Ralf Stark

Side Loads in Subscale Dual Bell Nozzles

TODAY’S European heavy lifter Ariane 5 features a parallel staged design, where a cryogenic main stage is supported by two solid boosters generating the main part of the liftoff thrust. Its original objectivewas to deliver heavy payloads to a low Earth orbit. Nowadays Ariane 5’s dual GTO payload capability is in focus. In opposition to tandem-staged rocket systems, like Ariane 4, the main stage engine Vulcain 2 has to be ignited on the ground for security reasons to assure proper running before solid boosters’ ignition and rocket takeoff. Because of this design concept, the main stage engine has to fulfill a wide range of operation conditions, from sea level to near vacuum. To reduce undesired side loads that would affect the engine, the rocket structure, and even the payload itself, the nozzle area ratio is limited, preventing flow separation at sea level. This area ratio limitation leads to performance losses as the engine’s exhaust flow is driven overexpanded at sea level and highly under expanded at high altitudes. To optimize the overall Isp of an engine during ascent, the use of altitude-adaptive nozzles, where the thrust generation is not only optimized at one specific altitude, comes into focus as the subsystem with the most promising performance gain. Different concepts were developed to circumvent the limitation in area ratio of conventional nozzles. The commonly discussed solutions are plug, extendible, and dual bell nozzles. The characteristic contour inflection of the dual bell nozzle divides the nozzle into base and extension (Fig. 1) and offers a one-step altitude adaptation. At sea level, the contour inflection forces the flow to separate controlled and symmetrically (Fig. 2). The base nozzle flows full and the extension is separated: the dual bell is operating in sea levelmode. Because of a smaller effective area ratio the sea level Isp increases compared with a conventional nozzle (Fig. 3). At the designed altitude theflow attaches abruptly to thewall of the extension down to the exit plane (Fig. 4). This transition to high-altitude mode results in a short time Isp loss but later on in a higher vacuum performance. The dual bell’s major advantage is the absence of anymoving parts. Only minor changes to the design and the structure of already operating rocket engines would be necessary. The concept of applying a contour inflection was first mentioned by Foster and Cowles [1] within a study on flow separation in supersonic nozzles. Various solutions were suggested to prevent uncontrolled flow separation. The onewith an inflection dividing the nozzle in two parts was later patented as the dual bell nozzle by Rocketdyne in 1968. The first experimental study was performed by Horn and Fisher [2] with different extension contour design approaches in cold flow subscale tests. The transition from one operating mode to the other is particularly of interest as the flow potentially separates asymmetrically within the extension, resulting in a strong side load peak. The dual bell topic was introduced in the late 90s into Europe’s community [3]. Hagemann et al. [4] presented in 2000 experimental cold as well as hot flow studies with respect to side load generation. One remarkable fact is that the side load peak during retransition (while the nozzle is shut down) was shown to be significantly higher than during transition. An opposite result is given in studies performed since (e.g., by Hieu et al. [5]) where the transition to highaltitude mode generates higher side loads. The experimental cold flow results [4] were recalculated at DLR, German Aerospace Center by Karl and Hannemann [6] using the inhouse code TAU. The transient simulations showed that the calculated side load peak during transition mainly depended on the nozzle Presented as Paper 2010-6729 at the 46th AIAA Joint Propulsion Conference, Nashville, TN, 25–282010; received 3November 2010; revision received 3 February 2011; accepted for publication 8 February 2011. Copyright

Experimental Flow Investigation of a Truncated Ideal Contour Nozzle

Various tests showed a significant side load peak for low nozzle pressure ratios during engine start-up and shut down phase. DLR Lampoldshausen carried out tests to examine the flow field in a truncated ideal contour nozzle for low NPR. For NPR 20 a slight concave shaped Mach disk was found. Its curvature is limited to the centre and its height trend correlates with measured side loads. A concave shaped Mach disk being responsible for re-attached flows at low NPR could be excluded. The experiments were accompanied by numerical simulations of the flow field on various pressure ratios with regards on the shock pattern. The predicted Mach disk shape compares well with the experiments.

Experimental and Numerical Study of Heat Flux in Dual Bell Nozzles

The characteristic contour inflection of a dual bell nozzle is the key to altitude adaption. In sea level conditions, it forces the flow to a symmetrical separation, limiting the side load generation and increasing the thrust. After the transition, under high altitude conditions, the nozzle flows full, increasing the vacuum thrust. A hot flow experimental study has been conducted at the German Aerospace Center on a planar dual bell nozzle. The wall temperature distribution has been measured at various depths for the determination of the heat flux through the wall. The region of the inflection is of particular interest for the adjustment of the conventional cooling methods of dual bell nozzles. The contour inflection leads to a local increase of the thermal loads. In addition to the tests, the flow behavior and thermal loads have been calculated with CFD method and compared with the experiment.

