were digital computers appearing on-board aircraft and missiles. The mass production started with the Minuteman II ICBM and its D37C computer. It was roughly the shape of a loaf of bread, was about a half cubic foot in volume, and weighed in at about 26 pounds. This was “micro” in the 1970s. It had a capacity of around 7,000 11 or 24-bit words.

By Paul E. Ceruzzi, MIT Press, 1989

Why was the D-37C the first mass produced small computer? There’s a book here on my bookshelf to answer that question. If the history of placing computers in missiles and aircraft interests you, you can still get an old copy on Amazon for around $40. See the picture abovefor details. Turning to page 93…

“… a change in requirements … made demands … that discrete circuits could not meet … a change in the U.S. strategic policy from massive retaliation to a flexible response. This meant a need to retarget and thus reprogram the missile up to the instant before launch.”

Each Minuteman II guidance system needed a couple thousand Texas Instruments integrated circuits and the initial production run included hundreds of missiles. Thus, ICBMs gave us the first mass-produced computer. This drove TI production costs down and set the stage for the ubiquitous home computer industry.

My first assignment out of the Academy in 1976 was as a pilot. This was also the point in time that Zilog introduced the Z80 chip to the world. In that assignment, fellow pilot Capt. Steve Brown and I helped introduce Strategic Air Command to small computers for the cockpit. See my July 1982 Combat Crew article “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 posted to Los Angeles Air Force Station and the Air Force’s 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 in the back of the space shuttle and loft spy satellites to high orbits. During both those periods I was an avid home computer hobbyist. For instance, I was programing my TRS80 computer (1977 to 1981) in Z80 assembly and machine language.

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

The D37C, on the other hand, kept its data on a rotating disk. However, the read and write heads were “nailed down” so the programmer had to know when the data was near the read or the write head. Programming occurred using “word times.”

Assembly language was a convenience, a step up from machine language which programs the Z80 chip directly in its own internal code. But the TRS80 was so small, I needed to resort to machine language to get certain efficiencies. This hobby got me “speaking chip” more fluently. I got comfortable with using storage registers, that is, where to put data and where to put instructions as an example. How to do binary math using “twos complement.”

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

It wasn’t a big stretch. My degree at the Academy was in Astronautical Engineering. But to compute those orbits and their perturbations, we needed 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 classes from the computer science professors, since I was interested in that area. Before I got my appointment to the Air Force Academy, I was signed up for CalPoly San Luis Obispo in a brand-new field called “Computer Science.” At CalPoly, it was in their math department. OK. Maybe I am ancient.

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

NAND Gate Logic (graphic by author)

On one end, it had 33 cards, mostly filled with NAND gates, as shown above I learned that we practically owned Texas Instrument’s chip production line back in the 1960s as these were being built. On the other end was a spinning disk with fixed read and write heads. As mentioned before, the software had to operate in “word times” to ensure read and write operations occurred when the heads were lined up with the data.

My “aha!” moment was when I realized that all those machine language instructions, the Z80 instruction set I had learned for the tiny Z80 chip, had their equivalents in those dozens and dozens of NAND gates spread across the bulk of the 33 modules. For instance, Boeing’s book, “D37C Logical Description,” provides hundreds of pages of the NAND gate connectivity needed to do, for instance, twos complement math. And don’t get me started on their wiring diagrams!! I’ll ensure a damper on any raucous party.

I learned that chips that didn’t go into Minuteman II made their way into the Apollo Moon Shot. So, ICBM engineers would often refer 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 of the D37C, but essentially the same. Minuteman II was retired in the 1990s; the last silo was imploded in 1997. All of the guidance electronic gear in the Minuteman III, including the D37D computers, were modernized (i.e. replaced) in 1999. A design challenge was to ensure the new computer could be just as “hard” to nuclear bomb radiation as the older version.

Now you know a tiny bit more of Cold War history 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 U.S. Air Force colonel and retired defense contractor born and raised in Wasco, Calif., and currently living in Ogden Utah. See charlesvono.com for his videos on various aerospace subjects.

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