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AA s the 21st century unfolds, a revolu-tionary engine technology is aiming to

fly craft at high Mach speeds and seamlesslyintegrate air-to-space operations. The super-sonic combustion ramjet, or scramjet, usesno rotating parts, will power vehicles hun-dreds of miles in minutes, and will makerapid global travel and affordable access tospace a reality.

These goals drew closer to achievementthis spring when the first scramjet-poweredaircraft flew on its own. On the afternoon ofMarch 27, an unpiloted X-43A, a NationalAeronautics and Space Administration(NASA) craft mounted on a Pegasus boosterrocket, dropped from a B-52 flying at 40,000 ftoff the coast of California. The rocket sent

the experimental aircraft soaringto its test altitude of 95,000 ft,where the X-43A separated fromits booster, and its scramjetengine fired for a planned 10-stest, achieving an incredibleMach 7, or 5,000 mph.

Data from that flight helped validate theconcept of a hypersonic craft with an air-breathing engine. More flights during thenext several years will expand on the engineand aerodynamics data obtained in March,and could put some scramjet vehicles inservice in less than a decade.

Scramjets will enable three categories ofhypersonic craft: weapons, such as cruisemissiles; aircraft, such as those designed forglobal strike and reconnaissance missions;and space-access vehiclesthat will take off and landlike airliners.

Scramjets have a longand active developmenthistory in the United States.

On the basis of theoretical studies started inthe 1940s, the U.S. Air Force, Navy, andNASA began developing scramjet engines inthe late 1950s. Since then, many hydrogen-and hydrocarbon-fueled engine programshave helped scramjet technology evolve toits current state. The most influential ofthese efforts was NASA’s National AerospacePlane (NASP) program, established in 1986to develop a vehicle with speed greater thanMach 15 and horizontal takeoff and landingcapabilities. The program ended in 1993,but the original NASP engine design, signifi-cantly modified by NASA, provided thefoundation for the power plant used duringthe X-43A’s recent flight.

The U.S. Air Force and Pratt & Whitneyground-tested the first uncooled hydrocar-

24 The Industrial Physicist

TECHNOLOGY

Scramjets integrateair and space

AUGUST/SEPTEMBER 2004© American Institute of Physics

Figure 1. The scramjet engine occupies theentire lower surface of the vehicle body (a). Asair passes through the forebody, internal inlet,

isolator, combustor, internal nozzle, and aftbody (b), itexperiences a complex series of shock waves that

change with speed and have to be carefullymanaged and integrated with the

power design.

The world’sfirst

scramjetengine to

demonstrateoperability at Mach 4.5–6.5

using conventional fuel

by Dean Andreadis

Aftbody Internal nozzle Combustor Isolator

Fuel injection

Fuel subsystem

Exhaust

Precombustionshock structure

Boundary layer separation

a

b

TIPAugSept modified 7/27/04 2:26 PM Page 26

bon-fueled scramjet engine at simulatedMach 4.5–6.5 in 2001. This collaborationalso demonstrated in 2003 a scramjetmade from nickel-based alloys and cooledby its JP7 jet fuel. The 2003 engine has thepotential to power future missiles, aircraft,and access-to-space vehicles. Last year, theDefense Advanced Research ProjectsAgency, U.S. Navy, Boeing, Aerojet, andJohns Hopkins University also ground-test-ed a scramjet engine, which was construct-ed primarily from nickel alloys, powered byJP10 jet fuel, and intended exclusively forhypersonic missiles.

What is a scramjet?In a conventional ramjet, the incoming

supersonic airflow is slowed to subsonicspeeds by multiple shock waves, created byback-pressuring the engine. Fuel is added tothe subsonic airflow, the mixture combusts,and exhaust gases accelerate through a nar-row throat, or mechanical choke, to super-sonic speeds. By contrast, the airflow in apure scramjet remains supersonic through-out the combustion process and does notrequire a choking mechanism, which pro-vides optimal performance over a wideroperating range of Mach numbers. Modernscramjet engines can function as both a ram-jet and scramjet and seamlessly make thetransition between the two (Figure 2).

