名詞 step rocket:

1. a rocket having two or more rocket engines (each with its own fuel) that are fired in succession and jettisoned when the fuel is exhausted
   同義詞：multistage rocket

1. One of the ways to make step rockets is to put the stages side by side at the bottom.
   制造多級(jí)火箭的一種方法是把各級(jí)火箭并排放在底部。