New similarity principles are introduced to facilitate the design of a new liquid propellant combustion chamber providing that data of a well designed motor is available. Deduction of these principles is based on the defining of six new factors and the numerical analysis of the droplet combustion data The investigated fuels are Hydrazine, Hydrazine Hydrate, Monomethyl Hydrazine, Unsymmetric Dinitthyl Hydrazine and Kerosene; with Nitric Acid as an Oxidizer. The dimensions of combustors burning each of the Hydrazine's fuels are related to the dimensions of combustors burning Kerosine of the same droplet size. A design procedure involving these principles is introduced to develop an existing motor or to change its fuel.