When you want to spew out a raging fire, you have to solve a very difficult problem - how does the fire start? This problem not only disrupted the evolution of biological fire -breathing functions, but also plagued aerospace engineers for a long time. From the following examples, you can see that it is only more difficult for creatures to breathe fire than spaceflight. Long before the birth of modern rockets, Russian Konstantin Tsiolkovsky calculated that liquid oxygen + liquid hydrogen is the most efficient chemical energy fuel . But people have...