It slows you down :) The vacuum part was an emphasis on requiring a counterpart to change momentum. In an atmosphere you could flap your wings, or use electri

It slows you down :) The vacuum part was an emphasis on requiring a counterpart to change momentum. In an atmosphere you could flap your wings, or use electric fans, and argue what it means to ride a swing, but it would be besides the point, as in an atmosphere there is no need for propellant-less engines from the article.