Complex sentence: A spacecraft's engines are defined only by the amount of thrust they put out and amount of fuel they use, allowing anything from solar sails to conventional rocket engines to futuristic nuclear fission and fusion drives to be simulated.
Split into simpler sentences: A spacecraft's engines are defined only by the amount of thrust they put out and amount of fuel they use. This allows anything from solar sails to conventional rocket engines, to futuristic nuclear fission and fusion drives to be simulated.