Ariane 5 Performance Optimization Using Dual-Bell Nozzle Extension

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...

Reynolds Number Influence on Dual-Bell Transition Phenomena

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...

Gas density effects on dual-bell transition behavior

An experimental investigation was conducted to study the dual-bell transition behavior inside a high-altitude test facility under different back pressure environments. The high-altitude chamber was evacuated using an ejector nozzle and the dual-bell nozzle driving pressure, P 0N, was varied from 30 to 5.3 bar in subsequent test campaigns. As P0N was decreased, the dual-bell transition was observed to get delayed, and the transition nozzle pressure ratio increased by as much as 20% for the lowest driving pressure tested. Similar results were also observed for the dual bell retransition nozzle pressure ratio. For very low values of P0N (of 3.5 bar), transition did not occur at all. The delay in the dual-bell transition process to higher nozzle pressure ratio is primarily attributed to the increase in the width of inflection region with decrease in Reynolds number, which seems to control the dual-bell transition behavior. During these tests, it was also observed that the visible intensity of condensation shock decreased gradually with decreasing P0N and finally, for P0N � 17:4 bar, no condensation shock was observed. The decrease in gas density/reduced mass flow is also accompanied by a significant decrease in the amplitude of wall pressure fluctuations in the region of separation due to decrease in shock strength.

Advanced Flow Visualisation Techniques in Cold Gas Subscale Nozzles, a Comparison

Tests were conducted at DLR-Lampoldshausens cold gas subscale test stand P6.2 to visualise the separation in strongly over expanding nozzles. Several techniques were tested or developed like: Pressure Sensitive Paint, Backflow Frosting and Infrared Thermography. Pressure sensitive paint reacts with oxygen. The intensity of the emitted light depends on the partial pressure of the surrounding oxygen. As an operating gas nitrogen was used to prevent the reaction of the paint with oxygen in the attached flow region. This results in a light zone representing the attached flow and a dark zone representing the backflow of ambient air into the separated region. A clear visible contrast line visualises the physical separation of the flow. Backflow Frosting uses the behaviour of nearly saturated air at its due point. The saturated back flowing air passes the cooled nozzle wall downstream the separation line and hoarfrost forms out determining the physical separation. Infrared Thermography shows the deviation of the recovery wall temperature from its expected decreasing trend and therefore the start of the separation. All methods in combination give an impression of the separation zone. The visualisation results and the belonging pressure measurements are compared with an analytical model of the separation zone.

Numerical investigation of Dual Bell Nozzle Flow Field

The flow field of a cold flow dual bell has been numerically simulated with the DLR TAU code. The two operation modes and the transition from one mode to the other are investigated by varying the nozzle pressure ratio. A cold flow test campaign on a sub-scale nozzle model yields experimental data for validation of the simulation. Pressure measurements along the nozzle wall and schlieren optics provides information on the flow field in the nozzle. In addition, the models have been successively shortened to allow the observation of the shock system in the vicinity of the contour inflection.

Side Loads in Sub-scale Dual Bell Nozzles

The contour inflection of the dual bell nozzle forces the flow to a symmetrical and controlled separation in sea level mode. At a certain altitude, the transition to high altitude mode takes place: the flow attaches rapidly to the nozzle extension wall, down to the exit plane. During this transition, the separation point moves in the extension generating potential high side load peaks due to its asymmetrical position. A cold flow subscale test campaign has been conducted on three nozzle models at the German Aerospace Center to evaluate the generation of side loads in dual bell nozzles. The phenomenology is given for the different nozzle flow regimes. Both operating modes are related to very low side loads. Transition and retransition induce a strong short time peak. The phase of sneak transition, corresponding to a flow separation within the inflection region before the start of the actual transition, generates comparable side loads to separated conventional nozzles. The influence of the various geometrical parameters on flow behavior and side load generation was also investigated in this study. The extension length is shown to be the critical parameter for flow stability, transition duration and side load generation, leading to the necessity of a trade off for the optimization of the dual bell concept in rocket applications.

Side Loads in Dual Bell Nozzles, Part I: Phenomenology

nozzle flow regimes. The most important are strong short time peaks induced by transition and retransition from sea level to high altitude mode and vice versa. In sea level and high altitude mode the dual bell creates less side loads than a comparable conventional nozzle. An additional flow regime could be confirmed, addressed as sneak transition. Contrary to the accepted doctrine, the flow separation in sea level mode will not be continuously fixed at the contour inflection. Before the actual transition to high altitude mode takes place the flow separates within the inflection region. As a result a distinct phase of side loads comparable to separated conventional nozzles is given. A procedure to reduce the regime of sneak transition is proposed.