Characterizing material states in orbital debris impacts

Characterizing Material States in Orbital Debris ImpactsWilliam P. SchonbergCivil & Environmental Engineering DepartmentUniversity of Alabama in HuntsvilleHuntsville, Alabama 35899ABSTRACTThe key to conducting an accurate damage assessment of a structure that has been impacted by an orbital debrisparticle is the use ofa robust assessment methodology. To accurately determine total structural damage, a damage assessmentmethodology must include the effects of discrete impacts by solid debris cloud fragments as well as impulsive loadings due tomolten and vaporous debris cloud material. As a result, the amount ofdebris cloud material in each ofthe three states of mattermust be known to accurately assess total damage due to a high speed orbital debris impact. This paper presents a firstpnnciplesbased method to calculate 1) the amount ofmaterial in a debris cloud created by a perforating hypervelocity impact that is solid,molten, and vaporous, 2) the debris cloud leading edge, trailing edge, center-of-mass, and expansion velocities, and 3) theangular spread ofthe debris cloud material. The predictions ofthis methodology are compared with those of empirically-baseddamage assessment schemes as well as against numerical and empirical results obtained in previous studies of debris cloudformation.Keywords: damage, debris cloud, fragmentation, hypervelocity impact, lethality assessment, orbital debris, vaporization1.0 INTRODUCTIONAll earth-orbiting structures and vehicles are subject to high speed orbital debris particle impacts. These impacts candamage critical structural systems and components, which in turn can result in mission failure and, in the case ofthe SpaceStation Alpha, even possibly loss oflife. In order to guard against such catastrophic failures, vehicles and structural systemsdesigned to operate in earth orbit must contain protective systems, such as 'debris shields' or 'bumpers'. The concept ofusing asingle thin plate placed at a short distance ahead ofa primazy structural system or component as a means of mitigatinghypervelocity impact damage was originally proposed in J9471, Since then, such a system has been shown to be highly effectiveand efficient in increasing the damage resistance of manned and unmanned space systems. However, in the event that an on-orbit impact does occur, it is imperative to be able to accurately assess the damage sustained by the impacted vehicle or structure.In military space applications, this information can be used to determine whether or not the vehicle has been destroyed or if itstill poses a threat. In civilian space applications, this information can be used to determine whether or not a structure has beenrendered useless or uninhabitable, or if it has only been slightly damaged and is salvageable.The key to conducting an accurate damage assessment of a structure that has been impacted by an orbital debrisparticle is the use of a robust assessment methodology. The methodology should incorporate all ofthe significant responsecharacteristics and damage mechanisms that would result from such an impact. The response ofa structure to a high speedimpact can be said to consist of two basic and distinct types of response: 'local response' and 'global response'. For high speedimpacts, material damage associated with local response occurs within the first several tsec and is limited to a volumeimmediately adjacent to the impact site. Global response can refer to any one ofa number of global phenomena that occur overa longer period oftime (i.e. on the order ofmilliseconds), under less intense loads, and over a much larger area ofthe structure.Typical global responses include the denting, buckling, tearing or catastrophic dismemberment of internal components.In high speed orbital debris impacts, one or more debris clouds are created during the initial impact on a bumper orouter wall. These debris clouds spread out as they move through voids and eventually impact an inner wall or interiorcomponent of the structure. Depending on the impact velocity and the relative material properties of the projectile and theimpact structural component, these debris clouds can contain solid, melted, and vaporized projectile and plate materials. Thelevels of melt and vaporization within the debris clouds in turn determine the nature of the loads transmitted to variousstructural components.