Propulsion efficiency is Isp•s, where effectivespecific impulse (Isp) is determined by theratio of thrust to drag.

The scramjet provides the most integrat-ed engine–vehicle design for aircraft andmissiles. The engine occupies the entirelower surface of the vehicle body (Figure1a). The propulsion system consists of fivemajor engine and two vehicle components:the internal inlet, isolator, combustor, inter-nal nozzle, and fuel supply subsystem, andthe craft’s forebody, essential for air induc-

tion, and aftbody, which is acritical part of the nozzlecomponent.

The high-speed air-induc-tion system consists of thevehicle forebody and inter-nal inlet, which capture andcompress air for processingby the engine’s other com-ponents. Unlike jet engines,vehicles flying at high super-sonic or hypersonic speedscan achieve adequate com-pression without a mechan-ical compressor. The fore-body provides the initialcompression, and the inter-nal inlet provides the finalcompression. The air under-goes a reduction in Machnumber and an increase inpressure and temperature asit passes through shockwaves at the forebody andinternal inlet (Figure 1b).

The isolator in a scramjet is a critical com-ponent. It allows a supersonic flow to adjustto a static back-pressure higher than theinlet static pressure. When the combustionprocess begins to separate the boundarylayer, a precombustion shock forms in theisolator. The isolator also enables the com-bustor to achieve the required heat releaseand handle the induced rise in combustorpressure without creating a condition calledinlet unstart, in which shock waves preventairflow from entering the isolator.

The combustor accepts the airflow andprovides efficient fuel–air mixing at severalpoints along its length, which optimizesengine thrust. The expansion system, con-sisting of the internal nozzle and vehicle aft-body, controls the expansion of the high-pressure, high-temperature gas mixture toproduce net thrust. The expansion processconverts the potential energy generated bythe combustor to kinetic energy. The impor-tant physical phenomena in the scramjetnozzle include flow chemistry, boundary-layer effects, nonuniform flow conditions,shear-layer interaction, and three-dimen-sional effects. The design of the nozzle has amajor effect on the efficiency of the engineand the vehicle, because it influences thecraft’s pitch and lift.

Operations An air-breathing hypersonic vehicle

requires several types of engine operationsto reach scramjet speeds. The vehicle mayutilize one of several propulsion systems toaccelerate from takeoff to Mach 3. Two


Figure 2. Propulsion efficiency decreases with speed as we progress from turbojetsthrough ramjets and scramjets to rockets; hydrogen is more efficient than jet fuel.

Turbojets

8,000

6,000

4,000

2,000

0

Ramjets

Scramjets

Rockets

0 5 10 15 20Mach number

Hydrogen fuel

JP fuel

Prop

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Rocket-basedcombined cycleTurbine-basedcombined cycle

Rocket

Developed

In development

Future

Advanced Illustration Ltd.

Internal inlet Forebody

Vehicle body

Vehicle bow shock

Flow

tion

Shock/boundary-layer interactions

TIPAugSept modified 7/27/04 2:26 PM 7

Technology

examples are a bank of gas-turbine enginesin the vehicle, or the use of rockets, eitherinternal or external to the engine.

At Mach 3–4, a scramjet transitions fromlow-speed propulsion to a situation in whichthe shock system has sufficient strength tocreate a region(s) of subsonic flow at theentrance to the combustor. In a conventionalramjet, the inlet and diffuser decelerate theair to low subsonic speeds by increasingthe diffuser area, which ensures completecombustion at subsonic speeds. A converg-ing–diverging nozzle behind the combustorcreates a physical throat and generates thedesired engine thrust. The required chokingin a scramjet, however, is provided withinthe combustor by means of a thermal throat,which needs no physical narrowing of thenozzle. This choke is created by the rightcombination of area distribution, fuel–airmixing, and heat release.

During the time a scramjet-powered vehi-cle accelerates from Mach 3 to 8, the air-breathing propulsion system undergoes atransition between Mach 5 and 7. Here, amixture of ramjet and scramjet combustionoccurs. The total rise in temperature andpressure across the combustor begins todecrease. Consequently, a weaker precom-bustion system is required, and the precom-bustion shock is pulled back from the inletthroat toward the entrance to the combustor.

