C. Knowlen

Operational characteristics of the thermally choked ram accelerator

Operational characteristics of the thermally choked ram accelerator, a ramjet-in-tube device for accelerating projectiles to ultrahigh velocities, are investigated theoretically and experimentally. The projectile resembles the centerbody of a conventional ramjet and travels through a stationary tube filled with premixed gaseous fuel and oxidizer at high pressure. The combustion process travels with the projectile, its thermal choking producing a pressure field which results in thrust on the projectile. The results of experiments with 45-75 gm projectiles in a 12.2 m long, 38 mm bore accelerator, using methane-based propellant mixtures, are presented in the velocity range of 1150-2350 m/s. Acceleration of projectiles with staged propellants and transitions between different mixtures are investigated and the velocity limits in several propellant mixtures are explored. Agreement between theory and experiment is found to be very good.

Experimental investigation of ram accelerator propulsion modes

Experimental investigations on the propulsive modes of the ram accelerator are reviewed in this paper. The ram accelerator is a ramjet-in-tube projectile accelerator whose principle of operation is similar to that of a supersonic air-breathing ramjet. The projectile resembles the centerbody of a ramjet and travels through a stationary tube filled with a premixed gaseous fuel and oxidizer mixture. The combustion process travels with the projectile, generating a pressure distribution which produces forward thrust on the projectile. Several modes of ram accelerator operation are possible which are distinguished by their operating velocity range and the manner in which the combustion process is initiated and stabilized. Propulsive cycles utilizing subsonic, thermally choked combustion theoretically allow projectiles to be accelerated to the Chapman-Jouguet(C-J) detonation speed of a gaseous propellant mixture. In the superdetonative velocity range, the projectile is accelerated while always traveling faster than the C-J speed, and in the transdetonative regime (85–115 % of C-J speed) the projectile makes a smooth transition from a subdetonative to a superdetonative propulsive mode. This paper examines operation in these three regimes of flow using methane and ethylene based propellant mixtures in a 16 m long, 38 mm bore ram accelerator using 45–90 g projectiles at velocities up to 2500 m/s.

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.

Ram Accelerator Operating Limits, Part 1: Identification of Limits

Operational limits of the thermally choked ram accelerator are investigated. A quasisteady, one-dimensional model of the ram accelerator predicts it should be able to operate when the projectile Mach number is sufficient to maintain supersonic flow past the projectile throat and the heat release is sufficient to stabilize a normal shock upon the projectile body, but not enough to drive the shock over the projectile throat. These limits to operation can be expressed as relations of Mach number and heat release Q, and together they define a theoretical envelope of operation in the Q-M plane. The corresponding experimental envelope was investigated by injecting projectiles at different Mach numbers into methane/oxygen/ nitrogen and hydrogen/oxygen/methane mixtures at 25 and 50 atm, respectively. The results indicated a broad range of mixtures that were able to accelerate the projectile through the Chapman-Jouguet (CJ) detonation speed of the mixture. In the more energetic mixtures, the normal shock wave surged forward and immediately unstarted the projectile as it entered the test section. Unstarts were also observed when the projectile was accelerated beyond the CJ detonation speed, but because of the projectiles long intube residence time, these unstarts are believed to have been structural, not gasdynamic in nature.

Initiation of combustion in the thermally choked ram accelerator

The methodology for initiating stable combustion in a ram accelerator operating in the thermally choked mode is presented in this paper. The ram accelerator is a high velocity ramjet-in-tube projectile launcher whose principle of operation is similar to that of an airbreathing ramjet. The subcaliber projectile travels supersonically through a stationary tube filled with a premixed combustible gas mixture. In the thermally choked propulsion mode subsonic combustion takes place behind the base of the projectile and leads to thermal choking, which stabilizes a normal shock system on the projectile, thus producing forward thrust. Projectiles with masses in the 45-90 g range have been accelerated to velocities up to 2650 m/sec in a 38 mm bore, 16 m long accelerator tube. Operation of the ram accelerator is started by injecting the projectile into the accelerator tube at velocities in the 700 - 1300 m/sec range by means of a conventional gas gun. A specially designed obturator, which seals the bore of the gun during this initial acceleration, enters the ram accelerator together with the projectile. The interaction of the obturator with the propellant gas ignites the gas mixture and establishes stable combustion behind the projectile.

