About 10 years ago, MRC made what they called FX engines to go with their Concept II line of Estes-class kits. The "engines" had no impulse; their sole function was to dump out smoke for roughly 6 seconds. They were about a 1/4 inch in diameter and an inch or so long.
The Concept II kits that they were made for had interchangeable motor mounts; you could switch out mounts for a single 18mm engine [Estes A-C], an 18mm engine and two FX engines, or a single 24mm engine [Estes D]. Incredibly, I still have the brochure about this line of kits.