This morning a group of Computer History Museum volunteers went on a tour of the USS Pampanito, a WWII-era Balao class Fleet submarine moored in San Franciso. The normal tour is very interesting, and I’d definitely encourage anyone interested in naval history and technology to go, but for us the highlight was a specially-arranged tour of the conning tower, normally off limits to the public for safety reasons. This allowed us to see the Mark III TDC (Torpedo Data Computer), an amazing analog computer used for computing torpedo headings from visual or sonar contact.
The TDC worked much better than the systems in use by our foes, at least partially because it tracked the target in real time, taking into account the submarine’s velocity, while the foes’ systems apparently did not.
Also of interest, and visible on the public tour, is the ECM Mark II cipher machine. While this was a rotor-based machine that in principle this machine performed the same functions as the famous German Enigma cipher machine, it was much more sophisticated. Instead of advancing the rotors one position at a time like an odometer, it had a second set of rotors that controlled the advances of each of the main rotors.
It occurs to me that this dual-rotor system is not unlike one of the methods used in more recent times to generate pseudo-random numbers. LFSRs (Linear Feedback Shift Registers) can be used as PRNGs, but have many undesirable properties. Sometimes one LFSR is used to control the step rate of another LFSR in order to improve the “randomness” of the results.
Many people on the tour were heard remarking about how incommodious the submarine is. Having heard for many years that they are quite cramped, I actually thought the real thing was not as bad as I’d imagined. But I still wouldn’t want to serve on one for months at a time.