It has long been known that heat, UV light, or protonization will cause the bond in Leuco dye's to break. The underlying microscopic mechanism is a still a matter of discussion. It stands to reason that a local model should correctly predict the temperature behavior of the heat capacity at the thermochromic phase transition. Here we develop a tight binding (TB) model for the variation of the energy of the bond as it breaks as a function of temperature. We use a statistical mechanical approach to incorporate this variation in a partition function, calculate a free energy, and obtain the heat capacity of the phase transition. A distribution of such heat capacities is shown to fit the data for the leuco dye. The heat capacity distribution is compared to the experimentally determined heat capacity by differential scanning calorimetry (DSC).

Switchable coatings could reduce thermal control costs by accessing the infrared Solar spectrum. Thermochromic liquid crystals and Leuco dyes were studied using diffuse reflectance from 15–40°C towards development of infrared switching materials.

The elects of the micrometer-scale surface morphology of polyoxymethylene on propulsion performance induced by transversely excited atmospheric CO2 laser pulse ablation were investigated. Two methods were examined for producing a surface structure on the same order as the laser wavelength, 10.6 mu m: hydraulic compaction of powder and etching using microelectromechanical systems technology. With the compacted powder, the fluence for maximum momentum coupling coefficient C-m shifted to a smaller value of 7 J/cm(2) from the corresponding value of 15 J/cm(2) for bulk material. However, mass consumption was increased, thereby lowering specific impulse I-sp. Three surface patterns were examined with the microelectromechanical systems etching, obtaining an up to 20% increment, both in C-m and I-sp.

In research on beamed energy propulsion, the momentum coupling coefficient cm is a central figure of merit to characterize a propulsion system. The determination of cm is based on the measurement of imparted impulse and laser pulse energy. Nevertheless, the knowledge of laser pulse length, laser spot area and ablated mass is of great importance for the comparability of experimental results in laser ablative propulsion. The use of a great variety of measurement techniques for these parameters throughout the scientific community implies the risk of misunderstandings and might impede the comparability of results. In this paper, we present critical issues concerning the measurement of the aforementioned key parameters with respect to possible standardization issues. As an example, a simple laser propulsion experiment will be presented and compared with an experimental model from a different research group.

An optical gas-detection sensor safely monitors pressurized systems (such as cryogenic tanks) and distribution systems for leaks. This sensor system is a fiber-coupled, solid optical body interferometer that allows for the miniaturized sensing element of the device to be placed in the smallest of recesses, and measures a wide range of gas species and densities (leaks). The deflection of the fringe pattern is detected and recorded to yield the time-varying gas density in the gap. This technology can be used by manufacturers or storage facilities with toxic, hazardous, or explosive gases. The approach is to monitor the change in the index of refraction associated with low-level gas leaks into a vacuum environment. The completion of this work will provide NASA with an enabling capability to detect gas system leaks in space, and to verify that pressurized systems are in a safe (i.e. non-leaking) condition during manned docking and transit operations. By recording the output of the sensor, a time-history of the leak can be constructed to indicate its severity. Project risk is mitigated by having several interferometric geometries and detection techniques available, each potentially leveraging hardware and lessons learned to enhance detectability.

2in atmospheric air. A transversely excited atmospheric CO2 laser operating at 10:6 � m with approximately 100 ns pulse length and about 3–20 J output pulse energy was used to ablate prepared polymer samples with single pulses of laser energy. Measurements of parameters such as ablated mass per spot area � ,

A space-based laser ablation propulsion system has the potential to produce a paradigm shift in the management of space resources. This paper outlines practical considerations for in-space use of a laser ablation propulsion system for generating an attractive force between two space objects. Limitations of energy delivery to the target are determined, and mass removal and impulse generation are calculated as functions of range for a variety of laser wavelengths. A unique in-space application is proposed for a reversed-thrust system: emergency retrieval of astronauts and tools on-orbit.

We report on an international cooperation between Nagoya University (NU), Japan and DLR Stuttgart, Germany on scaling issues in laser ablative propulsion. Lessons learned from collaborative work in the laboratory will be summarized with respect to the comparability of experimental methods and corresponding standardization issues. With the background of previous experimental research at the University of Alabama in Huntsville (UAH), experimental work with CO2 lasers in a moderate (NU) and high (DLR) pulse energy range on laser ablation of POM is presented. Profilometry results of target surfaces are compared with fluence distributions from beam propagation modeling. Ablation from flat targets is reported with respect to energy and area scaling and compared with results from ablative propulsion employing parabolic nozzles.

The familiar Bouguer-Lambert-Beer absorption law, often called Beer's law, is an essential component of many laser ablation propulsion models. However, its treatment in non-planar conditions requires a consideration of irradiation geometry. Forms of the absorption law are derived for cylindrical and spherical normal incidence geometries, and for conical nozzles with flat and cylindrical targets. The results indicate that use of a concentrating nozzle optic with a transparent target could provide increased impulse generation for laser propulsion. This improvement would be accomplished using a combination of chosen optics and a transparent target material to generate highly confined ablation in-volume. The surface fluence and ablation depth on a cylindrical target in a parabolic optical nozzle is also derived, and the results are compared to literature raytracing model results and profilometry data, respectively.

Polyoxymethylene (POM) has been widely studied as a promising laser propulsion propellant when paired to CO2 laser radiation. POM is a good test case for studying ablation properties of polymer materials, and within limits, for study of general trends in laser ablation-induced impulse. Mechanisms such as vaporization, combustion, and plasma are evaluated and a description is made of the link between the fluence of the beam and the resulting temperature of the target. For characterization of propulsion parameters, almost all previous studies of POM considered limited ranges of ambient pressure and incident fluence. As a result, despite many studies, there is no general understanding of POM ablation that takes into account pressure, spot area, fluence, and effects from confinement and combustion. This paper reviews and synthesizes CO2 laser ablation propulsion work using POM targets in order to make preliminary steps to address this deficiency. Previously published data is compared in terms of ablated mass ...

Various measurement techniques have been used throughout the over 40-year history of laser propulsion. Often, these approaches suffered from inconsistencies in definitions of the key parameters that define the physics of laser ablation impulse generation. Such parameters include, but are not limited to the pulse energy, spot area, imparted impulse, and ablated mass. The limits and characteristics of common measurement techniques in each of these areas will be explored as they relate to laser propulsion. The idea of establishing some standardization system for laser propulsion data is introduced in this paper, so that reported results may be considered and studied by the general community with more certain understanding of particular merits and limitations. In particular, it is the intention to propose a minimum set of requirements a literature study should meet. Some international standards for measurements are already published, but modifications or revisions of such standards may be necessary for application to laser ablation propulsion. Issues relating to development of standards will be discussed, as well as some examples of specific experimental circumstances in which standardization would have prevented misinterpretation or misuse of past data.