The Nuclear Engine has a ISP of 800, meaning that 1 litre of 'fuel', (Liquid Fuel and Oxidiser mixed in the ratio 0.9:1.1) be propelled 800 arbitory distance units from the reaction chamber. If two Nuclear Engines are sharing the same fuel tank, then what is the total ISP, and thrust, of them both? My thinking currently follows that: 1 litre of 'fuel' must be split between 2 engines for the same ISP, therefore the ISP of each engine is halved, but the thrust is added. So the engines together would have an ISP of 800, yet a thrust of 100kN. What would happen if more engines were added?