We have been relying on the same outdated technology of solid fuel propellants witch date back to china`s first gunpowder rockets. I believe that the answer lies in magnetic and electrical energy. If we were to charge particles and rotate them until they nearly reach light speed as they do to split atoms. They then could be released and directed the force would be great but with very little mass to start with. I have also heard as a particle reaches such high seed that it actually weighs more can any one confirm this