Also the "Free Piston" engineJust a modern take on the “Screamin Demon”. The Demon used the free piston as a ”gas generator“ the “Work“ was thrust from the exhaust. It was reported to be horrifically loud so I’m betting this will be too, so add a fair amount of weight for a muffler…
The gap between the energy density of fuel and batteries is closing,and the rate of closure is accelerating.Fuel is useualy lighter,exceptions are the high altitude electric solar planes with endurance that is truely amazing,the mars flyer,rovers,there are going to be more.I am curious why such a large battery. Fuel for the engine is always lighter than an equivalent energy battery. Maybe a smaller battery to assist with take-offs and climb outs.
I built a free piston engine a while back. It really was a screamer and I can see why the muffler would have to be significant to make the drone acoustically stealthy.