Hi, I got this fun idea. Has anyone ever consider that heat radiation could be an efficient propellant for a spaceship? Imagine a spherical spaceship, half of which is hot and with a large emissivity coefficient (say almost 1), and the other half is cold and with low emmisivity. A naive calculation with the approximations above gives me a net power of this engine: P almost equal to sigma 2 Pi R^2 T_hot ^4 which can be quite large. Of course, inside the spaceship, energy must be produced to keep the hot surface at constant temperature. One the main advantage I see here, is that you can use nuclear energy to provide with the required heat, and this is much more efficient than chemical energy. So it seems to be a not so crazy idea! :) :) Comments very welcome, thanks!