Fluctuating side-load measurements in overexpanded subscale rocket-nozzles

Abstract

A test campaign aiming at the measurements of side loads induced by the flow separation in overexpanded subscale nozzles has been performed in the R2Ch blowdown wind tunnel of the Onera Chalais-Meudon center. These tests have characterized the influence of the nozzle contour regarding the side load levels. Two axisymmetric nozzle models have been tested: one having a truncated ideal contour (TIC), the other being designed with a thrust-optimized contour (TOC) and having the ability to simulate wall film injection. The principle of the side load measurement relies on high-sensitive dynamic strain-gauges sticked on a stiff tube. The chosen tube stiffness ensures that the vibration eigenfrequencies of the tube are high enough to avoid interference with aerodynamic low frequencies. The tube displacements are so tiny that parasitic side loads due to tube deformation are negligible. The TIC nozzle, which gives the highest levels of side-loads, is characterized by one side-load peak when nozzle pressure ratio is about 44. The TOC nozzles show two side-load peaks, which correspond to the transition between two flow separation regimes. An evaluation of the low-frequency aerodynamic side-loads is obtained if we analyze only the frequency band beneath the tube eigenfrequencies. Aerodynamic side-loads keep the same trends that the measured side-loads, but are reduced of about 50%. Nomenclature L : throat-to-exit longitudinal length of the nozzle : global rms side-load moment My : side-load moment relative to the transverse axis Y Mz : side-load moment relative to the vertical axisZ NPR : nozzle pressure ratio, psj I pa pa : pressure in the test chamber (or test caisson) pst : stagnation pressure t : time Vsu, V^ : NPR variation speed, A( /?^ /p a ) /A t , during start-up and shut-down, respectively X : abscissa in streamwise direction Introduction A rocket first-stage engine nozzle has to operate from ground atmospheric conditions to extraatmospheric altitude conditions characterized by nearly vacuum levels. Ideally, to benefit from the complete gas expansion during all the ascent, a nozzle should have its adaptation regime at these extra-atmospheric altitudes, and should permanently operate in strongly overexpanded flow regime at low altitudes. * Research scientist Technical engineer * Research engineer 55 Engineering school student Copyright © 2001 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. As the flow separation induced by the overexpansion regime has a fluctuating and threedimensional character, unsteady non-symmetrical pressure efforts F(t), exerting on the nozzle wall, are generated. This leads to hazardous nozzle side loads (see Fig. 1) which could damage the engine structure. It is the reason why the expansion ratio of rocket engine nozzles is fixed today at a maximum value where the nozzles, although slightly overexpanded, are still running in a full flowing regime. Instantaneous separation lines F(t+At) Fig. 1 Side-load phenomenon in an overexpanded