an overview of nasa’s high-speed research program · 13. supplementary notes see also adm002333....

ICAS 2000 CONGRESS

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AN OVERVIEW OF NASA’S HIGH-SPEED RESEARCHPROGRAM

Dr. Alan W.WilhiteDirector, Systems Management Office

NASA Langley Research Center

Dr. Robert J. ShawDirector, Ultra-Efficient Engine Technology Program

NASA Lewis Research Center

Abstract

The National Aeronautics and SpaceAdministration sponsored a joint High-SpeedResearch Program with United States airframeand propulsion companies to provide the criticalhigh-risk technologies for a Mach 2.4, 300passenger civil transport. Laboratory andmedium-scale tests provided promising results inmeeting both the environmental and economicgoals for a future High-Speed Civil Transport.However, before the technologies weredemonstrated at large and full scale, theprogram was cancelled because of globaleconomic concerns of the U.S. transportindustry.

1 Introduction

The U.S. initially dominated the world’s civil jettransport market, but over the past 15 years theEuropean Airbus Industrie has been eating intothe U.S. market share. In the past few years,Airbus has captured half of the civil largetransport market share. The Europeans arecontinuing the offensive with a very largetransport (>500 passengers) on the drawingboard and the development of technologies for asecond generation Concorde. Although theConcorde was not an overwhelming financialsuccess for the manufacturers, it was atechnological and systems integration marvel forits time and provided the confidence and

foundation for the formulation of the multi-nation Airbus Industrie. Airbus was formed in1970, one year after the initial flight of theprototype Concorde.

The U.S. industry has shelved the plans formegaliners stating a lack of market for that sizeairplane; however, they were keenly interested ina more productive, mid-size (300 passenger)airliner that could leap frog the current transportindustry. Speed is one way in whichproductivity can be dramatically improved.Compared to a megaliner, a mid-size supersonicairplane can fly the same number of passengersusing two round trips while halving passengertravel time and provides more market flexibilitywith its reduced passenger capacity. Asupersonic airplane can be used as a mid-range,large-capacity transport saving over 6 hours fromLos Angeles to Tokyo or as a long-range, mid-size transport by using short service stops savingover 10 hours from Los Angeles to Sydney.

Congress cancelled the U.S. SupersonicTransport (SST) program in March 1971.Cancellation justification was based on bothenvironmental and performance issues.Environmentally, many countries outlawedsupersonic flight overland because of the sonicboom, thus severely restricting projected marketpenetration; atmospheric scientists predictedcatastrophic reductions of ozone from engineemissions severely restricting fleet size; aircraftregulators wanted the engines that were designed

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14. ABSTRACT The National Aeronautics and Space Administration sponsored a joint High-Speed Research Program withUnited States airframe and propulsion companies to provide the critical high-risk technologies for a Mach2.4, 300 passenger civil transport. Laboratory and medium-scale tests provided promising results inmeeting both the environmental and economic goals for a future High-Speed Civil Transport. However,before the technologies were demonstrated at large and full scale, the program was cancelled because ofglobal economic concerns of the U.S. transport industry.

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Dr. Alan W. Wilhite, Dr. Robert J. Shaw

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for supersonic flight to meet subsonic noisecertification standards; and health officials wereconcerned about the effects of high-altitudeatmospheric radiation. In addition, performanceissues that were cited for the cancellationincluded the need for more efficient lift-to-dragratio for both subsonic and supersonic flight;sufficient thrust from propulsion at bothsupersonic and subsonic speeds with low noiseand efficient fuel consumption; airframestructures and materials with greater strengthwith less weight, and system integrationtechniques to maximize airplane efficiency.

Beginning in 1989, NASA and industryinvestigated the potential of a High-Speed CivilTransport (HSCT), the airplane specifications,and required technologies. The existing Mach 2European Concorde and Russian Tu-144airplanes are not environmentally acceptable oreconomically viable. The original U.S. SSTdesign was planned for Mach 2.7, but itstitanium structure was too heavy. In 1989, theNational AeroSpace Plane Program wasinvesting in Mach 25 technology for both Earth-to-Orbit transport and “Orient Express” civiltransport applications. At these speeds, therequired hydrogen fuel would dictate extremechanges in existing airport infrastructure and theairplane efficiency would be limited because itwould rarely see cruising speed with longacceleration and deceleration times required forpassenger comfort. The studies concluded thatan airplane launched in the early 21st centuryshould be compatible with current airports, usejet fuel, and be within a 10- to 15-yeartechnology reach. Both Boeing and McDonnellDouglas converged on a Mach 2.4, 300passenger, 5000 nautical mile airplane as a focusfor technology development.

