A method has been developed for real-time stability margin measurement calculations. The method relies on a tailored-forced excitation targeted to a specific frequency range. Computation of the frequency response is matched to the specific frequencies contained in the excitation. A recursive Fourier transformation is used to make the method compatible with real-time calculation. The method was incorporated into the X-38 nonlinear simulation and applied to an X-38 robustness test. X-38 stability margins were calculated for different variations in aerodynamic and mass properties over the vehicle flight trajectory. The new method showed results comparable to more traditional stability analysis techniques, and at the same time, this new method provided coverage that is more complete and increased efficiency.Bosworth, John T. and Stachowiak, Susan J.Armstrong Flight Research CenterREAL TIME OPERATION; ROBUSTNESS (MATHEMATICS); STABILITY TESTS; X-38 CREW RETURN VEHICLE; CONTROL SYSTEMS DESIGN; FREQUENCY RANGES; FREQUENCY RESPONSE; NONLINEARITY; SIMULATION; RUDDERS; TRAJECTORIES; MONTE CARLO METHOD; EXCITATION; FAST FOURIER TRANSFORMATIONS...