The
Object Reentry Survival Analysis Tool (ORSAT) is the primary NASA computer code for predicting the reentry survivability of satellite and launch vehicle upper stage components entering from orbital decay or from controlled entry. The prediction of survivability is required in order to determine the risk to humans on the ground. According
to NASA Safety Standard 1740.14,
Guidelines and Assessment Procedures for Limiting Orbital Debris,
this impact risk, which is based on the predicted total debris casualty area, orbit inclination, and year of reentry, should be less than 1:10,000. ORSAT is frequently used for a higher-fidelity survivability analysis after
the NASA
Debris Assessment Software (DAS)
has determined that a spacecraft is possibly non-compliant with the
NASA Safety Standard.
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Example of
ORSAT used in predicting the Upper Atmosphere Research Satellite (UARS) spacecraft reentry breakup. Demise altitude vs. downrange evaluated for nearly all of the UARS components.
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The
ORSAT code uses integrated trajectory, atmospheric, aerodynamic, aerothermodynamic, and thermal/ablation models to perform a complete satellite or launch vehicle upper stage component analysis in determining the impact risk. A 3-degree-of-freedom trajectory is used with either the 1976 U.S. standard atmosphere or MSISe-90 atmosphere for spheres, cylinders, boxes, and flat plates for a total of 10 types of spinning and tumbling modes. Drag coefficients of these objects are considered from hypersonic speed down to subsonic speed to obtain kinetic energy of objects at ground impact. Stagnation point continuum heating rates are obtained for spherical objects and are adjusted for other bodies and for rarefied flow regimes. Both lumped mass and 1-D heat conduction models may be used to compute the surface temperature. The object is considered to demise when its absorbed heat reaches the material heat
of ablation.
Thermal
properties for over 75 materials are included in a database in ORSAT, with temperature-varying properties listed for thermal conductivity, specific heat, and surface emissivity. A parametric option may be run for objects that are on the threshold of demise or survival. Parameters such as oxidation efficiency, initial temperature, surface emissivity, number of layers, dimensions, or breakup altitude may be varied in a single run to obtain the critical demise/survival point of a component. After the best engineering judgment is applied with the parametric analysis, the total debris casualty area is computed. The impact risk is then determined to see if the satellite or upper stage is compliant with the NASA Safety Standard
1740.14.
