An innovative, low-weight shield system has been developed by NASA Johnson Space Center (JSC) and Marshall Space Flight Center (MSFC) engineers to enhance the protection of conventional Whipple shields. This shield, the "Stuffed Whipple" shield, includes a flexible blanket combining Nextel TM ceramic fabric and Kevlar TM fabric (or "stuffing") between the aluminum bumper and rear wall of a Whipple shield. The Stuffed Whipple (SW) shield is particularly effective if shield standoffs are short (i.e., shield spacing to projectile diameter ratios of 15 or less). Alternative shields using aluminum, Nextel TM or Kevlar TM alone as the intermediate bumper were tested but did not provide the same level of protection performance for the weight as a combination of Nextel TM and Kevlar TM. Hypervelocity impact (HVI) testing with greater than lg aluminum projectiles using Light-Gas Guns (LGG) up to-7 km/sec and Shaped-Charge Launchers (SCL) up to ~11 km/sec were used in the development program. NOTATION d 0c P m S t3 t 0 V Projectile diameter (cm) Projectile diameter causing failure; i.e., "critical" particle that just results in complete penetration of the shield's rear wall (cm) Density (g/cm 3) Areal density (g/cm 2) Overall spacing from the front of outer bumper to the back of rear wail (cm) Rear wail yield stress (ksi) Thickness (cm) Impact angle measured from surface normal (deg) Projectile speed (km/sec) Subscripts: b All bumpers (Nextel TM, mesh, and aluminum) and intermediate layers (MLI and Kevlar TM) p projectile w rear wail
Following the breakup of the Space Shuttle Columbia during reentry a NASNContractor investigation team was formed to examine the probable damage inflicted on Orbiter Thermal Protection System elements by impact of External Tank insulating foam projectiles. Our team was to apply rigorous, physics-based analysis techniques to help determine parameters of interest for an experimental test program, utilize validated codes to investigate the full range of impact scenarios, and use analysisderived models to predict aero-thermal-structural responses to entry conditions. We were to operate on a non-interference basis with the Impact Testing Team, and were to supply significant findings to that team and to the Orbiter Vehicle Engineering Working Group, being responsive to any solicitations for support from these entities.The authors formed a working subgroup within the larger team to apply the Smooth Particle Hydrodynamics code SPHC to the damage estimation problem. Numerical models of the Orbiter's tiles and of the Tank's foam were constructed and used as inputs into the code. Material properties needed to properly model the tiles and foam were obtained from other working subgroups who performed tests on these items for this purpose. Two-and three-dimensional models of the tiles were constructed, including the glass outer layer, the main body of LI-900 insulation, the densified lower layer of LI-900, the Nomex felt mounting layer, and the Aluminum 2024 vehicle skin. A model for the BX-250 foam including porous compression, elastic rebound, and surface erosion was developed. Code results for the tile damage and foam behavior were extensively validated through comparison with Southwest Research Institute foam-on-tile impact experiments carried out in 1999. These tests involved small projectiles striking individual tiles and small tile arrays.Following code and model validation we simulated impacts of larger foam projectiles on the examples of tile systems used on the Orbiter.Results for impacts on the main landing gear door are presented in this paper, including effects of impacts at several angles, and of rapidly rotating projectiles. General results suggest that foam impacts on tiles at about 500 mph could cause appreciable damage if the impact angle is greater than about 20 degrees. Some variations of the foam properties, such as increased brittleness or increased density could increase damage in some cases. Rotation up to 17 rps failed to increase the damage for the two cases considered. This does not rule out other cases in which the rotational energy might lead to an increase in tile damage, but suggests that in most cases rotation will not be an important factor. BACKGROUND
varying solar wind, electromagnetic fields, and particles that iutcract with the planets and the galaxy. The Earth's magnetic field and upper atmosphere provide the shielding from the Sun's external influences, and they also form a coupled system with the Sun and the heliosphere. The Sun-Earth Connection (SEC) [ I ] theme in the NASA Office of Space Science (OSS) has a goal to understand the Sun, heliosphere, and planetary environments as a single connected system.The SEC Roadmap [2] strategy is to develop a synergistic set of missions that will allow us to discover and understand the connected Sun-Earth system in the spatial, temporal, and spectral domain from optimal vantage points in space. These missions will provide the remote and multipoint in situ measurements from unique vantages to support the primary SEC scientific goals and objectives. These missions will image the Sun's atmosphere in t h e dimensions by including high latitude and polar regions;
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