LiquidPiston's “inside-out” rotary X-Engine wins Army research contract
>We're running engines for tens of hours, dozens of hours, we're not yet in the hundreds of hours where we want to be.Durability and reliability  and maintenance are the issues with these type engines.For some military  applications that's not an issue. If you want to build large number of cheap lightweight drones for high intensity warfare, durability is secondary. They will be shot down eventually anyway. Operating smaller number for training purposes during peacetime is not that expensive.