Dynamic stability of hammerhead launch vehicles at transonic Mach numbers

Ericsson's (1986) quasi-steady approach is used to assess the dynamic stablity of two typical launch vehicles with hammerhead payloads and compare it with the aeroelastic stability of similar launch vehicles available in the literature. The aerodynamic damping at transonic Mach numbers is calculated taking into account the highly nonlinear local aerodynamic load that can occur due to shock-induced flow separation. It is shown that destabilizing aerodynamic damping is the result of such nonlinear loads occurring in an on-off fashion during bending vibration of the launch vehicle. Instability proneness of a particular mode is found to be greater if the vehicle executes bending oscillations about near-zero angles of attack. Design solutions are suggested for averting the flow phenomenon driving the instability.