Thesis (M.S. in Electrical Engineering)--Naval Postgraduate School, June 2008. / Thesis Advisor(s): Julian, Alexander L. ; Maier, William B. "June 2008." Description based on title screen as viewed on August 29, 2008. Includes bibliographical references (p. 69). Also available in print.

Homogenous polymer materials, such as bulk polyester or high-density polyethylene (HDPE), are not commonly associated with armor materials in their raw, unmodified form due to their poor performance at typical ballistic impact velocities. However, projectile penetrations into homogenous polymeric materials have been shown to correlate strongly to the highly temperature-dependent viscoelastic properties such as elastic storage modulus and loss modulus. Ballistic trials conducted at room temperature showed that these two parameters statistically account for a large percentage of the variation in ballistic performance between different polymers. The purpose of this study is to determine the correlation of viscoelastic properties to ballistic resistance when the temperature of the polymer targets is altered above and below room temperature. The ultimate goal is to use these data to determine which materials would perform best against ultra-high velocity impacts, such as the case of micrometeoroid impacts with spacecraft.

Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, June 2008. / Thesis Advisor(s): Kwon, Young W. "June 2008." Description based on title screen as viewed on August 26, 2008. Includes bibliographical references (p. 51). Also available in print.

Thesis (M.S. in Electrical Engineering)--Naval Postgraduate School, March 2007. / Thesis Advisor(s): Alexander L. Julian, William B. Maier. "March 2007." Includes bibliographical references (p. 53). Also available in print.

No description available.

With the growing threat of orbital debris impacts to space structures, the development of space shielding concepts has been a critical research topic. In this study, numerical simulations of the hypervelocity impact response of stacked aluminum 6061-T6 sheets were performed to assess the effects of layering on penetration resistance. This work was initially motivated by set of experimental tests where a stack of four aluminum sheets of equal thickness was observed to have a higher hypervelocity ballistic resistance than a monolithic aluminum sheet with the same total thickness. A set of smoothed particle hydrodynamic simulations predicted a 40% increase in the ballistic limit for a 6-layer target compared to a monolithic sheet. In addition, the effect of variable sheet thickness and sheet ordering on the impact resistance was investigated, while still maintaining a constant overall thickness. A set of thin layers in front of a thick layer generally lead to a higher predicted ballistic limit than the inverse configuration. This work demonstrates an increase in the performance of advanced space shielding structures associated with multi-layering. This suggests that it may be possible to dramatically improve the performance of such structures by tailoring the material properties, interfaces, and layering concepts.

Energy dissipation during penetration is an important consideration in materials selection for lightweight armoring to protect against hypervelocity impacts (HVIs). Impact-induced glass transition in polymeric materials has been observed to increase energy dissipation during penetration. Incorporating unconventional armor materials like polymers could improve performance in these types of applications. A series of HVIs was performed, with impact velocities over the range of 2-7 km/s, on samples of ultra-high molecular weight polyethylene and poly(methyl methacrylate). A relationship between back face debris cloud velocity and impact velocity was developed for each material. Damage zone sizes were compared, offering insights into the effects of molecular architecture on stress delocalization and energy dissipation during hypervelocity perforation. Thermal analysis of the two material systems provides quasi-static glass transition temperatures, as well as melting and crystallization temperatures. The apparent failure mechanisms, in conjunction with thermal analysis, were used to explain the relative performance of each material.

Since the 1940s, the study of supersonic and hypersonic flow has posed significant challenges due to the variable aerodynamic characteristics and alterations in air properties at such high speeds. Hypervelocity wind tunnels have been instrumental in addressing gaps in this field, yet no existing facility can fully replicate true hypersonic conditions. The primary obstacle lies in sustaining the high enthalpies and targeted total conditions necessary for authentic supersonic and hypersonic environments. This paper focuses on the development of a mixing manifold section for a high enthalpy, long-duration hypervelocity wind tunnel designed to provide clean airflow and accurately replicate true hypervelocity conditions for extended run times. Research was done over a wide range of both computational designs and their experimental counterparts to determine the most effective design that replicates the conditions needed for the full wind tunnel.

