JOURNAL OF SPACECRAFT AND ROCKETS

Vol. 38, No. 6, November–December 2001

Hyper-X Aerodynamics: The X-43A Airframe-IntegratedScramjet Propulsion Flight-Test Experiments

I N 1996 NASA initiated the Hyper-X program as part of an ini-tiative to mature the technologiesassociatedwith hypersonicair-

breathing propulsion. The primary goals of the Hyper-X programare to demonstrate and validate the technologies, the experimen-tal techniques, and the computational methods and tools requiredto design and develop hypersonic aircraft with airframe-integrated,dual-mode scramjet propulsion systems. Hypersonic airbreathingpropulsion systems, studied in the laboratory environment for over40 years, have never been � ight tested on a complete airframe-integrated vehicle con� guration. This series of papers outlines theground-based aerodynamic analysis efforts, including the experi-mental wind-tunnel test programs and the computationally baseddesign and assessment activities, which led to the � rst Hyper-XX-43A vehicle � ight test. Six of the seven papers in this serieswere � rst presented in a special invited papers session at the 18thAIAA AppliedAerodynamicsConferenceheld in August2000.Theseventh paper, which has a more obvious scramjet propulsion testprogram emphasis, was presented at the 10th AIAA InternationalSpace Planes Conferenceheld in April 2001. It is includedhere as ameansbywhich tohighlighttheaeropropulsiveinteractionsandcou-pling for this particular class of vehicle with an airframe-integratedscramjet propulsion system. Collectively, these papers provide avery brief overview of the broad effort executed to de� ne and de-velop theaerodynamicdatabasesin supportof the � ight-testelementof the Hyper-X program.They offer insights into the aerodynamics-related engineering challenges directly associated with the X-43AResearch Vehicle � ight test, as well as documenting several of thekey advances made to the state of the art in ground-basedairframe-integrated scramjet propulsion system testing and analysis and inthe area of hypersonic boundary-layer trip design.

Although the � rst � ight-test attempt, conducted in June 2001,endedprematurelyin a failureduring the rocket-assistedboost to thescramjet test condition, two additional expendable X-43A vehiclesare currently being readied for � ight tests. The eventual successful� ight testing of these vehicles will provide a unique opportunityto obtain hypersonic aerodynamic data on a slender-body, nonaxi-symmetric airframewith an airframe-integratedscramjetpropulsionsystem. Because of the highly integrated nature of the propulsionsystem with the airframe, the traditional distinctions between vehi-cle aerodynamicsand propulsionare blurred. Therefore in additionto the scramjet operational and performance data that will be ob-tained, a tremendous amount of aerodynamic data will be gatheredduring the � ight tests, so that a more complete understandingof theaeropropulsive interactions in � ight can be formulated.

The � rst in this seriesof sevenpapersprovidesa generaloverviewof the Hyper-X program and the � ight-test mission sequence. Theemphasis here is on the overall aerodynamicdatabase developmentactivities in support of the Hyper-X X-43A � ight-test program. Asummary is provided of the overall ground-based wind-tunnel testprogram and parallel computational � uid dynamics (CFD) analysiseffortsconductedto supporttheentireX-43A� ight-testmissionpro-� le.This is followedby some speci� c detailson theX-43A ResearchVehicle’s aerodynamic characteristics, including the direct and in-direct effects of the airframe-integratedscramjet propulsion systemoperation on the basic airframe stability and control characteristics.More speci� c details on all of the subject areas are provided in thefollowing papers.

The second and third papers focus speci� cally on the aerody-namics associated with the stage separation event. Although theultimate goal of the � ight test is to obtain data at and around thehypersonic scramjet test conditions,deliveringthe X-43A ResearchVehicle to the test point presents a number of unique challenges.Byfar the highest-risk element of the mission was deemed to be thestage separation event, in which the X-43A Research Vehicle must

separate from the � rst-stage rocket booster at the extreme environ-mental conditionsassociatedwith � ight at Mach seven and dynamicpressure in excess of 1000 lb/ft2. To reduce the risk associatedwiththe stage separation event, a comprehensive wind-tunnel test pro-gram and computationaleffort using state-of-theart CFD codes andcapabilities was undertaken. The second paper describes in detailthe stage separation wind-tunnel test program and offers some in-sight into the complexities of testing two nonaxisymmetric bodiesin close proximity. The third paper provides details of the stageseparation computational activities, which included benchmarkingof codes with experimental data and extending the aerodynamicdatabase with solutions when experimentalmethods could not pro-vide results or suf� cient detail. The computational activities alsohelped to provide a more thorough and detailed understanding ofthe complex � ow structure between the two bodies throughout theseparation event.

