The term 'rocket engine' is composed of two crossword solutions: 'RETRO' (5 letters) and 'BOOSTER' (7 letters). A rocket engine is a type of propulsion system that uses stored propellants to generate thrust in the form of hot gases. RETRO rockets are used to slow down a spacecraft, while boosters are used to increase the speed of a spacecraft. Together, these two components make up a rocket engine, which is used to propel a spacecraft through space.