New concept: What about if we are to depend from external forces? So we cannot produce any kind of propulsion from our ship.
Previous question:
I know that there are two main ways to gain speed in the emptiness of space. One is by burning fuel and therefor accelerating the spaceship. The other one is by using slingshot effects (gravity assists) where the spaceship gets a boost in speed. Are there any other ways to gain speed in empty space?