John E. Sinko

Conical nozzles for pulsed laser propulsion

A CO2 laser of 300 ns pulse length, operating at 10.6 μm wavelength and from 1-4 J pulse energy was used to ablate carbon-doped Delrin® (polyoxymethylene, or POM) targets in a set of conical aluminum minithrusters at standard temperature and pressure. Nozzles with lengths ranging from 0.5 - 5 cm were used (corresponding to expansion ratios of about 4 to 16), as well as a bare sample with no nozzle. A piezoelectric force sensor was used to record the imparted impulse for fluences in the range of 1-100 J/cm2 for each thruster. The effect of increasing the expansion ratio on the impulse generation for single pulse laser propulsion experiments will be described. The study will also clarify the effect of confining air from an ambient atmosphere in augmenting impulse generation.

Critical fluence effects in laser propulsion

The fluence dependence of the laser ablation of selected polymers was studied within the range from 1-150 J/cm2. A TEA CO2 laser operating at 10.6 μm with 300 ns main pulse length and up to 20 J pulse energy was used to ablate prepared polymer samples with single pulses of laser energy. Measurements of parameters such as the ablated mass per spot area (Δma), momentum coupling coefficient (Cm), specific impulse (Isp), and internal efficiency (ηi) will be plotted as functions of fluence. Critical threshold effects observed throughout the experiments will be described in detail.

Laser Propulsion with Liquid Propellants Part I: an Overview

Despite years of research, laser propulsion on liquid propellants has yet to achieve specific impulses greater than tens of seconds. It is well established that liquids, when used as propellants, can provide coupling coefficient (Cm) on the order of 100–1000 dyne/W. However, the specific impulse (Isp) has proven to be significantly inferior to that for solid propellants. This paper will examine the various laser propulsion schemes based on ablation of liquid propellants and intended primarily for space applications. The principal shortcomings associated with liquid propellants will be outlined.

Laser Propulsion with Liquid Propellants Part II: Thin Films

Thin films of a liquid propellant have been studied as a potential way to boost thrust for laser propulsion applications. A TEA CO2 laser with 300 ns pulse width was operated at up to 20 J pulse energy to produce irradiances at the target on the order of 1–1200 MW/cm2 to ablate various systems of thin films on Delrin® substrates. In this study, time‐resolved force sensors and ICCD imaging techniques were used to determine how an addition of thin liquid films to solid substrates affects propulsive properties such as momentum coupling coefficient, specific impulse, and internal efficiency. Transparent (hexane) and absorbing (ethanol and water) thin films were formed above Delrin® substrates for the laser system operating at 10.6 μm. Thickness effects on the hexane‐Delrin® system will be examined. An analysis will be made of the possible routes for force generation, and the general properties, benefits, and shortcomings of liquid thin film structures will be summarized with regard to laser propulsion.

Sphere-Wall Impact Experiments with Piezoelectric Force Sensors

Measurement of impulse imparted to a target from μs‐timescale laser ablation events is often performed with piezoelectric force sensors. For pulsed laser ablation with a target resting on the force sensors, an effect can occur for a vertical thrust stand in an exhaust‐up configuration that results in measurement of about twice the actual imparted impulse. A CO2 laser operating at 10.6 μm wavelength, 300 ns pulse length, and up to 20 J pulse energy single shots was used to ablate samples of PCTFE. Force sensor measurements of the imparted impulse were compared to tests with a ballistic pendulum over a variety of fluences. The theoretical impulse delivered by the impacts of 6 mm diameter spheres of aluminum, steel, POM, and PTFE on the force sensor were studied, and the coefficients of restitution were measured for the targets. Practical issues for measurement of ablation‐imparted impulse with piezoelectric sensors are discussed.

Laser-Driven Mini-Thrusters

Laser‐driven mini‐thrusters were studied using Delrin® and PVC (Delrin® is a registered trademark of DuPont) as propellants. TEA CO2 laser (λ = 10.6 μm) was used as a driving laser. Coupling coefficients were deduced from two independent techniques: force‐time curves measured with a piezoelectric sensor and ballistic pendulum. Time‐resolved ICCD images of the expanding plasma and combustion products were analyzed in order to determine the main process that generates the thrust. The measurements were also performed in a nitrogen atmosphere in order to test the combustion effects on thrust. A pinhole transmission experiment was performed for the study of the cut‐off time when the ablation/air breakdown plasma becomes opaque to the incoming laser pulse.

Constant‐Fluence Area Scaling for Laser Propulsion

A series of experiments was conducted on polyoxymethylene (POM, trade name Delrin®) propellants in air at atmospheric pressure. A TEA CO2 laser with maximum output power up to 20 J was used to deliver 300 ns pulses of 10.6 μm radiation to POM targets. Ablation at a constant fluence and a range of spot areas was achieved by varying combinations of the laser energy and spot size. Relevant empirical scaling laws governing laser propulsion parameters such as the momentum coupling coefficient (Cm) and specific impulse (Isp) for spot areas within a range of about 0.05–0.25 cm2 are presented. Experimental measurements of imparted impulse, Cm, Isp, and ablated mass per pulse were made using dynamic piezoelectric force sensors and a scientific balance. Finally, Schlieren ICCD imaging of shock waves and vapor plumes was performed and analyzed.

Time-resolved force and ICCD imaging study of TEA CO 2 laser ablation of ice and water

Time-resolved force measurements and Intensified Charge-Coupled Device (ICCD) imaging techniques were applied to the study of force generation in the laser ablation of water and ice. A transversely excited atmospheric (TEA) CO2 laser operated at 10.6 μm, 300 ns pulse width, and up to 20 J pulse energy was used to ablate water and ice held in various containers. Net imparted impulse and coupling coefficient were derived from force sensor data and relevant results will be presented for ice and water. ICCD imaging was used in conjunction with time-resolved force measurements in order to determine the dominating physical mechanism under which the thrust is produced. The effect of shock wave generation and propagation, as well as its contribution to the overall impulse imparted to the targets, was examined from the comparison of the timelines for the pertinent phenomena. The process of mass removal was investigated for each case, and specific impulse and efficiency were calculated from the data. Differences in the force-time curves for ice and water will be presented and discussed. Ballistic experiments were conducted in order to corroborate the force measurements.

Ablation of Liquids for Laser Propulsion With TEA CO2 Laser

Time‐resolved force sensing and intensified charge‐coupled device (ICCD) imaging techniques were applied to the study of the force generation mechanism for laser ablation of liquids. A Transversely Excited at Atmospheric pressure (TEA) CO2 laser operated at 10.6 μm, 300 ns pulse width, and 9 J pulse energy was used to ablate liquids contained in various aluminum and glass vessels. Net imparted impulse and coupling coefficient were derived from the force sensor data and relevant results will be presented for various container designs and liquids used. ICCD imaging was used in conjunction with the dynamic force techniques to examine dependencies on absorption depth, irradiance, surface curvature, and container geometry. ICCD imaging was also used to determine whether surface or volume absorption should be preferable for laser propulsion using liquid propellants. Finally, ballistic experiments were conducted in order to verify the dynamic force data and lend additional evidence as to the predominant methods of force generation.

A Conceptual Tree of Laser Propulsion

An original attempt to develop a conceptual tree for laser propulsion is offered. The tree provides a systematic view for practically all possible laser propulsion concepts and all inter‐conceptual links, based on propellant phases and phase transfers. It also helps to see which fields of laser propulsion have been already thoroughly explored, where the next effort must be applied, and which paths should be taken with proper care or avoided entirely.