To evaluate the impact of dual-bell nozzles on the payload mass delivered into geostationary transfer orbit by Ariane 5 Evolution Cryotechnique Type A (ECA), detailed studies were conducted. For this purpose, a multitude of Vulcain 2 extension contours were designed. The two variation parameters were the starting point and the inflection angle of the nozzle extension. As the most upstream starting point, the position of the turbine exhaust gas injection was chosen. Geometrical restrictions were imposed by the launch pad ELA 3. Considering these parameters, an analytical and a numerical method were applied to predict the impact of the dual-bell nozzle on the payload mass. The analytical approach yields a correlation between specific impulse, nozzle mass, and payload mass increment. The numerical approach was conducted applying German Aerospace Research Center’s trajectory simulation code Trajectory Optimization and Simulation of Conventional and Advanced Transport Systems. Both calculation procedures yield...

Ariane 5 Performance Optimization Using Dual-Bell Nozzle Extension

An experimental investigation was conducted to study the effect of ambient pressure fluctuations on the dual-bell
transition behavior. Ambient or backpressure fluctuations of different frequencies (1.0, 1.5, and 3 Hz) were artificially introduced inside the high-altitude simulation chamber using three magnetic valves. The dual-bell transition behavior was found to be very sensitive to backpressure fluctuations of magnitude >20%, which triggered a flip-flop phenomenon that was observed to continue as long as the fluctuations prevailed. Increasing the throttling-up rate also did not seem to help much in achieving a stable dual-bell transition within the scope of the present tests. This, too, was not favorable, as it can lead to a delay in transition. These test results suggested that the dual-bell nozzle can present problems in real-flight operation, especially when the launcher experiences the buffeting phase of flight. The solution
perhaps lies in increasing the dual-bell stability parameter to a value greater than the magnitude of backpressure fluctuations so as to avoid the nozzle pressure ratio range that will set the dual bell into a nonstop flip-flop mode during buffeting.

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Effect of Ambient Pressure Fluctuations on Dual-Bell Transition Behavior

An experimental investigation was conducted to study the Reynolds number influence on dual-bell transition behavior for tests inside a high-altitude simulation chamber. For the range of nozzle supply pressures tested, the nozzle Reynolds number is seen to gradually decrease from a relatively high value (of the order of 107 for tests in sea-level atmospheric conditions) toward the transitional range (lower side of 106 for tests inside the high-altitude chamber). This influences the width of the inflection region, which is seen to decrease with an increase in nozzle Reynolds number. Because of the smaller negative pressure gradient experienced during sneak transition with a decrease in nozzle Reynolds number, the separation point is seen to move into the region of wall inflection much earlier and tends to stay in the region of wall inflection for a relatively longer time. Although the time duration of final transition remains more or less constant for different nozzle supply pressure values, the time durati...

Reynolds Number Influence on Dual-Bell Transition Phenomena

An experimental investigation has been carried out to study the changes in the exhaust flow development of a thrust-optimized parabolic nozzle when it is tested in two different test environments, namely, under sea-level conditions and in a low-pressure environment inside a high-altitude test facility. For tests inside the high-altitude simulation chamber, the incipient separation location is found to be located considerably upstream relative to its location under sea-level tests. Furthermore, it is also observed that the location of normal shock for tests inside the high-altitude chamber is slightly downstream of its position in sea-level tests. These flow modifications shift the momentum imbalance along the overexpansion and reflected shocks in such a way that only free shock separation condition is favored (both during startup and shutdown transients) for all test conditions inside the high-altitude facility. As a result, flow transitions such as free shock separation to restricted-shock separation and vice versa, observed in the present tests under sea-level conditions, are not at all observed for tests inside the high-altitude test facility. The primary cause responsible for such a discrepancy in the observed flow conditions is the significantly 
reduced mass flow or density of the test gas [due to the lower driving pressure (P 0) required to achieve similar nozzle pressure ratio] inside the high-altitude chamber. The reduced mass flow also prevents the gas to nucleate earlier, and hence delays the onset of nitrogen condensation to a much higher nozzle pressure ratio.

Cold Gas Testing of Thrust-Optimized Parabolic Nozzle in a High-Altitude Test Facility

An experimental investigation has been carried out to study the linear-plug-nozzle flowfield development with and without freestream flow. The effect of truncation (40%), plug contour, and side plates on base pressure development has been also studied. Relative to no freestream condition, the higher outward deflection of the initial portion of the free-jet boundary in the presence of freestreamprevents the overexpansion shock from the upper portion of the inner nozzle to impinge on the plug surface, modifying the flow development downstream of the overexpansion shock. This reduces the altitude-adaptive character of the plug nozzle and also influences the base pressure value, which is seen to decrease with increase in freestreamMach number. The nozzle pressure ratio for base wake closure is also found to be sensitive to the plug configuration. Contouring the plug significantly enhances the base pressure value, but a smaller base initiates an early wake closure. The addition of side plates results in an early base wake closure in each case. The characteristic frequency of base pressure fluctuations is found to be strongly dependent on plug configuration and base region size.

Linear-Plug Flowfield and Base Pressure Development in Freestream Flow

An experimental investigation has been carried out to study the effect of test environment on transition characteristics and the flow unsteadiness associated with the transition modes of a dual-bell nozzle. Cold-gas tests using gaseous nitrogen were carried out in (i) a horizontal test-rig with nozzle exhausting into atmospheric conditions and, (ii) a high altitude simulation chamber with nozzle operation under self-evacuation mode. Transient tests indicate that increasing δP0/δt (the rate of stagnation chamber pressure change) reduces the amplitude of pressure fluctuations of the separation shock at the wall inflection point. This is preferable from the viewpoint of lowering the possible risk of any structural failure during the transition mode. Sea-level tests show 15–17% decrease in the transition nozzle pressure ratio (NPR) during subsequent tests in a single run primarily due to frost formation in the nozzle extension up to the wall inflection location. Frost reduces the wall inflection angle and hence, the transition NPR. However, tests inside the altitude chamber show nearly constant NPR value during subsequent runs primarily due to decrease in back temperature with decrease in back pressure that prevents any frost formation.

C. Nuerenberger-Genin

Papers

Cold-gas experiments to study the flow separation characteristics of a dual-bell nozzle during its transition modes