Kenneth A. Herren

Effect of air pressure on propulsion with TEA CO 2 laser

The assessment of energy partition between air and solid propellant has been conducted using a TEA CO2 laser. The experiments were performed by focusing output pulses of the laser (200 ns pulsewidth at 10.6 μm wavelength and ~10.6 J pulse energy) on aluminum targets mounted on a ballistic pendulum. Coupling coefficients and mass removal rates were determined as functions of air pressure, which varied from 1 atm to 3.5 mTorr. The data from both coupling coefficients and mass removal rates show that there is a sharp transition region ranging between 1.0 and 10 Torr. In this region the momentum imparted to the target via air breakdown appears comparable and, at higher pressures, dominating the momentum due to the breakdown on the target surface.

Effects of Time Separation on Double‐Pulsed Laser Ablation of Graphite

This work continues on previous investigations of elementary propellants for Ablative Laser Propulsion (ALP). The details on experimental methods used for alignment of a non‐collinear pulse splitting apparatus are presented. Spatial and temporal coincidence of 100‐ps wide pulses is demonstrated and the first data is reported on the pulse separation effects studied by means of time‐of‐flight (TOF) energy analyzer on graphite. The data includes ion velocity and number density, measured as functions of pulse separation. Possible models of observed phenomena are discussed.

Bidirectional reflectance distribution function measurement of molecular contaminants in the ultraviolet, vacuum ultraviolet, and visible ranges

Bidirectional reflectance distribution function measurements of optical surfaces both before and after molecular contamination have been done using UV, VUV, and visible light. Molecular contamination of optical surfaces from outgassed material has been shown in many cases to proceed from acclimation centers, and to produce many roughly hemispherical “islands” of contamination on the surface. The products of this outgassing will inevitably migrate throughout the surrounding area and adhere to any convenient surface, including optical elements in the system.

Ion Milling of Sapphire

The ion figuring system at the Marshall Space Flight Center has been successfully used for at least three previous investigations into the ion milling of metals. The research was directed toward improving the surface quality of X-ray directing optics. These studies were performed on surfaces that were already hand polished to an excellent surface quality and were intended to remove the residual unwanted figure left by those techniques. The ion milling was typically carried out on test surfaces or mandrels that were several centimeters in width and length. The good thermal conductivity of the metal samples allowed the ion beam to be directed onto the sample for an indefinite period of time. This is not true of sapphire or most electrical insulators and problems have arisen in recent attempts to ion mill thin samples of sapphire. The failure and fracture of the material was likely due to thermal stresses and the relatively low thermal conductivity of sapphire (compared to most metals), These assumed stresses actually provided the key as to how they might be monitored. A thermal gradient in the sapphire sample will induce an effective index of refraction change and because of the shape constraint and the crystal structure and simple thermal expansion, this index change will be nonuniform across the sample. In all but simple cubic crystal structures, this leads to a spatially nonuniform optical retardance induced on any polarized optical beam traversing the sample, and it is this retardance that can be monitored using standard polarimetric procedures.

Specific Impulse Definition for Ablative Laser Propulsion

The term “specific impulse” is so ingrained in the field of rocket propulsion that it is unlikely that any fundamental argument would be taken seriously for its removal. It is not an ideal measure but it does give an indication of the amount of mass flow (mass loss/time), as in fuel rate, required to produce a measured thrust over some time period. This investigation explores the implications of being able to accurately measure the ablation rate and how the language used to describe the specific impulse results may have to change slightly, and recasts the specific impulse as something that is not a time average. It is not currently possible to measure the ablation rate accurately in real time so it is generally just assumed that a constant amount of material will be removed for each laser pulse delivered. The specific impulse dependence on the ablation rate is determined here as a correction to the classical textbook definition.

Initial Demonstration of Ablative Laser Propulsion

Ablative Laser Propulsion (ALP) is defined by a direct momentum transfer due to mass ablated from a solid target. Studies of this concept have yielded specific impulses (Isp) and coupling coefficients (Cm) of 1000 – 5000 s and 2 – 8 dynes/Watt respectively. These parameters were used to design and test the first ALP‐vehicles on the laboratory scale. Two models of the first ALP‐vehicles were made from electroplated nickel (mass 35 mg) and Kapton (10 mg). Initial tests in vacuum and in air were attempted using 100‐ps wide, 35 mJ laser pulses at 532 nm wavelength. The Isp and Cm deduced from these tests are in good agreement with previously reported figures.