A liquid propelled rocket engine comprising a unique pintle injector, radial injector plate and ablative insert to improve combustion stability and reduce harmonic disturbance within the chamber. The injector plate is generally in the shape of a ring surrounding a combination spud and pintle injector, the two allowing at least one liquid propellant and one liquid oxidizer to be injected into the chamber. The injector plate has an array of injectors for injecting liquid propellant into the chamber and a series of cavities within allowing the harmonic disturbance to be damped and providing a more stable combustion environment. The number of cavities and the unique pintle injector's length and width are chosen to reduce harmonic disturbance in the longitudinal, radial and lateral modes. A method for designing engine components and tuning harmonic frequencies within a combustion chamber is also disclosed.