The Fission Sail is an odd little idea suggested by the late Dr. Robert Forward to augment existing other sailing propulsion systems such as solar sails.

His suggestion was to create a two layered material as part of the sail. The layer toward the forward direction of the ship would be very absorbent of stray particles. The layer toward the backside of the ship would be made of decaying radioactive material. As the natural decay occurred it would emit random particles in all directions. Those particles ejected from the radioactive layer backward would add a low constant thrust to the ship. Those emitted from that layer to the fore of the ship would immediately impact the absorbing layer. The impact with the absorbing layer would add thrust to the material. 

The overall effect would be a constant low level vector of thrust to the ship, despite of the performance of the other sail mechanism.  The headache behind this method would be its constant output. The ship would have figure out a way to stow the sails in such a way to decrease this constant thrust when trying to station keep, but there is still the need to handle constant source of radiation on the ship. 

Then there is the need to create a material sail out of radioactive material. The handling of the material for manufacturing need some safety concerns, as would the general maintenance needs of the ship. 

Overall it is a creative idea, trying to utilize every possible method to achieve ‘free’ acceleration, but it would bring complications with it. Due to the low propulsion to a pure sail craft, the likely spacecraft will be mostly robotic. This could make it easier to deal with safety concerns of radioactive material being such an integrated part of the ship.