Complex sentence: A gas generator usually refers to a device, often similar to a solid rocket or a liquid rocket that burns to produce large volumes of relatively cool gas, instead of maximizing the temperature and specific impulse.
Split into simpler sentences: A gas generator usually refers to a device, often similar to a solid rocket, that burns to produce large volumes of gas. It is similar to a solid rocket, but is usually designed to produce large volumes of cool gas, instead of maximizing the temperature and specific impulse.