When examining the different spacecraft concepts described in this book that utilise a nuclear pulse engine, it will be clear that there is a range of parameters which are strongly dependent on the spacecraft design but also the type of mission. This also depends on the mission requirement such as a 100-days Mars mission versus a 100-year interstellar mission, and so it should be no surprise that the engine performance is also different. In general, an interstellar mission will be many orders of magnitude higher in terms of propulsion requirements when discussing performance. This is why interstellar flight is so difficult compared to interplanetary flight.