Based on the market and technologyprojections of a HSCT, NASA started the two-phase High-Speed Research (HSR) technologyprogram in 1990 with the civil transport industry-- Boeing, McDonnell Douglas, General Electricand Pratt and Whitney. The $280M Phase I

Program focused on the development oftechnology concepts for environmentalcompatibility. With the successful completion ofPhase I, the $1400M Phase II Program started in1993. This Phase was to demonstrate theenvironmental technologies and define anddemonstrate selected high-risk technologies foreconomic viability.

However, because of global economics andthe U.S. industry focus on keeping their subsonicmarket viable, the HSR program was cancelledin 1999. At this point in the program, thetechnology selections were made for final full-and large-scale demonstrations based onmedium-scale ground tests and flight tests. Thefollowing is a status report of the program whenthe program was cancelled.

2 Environmental Compatibility

2.1 Atmospheric Impact and EmissionReductionThe most publicized environmental concernabout supersonic flight is the depletion ofstratospheric ozone. Dr. Harold Johnston led agroup of atmospheric scientists in the SSTCongressional hearings of the early 1970s thatraised the potential of the significant depletion ofthe Earth's ozone shield to a global concern.

Ultraviolet rays break down stratosphericozone into molecular and atomic oxygen. Thesemolecules are later reunited in a reaction thatforms new ozone that maintains a sufficient layerto protect the Earth from ultraviolet rays. Thecurrently proposed supersonic transport cruisealtitude, where nitrogen oxides (NOx) would beintroduced from the engine exhausts, is about60,000 feet, near the maximum density ofstratospheric ozone. These nitrogen oxides, withenergy from ultraviolet rays, break down anddeplete the amount of ozone by formingmolecular oxygen.

The Atmospheric Effects of StratosphericAircraft element was formed in Phase I of the

AN OVERVIEW OF NASA’S HIGH-SPEED RESEARCH PROGRAM

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HSR Program to address the ozone issue. Aninternational team of scientists developed globalatmospheric models to predict the impact ofnitrogen oxides, water vapor, and other exhaustemissions on ozone chemistry and climatechange. These 2- and 3-dimensionalphotochemical transport models were calibratedthrough laboratory chemical kinetics tests andatmospheric observations. A converted U-2 spyplane called the ER-2 (Figure 1) and balloonsprovided global coverage to identifyphotochemical, radiative, and dynamic featuresof the stratosphere. The ER-2 was also used tosniff the exhaust of the Concorde providingvaluable data for near-field atmosphericinteractions. As input to these atmosphericmodels, a global engine exhaust emissionsdatabase was defined by city-pair operationalscenarios for the HSCT and corresponding flighttrajectories between these airports.

Figure 1. ER-2 atmospheric research airplane.

Results from five 2-dimensionalatmospheric models showed a range of -0.7percent to +0.1 percent ozone impact, wherepositive is creating ozone, based on a maturefleet of 1000 HSCT’s flying at Mach 2.4 and acombustor emissions index of 5 grams of NOxper kilogram of fuel burned. These resultsaffirmed the combustor goal of an emissionsindex of 5 as compared to approximately 20 forthe Concorde. Further research enhancedunderstanding of both chemical and transportmechanisms as recommended by the National

Research Council with an emphasis on additionalatmospheric observations to assist modelvalidation.

The high level of NOx emissions fromcurrent combustors are due to burning fuel nearstoichiometric air-to-fuel ratios; the key toreduced combustor emissions is to burn eitherfuel rich or lean. One concept was a two-stagecombustor design called rich burn, quick quench,lean burn design where fuel is initially burnedrich, air is then added, and the products areburned lean. The quick air quench mixing withthe rich burn products was the challenge. Thesecond concept was the pre-mixed, pre-vaporized lean burn combustor where flamestability and hardware complexity dominatedresearch. Both concepts were tested in flametube laboratory tests at NASA Glenn ResearchCenter and reduced scale sector tests showingemission indices less than 5 grams NOx perkilogram of fuel at the propulsion industryfacilities.

For both combustor concepts, liner materialwas also a challenge because active cooling withair changes the mixing and chemistry that arecritical for low NOx. Thus, ceramic matrixcomposites was the leading candidate materialfor the 3500°F environment and 9000-hour liferequirements. These composites have beendemonstrated at design temperature and nearmechanical load conditions using accelerated testtechniques.

