Tindallgrams

Using it wasn't terribly exciting; the rope memory for a program was broken up into six rope modules that could be installed into and removed from the back of the AGC pretty easily with a screwdriver. Dedicated rope modules were only really used for flight and for completed test programs. For the most part during development they made use of "core rope simulators", that simulated the electrical properties of a core rope memory, but read data from a traditional coincident current ferrite core stack. This let them much more easily and quickly test programs out on hardware, without going through the pain of shipping a release out to the factories to manufacture.

The long and expensive assembly process caused last-minute changes before mission to be, in some cases, a bit hacky. They would do their best to localize changes to a single module, if possible, so that they would only have to re-manufacture one of them instead of all six. The Apollo 11 LM thus flew with 3 modules of Luminary 97, 2 from Luminary 99, and 1 from Luminary 99 Rev. 1. And this page from the Apollo 5 software Sunburst 120 shows how messy that could get: https://archive.org/stream/yulsystemforagcr00nasa#page/n485/...

Rope memory led to one of the most interesting "binary" output formats from an assembler that I've ever come across. The assembler (https://archive.org/details/yulsystemsourcec00hugh), upon successfully assembling a program, could punch a paper tape for manufacturing. The tape didn't contain the words of the program, directly. Rather, it contained commands for a special machine that was designed to help with the construction of the rope modules. The machine would position a small loop in front of the next core a wire was supposed to be threaded through. One of the two operators would pass a needle and wire through the loop and through the core to the operator on the other side, and the loop would then be repositioned in front of the next core the wire was to pass through. This video describes the process in more detail and shows it being done: https://www.youtube.com/watch?v=YIBhPsyYCiM

Preparing it nowadays is a bit challenging. Contrary to most descriptions you'll find online, rope memory is not "just" simple transformer coupling between the drive lines and the sense lines. The operation is a lot closer to the concept of coincident current ferrite core memory. Just like regular ferrite core memory, rope memory relies on the switching action of cores with "square" hysteresis loops. When a particular word is being read out of memory, a single core is addressed (each core stores the data for twelve 16-bit words). This core is "set", or driven to one magnetic polarization, by a set current, with all other cores being held by a series of inhibit lines. After a short time, the core is then "reset" to its original polarization by flowing current through the other direction. The changing magnetic field of the core couples into the sense lines that go through the core (and not onto those that don't), which are run into a traditional sense amplifier circuit.

Anyways, the cores used were (as far as I've been able to gather) metal tape cores, wound with 1/8mil thick 4-79 molybdenum permalloy tape. Similar stuff is still made today, but it's not super easy to come across. Each of the six modules contains 512 of them, so you'd need 3072 cores total if you wanted to weave one of the full manned flight programs.

https://www.quora.com/What-is-the-astronaut-fatality-rate

The most dangerous jobs in the US, logging, has only 60k workers and a mortality risk of about 2.7% over a 30-year career.

https://priceonomics.com/how-the-lumberjack-became-such-a-de...

The reason it's bad to spend so much on astronaut safety is because the cost is so enormous and because the people at risk are very well-informed and participate voluntarily.