Foam-on-Tile Impact Modeling for the STS-107 Investigation

Following the breakup of the Space Shuttle Columbia during reentry , a NASA/Contractor impact modeling team was formed to examine the probable damage inflicted on Orbiter Thermal Protection System (TPS) elements by impact of External Tank (ET) cryoinsulation debris . Our team was tasked to apply “physics based ” analysis techniques to help determine parameters of interest for the experimental test program in preparation at Southwest Research Institute (SwRI), and to utilize validated codes to investigate the full range of impact scenarios beyond what could be tested within a reasonable timeframe. The term “physics -based” was used to indicate that t he physics of the event – projectile mass, velocity, and impact angle – would be used to develop the transient loading profile. The team had members from several NASA centers, Sandia National Laboratory, academia, and private industry. The analysis team r eported to the Orbiter Vehicle Engineering Working Group, gave input to the impact test team, and presented pre -test predictions at the Main Landing Gear Door and Wing Leading Edge impact test readiness reviews. The authors formed a working sub -group with in the larger analysis team to apply the Smooth Particle Hydrodynamics code SPHC to the damage estimation problem. We obtained material properties needed to numerically model the TPS tiles and ET foam from tests performed by other organization s w ithin the analysis team, and from existing materials data bases . We constructed t wo - and three -dimensional tile models , and developed a model o f BX -250 foam , which includ ed porous compression, elastic rebound, and surface erosion characteristics . We validated our code results for the tile damage and foam behavior through extensive comparison with SwRI foam -on -tile impact experiments carried out in 1999. These tests involved small projectiles striking individual tiles and small tile arrays. The foam used in the 199 9 SwRI tests was a different cryoinsulation from that used on the ET, but with similar density and mechanical propert ies . Following detailed comparison of experimental and model results , we simulated impacts of larger foam projectiles on examples of tile systems used on the Orbit er. Results for impacts on the M ain Landing Gear Door are presented in this paper, i ncluding 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 incre ase 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