Lateral-directional coupling and unsteady aerodynamic effects of hypersonic vehicles

Results from a joint research program of the Institutes of Fluid Dynamics and of Flight Mechanics and Flight Control of the Technical University of Munich on a two-stage space transportation system are presented. The unsteady aerodynamics resulting from an orbital stage at stage separation flow conditions at Mach number 6.8 are first discussed. Unsteady flowfields associated with yaw and roll oscillations of the orbital stage at a certain distance above the carrier stage are investigated. The calculations are based on a finite volume method for real-time solutions of the unsteady Euler equations. The results focus on pressure distributions and aerodynamic coefficients providing an essential means for carrying out stability and control investigations. In the second part, flight mechanics stability and control problems are addressed considering key issues of lateral - directional dynamics. Inherent vehicle characteristics can show specific stability and flying quality deficiencies in hypersonic flight, concerning a partially unstable dutch roll mode with high roll-yaw coupling and weak roll damping. There is also a coupling of the roll and spiral poles for certain configurations, yielding a slow, eventually unstable oscillation called lateral phugoid. Reasons and effects for the stability deficiencies are discussed, including conditions of existence for the lateral phugoid.