The effect of roughness on hypersonic boundarylayer transition has been studied for three primary purposes: to trip a laminar layer to turbulence, to determine whether naturally occurring roughness is expected to cause early transition, and to determine the largest allowable roughness that will not affect the location of transition. Roughness is often divided into two classes: isolated roughness in which each protuberance can be considered separately, and distributed roughness similar to sandpaper, in which the roughness elements are many and are not considered separately. The effects of roughness on hypersonic transition are reviewed, considering the physics of the process, known parametric effects, some of the common correlations, and a few case studies. The three or more modes by which roughness can affect transition are outlined. At hypersonic edge Mach nunbers, it requires very large roughness heights to affect transition. Various correlations are often used to estimate the effect of roughness; several of these are described, although none provide good agreement with all the data.

Effects of Roughness on Hypersonic Boundary-Layer Transition

A novel technique for generation of a high flux of energetic (nominal ~5 eV) oxygen atoms has been demonstrated in the laboratory. The generation technique involves a laser-induced breakdown of molecular oxygen followed by a rapid expansion of the recombining plasma, resulting in the production of a flux of nearly monoenergetic oxygen atoms. We have developed consistently high velocity streams of oxygen atoms in an evacuated hypersonic nozzle. The oxygen flow is introduced into the nozzle through the mediation of a fast (~ 100 IJLS) pulsing valve and broken down with a CO2 laser. We have measured average oxygen atom velocities of ~ 5 to 13 km/s with an estimated total production of 10 18 atoms per pulse over pulse durations of several jis. Although these were single pulse measurements, scaling to a repetitively pulsed device appears straightforward. The O-atom source was used to irradiate sample targets, including Al, Fe, and polyethylene, and target-materialspecific radiation was observed above the target surfaces, indicating energetic O-atom induced mass removal.

A high flux source of energetic oxygen atoms for material degradation studies

: A laser propelled Lightcraft vehicle has been successfully flown in a series of experiments conducted at the High Energy Laser Systems Test Facility (HELSTF), White Sands Missile Range (WSMR), NM. The flight tests, conducted under a joint USAF/NASA flight demonstration program, used a single optimized geometry scaled over a range of sizes designed to fly on the 10 kW Pulsed Laser Vulnerability Test System (PLVTS) pulsed carbon dioxide laser. The axisymmetric Lightcraft vehicles were propelled by airbreathing, pulsed-detonation engines with an infinite fuel specific impulse. Schlieren and shadowgraph pictures were taken as a function of time, and laser beam propagation studies were conducted with different focal length telescopes. Spin-stabilized free-flight launches outside the laboratory have been accomplished to altitudes approaching 30 m (100 ft).

Ground and Flight Tests of a Laser Propelled Vehicle

Recent experimental data have indicated that when highly reflective metal targets are irradiated by pulsed lasers in an air environment, a larger fraction of the incident laser energy is coupled into the target when a plasma is ignited above the surface than when plasma formation does not occur. Models of the plasmadynamics and subsequent energy transfer to the surface by radiation and conduction are constructed in order to predict the thermal coupling coefficient (coupled energy/laser energy) for pulsed laser irradiation of a metal target. It is found that the largest fraction of the incident laser energy is coupled into the target when a laser-supported combustion wave is ignited in close proximity to the surface. Energy transfer is via reradiation from the absorbing air plasma. Techniques for obtaining maximum thermal coupling are suggested as a result of the theoretical models. Results are compared with both computer simulations of the plasmadynamics and existing experimental data.

Plasma Energy Transfer to Metal Surfaces Irradiated by Pulsed Lasers

Laser propulsion, a unique new system in which thrust is produced by the absorption of remote laser energy, has several distinct advantages over existing forms of propulsion. This article is intended as an overview of the status of laser propulsion based on a review of the scientific literature. Included in the discussion is an evaluation of the advantages and limitations of laser propulsion, as well as the types of missions for which it can best be used. The various types of proposed thrusters are also discussed, focusing on operation, current research, and unresolved problems. Also included is a thorough list of references documenting the research in this relatively young field.

