I wrote my memoir, The Step, to draw attention to the unsung heroes of the Apollo Program. The men and women who made up the technical team, instead of the astronauts who have historically received all the attention and the glory. I’ve been promoting my book and giving book talks for over a year now. I always mention the effort that was made to assemble this incredibly impressive, intelligent and hardworking scientific team that made this seemingly impossible feat, of landing a man on the moon, attainable.
Last weekend I had the pleasure of addressing Mensa at their Annual Gathering in Hollywood, Florida. I ended my talk, as I usually do, with a Question & Answer session. One man named Bob Tewchuk raised his hand and asked, “Seems there is a rumor that 90% of the Apollo mission computer was code redundant. That is, 10% of the code contained the instructions, and 90% was used for error checking and to ensure there were no errors or crashes. A friend of mine told me this years ago, and I’d like to know if it’s true.”
I worked in PR not in computer programming, so I didn’t know the answer to his question. But I knew someone who would! I took down Bob’s information and promised I would get back to him with an answer. I forwarded his question to two of my former Apollo coworkers and friends, Kenneth Clark and Jim Handley. Here’s Ken’s reply:
“The term “code redundant” implies that there is code that is redundant for some reason such as to recompute a value for which the answer is known in order to verify correctness. I doubt there was any of that in the flight computers and know for a fact there was none in the ground computers. A second form might be some form of redundancy in hardware with identical software in the redundant hardware and some sort of voting logic to determine which hardware was correct. The Launch Vehicle Digital Computer used triple modular redundancy (TMR) logic, but I don’t believe the code was replicated. The Saturn Ground Launch Computers were not TMR. However, the Mobile Launcher Computer did contain redundant set of code which was switched to if the primary memory encountered a parity error or no instruction alarm during execution. I don’t know if the Apollo Guidance Computers contained any form of redundancy and don’t see any evidence of any in my investigation on the internet.
On the subject of error checking, not even close to 90% of the code would be allocated to that task. The amount of memory in any of the computers made it absolutely impossible for there to be much if any code in the computers to be used for error checking. The error checking that existed was to determine if an operation requested or commanded by a program completed successfully. There were some checks even in the Lunar Lander to report on unexpected errors. An example of this was the Lunar Module program alarms minutes into the landing sequence (Error codes 1201 & 1202).
Memory in the computers was mostly magnetic core. Here are some examples of the memory sizes used in the computers
Saturn Ground Launch Computers (RCA 110A) – 32 K 24 bit words + 1 parity bit
Instrument Unit Launch Vehicle Digital Computer – 32 K 28 bit words including 2 parity bits
Apollo Guidance Computers — 2048 K words of erasable magnetic core memory and 36 K 16 bit words of read-only core rope memory.
Note: There were 2 Apollo Guidance Computers in the spacecraft. One in the Command Module and one in the Lunar Module.
Hope this helps,
Amazing that over fifty years later his memory is so precise. I called him a genius in my book and clearly he’s living up to the title.
The remarkable triumphs of that team have changed the trajectory of our country forever. The greatest minds of the time collected and working together were ever so powerful. Unfortunately, when Apollo ended the team was disbanded. Many of the brilliant scientists and engineers were sent packing or to sell typewriters.
Our space program has been stuck on the back shelf for years. As we stagnate China, Russia and private companies make leaps and bounds forward. It’s about time we revisit our treatment of the space program and recognize it’s importance in securing our future stability and respectability as a nation. I can think of two guys who would be perfect to head up the scientific team!