While my first attempt at thruster flames yielded a halfway acceptable result, I was never really satisfied with it. The flames in Descent 3 just looked better, and I wanted something like that. After some pondering on 3D math and some experimentation I finally managed to come up with something I really like:

What I also never really liked was that robots did not have thruster flames. The main problem here was to properly determine position and orientation of the robots' thrusters. For missiles and the player ships I could use a simple approximation, but that didn't work at all for robots due to the great variety in thruster position and facing. The most promising ansatz was to somehow use the flickering glow texture that got applied to robot thrusters, and that indeed worked, so here they are:

Not all robot models have a glowing thruster, so not all of them will show thruster flames, and you may notice them suddenly popping up when first seeing a robot, but these are minor backdraws and will probably go rather unnoticed. Turn the new thruster flames on by selecting 2D thruster flames in the effect render options menu.