The initial results from the damage assessment process were promising, but questions still remained about the material response. An arc-jet test was designed specifically to address this concern. The scanned damage was machined directly into an existing, pre-instrumented tile array and tested in an approximate flight environment. Arc-jets are particularly well suited to this type of testing, but a key question is how the test conditions relate to the true flight conditions. The high-fidelity analysis process was able to help here as well by simulating the as-tested configuration.
Based on the results of the complete aerothermal/thermal/stress analysis cycle, the decision was made to reenter the Orbiter as-is. The cavity is shown post-flight in Figure 5. It is clear from the figure that the damage did not progress during reentry. The correct decision was made.
It is worth mentioning, however, that a repair effort was being pursued in parallel to the nominal damage, assessment process. In the event a repair was warranted the urgent analysis process undoubtedly would have been engaged again to help assess and define repair requirements. This places a large burden on the analysis community, as they must carefully evaluate many possible scenarios. However, given the compressed timeline imposed by manned spaceflight with limited consumables, there is no alternative to this seeming chaotic, parallel-path approach.
The rapid aerothermal analysis capability put in place during NASA’s return-to-flight efforts has proven a, critical component of the damage assessment process which aims to assure the Shuttle is “go” for reentry. On multiple occasions, the Orbiter aerothermal analysis team has demonstrated the ability to meet the aggressive schedule demanded by real-time space operations support. In the case of STS-118, insights gained through this capability helped demonstrate that repair was not necessary, allowing the primary mission objectives to be achieved while ensuring crew safety. Given that Shuttle flights typically carry seven crewmembers, are estimated at $500 million a piece, and each Orbiter costs in excess of $1 billion, it is hard to underestimate the programmatic value of making the right decision in such circumstances.
Instituting this capability required the efforts of many people over a period of years. Key to its success was the dedication of these individuals and the tireless efforts of the overall team. The capability that has been put in place continues to evolve and benefits from experience gained each flight. We believe this is a critical aspect of using urgent computing to support high-stakes, real-time decisions. In our experience, it required three full-up system tests (in the form of pre-flight mission simulations) to effectively shake out the process, to illustrate strengths, and to identify and address weaknesses.
A highly automated process, robust quality control procedures, and dedicated, on-demand access to world-class resources are all prerequisites that help enable this capability. Equally important, and perhaps more surprisingly, are the human factors involved. Our experience is that timely generation of accurate results is critical, but proper interpretation and communication of those results is equally as critical. For our application, we require that analysis leads be co-located with the end users of the analysis data.
2Tang, C., Saunders, D., Trumble, K., and Driver, D., “Rapid Aerothermal Simulations of Damage and Repair during a Space Shuttle Mission,” AIAA Paper No. 2007-1783, April 2007.
3Campbell, C., Driver, D., Alter, S., Fasanella, E., Wood, W., and Stone, J., “Orbiter Gap Filler Bending Model for Re-Entry,” AIAA Paper No. 2007-0413, Jan. 2007.
4 Everhart, J., “Supersonic/Hypersonic Laminar Heating Correlations for Rectangular and Impact-Induced Open and Closed Cavities,” AIAA Paper No. 2008-1283, January 2008.
5 Gnoffo, P. A. and Cheatwood, F. M., “User’s Manual for the Langley Aerothermodynamic Upwind Relaxation Algorithm (LAURA),” NASA Technical Memorandum TM-4674, National Aeronautics and Space Administration, 1996.
6Wright, M.J., Candler, G.V., and Bose, D., “Data-Parallel Line Relaxation Method for the Navier-Stokes Equations,” AIAA Journal, Vol. 36, No. 9, 1998, pp. 1603-1609.
7Reuther, J., McDaniel, R., Brown, J., Prabhu, D., Saunders, D., and Palmer, G., “External Computational Aerothermodynamic Analysis of the Space Shuttle Orbiter at STS-107 Flight Conditions,” AIAA Paper No. 2004-2281, June 2004.
8External Aerothermal Analysis Team, “Smooth Outer Mold Line Aerothermal Solution Database for Orbiter Windside Acreage Environments During Nominal Entry Conditions,” NASA Johnson Space Center Engineering Note EG-SS-06-01, 2005.