Hi TS, welcome to the forum. And yes, that is possible. You just needed a few more settings.
I added some comments in the code… to tell what I did.
First, I/we set gravity to 0,0,0 to keep particles from falling toward ground.
Then, I/we adjust the min/max emitbox to be located at the wide end of the rocket. Those emitBox settings are positional vector3… emit particles from where to where… on the rocket mesh itself. Therefore, it also sets the “diameter” of the rocket’s thrust nozzle (currently set to 0 by 0). See lines 66/67.
Then, we adjust our direction1/2 to spray particles in the correct direction (these values take a little special directional vector thinking… which is somewhat different than positional vectors.) See lines 60/61.
Also added some particle colors in lines 70-73, just for fun. They “evolve” as the particle ages.
I hope this is helpful. Ask questions as needed, or, maybe mark as solved… your call.
Note to core folk: We MIGHT have a little bug, here. It appears that the emitRate is increasing by itself, over time. It could be a Wingnut mistake, though (common). Concerned folk, give opinions about that, here, too. I’m sure TS won’t mind a little extra talking on his thread.