A hybrid rocket motor includes a storage tank which stores an oxidizer under relatively low pressure, a turbopump preferably directly coupled to an outlet of the storage tank which pressurizes the oxidizer to a relatively high pressure, a combustion chamber including a solid fuel, and an injector between the turbopump and combustion chamber through which the oxidizer is injected into the combustion chamber. According to a preferred aspect of the invention, the turbopump is operated by an expander cycle of a heat exchanger. According to another preferred aspect of the invention, the fluid flowing through the heat exchanger is oxidizer tapped from the storage tank. A barrier is maintained between an oxidizer feed line from the turbopump and the injector until sufficient pressure is created by the turbopump to pump the oxidizer at the requisite pressure into the injector.