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 © 2011 by DLR, German Aerospace Center. Published by the American Institute of Aeronautics and Astronautics, Inc. , with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the 10. 00 per-copy fee to theCopyright Clearance Center, Inc. , 222RosewoodDrive, Danvers, MA01923; include the code 0748-4658/ 11 and 10. 00 in correspondence with the CCC. Research Scientist, Institute of Space Propulsion, Langer Ground. Head of Nozzle Group, Institute of Space Propulsion, Langer Ground. JOURNAL OF PROPULSION AND POWER Vol. 27, No. 4, July–August 2011