"Everyman's Jet Engine" - Why Valveless Pulsejets? No moving parts of any kind Unlike all other propulsion engines (other than the ramjet), valveless pulsejets contain absolutely no moving parts. Because of this, there are no bearing surfaces in the usual sense of the term. The internal geometry of the engine remains absolutely unchanged throughout all phases of the engine cycle. This is extremely important, because it provides both simplicity and lightness. A lot of the mass of ordinary internal combustion engines is just extra material needed to support internal moving parts, with associated extra complexity and production costs. I mentioned the ramjet -- an engine just as simple as the valveless pulsejet (and more compact, as well). The problem is, the ramjet won't run as a stationary engine; you have to use another propulsion system to get it up to some threshold speed that will allow it to begin operating. Thus, a design that seems simple actually demands greater complexity (outside of the engine proper) to start working as a propulsion system. Because of this, ramjets have little practical use for the hobbyist (though they are certainly interesting and have their devotees, of course).