Concepts and status of laser-supported rocket propulsion

A fluid mechanical model is developed to assess the performance of a rocket that is propelled by the absorption of radiant energy from a remotely stationed, repetitively pulsed laser. The model describes the flow within a conical nozzle that is subjected to point energy depositions at the apex of the cone. A similarity solution is obtained and the specific impulse and energy efficiencies that may be achieved with such a device are determined. Fluid mechanical constraints limit the range of pulse repetition rates that may be utilized. Preliminary design considerations indicate that a specific impulse of 800 sec or greater may be achieved with both a laboratory and a full-scale device. A two pound laboratory rocket can be accelerated at 10 #'s with a 15 joule laser pulsed 25,000 times per sec. A one ton rocket will require a megajoule laser operating at 350 pulses per sec to achieve an equivalent acceleration. A,a D* dm

The Fluid Mechanics of Pulsed Laser Propulsion

A coated substrate manufactured by applying a layer of a material to the substrate and generating thermal and pressure waves in the layer by exposing the layer to high intensity, short duration laser radiation, and the process of manufacturing such a coated substrate. The laser radiation is applied in an intensity range that creates an instantaneous surface vaporization of the layer material that in turn drives a pressure wave into the layer. The pressure wave interacts with the layer-substrate interface to create bonding between them of varying strengths and qualities depending on the intensity and duration of the initial laser pulse. A thermal wave is created in some regimes of operation, or results from compressional heating of the layer by the pressure wave, and is of sufficient energy to contribute to the bonding at the interface. The coating and the process for its creation has application in diverse areas where surface properties of a particular color, hardness, corrosion resistance, abrasion resistance, electrical conductivity, among others are desired.

Method for bonding using laser induced heat and pressure

: A fluid mechanical model is developed to assess the performance in both finite background pressure and vacuum environments of a rocket that is propelled by the absorption of radiant energy from a remotely stationed, repetitively pulsed laser. The model describes the gaseous propellant flow within a conical nozzle that is subjected to a series of point energy depositions at the apex of the cone. An equivalence between conical and parabolic nozzles is discussed for finite background pressure operation. The model predicts laser parameters necessary to achieve high specific impulses, i. e., 600 to 1000 sec. Scaling laws for high thrust - high specific impulse rocket systems are discussed.

Pulsed laser propulsion

Analytic solutions are presented for the time to plasma shielding above a laser-irradiat ed surface in an atmosphere. The time scale for ignition of a laser-supported combustion wave is determined as a function of laser intensity, material, spot size, and wavelength. The mechanism for absorption wave ignition involves surface vaporization, rapid heating of the target vapor and subsequent ignition in either the vapor or surrounding air. It is shown that over a range of intensities the vapor properties up to the time for ignition can be calculated by uncoupling the heating from the gas dynamics. The time history of the heating effects is obtained by doing a perturbation upon the flowfield. The flow is either one-dimensional planar or two-dimension al axisymmetric depending upon the ratio of an appropriate absorption time scale to a characteristic flow time. The onedimensional planar analysis including the effects of unsteady surface vaporization is summarized, and the details of the two-dimensional analysis are presented. Results are compared with published computer solutions and existing experimental data for the time to plasma initiation above metal surfaces irradiated with 10.6 /< and 5 \L laser radiation.

Analytic Solutions for Laser-Supported Combustion Wave Ignition above Surfaces

: Radiative ionization patterns in the precursor of the shock preceding a blunt body immersed in a hypersonic monatomic gas flow are obtained. Based on earlier work that provides for the uncoupling of the radiative transfer from the gas dynamics, a plausible model of the shock structure and shock layer radiation is proposed. The rate equation for the production of electrons in the precursor is dependent only upon photon absorption because ionizing and neutralizing collisions are negligible; it is solved in the near precursor where one-step radiative ionization dominates. Geometrical complexities introduced by the multidimensional shock are encountered along with the spectral difficulties associated with nonequilibrium radiative ionization. Spatial calculations for the degree of ionization in argon throughout the near precursor are presented for a downstream degree of ionization on the stagnation streamline of 0.8, an upstream temperature of 300K, and an upstream pressure of .001 atm. Intriguing patterns of constant electron density reveal a 'concentric sphericity effect' close to the shock axis of symmetry. In addition, asymptotic solutions illustrate the failure of a radiative point source description of the shock layer to yield accurate solutions illustrate the failure of a radiative point source description of the shock layer to yield accurate solutions off the axis of symmetry. (Author)

Anthony N. Pirri

Papers

The Fluid Mechanics of Pulsed Laser Propulsion

Method for bonding using laser induced heat and pressure

Pulsed laser propulsion

Analytic Solutions for Laser-Supported Combustion Wave Ignition above Surfaces

Radiative ionization patterns in cold precursor of axisymmetric detached shock