<p dir="ltr">Here we present two studies of different aspects of meteoritic impacts. The first study is about the behavior of ejecta plumes after a hypervelocity impact onto a body with an atmosphere. The second study looks at the effect vaporization has on meteoroids as they descend through Earth's atmosphere, specifically the effect permeability and meteor size have on the vaporization during their explosive fragmentation.</p><p dir="ltr">Atmospheres play an important role in ejecta deposition after an impact event. Many impact experiments and simulations neglect the effect of atmospheres. In the first study, we simulate ejecta plumes created by craters with transient diameters of 2 km and 20 km on Mars and Earth to show the difference atmospheric density and crater size have on the strength of the interaction. The interaction of ejecta with an atmosphere is explored in this study using a two-fluid hydrocode that simultaneously simulates ejecta and atmospheres as coupled, continuum fields to correctly capture the transfer of mass, energy, and momentum between the two. Here we study the effect of vaporization of plume material as well as the effect of the bow shock. We find that only the fastest ejecta is vaporized with a peak vaporized mass of 2.5x10⁵ kg, 3.5 s after the impact in our 2 km diameter Terrestrial crater. Terrestrial meteorites are preferentially formed from the fastest ejecta. However, that fastest ejecta is mostly vaporized in our simulations, so to form a Terrestrial meteorite there must be a sufficiently large impact for solid material to be ejected and not vaporize. Thus, we place a lower limit of 33 km on the size of crater needed to generate terrestrial meteorites, but the crater size needed could be substantially larger. The bow shocks in our simulations result in lofting of ejecta, especially vaporized material, in the wake of the impactor. We find that Mars' thin atmosphere slows the ejecta but does not significantly change the trajectory of the plume. Earth's atmosphere can stop and entrain ejecta particles to suspend heated material long after the majority of material has already been deposited, resulting in 4x10¹⁰ kg of material being suspended in the atmosphere 100 seconds after the impact for a 2 km diameter crater. For larger craters, we find that Earth's atmosphere has a more limited effect and ejecta more closely follows a ballistic trajectory.</p><p dir="ltr">The 1908 Tunguska bolide event and the 2013 Chelyabinsk bolide event underscore the potential damage posed by relatively small meteoroids as compared to the dinosaur-killing Chicxulub meteoroid. In this study, we model Tunguska- and Chelyabinsk-sized bolide events, extending the work of Tabetah and Melosh (2018) by exploring a larger parameter space and introducing the novel feature of material vaporization. Building upon their findings that the porosity and permeability of a meteoroid significantly influence fragmentation, we investigate additional factors such as meteoroid size, entry speed, and entry angle. Furthermore, we demonstrate that vaporization plays a crucial role, lowering the fragmentation height by extracting energy through latent heat. We find that a larger meteoroid size or higher entry speed increases the amount of vaporization that occurs while lowering the altitude of disruption of the meteoroid, and that a shallower entry angle decreases the amount of vaporization and increases the altitude of disruption. Our study not only refines the understanding of bolide events but also introduces a novel perspective with potential implications for planetary science and impact risk assessment.</p>

L’étude des impacts hypervéloces (IHV) est essentielle dans de nombreux domaines tels que l’aérospatial, la cosmologie ou l’armement. Pour les reproduire en laboratoire, il est usuel d’utiliser des lanceurs à gaz ou à poudre. Toutefois, ce type de moyen se limite à des vitesses d’impact de l’ordre de 10 km/s pour des projectiles millimétriques. Afin d’étudier des vitesses plus élevées, il faut se tourner vers des moyens alternatifs. Dans cette étude, nous démontrons qu’une analogie est possible entre irradiations laser et IHV. Pour parvenir à ce résultat, des données expérimentales ont été obtenues sur le lanceur HERMES et sur l’installation laser GCLT. Deux matériaux cibles ont été considérés : l’aluminium 6061-T6 et l’EDM3, un graphite poreux. Par simulation numérique, nous avons caractérisé spatialement et temporellement les champs de pression générés en surface des cibles par un projectile et par un laser. Cela nous a permis de proposer et de valider une procédure permettant de lier IHV et essais laser. Pour finir, une campagne expérimentale été réalisée sur l’installation laser du LULI2000 afin d’étudier des vitesses d’impact pouvant atteindre 32 km/s. / The study of hypervelocity impacts (HVI) is essential in many fields such as aerospace, cosmology or defense. To reproduce them in laboratory, it is usual to use gas or powder launchers. However, this type of facility is limited to impact velocities under 10 km/s for projectiles of millimeter size. In order to study higher velocities, it is necessary to consider alternative means. In this study, we demonstrate that an analogy is possible between laser irradiations and HVI. To do this, experimental data were obtained on the HERMES launcher and the GCLT laser facility. Two target materials were considered: 6061-T6 aluminum and EDM3, a porous graphite. By numerical simulation, we spatially and temporally characterized the pressure fields generated on the surface of the targets by a projectile and a laser. It allowed us to propose and validate a procedure to link HVI and laser shots. Finally, an experimental campaign was carried out on the LULI2000 laser facility to study impact velocities up to 32 km/s.