2.2 Community NoiseCommunity noise is the dominant constraint inthe selection of engine cycle and airframeconfiguration designs. Any future supersonicairliner will operate from existing internationalairports and must meet local airport communitynoise requirements and national/internationalnoise certification regulations similar to subsonicairliners. Because the HSCT was projected notto operate until after the turn of the century,subsonic noise reduction technologies and thepotential for more stringent regulations were


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being closely followed to formulate therequirements for supersonic technologydevelopment. A final conclusion is that anyfuture airplane including the HSCT must be atleast as quiet as the fleet average at productionstart date.

The primary noise source for takeoff is fromthe high-speed, hot engine jet exhaust. Theturbojet cycle used in the Concorde has excellentsupersonic performance but also has high noiseas opposed to high-bypass-ratio turbofans usedby subsonic commercial transports designed forsubsonic performance and low noise. Acompromise cycle called the mixed-flowturbofan was selected for the HSR technologydevelopment. Although this cycle does have amoderate bypass ratio to slow the jet, a mixer-ejector nozzle must also be included to furtherreduce noise.

This nozzle entrains outside freestream air,which is mixed with the core jet exhaustresulting in a slower and cooler exhaust jet thatreduces noise by 16 decibels out ofapproximately 120. During supersonic cruise,external air entrainment is not required, thuseliminating the drag and reduced efficiencyassociated with the noise suppression mode.Small-scale wind-tunnel and large-scale (Figure2) nozzle tests demonstrated the required noise

Figure 2. Large-scale nozzle test.

attenuation while meeting performancerequirements. To keep nozzle weight at aminimum, advanced materials andmanufacturing processes were being developedincluding thin wall castings of superalloys forthe mixer, gamma titanium aluminides for theflap, ceramic matrix composite acoustic tiles forreducing mixing noise, and thermal blankets toprotect the nozzle backside materials.

For approach noise where the engines arethrottled back and have reduced jet velocity,turbomachinery noise is a key contributor tooverall aircraft noise. To address thisenvironmental compatibility challenge, NASAand industry looked at both mixed compressioninlet and low-noise fan concepts. Mach 2.4operation requires a mixed compression inlet forefficient propulsion system operation where theshock structure is managed both externally andinternally. Stability, high recovery, and lowdistortion in the inlet must be balanced with low-noise, operability, and complexity. Two-dimensional bifurcated, axisymmetric translatingcenterbody, and variable diameter centerbodyinlets were being considered. Small-scale wind-tunnel tests, operability tests, and duct spacingground tests of the Tu-144 engine provided datafor the selection, design, and validation testingfor performance and operability through thespeed regime and approach acoustics.

Additional noise reduction can be attainedwith high-lift leading and trailing edge wingsystems which reduce thrust required for takeoff,climbout, and landing. These advanced high-liftconcepts more than double the low-speed lift-to-drag ratio relative to the Concorde whencombined with advanced landing and takeoffprocedures such as automatic flap and throttlesettings.

2.3 Sonic BoomAt supersonic speeds, an aircraft produces shockwaves that propagate to the ground, creatingsudden pressure changes that may be perceivedas a startling and annoying noise. To minimize


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human disturbance, HSCT supersonic flight willonly take place over the oceans.

In response to policies detailed in the U.S.Marine Mammal Protection Act and the U.S.Endangered Species Act, research on marinemammal behavioral response to sonic boomswas conducted. The National Zoo and Hubb’sResearch Institute conducted studies of thewildlife response to sonic boom events andlevels. In previous studies, biologists have shownthat wildlife will quickly habituate to the booms.These results are also shown by anecdotalexperiences such as the utilization of air canon inChesapeake Bay to frighten sea gulls away whenpulling in the fishing nets and underwaterexplosions near the Ballard Locks in Seattle todiscourage Hershel the sea lion from eating thesteelhead salmon on their way to spawn.Unfortunately, these devices soon became the“dinner bell” for Hershel and his buddies.

Other concerns were the noises introducedinto the oceans that may affect marine mammalbehavior by interfering with ability to detect callsfrom members of their own species. Numerousstudies have already been performed to examinethe effects of noise from human activitiesincluding ship props, underwater drilling for oiland gas recovery, and offshore constructionoperations. Because a major part of the energyassociated with the HSCT sonic boom would bedeflected off the ocean’s surface, the noise levelsand frequency spectrum generated by thesemarine sources are potentially much moredetrimental to underwater marine life than sonicboom phenomena.

