These are the guys trying to make an AeroSpike engine. This looks to be a test of that! Super cool!
There is video, but I can only find on Twitter presently: https://twitter.com/stoke_space/status/1703569700540883195
From the wiki page linked here, this looks like a toroidal spike. I wonder what sort of gas they use for the base bleed.
Several versions of the design exist, differentiated by their shapes. In the toroidal aerospike the spike is bowl-shaped with the exhaust exiting in a ring around the outer rim. In theory this requires an infinitely long spike for best efficiency, but by blowing a small amount of gas out of the center of a shorter truncated spike (like base bleed in an artillery shell), something similar can be achieved.
Pretty sure those are several small distinct engines, but at high altitude with a large expansion ratio it will kind of work like an aerospike
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Everyday Astronaut did a video on the vehicle. it shows a test fire but not the hop (have seen that on YouTube as well). Worth a watch: https://m.youtube.com/watch?v=EY8nbSwjtEY.
Edit: Clarity
