The most fast idea of space engine from already known ones is nuclear space engine. Liquid hydrogen is heated by nuclear
heat source to temperature not higher than 3000 degrees. It is impossible to make heat exchanger which sustain higher temperature.
At this temperature hydrogen molecules moves with average speed 5 km/s. Being thrown away from nozzle with this speed
hydrogen accelerates spacecraft not faster than 10 km/s if mass of hydrogen is 2/3 of total mass of spacecraft.
However in all cpacecrafts cascade scheme is used. This means that big engine accelerates small engine instead of
spacecraft. Small engine starts from big engine after big engine used up all its fuel. Small engine accelerates
very small engine. And then very small engine accelerates a spacecraft. So we have 10+10+10=30 km/s.
My idea is to use nuclear heatsource not for heating hydrogen but for heating itself and radiating this heat from
heatsource surface. Let's calculate... Today it is possible to obtain higly enriched uranium. Uranium gives 0.9 Mev/nucleon
of energy. Lets suppose than only 0.5 Mev/nucleon we can obtain. Nucleons weight is 1000 Mev. So if we through away 0.5
Mev of energy by heat radiation i.e. with speed of light we accelerate nucleon to 150 km/s. If mass of fuel is 2/3 of
total spacecraft mass like in previous example we have 100 km/s instead of 10 km/s. It is 10 times faster. So using of
cascade scheme gives 300 km/s or thousand part of speed of light. This means that 4000 years we need to achieve
nearest star.
