Hydro-formed "Escopette"-style valveless pulsejet?
They run as efficiently as a turbojet (w.r.t. specific fuel consumption), and Mach 0.7 is way beyond your "couple hundred miles an hour".
It will be burnt up by the time it runs out of fuel, though.
For a good one it has to be hydro-formed out of seamless pipe, which is likely hot-stretched (like wire drawing, but using an induction heating coil in place of the die, and independently controlling the feed-rate and the pull-rate) in advance to retain consistent wall thickness in the engine despite being cross-section after hydro-forming. Also probably incremental hydro-forming with grain-structure-fixing re-heating between stages.
The hard part is just that most of the work is in making the hydro-forming dies/tools, not then using them to cheaply produce more engines.
Fascinating, haven't heard of this. Some googling suggested there is some active research in this area, and at least some active use by hobbyists and by the military in target drones. Seems like a promising angle for a cheaper cruise missile, at least on the engine front.
It will be burnt up by the time it runs out of fuel, though.
For a good one it has to be hydro-formed out of seamless pipe, which is likely hot-stretched (like wire drawing, but using an induction heating coil in place of the die, and independently controlling the feed-rate and the pull-rate) in advance to retain consistent wall thickness in the engine despite being cross-section after hydro-forming. Also probably incremental hydro-forming with grain-structure-fixing re-heating between stages.
The hard part is just that most of the work is in making the hydro-forming dies/tools, not then using them to cheaply produce more engines.