Overview of the subdetonative ram accelerator starting process

The ram accelerator requires a conventional gun to initially boost the projectile to supersonic entrance velocity. An experimental investigation has been undertaken to improve the understanding of transition from the conventional gun to the ram accelerator and initiation of the thermally choked propulsive mode, referred to as the starting process. Developing a robust starting process is instrumental for utilizing the ram accelerator in a variety of applications. Four possible outcomes of a start attempt have been identified. A successful start is achieved when supersonic flow is maintained throughout the diffuser, and the normal shock system is stabilized on the projectile body through propellant energy release. A sonic diffuser unstart is caused by conditions upstream of the throat resulting in subsonic flow in the diffuser. A wave fall-off occurs when insufficient energy is released from the propellant to keep the shock system on the body from receding behind the base. A wave unstart is caused by conditions downstream of the throat resulting in disgorgment of the shock system on the body into the diffuser. Experimental results are presented, along with a discussion of the factors involved that determine which of these outcomes actually occurs.

Ram accelerator operation in the superdetonative velocity regime

Preliminary experiments on superdetonative propulsive cycles at a ram accelerator facility using projectiles fabricated from aluminum and titanium alloys have demonstrated that acceleration is possible at velocities greater than the Chapman-Jouguet (CJ) detonation 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 experimental projectile geometry is predicted to be about 1.5 times the CJ speed of the propellant mixture. It was found 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. (Author)

Unsteady effects on ram accelerator operation at elevated fill pressures

Experiments show that as the projectile acceleration of the ram accelerator is increased, by increasing the propellant fill pressure or reducing the projectile mass, its performance begins to deviate significantly from that predicted by the widely used quasi-steady control volume model. At high fill pressures, experimental velocity‐ distance data are overpredicted by the quasi-steady model for thrust determination when using a real-gas equation of state for the combustion products. The primary reason for this behavior is that the mass of the propellant accumulating in the control volume at high fill pressure approaches the mass of the projectile itself. A revision to the control volume model to account for unsteady flow effects indicates that the thrust coefficient vs Mach-number profile obtained for high-pressure conditions is consistently lower than that obtained with the quasi-steady model. This deviation correlates with experimental results obtained in a 38-mm-bore ram accelerator at fill pressures in the range of 15‐20 MPa. The best agreement with high-pressure experimental data is obtained using the unsteady modeling approach in conjunction with the heat release vs Mach-number profile calculated using the Boltzmann equation of state.

Ram Accelerator Operating Limits, Part 2: Nature of Observed Limits

The causes of unstarts, which limit the operation of ram accelerators, are investigated. Prior experiments have identified distinct limits as functions of the projectile Mach number and the energy content of the propellant mixture. When a ram accelerator projectile enters a very energetic mixture, a normal shock is immediately driven in front of it. The mechanism of this unstart was examined by stripping the driving combustion wave from the projectile before it entered the test mixture. The projectile in this case was able to coast supersonically without unstart, which suggests the unstart mechanism is a surge of the driving combustion wave past the projectile throat. In less energetic mixtures, projectiles are able to accelerate beyond the detonation speed of the mixture before unstarting. This unstart mechanism was investigated by changing the projectile material from Mg to Al and from Al to Ti, which allowed the projectile to accelerate to even higher Mach numbers. These results show the unstart mechanism in these cases to be a projectile structural effect, not a gasdynamic phenomenon. An experiment with an all-Ti projectile was able to withstand the intense heat transfer environment in the ram accelerator.

Applications of the ram accelerator to hypervelocity aerothermodynamic testing

A ram accelerator used as a hypervelocity launcher for large-scale aeroballistic range applications in hypersonics and aerodynamics research is presented. It is an in-bore ramjet device in which a projectile shaped like the centerbody of a supersonic ramjet is propelled down a stationary tube filled with a tailored combustible gas mixture. Ram accelerator operation has been demonstrated at 39 mm and 90 mm bores, supporting the proposition that this launcher concept can be scaled up to very large bore diameters of the order of 30-60 cm. It is concluded that high quality data obtained from the tube wall and projectile during the aceleration process itself are very useful for understanding aerothermodynamics of hypersonic flow in general, and for providing important CFD validation benchmarks.