That takes me back 50 years!
My closest friend was seriously into aero-modelling. By some means, he had acquired a miniature *ram jet* to fit on a control line aircraft.
I think the combustion chamber explosion closed off the fuel/air (petrol) intake by distorting a thin steel flutter valve and forcing the gasses down the tail pipe. Ram air pressure was then intended to re-open the valve and admit more fuel/air >>>>>>
Stationary, on the ground, this did not work but he then hit on the idea of using his father's garden spray as a source of compressed air. The noise was unbelievable and was terminated by his mother, flying out of her kitchen door and vetoing any further experimentation:-)
I suppose Doodle bugs were not long out of her memory:-)
regards