Introducing the SM Fission Fragment Rocket! It has poor thrust, it is massive, it can not be refueled after launch, no addition fuel tanks are possible, you have to provide it with a source of electricity and it generates a truly frightening amount of waste heat. Why would you possibly want it?? 500,000 Isp. Now, before all the purists get all ornery with me, go read this. If some of you people would be kind enough to try this out (make sure to use the included heat shield) and give me some feed back, I would appreciate it. I am considering lowering the Isp and increasing the thrust, probably by a factor of 2 for both if I do. Spaceport download link is embedded in the photo. Have fun.