Ram Jet/Linier Pulse Engine's.

Although these crude looking engines don't look like much when they are fired up on the ground with a near zero (14.7) intake pressure, for some of them that all changes when the intake velocity reaches over 450mph or 40psi and higher (not including the HK Red Head engine). These linier valve less pulsejet engines become Ram Jet Engines once the intake pressures are higher than the explosion pressures experienced inside the combustion chamber.