Effects of Shear-Layer Reattachment on the Nozzles of a Launch Vehicle

Abstract

Wind-tunnel tests in the Mach number range of 0.18-2.5 showed the reattachment of shear layer from the strap-on boosters onto a point on the canted nozzles of a multibooster launch vehicle model. Such a reattachment can cause dynamic side loads on the nozzles and the associated gimbal actuators. A mean flow model of the forcing function was construed based on qualitative studies. The forcing function associated with the external excitation up to 550 Hz was quantified using specially designed strain-gauged nozzles that had a natural frequency of more than 700 Hz. With gimbaling, magnitudes of mean moments at the simulated location of the gimbal actuator were found to increase while the fluctuations decreased. Simple passive "load alleviation devices," which prevent local flow separation, deflect the trajectory of the separating shear layer and avoid reattachment on the nozzle were found to substantially reduce the flow-induced mean moments. A short semicircular segment that envelopes the boat tail of the strap-on boosters was found to be the most effective device for reducing the mean moments, as well as flow-induced unsteadiness on the nozzles at the location of gimbal actuators. The nozzle response indicates occurrence of discrete frequencies of about 103 and 466 Hz in the forcing function.