Abstract: A low-cost rocket or thruster has a low-cost propellant injector, in which fluid fuel and oxidizer are injected into a combustion chamber. The walls of the combustion chamber are protected from the high temperatures of the combustion by a grain of solid propellant, the surface of which tends to melt andor vaporize in the presence of combustion temperatures, and thereby protects the walls of the chamber. The low-cost propellant injector may not mix the fluid fuel and oxidizer effectively, so that pockets of noncombusted gas may occur within the chamber. The ratio of fluid fuel and oxidizer is selected to be slightly oxidizer-rich, so that any pockets of unburned gas tend to be oxygen-rich. When the pockets come into contact with the solid fuel, the excess oxygen combusts with the gaseous solid fuel, and when the mixture is near stoichiometric, the fluid fuel combusts.