For more information, contact Joel W. Sills, Langley Research Center, joel.w.sills@nasa.govDownload the PDF version NASA’s Chemical Equilibrium with Applications (CEA) code is a foundational tool for propulsion system analysis. It provides equilibrium chemistry, rocket performance, shock, and detonation calculations used across NASA and the broader aerospace community. NASA Engineering and Safety Center (NESC) Activity TI-22-01730 modernized the legacy CEA2 Fortran code into CEA v3, a Fortran 2008, object-oriented software package with expanded interface support, updated thermochemical data, improved maintainability, and substantially improved workflow integration. The modernized code preserves backward compatibility with legacy CEA input workflows while enabling direct use from modern analysis environments, including Python, C, MATLAB, and automated design studies.Why Complex Modes Can Appear at Low Damping Classical modal analysis often treats lightly damped structures as nearly proportional systems with real mode shapes. That interpretation can become misleading when two modes are closely spaced. In modal coordinates, nonproportional damping introduces an off-diagonal coupling term ε. The relevant question is therefore not simply whether damping is small, but whether the coupling is large relative to the baseline half-frequency separation δ=(ω2 -ω1 )/2.Reduced Perturbation FrameworkFor a closely spaced pair, define the central eigenvalue λ0 ≈-ζω +iω and write the coupled eigenvalues as λ1,2=λ0 ±s. Under lightdamping and close-mode asymptotic assumptions, the local quartic eigenvalue problem reduces to s²=ε²/4-δ². This single relation determines the direction of eigenvalue separation in the complex plane, as summarized in Figure. 1. For 0