Propulsion system sizing methodology

Abstract

The simplified rocket equation ΔV = g × Isp × Ln(W total/W burnout) has two very important variables; propellant specific impulse and propellant mass fraction. With this equation, the delta velocity capability for rocket propulsion systems can be predicted during the conceptual phase of program definition. At Pratt & Whitney Rocketdyne (PWR) a methodology and system analysis process has been developed that combines System Design, Combustion Analysis, Structural Analysis, and Materials Engineering. With this process the cycle time for analysis and component definition for the propulsion system is greatly reduced. The variables which influence the calculation of propellant specific impulse and propellant mass fraction are incorporated in this process. This spreadsheet (Microsoft™ Excel) based process integrates tens of thousands of lines of NASA's "Chemical Equilibrium and Transport" FORTRAN code developed for calculating combustion and exhaust properties, C-Star, and propellant specific impulse. This greatly improves the accuracy over the "frozen state" ideal case (i.e., Sutton equations). Propellant and pressurization feed systems and detailed components are sized for accurate weight predictions, a necessity for predicting realistic propellant mass fraction. This optimization process is used to determine sensitivities between key performance parameters. Parameters such as mixture ratio, expansion ratio, and system operating pressure, can be traded against system performance (i.e., delta velocity, system weight, and acceleration. This process is anchored to an extensive database of liquid propulsion systems and components that have been developed to flight worthiness at PWR over the past 20 years. There is also a link to a generic 3-D computer aided design (CAD) model. Key geometric parameters are passed between the Excel spreadsheet and the parametric 3-D solid model. This versatility provides interface and envelope evaluations plus detailed mass properties. Finally, there is an existing component database that can override the optimization to determine the overall performance impact of using high Technology Readiness Level (TRL) components. The key performance parameters of the existing propulsion system components (i.e., thrusters, propellant tanks, pressurant tanks, regulators, etc.) are used to determine the influence on performance from optimum.