Effects of unsteady flow and real gas equations of state on high pressure ram accelerator operation

An experimental and theoretical investigation of the conditions which enable thermally choked ram acceleration at fill pressures greater than 5 MPa is presented.; A set of experimental parameters was determined which enabled projectiles to be accelerated continuously in propellants at 20 MPa for distances up to 4 m. The operating conditions which permit thermally choked operation at 20 MPa are considerably different from those at 5 MPa and below; the effects of initial velocity, propellant composition, projectile design, and obturator design on high pressure operation were investigated. During thermally choked operation at high pressure, the velocity-distance profile is overpredicted by a quasi-steady control volume approach for thrust determination.; A revision to the control volume model accounting for unsteady flow effects was developed and presented here. The unsteady model indicates that the thrust coefficient-Mach number profile obtained for high pressure conditions is consistently lower than that obtained with the quasi-steady model, due to unsteady momentum transfer to the gas in the control volume surrounding the projectile, an effect considered negligible at 5 MPa and below. This analytical deviation correlates with high pressure experimental results. When the unsteady model incorporates the heat release behavior predicted for a real gas equation of state, good agreement is obtained with experimental velocity-distance data. A model for predicting sonic diffuser unstart under unsteady flow conditions is also presented; the model predictions agree well with experimental results.; Both models indicate that the mass of fluid in a control volume, when on the order of the mass of the surrounding system, has a significant effect on the body forces acting on the system under unsteady flow conditions. The unsteady models quantify this effect by showing that thrust in an unsteady flow propulsion system is not directly proportional to pressure, and that supersonic diffusers unstart at a higher velocity when decelerating. These models are applicable to other supersonic internal flow systems, such as ramjet engines and supersonic diffusers and nozzles.