Influence of the Combustion Chamber Geometry on the Performance of a Gel Propulsion System

Abstract

Gelled fluids are homogeneous mixtures of a liquid and a gelling agent and behave like a solid at rest but can be liquefied if a sufficiently high shear stress is applied. Over the past years a growing interest in gelled propellants for rocket and ramjet propulsion applications is observable worldwide because of safety and performance benefits. Due to the non-Newtonian flow behavior of their propel-lant, gel rocket motors combine the advantages of liquid engines e.g. throttling and restart capabilities with and simple handling and storage characteris-tics typical for solid motors. In Germany, within the German Gel Propulsion Technology Program (GGTP), which was started in 2001, necessary technologies to build a green gel propellant rocket engine are developed. Using the latest develop-ment of a gelled monopropellant, investigations on the performance, combustion pressure and the required combustor size thus residence time have been carried out. Capacitively cooled stainless steel combustion chambers with an inner diameter of 20.6 mm and a variable length of 80 mm up to 400 mm have been used. In combination with different nozzles featur-ing a throat diameter of either 5 mm or 6 mm a characteristic length L* (chamber volume to throat area ratio, Vc/At) between 1 m and 7 m could be realized. During tests, the mass flow rate has been varied between 0.058 kg/s and 0.124 kg/s resulting in a combustion pressure of up to 9 MPa. The re-sults obtained comprise the combustion perfor-mance (characteristic velocity, c*) as well as the operational envelope for a self-sustaining combus-tion for the current gel propellant and injector type. The experimental data serves as reference for future test and flight combustion chambers and as base for the development of design and scaling rules for gel rocket motors.