From reading that post, it seems like the soft system is the way to go, if you can figure out how to build in some compliance without too much weight, space, or complication. I can imagine doing something with spring loaded idler pulleys, but I don't know if that would provide enough movement. I'm wondering if there's some size below which these issues aren't as big of a problem. However, considering how small a BD-5 is, that's probably wishful thinking. So I also wonder why we don't hear about more trouble with things like the Ace redrive. Maybe I'm just not listening in the right place. If a redrive is necessary, I can imagine using two engines with coaxial props. One could fail while leaving the other intact. Not sure what it would sound like, though. Hard to tell how loud or obnoxious this is from a video: https://www.youtube.com/watch?v=gXsH6tjzgD0 The props would probably have to be larger than with a normal system, though, in order to maintain a reasonable disc area for use at low speeds.