The plate of steal is connected to the ballbearing gun in the sail and fan example. When the ballbearing gun fires a ballbearing, the gun will get pushed back, and thus will the plate. When the forward moving ballbearing hits the backwards moving plate, they will both stop by momentum conservation.But imagine replacing the fan with a ballbearing gun, and the sail with a plate of steel. The ballbearings will rebound off the plate and escape to the rear of the vehicle, producing thrust. We don't normally consider air to have inertia, but if you get enough of it moving, it certainly will.
Step one: ballbearing gets fired
gun+plate and ballbearing will have an equal and opposite momentum by momentum conservation
Step two: ballbearing hits the plate
since gun+plate and ballbearing have equal and opposite momentum, they will come to a halt when they collide, by momentum conservation
@A.T. : Is Dave's quoted post above the premise that you are defending ?