Rockets IV: Thermal rockets, nuclear and otherwise
See also parts I , II , III , V , and VI . Here we talk about applying some sort of external heat source to your propellant so it expands a bunch and then shoots out of the rocket. Technically chemical rockets are thermal rockets too but I'm trying not to be pedantic here. There are lots of things you can use as your source of heat. NASA has done a lot of studies with using a nuclear reactor to heat some propellant directly. If you've got a big parabolic mirror you can focus sunlight on your engine and heat it that way. If you've got a friend nearby with a big laser they can use it the same way. The constraints for these sorts of rockets aren't the same as for the last two sorts of rockets. You don't have a fixed ratio of energy to propellant that chemical rockets have so the v e isn't fixed that way. Energy is a concern as with electric rockets but not as large a one since its much easier to turn an energy source into heat than it is to