Blunt nose cones used in launch vehicles can excite buffet due to shock oscillations over the payload fairing on the leeward side at transonic Mach numbers and small angles of attack. Wind-tunnel tests show that the overall aerodynamic characteristics, such as the pitching moment and location of the center of pressure, experience sudden jumps at these conditions. Thus, the longitudinal stability and controllability of a launch vehicle, as well as unsteady loading on the payload fairing, can be affected. The cause of these oscillations is found to be interactions between the streamwise flow and reverse flow induced by a pair of counter-rotating vortices over the payload fairing. An aerospike was found to prevent the formation of counter-rotating vortices, shock oscillations, as well as jumps in overall aerodynamic characteristics, with no significant changes in the drag coefficient. The observations are supported by high-speed shadowgraphs and surface-flow visualizations. Computational fluid dynamics studies suggest that the shear layer from the tip of the aerospike envelops a significant region of the heat shield in a reverse flow due to the formation of a separation bubble. Mean-flow models of the topologies of the flowfield for the model without and with aerospike are proposed.