Ambient Pressure Dependence of Laser-Induced Impulse onto Polyacetal

Abstract

L ASER ablation can be usefully employed to generate a propulsive impulse on an object not only in the atmosphere but also in vacuum [1–3]. Larson et al. [4] proposed a launch vehicle propelled by repetitively pulsed laser ablation. Several authors [5–9] show that favorable propulsion performance in the atmosphere can be achieved with a polymer material, polyacetal, which is commercially named “Delrin” or is abbreviated as “POM.” The ablated gas from this material does not contain much air pollutant. Targeting space applications, the laser-ablative-propulsion performance of metals and polymers at low ambient pressures has been intensively investigated [10–17]. For many kinds of metals and polymers, Cm ranges from 10 to 100 N-s=J, and depends strongly on the characteristics of the laser pulse. According to measurements byGregg andThomas [10], metallic materials have an optimum laser intensity that maximizes Cm. Phipps et al. [12] formulated experimental Cm characteristics of aluminum alloys and several polymers in terms of the intensity, width, andwavelength of the laser pulse.Cm can be further increased by utilizing the so-called “volume absorber” [13] or layered target [14]. Recent measurements by D’Souza and Ketsdever [17] of Cm for polyacetal at low ambient pressure using a frequency-doubled Nd:YAG pulsed laser showed peak of 110 N-s=J at a laser intensity of the order of 10 W=cm. Several authors investigated the influence of ambient pressure on Cm [18,19]. Pakhomov et al. [18]measured the impulse generated on an aluminum surface irradiated by a CO2 laser pulse whose pulse width was 200 ns; Cm decreased monotonically with decreasing ambient pressure. In Dufresne et al.’s surface-pressure measurement [19], the ambient-pressure dependence of Cm for aluminum was quite sensitive to the laser pulse width. For polymer materials, Beverly and Walters [20] measured the CO2-laser-induced shock pressure in cellulose acetate and polymethylmethacrylate. The peak shock pressures exhibited the complex dependence on the ambient pressure. The ablative impulse dependence of materials other than aluminum warrants further investigations. To quantitatively evaluate the feasibility of laser-ablativepropulsion systems, the propulsive performance of polyacetal over a wide range of ambient pressure needs to be known. In the present paper, the influence of the ambient pressure on the impulse characteristics of polyacetal is investigated experimentally using a transversely excited atmospheric (TEA) CO2 pulse laser.