Sonic boom research was initially directedtoward reducing boom pressure signature toacceptable levels for flight over land bymaximizing the distribution of lift over theairframe. Configurations were designed bycomputational fluid dynamics methods that werevalidated for the prediction of the near-fieldpressure field and boom propagation through theatmosphere with wind tunnel tests and flight andground measurements of the SR-71 Blackbird.

Significant reductions of booms wereaccomplished; however, the configurations wereso radical that economical viability wasprecluded. These methods were then used tocharacterize the boom pressure field for thedevelopment of operational requirements toavoid populated islands and establish distancesfor deceleration to subsonic speeds for overlandflights.

2.4 RadiationHigh-altitude atmospheric radiation is of galacticor solar origin. Galactic cosmic rays arecomplex, heavy ions of high energy andpenetrate deep into the Earth’s atmosphere.Solar cosmic rays are generated by solar flaresand are less penetrating, but may be very intenseover short periods of time. Secondary neutronsare generated in the atmosphere by both sourcesand can be biologically damaging depending onthe dosage.

At supersonic cruise altitudes, the radiationdose is double that of a subsonic airliner flying at40,000 ft. But because the trip time is halved,the total trip dosage is approximately the same.The major concern is with the exposure levels ofthe flight crew, especially pregnant members.Radiation exposure can be managed by crewrotation scheduling based on validated radiationprediction methods and measurement.

Currently, there are substantial uncertaintiesin the knowledge of the radiation in the upperatmosphere and changes in latitude whereradiation is higher at the poles because of themagnetic field of the Earth. The NationalCouncil on Radiation Protection andMeasurements has identified the criticalscientific questions that must be answered toprovide a sound scientific basis for atmosphericradiation prediction. An ER-2 was equippedwith a suite of instruments provided by aninternational team for characterizing theradiation environment. The solar minimumenvironment (maximum radiation) was measuredto provide the worst case scenario.


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3 Economic Viability

NASA is not building the High-Speed CivilTransport nor developing a prototype airplane.NASA is in partnership with the U.S. Industry todevelop the most critical, high-risk technologiesas the foundation for the industry design anddevelopment of an economically viable HSCT.

3.1 AerodynamicsAerodynamic research provided potentialconcepts and validated analytical design andoptimization methods for airplane configurationdevelopment and ride quality enhancement.These design methods were used for dragreduction and for predicting aeroelastic stabilityand control characteristics, which are necessaryfor the design of safe, controllable, andeconomically viable HSCT configurations.

State-of-the-art computational fluiddynamics methods were coupled withoptimization techniques that adjust wing,fuselage, nacelle, and empennage geometry toreduce drag by over 10 percent relative tooptimized linear design methods at supersonicspeeds (Figure 3). These methods were extendedfor multipoint optimization to provide maximumrange for the airplane with a reference mission of85 percent supersonic over water and 15 percentsubsonic over land. In addition, methods topredict aeroelastic effects of models under loadwere developed and validated in supersonic andhigh-Reynolds-number subsonic wind-tunneltests at NASA’s Langley and Ames ResearchCenters. Test techniques were improved such aspressure sensitive coatings, Reynolds numberscaling, and corrections for model supportsystems.

Another method to achieve high-speed dragreduction is laminar flow control. Significantdrag reductions were demonstrated withsupersonic laminar flow control (SLFC) at Mach2.0 on an F16-XL (Figure 4) at NASA DrydenFlight Research Center.

Figure 3. CFD optimization methods using over 400variables are used for configuration design

Figure 4. Laminar flow control system demonstratedon a F16-XL at Mach 2.0.

Supersonic laminar flow was achieved witha left-wing glove with 10 million laser-cut holesthrough which a suction system controls thelaminar boundary layer to prevent turbulence.Data analyses tools were successfully applied tocalculate suction distributions and boundary-layer stability characteristics from flight data.


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Supersonic laminar flow control has the potentialto improve the aerodynamic performance of theHSCT by 10 percent. This concept was droppedfrom the program because further technologydemonstrations of the subsystems, wing skinwith holes through high-temperature composites,and a longer chord demonstration of laminarflow was required to reduce risk for applicationto a commercial airplane.

High-speed aircraft present unique guidanceand flight control challenges. Research inguidance and flight control technology provideda simulation of baseline HSCT airplanedynamics that was used to develop both rigid andelastic airplane control concepts and methodsthat was applicable to practical HSCT design.Both ground-based and in-flight simulation wasused to validate flying qualities criteria and flightcontrol system design criteria. An integratedguidance, control, and display system for safeand efficient HSCT operations in tomorrow’sairspace system was developed. Envelopeprotection strategies and certification criteriawere addressed in these systems. Designmethods were developed that were used toensure Level 1 flying qualities and robustperformance over the flight envelope for aslender flexible aircraft with both rigid andpossibly elastic instabilities.

