The specific impulse of an open-cycle gas generator engine such as Vulcain is inherently limited, while for the closed-cycle, staged-combustion design, it increases with the combustion pressure.

• 一个火箭引擎的总体效率由其特定推力来衡量，推力（动力改变-或单摆，发动机速度提升）能完成燃料的每个单元燃料（的燃烧）。