Disclosed is a pneumatically powered high-pressure and lightweight fluid pump. The pump is useful for pumping fuel for liquid rocket engines and for pumping water, such as for fire suppression. During operation of the pump, liquid is drained from a tank into a pump chamber and the chamber is then pressurized to deliver fluid. The chamber is then refilled from the main tank. An auxiliary chamber supplies fuel while the main chamber is being filled, thereby a steady stream is delivered from the pump. The auxiliary chamber is refilled from the tank while the main chamber is delivering fluid. The design results in substantial weight savings over a system in which the main tank is pressurized or a system with two pump chambers of similar size. The auxiliary chamber of the present disclosure has a smaller capacity than the main chamber. The main chamber is designed to be filled much faster than it is emptied, therefore the smaller auxiliary chamber supplies fluid only during the time while the larger main chamber is being filled.