I remember an anecdote our robotics lecturer told our university class in 1995, which was about how in the west we try to make expensive things that are the absolute best of technology and how the other side didn't have that luxury and relied on ingenuity.

He described a cold war Russian missile they had somehow obtained and were tasked with trying to reverse engineer. Ostensibly, it was thought to be a heat seeking missile, but there seemed to be no control or guidance circuitry at all. There was a single LDR (light dependent resistor) attached to a coil which moved a fin. That was it. Total cost for the guidance system maybe a couple of dollars, compared to hundreds of thousands for the cheapest guidance systems we had at the time.

The key insight was that if you shined a light at it, the fin moved one way and if there was no light the fin moved the opposite way. That still didn't explain how this was able to guide a missile, but the next realisation was that the other fins were angled so when this was flying (propelled by burning rocket fuel), the missile was inherently unstable - rotating around the axis of thrust and wobbling slightly. With the moveable fin in place, it was enough to straighten it up when it was facing a bright light, and wobble more when there was no bright light. Because it was constantly rotating, you could think of it as defaulting to exploring a cone around its current direction, and when it could see a light it aimed towards the centre of that cone. It was then able to "explore the sky" and latch on to the brightest thing it could see, which would hopefully be the exhaust from a plane, and so it would be able to lock on, and adjust course on a moving target with no "brain" at all.

That's roughly how the original Sidewinder worked. The original concept was to reduce near-misses. If the pilot could get on the target's tail and aim at the engines, it usually got a hit. That was the same task as getting into firing position for guns. Hit rate about 8% in combat.

Later versions allowed launches from longer ranges and from off-angles.

I believe there was a similar weapon being developed in the west, only recently, which involved a missile with contra rotating halves joined by a clutch. The fixed fins caused it to always steer one way. It flew straight by releasing the clutch to spin up the front half, negating the steering effect. Grabbing the clutch caused it to stop spinning and veer off in one direction.

Presto! Two axis continuous flight control with a 1-bit input.

Edit: my memory wasn’t far off. It’s Starstreak: https://en.wikipedia.org/wiki/Starstreak

Strike a light in front of a parked but otherwise active fin guided heat-seeker and its freaky to watch it come alive like a lazy beagle eyeing a treat.

That frugal, creative mindset is also the default for people of modest income everywhere in the world - borne of necessity.

This is shockingly similar to microbial motility mechanisms. Like random walk plus chemotaxis.

This sounds like the early Sidewinder or other 1940's/1950's attempts at infrared homing missiles.

Unless it was nighttime or the engagement happened at low altitude on a cloudy day, wouldn’t that usually lock onto the sun?

Similar to how moths guide themselves toward light

> That still didn't explain how this was able to guide a missile

That does explain why it lands on civilian areas tho

Incredible

that sounds like crude-fied version of first Sidewinders.

> with no "brain" at all

It seems this is how Russia moves in general. Hopefully, this will end at some point.