As speeds increase beyond Mach 5, theuse of supersonic combustion can provide

higher performance (Figure 3). Engine effi-ciency dictates using the ramjet until Mach5–6. At around Mach 6, decelerating airflowto subsonic speeds for combustion resultsin parts of the airflow almost halting, whichcreates high pressures and heat-transferrates. Somewhere between Mach 5 and 6,the combination of these factors indicates aswitch to scramjet operation.

When the vehicle accelerates beyondMach 7, the combustion process can nolonger separate the airflow, and the engineoperates in scramjet mode without a pre-combustion shock. The inlet shocks propa-gate through the entire engine. Beyond Mach8, physics dictates supersonic combustionbecause the engine cannot survive the pres-sure and heat buildup caused by slowing theairflow to subsonic speeds.

Scramjet operation at Mach 5–15 pre-sents several technical problems to achiev-ing efficiency. These challenges include

fuel–air mixing, manage-ment of engine heat loads,increased heating on lead-ing edges, and develop-ing structures and mate-rials that can withstandhypersonic flight.

When the velocity ofthe injected fuel equalsthat of the airstream enter-ing the scramjet combus-tor, which occurs atabout Mach 12, mixingthe air and fuel becomesdifficult. And at higherMach numbers, the hightemperatures in the com-bustor cause dissociationand ionization. These

factors—coupled with already-complex flowphenomena such as supersonic mixing, iso-lator–combustor interactions, and flamepropagation—pose obstacles to flow-pathdesign, fuel injection, and thermal manage-ment of the combustor.

Several sources contribute to engine heat-ing during hypersonic flight, including heat-ing of the vehicle skin from subsystems suchas pumps, hydraulics, and electronics, aswell as combustion. Thermal-managementschemes focus on the engine in hypersonicvehicles because of its potential for extremelyhigh heat loads. The engine represents a par-ticularly challenging problem because theflow path is characterized by very high ther-mal, mechanical, and acoustic loading, aswell as a corrosive mix of hot oxygen andcombustion products. If the engine is leftuncooled, temperatures in the combustorwould exceed 5,000 °F, which is higher thanthe melting point of most metals. Fortunate-ly, a combination of structural design, mater-ial selection, and active cooling can managethe high temperatures.

Hypersonic vehicles also pose an extraor-dinary challenge for structures and materi-als. The airframe and engine require light-weight, high-temperature materials andstructural configurations that can withstandthe extreme environment of hypersonicflight. These include:• very high temperatures,• heating of the whole vehicle,


Figure 3. As the vehicle speed increases from Mach 3 to Mach 8, the isolator pressure ratio passes through apeak at Mach 6. As the shock train and boundary layer retreat, the modes change from dual-mode ramjet todual-mode scramjet to pure scramjet mode.

Dividing an object’sspeed by the speed

of sound yields its Mach number, a rationamed for 19th-century Austrian physi-cist Ernst Mach, who laid out the princi-ples of supersonic speed. Mach 1 equalsthe speed of sound (transonic), and aMach number lower than 1 is subsonic.Supersonic covers speeds greater thanMach 1 up to Mach 5, and hypersonic isany speed greater than Mach 5.

MACH NUMBERS

Isol

ator

pre

ssur

e ra

tioM3.0

Flow

Internalinlet Isolator

M>1 M>1 M>1 M>1 M>1 M>1 M>1 M>1M<1

Combustor

Flow Flow

M4.0

M5.0

M6.0 M6.5

M7.5M7.0

M8.0Inlet unsta

rt

Maximum structural pressureControl throttlesetting to avoid

inlet unstart

Operate engineat full throttle

Boundary layerincipient sepa-ration limit

Dual-mode ramjet• Precombustion shock train

dominates isolator flowfield• Shock train terminates in

subsonic condition• Boundary layer is separated• Inlet close to critical

Dual-mode scramjet• Precombustion shock train pulls

back toward combustor entrance• Shock train terminates in super-

sonic condition• Boundary layer separation

decreases

Scramjet mode• Inlet shock continues through

isolator/combustor• Combustor supersonic throughout• Boundary layer fully attached• Precombustion shock-free

isolator

Inlet entrance Mach number


• steady-state and transient localized heat-ing from shock waves,

• high aerodynamic loads,• high fluctuating pressure loads,• the potential for severe flutter, vibration,

fluctuating, and thermally induced stresses, • erosion from airflow over the vehicle and

through the engine.With the completion of the successful

X-43A flight and the ground-testing of sev-eral full-sized demonstration engines, confi-dence in the viability of the hydrogen- andhydrocarbon-fueled scramjet engines hasincreased significantly. NASA plans tolaunch another X-43A this fall and fly it atMach 10, or 6,750 mph.

