Unsteady and coupling aerodynamic effects on the lateral motion in hypersonic flight

Results from a joint research program of the Institute of Fluid Dynamics and the Institute of Flight Mechanics and Flight Control of the Technische UniversitSit Miinchen on a two-stage space transportation system are presented. The first part of the paper deals with the unsteady aerodynamics of the orbital stage separating from the carrier stage at a Mach number of 6.8 while the second part contains related flight mechanics stability and control problems. In particular, unsteady flowfields are investigated associated with yaw and roll oscillations of the orbital stage at a certain distance above the carrier stage. The flowfield calculations are based on an explicit finite-volume-method for real-time solutions of the three-dimensional unsteady Euler equations. The discretization of the colnputational domain around the entire configuration is performed using a flexible multiblock grid generator. The results obtained focus on the pressure distributions as well as on the aerodynamic coefficients. They provide an essential mean to carry out stability and control evaluations discussed in the flight mechanics part. Consequently, key issues of hypersonic lateral dynamics and its significant. interdependencies with aerothermodynamics are considered. The inherent dynamic characteristics show specific stability and control deficiencies relating to a partially unstable dutch roll.mode with high roll-yaw-coupling in combination with a weak roll damping. Additionally, for certain configurations coupling of the roll and spiral poles appears to form a slow, eventually unstable oscillation called lateral phugoid. Decisive reasons and effects concerning the stability deficiencies addressed are identified by analyzing specific approximations for the lateral modes in hypersonic flight. In this regard, criterions concerning the existence of lateral phugoid are developed.