Space is the only place where these engines are of any practical use; a combination of endless solar power, no friction, not having to stay up in the air by con

Space is the only place where these engines are of any practical use; a combination of endless solar power, no friction, not having to stay up in the air by constantly fighting gravity and huge distances to travel means an engine with a tiny amount of thrust that can run for long periods of tiny with a tiny amount of fuel is useful. On earth you're likely not even going to be able to overcome friction and start moving.