Chloé Génin

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 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...

Numerical Investigation of Flow Transition Behavior in Cold Flow Dual-Bell Rocket Nozzles

The dual-bell nozzle is an altitude-adaptive nozzle concept. It combines the advantages of a nozzle with small area ratio under sea-level conditions and a large area ratio nozzle under high-altitude conditions. Reynolds-averaged Navier–Stokes and unsteady Reynolds-averaged Navier–Stokes simulations on two-dimensional axisymmetric grids were conducted at DLR, German Aerospace Center in Lampoldshausen to investigate the transition from one mode to the other of a dual-bell nozzle model with positive pressure gradient extension. A cold flow test campaign conducted at DLR’s cold flow test facility P6.2 provided validation data for the numerical approach. The present study investigates the influence of different turbulence models and feeding pressure gradients on the dual-bell flow transition behavior. Better results were achieved for the Spalart–Allmaras and Reynolds stress turbulence model. A clear impact of the feeding pressure ramp on the dual-bell transition pressure ratio and the flow separation position ...

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.

Numerical Model for Nozzle Flow Application Under Liquid Oxygen/Methane Hot-Flow Conditions

A numerical study is conducted to investigate the impact of different chemical reaction mechanisms on the behavior of reactive nozzle flow. Therefore, a 66-step chemical reaction mechanism for oxyg...

LOX/CH4 Hot Firing Dual Bell Nozzle Testing: Part I -Transitional Behavior-

A sub-scale test campaign has been conducted in the framework of cooperation between the German Aerospace Center (DLR) and the Japan Aerospace Exploration Agency (JAXA). Both partners had great experience in cold flow dual bell nozzle investigation. The present campaign took place at P6.1 test facility at DLR Lampoldshausen with a common combustion chamber. Two test specimens were tested with similar conditions and measurement systems. The conditions provided at the bench were the combustion of gaseous methane with liquid oxygen at pressure up to 60 bar. The combustion chamber pressure was varied up and down with constant ramps while keeping the mixture ratio constant. Objective of the study was, a part from demonstrating the precision of the design tool, to investigate the sensitivity of the transition to flow property variations (like the one due to the mixture ratio). The transition and retransition conditions, the range of hysteresis and the transition velocity were of particular interest for DLR side. The work of JAXA concentrates on heat flux and combustion instabilities and is presented in the second part of this work.

Transitional Behavior of Dual Bell Nozzles: Contour Optimization

The dual bell nozzle features two operation modes permitting an altitude adaption. The geometry of the nozzle extension defines the transitional behavior from one operating mode to the other. In particular, the wall pressure distribution along the extension contour is critical for a fast transition. A contour extension featuring a constant wall pressure (CP) or with a positive pressure gradient (PP) ensures, in theory, an optimal transition. Experimental and numerical researchs have been conducted at the German Aerospace Center to optimize the transitional behavior through extension geometry variation. Three nozzles have been investigated: a CP-reference contour and two PP-extensions. The positive wall pressure gradient in the extension leads to a better stability of the operation modes (wider range of hysteresis). The wall pressure gradient has also a critical influence on the sneak transition length and duration and should be therefore chosen accordingly.

Hot flow testing of dual bell nozzles

The dual bell nozzle offers a simple and efficient altitude adaption through its contour inflection, which insures a symmetrical and controlled separation at sea level and a large area ratio at high altitude. The conditions for the flow transition from one operating mode to the other have been intensively investigated in cold flow conditions. A test campaign has been conducted on two dual bell nozzle geometries to verify the theoretical prediction criterion and to investigate the influence of the temperature variations. Therefore, a thin walled axisymmetric and a planar nozzle were designed and tested under hot flow conditions. Wall temperature measurements have been made using both thermocouples and thermal imaging. The region of the contour inflection was of particular interest for the evaluation of thermal behavior in the nozzle wall.