3.2 Flight DeckTo reduce airplane weight and drag and improvepilot performance and safety, the droop nosewith forward cockpit windows may be replacedby large computer-generated displays (Figure 5).

The external vision team, with guidancefrom the Federal Aviation Administration,defined requirements and technology to providethe functional equivalent of forward sight.

Figure 5. – External vision concept.

During airport maneuvers, video camerason the nose gear and on top of the vertical tailprovide a panoramic view of the runway. Usingdifferential global positioning satellite (GPS)data coupled with a digitized map database of therunways, the pilot can track the current airplaneposition just like the Pacman video game. Aftertakeoff, the computer overlays the video imagewith speed, altitude, and any other personalizedselected data for the pilot. At cruise, thesurveillance system uses the Traffic CollisionAvoidance System (TCAS) and radar (to trackairplanes without TCAS) to warn the pilot ofpotential collisions. Also, one version of thenavigation system projects a lead airplane, usingGPS data and flight profile information, for thepilot to follow. On landing, football goal postsand other symbology aid the pilot for runwayline up and a flare indicator is used fortouchdown. For night vision, dynamic rangevideo sensors are augmented by X-band radar forobject detection. During weather conditions,radar is used to cut through the rain and fog, andthe video camera image is replaced with acomputer representation of the airport andsurrounding topology. The radar data can beprocessed during airport operations to detect

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hazards on the runway that cannot be seen, andthus provide greater safety.

A simplified system produced excellentresults with a conventional graphic displaymonitor used by the pilot on the NASA 737Transport System Research Vehicle that wasconfigured with a second cockpit and had nowindows in the middle of the airplane. The finalselected system was demonstrated on the groundwith the Surface Operations Research Vehicle(Figure 6) and in flight with the Calspan TotalIn-Flight Simulator (Figure 7).

Figure 6. – Surface operations research vehicle

Figure 7. Calspan Total In-Flight Simulator

3.3 Materials and StructuresThe fraction of the operating empty weight forairframe structure is much smaller for asupersonic transport than for conventional

subsonic commercial vehicles. This requires theuse of innovative structural concepts andadvanced materials to satisfy this stringentweight requirement. The operating environmentis also more severe because of the hightemperatures associated with the aerodynamicfriction heating caused by supersonic cruisespeeds.

The Mach 2.4 economically viable HSCTdrives the materials and structures technologydevelopment with a 60,000 hour durability at acycled 350°F skin temperature and a 30-percentreduction in weight relative to the Concorde.Conventional airplane materials such asaluminum and thermoset composites such asbismaleimides do not have the temperaturecapability, and titanium alloys are too heavy forthe entire airframe. Over 140 different materialswere analyzed to downselect to a handful ofmaterials for the enhancement of mechanicalproperties and fabrication processes.

Titanium was a prime candidate for themain wing box which required high-strength andfor the high-temperature-stagnation regions ofthe aerodynamic surface leading edges.Advanced titanium alloys were developed with agoal of 20-percent improvement in mechanicalproperties. Major technology challengesincluded the effects of thermomechanicalprocessing on optimum alloy compositions andthe manufacturing processes for reducing costsand risks.

To reduce weight of the fuselage, outboardwing, strake and empennage, polyimide carbonfibers matrix composites (PMC) were developed.A NASA patented polyimide resin called PETI-5 when combined with a vendor produced IM7fiber demonstrated mechanical properties greaterthan bismaleimides at 350°F. A “wet” prepregwas developed for laboratory hand layupstructures that required long cure times at highpressure in autoclaves to remove the volatilesand was demonstrated in the fabrication of large-scale panels (Figure 8).


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Figure 8. Large scale PETI-5 panel

At the end of the program, dry prepreg wasbeing developed that potentially had moreaffordable manufacturing processes such as resinfilm infusion and insitu robotic layup.

Durability isothermal tests after 55,000hours of a PMC showed no degradation, andPETI-5 had over 15,000 hours. Because of thecriticality of the durability data, the thermalmechanical fatigue tests were continued after theend of the program.

Summary

NASA’s High-Speed Research Program was ontrack to meet all of the environmental andeconomic goals established for the program.Technology was demonstrated in medium scaleground tests and flight tests. However, theprogram was cancelled in 1999 before the large-scale demonstration test articles were developedand tested.

an overview of nasa’s high-speed research program · 13. supplementary notes see also adm002333....

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