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An experimental investigation has been carried out to study the flow characteristics of a dual-bell nozzle during its transition modes. Cold gas tests using gaseous nitrogen were carried out in (i) a horizontal test rig with nozzle exhausting into atmospheric conditions and, (ii) also inside a high altitude simulation chamber with nozzle operation under self-evacuation mode. Transient tests indicate that increasing 0P0 /0t reduces the transition time as well as the amplitude of pressure fluctuations of separation shock at wall inflection point. Sea-level tests show 15-17% decrease in transition NPR during subsequent tests in a single run primarily due to frost formation in the nozzle extension up to the wall inflection location. This 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.

*† ‡ For a dual-bell nozzle to be employed in a future reusable launch vehicle, active control of working mode transition is essential. To establish a technique for such control, the effectiveness of secondary injection was investigated by conducting cold-flow tests. The secondary injection was introduced from the inflection position of the nozzle contour perpendicular to the thrust axis. As a result, the secondary injection successfully changed the transition condition. Based on the results, a system and a sequence to realize the active control of the transition between two working modes of a dual-bell nozzle are herein discussed.

Dual bell nozzles represent a possible solution to improve the performance of large liquid rocket engines for launcher first stages. During the low-altitude mode, the flow is separated and the separation line is located near the inflection of the nozzle wall, in a region characterized by a negative value of the wall pressure gradient and where side loads may occur. At high altitude the flow attaches to the nozzle wall and the full area ratio is used. The flow behavior during the transition between the two operating modes is analyzed by time-accurate simulations carried out on a simplified planar geometry and considering two configurations: one with a wall film cooling and the other without film. Aim of this study is to analyze the effect of the secondary gas injection on the dynamics of the separation point, the transition time and the velocity of the separation front.

The dual bell nozzle is a concept of altitude adaptive nozzles. The flow adapts to the altitude by separation at the wall inflection at low altitude, and full flowing at high altitude. To understand the phenomenology of the flow by the transition from sea level to high altitude mode, a series of tests have been made at the cold flow test bench P6.2 at DLR Lampoldshausen. Three nozzles with different geometries have been tested. Two of them were successively shortened and driven unter the same conditions for each extension length. This study yields the influence of the geometric parameters of the basis and the extension on the transition conditions. Furthermore a transition prediction is given.