In one of my favorite books, Tom Swift in The Race to the Moon he uses a spacecraft which is propelled by devisesdevices called repelatrons which are attached to the hull of the craft. They can be programmed to repel any substance as such forcing the craft in the opposite direction, they can also be used to stop projectiles, acting as a shield.
My question is weatherwhether this could be a feasible form of sub-light propulsion?