火箭推进器一般以某种形式大量存储在推进器容器里，过去被用来大量从火箭发动机喷射出以流体喷射物的形式，以产生推力作为推进。燃料推进剂往往与氧化剂推进剂燃烧产生大量非常热的气体。这些气体膨胀并从喷嘴喷出，不断加速，从火箭底部冲出产生推力直到火箭达到极高的速度。有时推进剂不会燃烧，但可以从外部加热都达到更好的效果。对于较小的实验推进器，使用压缩气体通过推进喷嘴喷出以推动飞船。

化学火箭推进剂是最常用的，火箭通过放热化学反应产生热气体达到推进目的。

在离子推进器中，推进剂是带电的原子，以磁性排斥的方式从飞船尾部推出。然而磁加速离子驱动器通常不被认为是火箭，而是一个使用电加热和磁喷嘴的类似级推进器。

Rocket propellant is a material used by a rocket as, or to produce in a chemical reaction, the reaction mass (propulsive mass) that is ejected, typically with very high speed, from a rocket engine to produce thrust, and thus provide spacecraft propulsion. Each rocket type requires different kind of propellant: chemical rockets require propellants capable of undergoing exothermic chemical reactions, which provide the energy to accelerate the resulting gases through the nozzle. Thermal rockets instead use inert propellants of low molecular weight that are chemically compatible with the heating mechanism at high temperatures, while cold gas thrusters use pressurized, easily stored inert gases. Electric propulsion requires propellants that are easily ionized or made into plasma, and in the extreme case of nuclear pulse propulsion the propellant consists of debris from nuclear explosions.