Systems and methods for subscale testing of rocket engine injector stability. The system includes a combustion chamber with telescoping throat that is continuously axially moveable via an actuator. A modular injector plate comprises one or more first rocket engine injector elements configured to inject one or more propellants, such as a fuel and an oxidizer, into the chamber. The injector plate and/or the telescoping throat may be continuously translated, to thereby continuously vary a combustion volume of the chamber and create a dynamically tunable downstream boundary. The injectors are thus exposed to acoustic modes of varying frequency, covering the range of acoustic modes expected in a full scale rocket engine. The injector plate is removably attached to an upstream end of the chamber for replacement of the first injectors with different, second injectors for subsequent testing.