The U.S. Air Force, Pratt & Whitney, andBoeing’s Phantom Works will conduct flighttests of a hydrocarbon-fueled scramjet in2007 or 2008. The tests—using an enginethat is relatively easy to manufacture—will

demonstrate significant acceleration, oper-ate the engine for several minutes at Mach4.5–6.5, and use sensors and computers tocontrol the engine and flight.

Demonstrating these technologies, alongwith additional ground- and flight-testexperiments, will pave the way for afford-able and reusable air-breathing hypersonicengines for missiles, long-range aircraft, andspace-access vehicles around 2010, 2015,and 2025, respectively.

Further readingAccess to Space Study: Summary Report;

Office of Space Systems Development, NASAHeadquarters, Washington, DC, 1994.

Curran, E. T.; Murthy, S. N. B.; Eds.Scramjet Propulsion; Progress in Astronau-tics and Aeronautics, vol. 189; AIAA: Wash-ington, DC, 2000.

Faulkner, R. F. The Evolution of the

HySET Hydrocarbon Fueled ScramjetEngine; AIAA Paper 2003-7005; AIAA:Washington, DC, 2003.

Heiser, W. H.; Pratt, D. T. Hypersonic Air-breathing Propulsion; AIAA: Washington,DC, 1994.

Kandebo, S. W. New Powerplant Key toMissile Demonstrator. Aviation Week SpaceTechnol., Sept. 2, 2002; p. 56.

McClinton, C. R.; Andrews, E. H.; Hunt,J. L. Engine Development for Space Access:Past, Present, and Future. Int. Symp. AirBreathing Engines, Jan. 2001; ISABE Paper2001-1074.

Both rockets and jet engines burn a mix-ture of fuel and an oxidizer to create

propulsive thrust. Their fundamental difference is that rocketscarry the oxidizer onboard, whereas jets obtain it from the air.Engines that use air are referred to as air-breathing engines.

In a turbojet, the air passes through a compressor, a burner, anda turbine, where some of the flow energy is extracted to drive thecompressor. In a turbofan engine, there is an additional shaft, anda fan in front that bypasses the engine and sends some high-speedair directly to the exhaust. In a supersonic ramjet, which has nomoving parts, pressure is boosted by reducing air moving through

the engine to subsonic levels before combustion. Scramjets areramjets that fly at hypersonic speeds and whose airflow throughthe combustor is supersonic. To meet the demands of supersonicflight, both engines eject small amounts of exhaust at high velocity.

A craft’s cooling and the systems needed to meet its missiontypically determine the choice between hydrogen and hydrocar-bon as its fuel. Missiles and short-range aircraft may use hydro-carbon fuels for their storability and volumetric energy density.Long-range cruise aircraft and rockets tend toward hydrogenbecause of its superior energy release per pound and heatabsorption capability.

ROCKETS AND JETS

Dean Andreadis is a specialist in flowpathaerothermal analysis and systems integra-tion at Pratt & Whitney Space Propulsion,Hypersonics, in West Palm Beach, Florida([email protected]).

B I O G R A P H Y

Adv

ance

d Ill

ustr

atio

n Lt

d.

Turbofan Turbojet Ramjet

CompressorCompressor

CombustionchamberFan

AirAir Air

Fuel inletFuel inlet

Fuel burners

Outer driveshaft(for compressor)

Inner driveshaft(for fan)

Driveshaft Bypass exhaust

Jetexhaust

Jetexhaust

TurbineTurbine

Combustionchamber

Combustionchamber

Venturi(propelling nozzle)


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