Flexible Launch Stability Analysis Using Steady and Unsteady Computational Fluid Dynamics

Abstract

LaunchvehiclesfrequentlyexperienceareducedstabilitymarginthroughthetransonicMachnumberrange.This reduced stability margin can be caused by the aerodynamic undamping one of the lower-frequency flexible or rigidbodymodes. Analysis of the behavior of a flexible vehicle is routinely performed with quasi-steady aerodynamic line loads derived from steady rigid aerodynamics. However, a quasi-steady aeroelastic stability analysis can be unconservative at the critical Mach numbers, where experiment or unsteady computational aeroelastic analysis showareducedorevennegativeaerodynamicdamping.Amethodofenhancingthequasi-steadyaeroelasticstability analysis of a launch vehicle with unsteady aerodynamics is developed that uses unsteady computational fluid dynamics to compute the response of selected lower-frequency modes. The response is contained in a time history of the vehicle line loads. A proper orthogonal decomposition of the unsteady aerodynamic line-load response is used to reduce the scale of data volume and system identification is used to derive the aerodynamic stiffness, damping, and mass matrices. The results are compared with the damping and frequency computed from unsteady computational aeroelasticity and from a quasi-steady analysis. The results show that incorporating unsteady aerodynamics in this way brings the enhanced quasi-steady aeroelastic stability analysis into close agreement with the unsteady computational aeroelastic results.