You don't strictly need one. There are designs that rely on exhaust pulse resonance for pressure. There are designs that use various kinds of mechanical valves.

You don't strictly need one. There are designs that rely on exhaust pulse resonance for pressure. There are designs that use various kinds of mechanical valves. It looks like (propane fuel) tesla valve pulse jets have been built, but I'm not seeing anything flight capable. https://www.youtube.com/watch?v=JYZhZ_qNKfE https://www.youtube.com/watch?v=JYZhZ_qNKfE