Adam P. Bruckner

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.

Experiments on picosecond pulse propagation in a diffuse medium

We present an experimental study of the pulse shape, delay, and spread of an optical pulse transmitted through a medium containing uniform latex microspheres using a mode-locked Nd:glass laser with a 20-psec pulse width. The delay of the arrival time of the peak intensity and the half-power pulse width for three different particle sizes are shown for an optical distance of up to 45.

Backscattering of a picosecond pulse from densely distributed scatterers

A theoretical and experimental study of the backscattering characteristics of a picosecond pulse scattered from a dense diffusing medium is presented. The theory uses a diffusion solution to the time-dependent equation of radiative transfer and the formulation of a picosecond range-gating technique. The experimental system consists of a high-power laser range-gating system, based on a picosecond Kerr-effect shutter. The results of experiments carried out on aqueous solutions of latex microspheres agree well with the theoretical calculations, not only in the pulse shape but in the relative magnitudes of the pulse height for different particle sizes and concentrations.

Picosecond light scattering measurements of cataract microstructure

A picosecond range-gated light scattering technique is used to measure the microstructure of cataracts in the living eye, with a resolution of the order of the wavelength of light. The cataract is illuminated with ultrashort pulses of light derived from a mode-locked Nd:glass laser. The pulses backscattered by the opacity are collected at several angles simultaneously by an array of fiber-optic light guides whose outputs are sequentially sampled by a multichannel picosecond Kerr shutter. This shutter transmits only that light which is scattered from the desired depth in the eye, while blocking from the detector any light scattered by foreground or background tissue and reducing multiple scattering effects. The size distribution of scatterers in the cataract is deduced from the angular distribution of scattering intensities by application of Mie theory. The results of experiments with suspensions of latex microspheres and with rabbit cataracts in vivo are presented. Cataract microstructure down to 0.5 microm has been measured in a rabbit eye and verified with electron microscopy.

Holocamera for 3-D micrography of the alert human eye

A holocamera that safely records holograms of the full depth of the alert human eye with a spatial resolution of ~20 microm is described. A single-mode argon-ion laser generating 2 W at 5145 A serves as the illuminating source. Holographic exposure times of 0.3 msec are achieved by means of a fail-safe electromechanical shutter system. Integrated retinal irradiance levels are well under the American National Standards Institute safety standards. Reconstructed real images are projected directly onto the vidicon faceplate of a closedcircuit TV system, enabling convenient scanning in the x-y-z dimensions of the reconstructed eyeball. Serially reconstructed holograms of cataractous rabbit eyes and normal human eyes are presented.

The ram accelerator - A chemically driven mass launcher

The ram accelerator, a chemically propelled mass driver, is presented as a viable new approach for directly launching acceleration-insensitive payloads into low earth orbit. The propulsion principle is similar to that of a conventional air-breathing ramjet. The cargo vehicle resembles the center-body of a ramjet and travels through a tube filled with a pre-mixed fuel and oxidizer mixture. The launch tube acts as the outer cowling of the ramjet and the combustion process travels with the vehicle. Two drive modes of the ram accelerator propulsion system are described, which when used in sequence are capable of accelerating the vehicle to as high as 10 km/sec. The requirements are examined for placing a 2000 kg vehicle into a 500 km orbit with a minimum of on-board rocket propellant for circularization maneuvers. It is shown that aerodynamic heating during atmospheric transit results in very little ablation of the nose. An indirect orbital insertion scenario is selected, utilizing a three step maneuver consisting of two burns and aerobraking. An on-board propulsion system using storable liquid propellants is chosen in order to minimize propellant mass requirements, and the use of a parking orbit below the desired final orbit is suggested as a means to increase the flexibility of the mass launch concept. A vehicle design using composite materials is proposed that will best meet the structural requirements, and a preliminary launch tube design is presented.

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.