Investigation of the aerothermodynamics of hypervelocity reacting flows in the ram accelerator

New diagnostic techniques for measuring the high pressure flow fields associated with high velocity ram accelerator propulsive modes was experimentally investigated. Individual propulsive modes are distinguished by their operating Mach number range and the manner in which the combustion process is initiated and stabilized. Operation of the thermally choked ram accelerator mode begins by injecting the projectile into the accelerator tube at a prescribed entrance velocity by means of a conventional light gas gun. A specially designed obturator, which is used to seal the bore of the gun, plays a key role in the ignition of the propellant gases in the subsonic combustion mode of the ram accelerator. Once ignited, the combustion process travels with the projectile and releases enough heat to thermally choke the flow within several tube diameters behind it, thereby stabilizing a high pressure zone on the rear of the projectile. When the accelerating projectile approaches the Chapman-Jouguet detonation speed of the propellant mixture, the combustion region is observed to move up onto the afterbody of the projectile as the pressure field evolves to a distinctively different form that implies the presence of supersonic combustion processes. Eventually, a high enough Mach number is reached that the ram effect is sufficient to cause the combustion process to occur entirely on the body. Propulsive cycles utilizing on-body heat release can be established either by continuously accelerating the projectile in a single propellant mixture from low initial in-tube Mach numbers (M less than 4) or by injecting the projectile at a speed above the propellant's Chapman-Jouguet detonation speed. The results of experimental and theoretical explorations of ram accelerator gas dynamic phenomena and the effectiveness of the new diagnostic techniques are presented in this report

Investigation of advanced propulsion technologies: The RAM accelerator and the flowing gas radiation heater

The two principal areas of advanced propulsion investigated are the ram accelerator and the flowing gas radiation heater. The concept of the ram accelerator is presented as a hypervelocity launcher for large-scale aeroballistic range applications in hypersonics and aerothermodynamics research. The ram accelerator is an in-bore ramjet device in which a projectile shaped like the centerbody of a supersonic ramjet is propelled in a stationary tube filled with a tailored combustible gas mixture. Combustion on and behind the projectile generates thrust which accelerates it to very high velocities. The acceleration can be tailored for the 'soft launch' of instrumented models. The distinctive reacting flow phenomena that have been observed in the ram accelerator are relevant to the aerothermodynamic processes in airbreathing hypersonic propulsion systems and are useful for validating sophisticated CFD codes. The recently demonstrated scalability of the device and the ability to control the rate of acceleration offer unique opportunities for the use of the ram accelerator as a large-scale hypersonic ground test facility. The flowing gas radiation receiver is a novel concept for using solar energy to heat a working fluid for space power or propulsion. Focused solar radiation is absorbed directly in a working gas, rather than by heat transfer through a solid surface. Previous theoretical analysis had demonstrated that radiation trapping reduces energy loss compared to that of blackbody receivers, and enables higher efficiencies and higher peak temperatures. An experiment was carried out to measure the temperature profile of an infrared-active gas and demonstrate the effect of radiation trapping. The success of this effort validates analytical models of heat transfer in this receiver, and confirms the potential of this approach for achieving high efficiency space power and propulsion

Unsteady 1-D thrust modeling with EOS effects for ram accelerator experiments at different bores

Advances made to the unsteady, one-dimensional (1-D) modeling of the thermally choked ram accelerator thrust-Mach number characteristics include the use of real-gas equations of state to account for the compressibility effects of the combustion products. Equations of state based on generalized empirical and theoretical considerations are incorporated into a 1-D computer code to predict the combustion end state equilibrium conditions when the propellant starts out a relatively high fill pressure (>2.5 MPa) and the projectile acceleration exceeds 100 km/s2. The objective of this work is to improve the unsteady 1-D model as a useful tool to predict the thrust of the thermally choked ram accelerator propulsive mode by utilizing key results from the more computationally intensive 2-D or 3-D simulations. New thrust-Mach number calculations compared with experimental data from 25-mm, 30-mm, 38-mm, 90-mm, and 120-mm-bore experiments are generally in good agreement until the point where enhanced accelerations are observed, presumably due to projectile material combustion. The results of this investigation indicate the need for more research on ram accelerator flow fields and the role projectile material may play in the combustion process

High Efficiency Energy Conversion Systems for Liquid Nitrogen Automobiles

This investigation of the use of cryogens as energy storage media for zero emission vehicles has found that using liquid nitrogen to liquefy the working fluids of one or more closed Rankine power cycles can be an effective means for increasing motive power. System configurations are presented which can realize a specific energy greater than 400 kJ/kg-LN2 (110 W-hr/kg-LN2) without relying on isothermal expanders. A zero emission vehicle utilizing such a propulsion system would have an energy storage reservoir that can be refilled in a matter of minutes and a range comparable to that of a conventional automobile

Compressibility effects of unreacted propellant on thermally choked ram accelerator performance

Thrust calculations of the thermally choked ram accelerator propulsive mode based on quasi-steady, one-dimensional modeling of the flow process have been quite successful in predicting the experimental velocity-distance profile when real gas corrections are applied to the combustion products of propellants at initial fill pressures up to 8 MPa. A further refinement of the modeling takes into account real gas corrections for the initial state at higher fill pressures. It turns out that the Redlich-Kwong equation of state accurately determines the thermodynamic properties of the unreacted propellant for fill pressures up to at least 20 MPa. Using this equation of state for the calculation of the sound speed for a typical ${\rm CH}_4/{\rm O}_2/{\rm N}_2$ propellant provides a 15% higher value at 20 MPa than that predicted for an ideal gas; this increase significantly affects the operating characteristics of the ram accelerator at a given velocity. The corresponding thrust maximum increases by 30%. This corrected theory is most appropriate under conditions of high pressure operation at relatively low acceleration levels; i.e., less than 10 000 g. The corrections to the aerothermodynamic equations that are discussed in this paper are fully generalized and can be applied using any equation of state

Ram accelerator operating characteristics at elevated fill pressures

An experimental investigation of the starting and operational characteristics of a 38-mm bore ram accelerator with propellant fill pressures greater than 5 MPa is in progress. Successful starts with 60 - 124 gm projectiles have been achieved using methane/oxygen/nitrogen propellants at fil1 pressures ranging from 6 to 15 MPa. At fill pressures of 8.5 MPa and above, it was found that projectiles having the nominal throat diameter (29 mm) required a minimum entrance velocity of 1250m/s, which is about 100m/s faster than the minimum needed to successfully start the ram accelerator at fill pressures below 7.5 MPa. Projectiles with a reduced throat diameter (25 mm) and a solid magnesium nose cone were successfully started at fill pressures up to 15 MPa with entrance velocities around 1300 m/s. The average accelerations achieved using high pressure stages were in general less than predicted for thermally choked operation by the Boltzmann equation of state

Investigation of obturator and ignitor effects on low velocity starting of the ram accelerator

An experimental investigation of the effects of obturator geometry, propellant chemistry, and onboard pyrotechnic ignitor systerns on the starting characteristics of the ram accelerator at low launch velocity has been conducted at the University of Washington 38-mm-bore facility. The ram accelerator was successfully started at entrance velocities as low as 760 m/s using stoichiometric methane/oxygen propellants with various levels of carbon dioxide dilution and obturator configurations at a fill pressure of 2.5 MPa. Experiments using an onboard pyrotechnic ignitor dernonstrated that ignition of the propellant and starting of the ram accelerator could occur without the presence of the obturator-driven normal shock