Prior to docking with the International Space Station on the third day of flight, the Orbiter executes a specific maneuver designed to aid in damage detection. The vehicle essentially performs a back flip while approximately 600 meters away from the Station. During this procedure two Station crewmembers perform photography of the vehicle. The imagery resolution is such that 7 cm damage can be identified anywhere on the vehicle, with damage as small as 2 cm identifiable in specific areas of interest. Imagery experts and hardware technicians provide the essential damage descriptions that are taken as input to a cross-disciplinary analysis. A composite lower-surface image that was obtained during Discovery’s return-to-flight is shown in Figure 1.
Each of the previously mentioned data acquisition tools is used on every mission. These data often provide the damage assessment team sufficient data to clear the vehicle for reentry. This is not always the case, however, and additional assets can be used to perform a focused inspection of a particular damage site that may be of concern. One such data set will be presented later.
It is at the end of flight day three when all of these data are available to analysts on the ground that the damage assessment process begins in earnest. From flight days three to five the coupled aerothermal-thermal-stress analysis process is performed for each identified damage site. The goal is to disposition each site as acceptable or unacceptable for reentry based on a set of well-defined structural and thermal limits. If a site is deemed unacceptable for reentry, it is the damage assessment team, in conjunction with on-orbit operations personnel, who work together to design, implement, and affect a repair procedure.
The first step in this process is to determine the aerothermal environment induced by a specific damage site. This includes any local changes in heat transfer that may result, as well as global effects such as early boundary-layer transition that may affect the downstream portion of the vehicle. Principally, empirically based correlations are applied to each site. These correlations are based on extensive test and analysis data that were performed pre-flight for physically relevant and geometrically similar conditions.4 As with any empirical correlation, however, questions of suitability for a particular case invariably arise and must be addressed. This is the primary area where high-fidelity analysis is used during the nominal process.
These aerothermal environments are then used as boundary conditions in transient thermal analysis for each site. The two primary goals of the thermal analysis are (i) to identify any material exceedances that may occur (e.g., exceeding allowable temperatures for aluminum structure), and (ii) to provide a damage-specific environment that can be used in stress analysis.
Assuming that a damage site has not exceeded material limits, the possibility still exists for local buckling due to thermal stress, for example. In this way the thermal environment is taken as input to a stress assessment that evaluates the potential for such effects. It is only when the end-to-end process is applied to a given site and presents no issues that the damage is deemed acceptable for reentry.
If the baseline process identifies an issue with any damage site, additional analysis is performed and the site is also considered as a candidate for on-orbit repair. It is in such high-risk scenarios that high-fidelity analysis and high-performance computing is particularly valuable.