The comprehensive series of wind-tunnel tests on the X-43AResearch Vehicle con� guration are described in detail in the fourthpaper. These tests providedairframe aerodynamicperformanceandstability and control data covering the entire � ight-test envelopefrom the hypersonic test conditions at and around the scramjet testpoint through the supersonic, transonic, and subsonic regimes theX-43A must � y through before completing the � ight-test mission.Brief descriptionsof the variouswind-tunnel facilities,wind-tunneltestmodels, and test techniquesand rationaleare provided,followedby select sample data from several of the tests conducted.

The � fth paper provides detail on the computational aspects ofthe coupled aeropropulsiveperformance prediction methodologiesemployedduring thepre� ight program.The highlyintegratednatureof this scramjet � owpath within the con� guration airframe presentsdistinctchallengesto the aerodynamicist:How to separateand prop-erlyaccountfor the scramjet-propulsion-inducedeffectson airframeaerodynamics and overall performance? In this paper a descriptionis provided of the computational analysis tools and methodologiesthat were utilized to develop the pre� ight predictions of the pow-ered aeropropulsive � ight characteristics. A brief examination ofthe detailedcomputationalpredictionsof the � ow� eld structuredur-ing the scramjet operation is provided, followed by some thoughtson future computational technology development requirements forhypersonic propulsion-airframeintegration analysis.

The sixth paper in this series provides the logical link betweenthe scramjet propulsion test and analysis activities and the over-all vehicle aerodynamics and performance database development.A series of powered airframe-integrated scramjet tests were con-ducted in the NASA Langley 8-Foot High Temperature Tunnel us-ing a full-scale airframe structure that duplicated the entire X-43Athree-dimensionalpropulsion� owpath surface and utilized an exactduplicateof the � ight engine. These tests replicated the Mach num-ber and enthalpy conditions expected at the design � ight-test point.This test series provided engine performance and operability dataand the � nal ground-baseddesign and database veri� cation bench-marks for the X-43A Mach seven � ight tests. Details are providedon the test series objectives, the facility and test article hardware,and results from the test, which helped to tie all of the aerody-namic and propulsion performance analysis predictions together,prior to obtaining the ultimate benchmark set of data from � ighttests.

The seventh and � nal paper deals speci� cally with the hyper-sonic boundary-layer trip design and development activity, whichwas integral to maximizing the scramjet propulsion system de-signed performance. To ensure robust scramjet engine operation,the boundary layer entering the engine inlet must be turbulent tominimize the possibilityof separated � ow conditions and inlet � owdistortion. Ingestion of a turbulent boundary layer increases inlet

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802 INTRODUCTION

operability and enhancesoverall scramjet engine performance.Cri-teria for natural boundary-layer transition indicate that some sortof arti� cial tripping mechanism must be employed on the X-43AResearch Vehicle forebody to ensure turbulent � ow conditions atthe inlet for the Mach seven scramjet test conditions. A programwas undertaken to develop and tailor a hypersonic boundary-layertrip design that would ensure turbulent � ow at the scramjet in-let to maximize engine performance while at the same time min-imizing aerodynamic performance losses resulting from the in-duced drag on the vehicle. Various trip designs were investigatedand tested in a number of hypersonic facilities. The overall hy-personic boundary-layer trip development test program and selec-tion criteria are described, and details of experimental results areprovided.

Even at this point in the Hyper-X program, prior to the � rst suc-cessful � ight test, a tremendous amount of knowledge has beengained through ground test and computational analysis efforts re-gardingscramjetpropulsionsystemandairframeaerodynamicinter-actions. Several hypersonic airbreathing propulsion follow-on pro-gram studies are currently underway and are utilizing the wealth ofknowledgeand lessons learnedin theground-basedtestingand com-putational analysis portions of the Hyper-X program. The Hyper-Xprogram remains strong and is committed to meeting its statedgoals througheventual successful � ight tests of the second and thirdX-43A vehicles.

Walter C. EngelundNASA Langley Research Center

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