Analysis of Ares Crew Launch Vehicle Transonic Alternating Flow Phenomenon

A wind-tunnel test of the Ares I-X rigid buffet model identified unusually large buffet loads. These loads were produced by an alternating flow phenomenon at the crew-module/service-module junction. The conical design of the Ares I-X crew module and the cylindrical design of the service module expose the vehicle to unsteady pressure loads due to the sudden transition between a subsonic separated and a supersonic attached flow about the cone-cylinder junction as the local flow randomly fluctuates back and forth between the two flow states. These fluctuations produce a square-wave-like pattern in the pressure-time histories, resulting in large-amplitude impulsive buffet loads. Subsequent testing of the Ares I rigid buffet model found lower buffet loads because the evolved Ares I design includes an ogive fairing that covers the crew-module/service-module junction, thereby making the vehicle less susceptible to the onset of alternating flow. An analysis of the phenomenon indicates that it is most severe at low angles of attack and exacerbated by the presence of vehicle protuberances. A comparison of impulsive loads derived from wind-tunnel and flight-test data for the Ares I-X indicates significant overpredictions in magnitude and duration of the buffet load.