Roberts W., Ordon R., Scharton T., Kuznetsov A.

Pulsejets are among the simplest propulsion devices, with no actively moving parts. These devices were originally conceived in the early 20th century and successfully used as the main propulsion system by the Germans in WW-II on their V-1 ‘Buzz bomb’. Following the war, there was considerable interest in pulsejets, particularly by the French and Americans. However, with dramatic improvements in gas turbine efficiency, a competing propulsion technology, the pulsejet fell out of favor in the mid 1960’s and has received very little attention until recently. Due to their simplicity, high thrust-to-weight ratio, and low cost, pulsejets may be attractive in some applications. It is therefore of interest to understand the acoustics, fluid mechanics, and chemical kinetics that governs these devices so that an optimization can occur. Historically pulsejets with valves have been modeled as 1/4 wavelength tubes, for which the operating frequency scales as the inverse of the tube length. However, for the valveless pulsejet of interest here, the inlet and combustion chamber configuration look acoustically like a Helmholtz resonator, for which the operating frequency is proportional to the inlet diameter. Therefore for the valveless pulsejet, the wave tube and Helmholtz models are combined. Through a series of experiments in which the length of the exhaust duct and the area of the inlet were systematically varied, it is determined that the valveless pulsejets work best when the frequencies of these two acoustic models coincide. And, of course, the acoustic coincidence frequency must be compatible with the characteristic times of the pulsejet fluid mechanics and chemical kinetics.