Some of the ram accelerator research topics being investigated at the University of Washington include:
- Low Velocity Starting
- Ventless Launch Tube Operation
- High Velocity Operating Characteristics
- Superdetonative Propulsion Mode
- High Pressure Operation
- Real Gas Effects on Performance Modeling
- Unsteady Effects on Performance Modeling
- Baffled Tube Ram Accelerator
Low Velocity Starting
Ram accelerator operation with low projectile entrance velocity (700 to 1000 m/s) is under investigation. Reducing the minimum entrance velocity allows relatively simple pre-launcher technologies to be applied for large-bore launchers. The primary variables that impact the "starting" of the ram accelerator at low Mach number are propellant mixture and projectile geometry. A successful "ram accelerator start" is defined as obtaining supersonic flow past the projectile throat while initiating and stabilizing combustion behind its base. Theoretical and computational studies outline the potential influence of propellant mixture and projectile geometry selection for these experiments. Results of various low velocity start experiments are provided in the publications.
Ventless Launch Tube Operation
Normal operation of the ram accelerator at the University of Washington involves the use of a perforated tube to vent the gun gases into an evacuated dump tank before the projectile enters the ram accelerator tubes. The need for a large dump tank and the equipment to evacuate it makes this venting process impractical for some potential applications of the ram accelerator. Experiments have shown that ventless operation is possible when gas gun blowby is minimized, gas gun barrel is highly evacuated, and if thick entrance diaphragms are used. Results of these experiments are available in several publications.
High Velocity Operating Characteristics
Operating characteristics of the ram accelerator thermally choked propulsive mode at high velocities is under investigation. Thermally choked propulsion occurs in subdetonative velocity regime where the ram projectile is moving faster than the sound speed of the propellant, but slower than its Chapman-Jouguet detonation speed (CJ speed). Earlier research in this velocity regime was performed in 1994 and 1995 by Imrich, wherein he systematically varied the projectile geometry to determine an optimum shape that would allow the projectile to be accelerated to near the detonation speed while minimizing the projectile's mass. Assuming the thrust is constant, this maximizes the projectile acceleration and, hence, its velocity in a fixed-length launcher. In other research, a standard projectile is accelerated through a variety of propellant mixtures to determine what combination of fuel, oxidizer and diluent produces the greatest thrust and widest operating Mach number range. Results of these studies have been published.
Superdetonative Propulsion Mode
Preliminary experiments with projectiles fabricated from aluminum and titanium alloys have demonstrated that acceleration is possible at velocities greater than the CJ speed of a gaseous propellant mixture. Projectile materials were found to play a significant role in these experiments. Theoretical modeling was successful in predicting projectile drag in nonreactive gas mixtures at hypersonic velocities. When this drag was subtracted from the ideal thrust of a supersonic combustion ram accelerator, the net thrust closely matched that measured in the experiments. The dependence of the maximum operating Mach number on both the projectile diameter and propellant heat release was examined. The peak velocity capability of the projectile geometry routinely used in the experiments is predicted to be about 1.5 times the CJ speed of the propellant mixture. This experimentally verified ram accelerator performance model indicates that the drag resulting from an increase in projectile diameter was more than offset by the corresponding enhancement in thrust, and that velocities of nearly twice the CJ speed are possible.
High Pressure Ram Accelerator Operation
High pressure operation is distinguished by the combustion products being at such elevated pressures that real gas effects are significant and direct performance scaling from lower pressure shots ceases to be accurate. Typically this occurs when fill pressures exceed 50 atm. Successful ram accelerator operation has been demonstrated at the UW with fill pressures up to 200 atm. The chronology of this experimental development is reported in the publications.
Performance Analysis Code Incorporating Real Gas Effects
A control volume analysis code was developed which incorporates a generalized equation of state to model the flow conditions inside the ram accelerator. Most prior performance analyses were conducted with 1-D codes using an ideal equation of state. However, the ram accelerator operates at higher pressures where the ideal gas equations are no longer valid. Not surprisingly, these codes typically underpredict the experimental results for the projectile's thrust and velocity. It has been recently demonstrated by other researchers that using a high pressure equation of state will provide excellent performance characteristics when compared with experiments. The code currently includes the following equations of state: ideal gas, Boltzmann, Percus-Yevick, and a Virial Expansion in which the Virial coefficients are obtained by the Lennard-Jones and Stockmayer potentials.
Performance Analysis Code Incorporating Unsteady Effects
When the ratio of propellant-to-projectile density is relatively high (i.e., > 0.2), the projectile experiences very high accelerations (> 30 kgee). Under these conditions, the acceleration of gas mass around the projectile must be accounted for in order to accurately predict the thrust in the thermally choked propulsive mode. Such modeling has successfully been accomplished and the results published (Bundy et al. 2004).
Baffled Tube Ram Accelerator
This is the newest and highest performing mode of ram accelerator propulsion yet proposed for gaseous propellants. A report of invention has been filed and new funding from WA state JCATI-WARP program has been provided to determine its effective operating envelope. Experimental results will be provided soon (by summer 2015).