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It maybe possible to make a cone shaped rock guard to clamp on to it

I thought the same thing. Obviously, there are places you would want to use it but if it charges as they say, then some clever person can make a rock guard very easily.

My first thought was to take a plastic gallon vinegar bottle, cut the bottom off and slip the tether thru the top and adjust the fit so the props wouldn't hit but the cone would deflect the rocks. Something cheap and DIY.