1. A PROPULSION SYSTEM COMPRISING A RADIONUCLIDE HEAT SOURCE WHICH CONSISTS OF A NEUTRON BOMBARDED SUBSTANCE WHICH HAS A RELATIVELY LOW GAMMA ACTIVITY AND CONTINUOUSLY EMITS A HIGH THERMAL HEAT, A FIRST ENGINE, A HEAT EXCHANGER IN THE ENGINE, CONDUIT MEANS FROM THE HEAT SOURCE TO THE HEAT EXCHANGER, PUMP MEANS IN SAID CONDUIT MEANS FOR TRANSFERRING HEAT FROM THE HEAT SOURCE TO THE FIRST ENGINE, LIQUID COOLANT IN THE CONDUIT MEANS, A HEAT DISSIPATION DEVICE INCLUDING A SECOND ENGINE SEPERATE FROM SAID FIRST ENGINE, AND VALVE MEANS IN THE CONDUIT MEANS TO DIRECT THE HEAT TO THE HEAT DISSIPATION