Nammo is close to completing a rocket engine that will make green space propulsion a reality.
Nammo’s hybrid rocket works by having the oxidizer separate from the solid fuel, and it is able to regulate its power by simply regulating the flow of the oxidizer. If necessary, it can even be turned off completely before re-igniting. The main benefit, however, is that only one fluid needs to be regulated, which makes the rocket far more stable and easy to work with. In addition neither of the fuel ingredients needs to be stored at high pressure, under extreme cooling or represents an explosive risk.
Nammo article: Going hybrid in space