I have been posting for years about the Sugar Shot to Space (SS2S) project, which employs dual-phase motors. That is, a motor that has two burn phases. Today, IEEE Spectrum reports on Multi-stage Micro Rockets for Robotic Insects. This work was done by a group from the Harvard Microrobotics Lab. Their tiny, three-stage, APCP-based motors appear to be about 2mm in diameter. The embedded video shows them in action and includes a thrust curve. The motors seem to perform better than their gliders but that might partially be because they are so small compared to the field of view.
The article also points to another micro-rocket project from Army Research Laboratory and the University of Maryland Microrobotics Lab. (I may have posted about this. So many posts, how can I be expected to remember them all?)