The theory behind the ramjet dates from the early years of powered flight, when French engineer René Lorin first thought of applying the compression-combustion-expansion thermodynamic cycle to aircraft propulsion in 1908. One advantage is that the technology is very simple since no moving parts are involved; the problem, however, was that such a jet engine required a minimum speed to kick in that was unattainable at the time, and the idea was shelved for a generation. It wasn't rediscovered until 1935 when René Leduc took another look at Lorin's work and decided to apply it experimentally.
Now in OTL Leduc hadn't completed his first prototype when France fell in 1940, and all further research was interrupted until well after the end of the war. But what if Lorin's work had been rediscovered earlier, say in the 1920s, and the technology had had time to mature by the beginning of the war? What practical applications, if any, might it have had? Could, say, ramjet-assisted interceptors have been feasible? What about guided or unguided rockets?
Now in OTL Leduc hadn't completed his first prototype when France fell in 1940, and all further research was interrupted until well after the end of the war. But what if Lorin's work had been rediscovered earlier, say in the 1920s, and the technology had had time to mature by the beginning of the war? What practical applications, if any, might it have had? Could, say, ramjet-assisted interceptors have been feasible? What about guided or unguided rockets?