1

MAE 4261: AIR-BREATHING ENGINES

Advanced Concepts

Mechanical and Aerospace Engineering Department

Florida Institute of Technology

D. R. Kirk

2

CONTENTS AND OVERVIEW

• Review

– What did we do in this course?

• Future / Advanced Topics

– What would be covered in a graduate course in air-breathing propulsion?

• SCRAMJETS

• Rocket-Based Combined Cycle Engines (RBCC)

• Pulse Detonation Engines

• Micro Propulsion

3

REVIEW OF MAE 4261• Review of governing equations of motion and energy

– General expressions for thrust and performance (continuity + F = d/dt(mv))

• General layout of an air-breathing engine– Inlet, compressor, burner, turbine, nozzle

• Air-Breathing engine performance parameters– Thrust, specific thrust, Isp, T/W, thermal, propulsive, overall efficiency– Relate to overall airplane performance

• Cycle analysis– Detailed examination of ideal and non-ideal Brayton cycle– Expression for thrust, Isp, etc. for ramjets, turbojets, turbofans

• Non-Rotating components– Inlets, nozzles, combustors, after-burners

• Energy exchange with turbomachinery– Euler turbine equation, velocity triangles– Implications for performance prediction and design

• Design of axial compressors and turbines– Radial variation (twist), axial area change, degree of reaction

• Component matching– Integrate compressor + burner + turbine on single shaft (gas generator)– Usefulness of compressor and turbine maps

4

FUTURE / ADVANCED TOPICS• Compressors

– Axisymmetric Design– Aspect Ratio, Radial changes in AR– Blade row performance, 3-D effects, blade row interactions, performance limitations,

tip clearance and vortical flow behavior– Instabilities: Rotating Stall and Surge

• Combustors– More detailed design on various geometries for performance and emissions, liner and

cooling flows, advanced concepts including pre-mixed combustion for NOx reduction– Instabilities, flame stability, turbulent flame speed, fuel control

• Turbines– Detailed disk design– Rotating seals– Detailed analysis of blade heat load prediction and turbine cooling– Flutter, vibration, fatigue, rotational dynamics and eccentricity

• Engine Matching– 2 and 3 spool engines, include effects of variable geometry

5

COMPRESSOR PERFORMANCE LIMITATIONS

• Compressor performance is limited by two important (and often highly detrimental) phenomena

1. Compressor stall

– Similar to stall on airfoils

– Can also be dynamic phenomena, called rotating stall

2. Compressor surge

– Static and dynamic phenomena that can lead to violent flow oscillations inside of device

6

COMPRESSOR STALL

• Usually not all blades are stalled

• Rotating stall refers to progression around blade annulus of stall pattern in which one or several adjacent blade passages are instantaneously stalled

• Zones of stall rotate at about ½ engine speed

7

ENGINE SURGE: H&P SECTION 7.5• If downstream blades are stalled may act as a throttle

– Variable mass flow ingestion capability because of variable downstream area• Leads to stable vs. unstable dynamic response of turbomachine• Compressor characteristics (compressor map) vs. throttle characteristics (operating line)

– IF slope of throttle line > slope of compressor line: Stable– IF slope of throttle line < slope of compressor line: Unstable (mass flow may go to zero)

8

ENGINE SURGE

9

SIMPLIFIED DYNAMIC MODEL

• Simplified representation called Moore-Greitzer model• Reference, “The Stability of Pumping Systems – The 1980 Freeman Scholar

Lecture, Journal of Fluids Engineering, Vol. 103.• Brilliance is in elegant simplicity of model (No CFD)

10

ROTATING STALL AND SURGE

11

HIGH CYCLE FATIGUE (HCF) IN GAS TURBINE ENGINES• HCF results from vibratory stress cycles at frequencies

which can reach thousands of cycles per second and can be induced from various aeromechanical sources (stall)

• Widespread phenomenon in aircraft engines that leads to premature failure of major engine components (fans, compressors, turbines) and in some instances has resulted in loss of total engine and aircraft.

12

WHAT IS GOING ON NOW?

• Integrated High Performance Turbine Engine Technology (IHPTET) program– Advanced material developments (ceramics)– Innovative structural designs– Improved aerothermodynamics– Drivers: Propulsion performance, reliability, and cost-

saving improvements

• Examples:– Turbomachinery

• Increase operating temperature and turbine life• Increase bearing lifetime and reduce their wear by

incorporating hydrostatic bearings (magnetic)• Reduction in number of compressor stages

– Improved injection, mixing and combustion modeling– High Cycle Fatigue– Burner-Turbine-Burner-Turbine Concepts

• http://www.pr.afrl.af.mil/divisions/prt/ihptet/ihptet.html

13

AEROENGINE CORE POWER EVOLUTION: DEPENDENCE ON TURBINE ENTRY TEMPERATURE [Meece/Koff]

14

FUEL CONSUMPTION TREND

1950 1960 1970 1980 1990 2000 2010 2020

JT8D

JT9D

PW4052

PW4084Fue

l Bur

n

Year

FutureTurbofan

• U.S. airlines, hammered by soaring oil prices, will spend a staggering $5 billion more on fuel this year or even a greater sum, draining already thin cash reserves

• Airlines are among industries hardest hit by high oil prices

NOTE: No Numbers

15

BURNER-TURBINE-BURNER (ITB) CONCEPTS

• Improve gas turbine engine performance using an interstage turbine burner (ITB)– With a higher specific thrust engine will be smaller and lighter– Increasing payload– Reduce CO2 emissions– Reduce NOx emissions by reducing peak flame temperature

• Initially locate ITB in transition duct between high pressure turbine (HTP) and low pressure turbine (LPT)

Conventional

Intra Turbine Burner

16

SIEMENS WESTINGHOUSE ITB CONCEPT

Tt4

17

UNDERSTANDING BENEFIT FROM CYCLE ANALYSISFrom “Turbojet and Turbofan Engine Performance Increases Through Turbine Burners, by

Liu and Sirignano, JPP Vol. 17, No. 3, May-June 2001

Conventional Intra Turbine Burner

18

2 additional burners 5 additional burners

UNDERSTANDING BENEFIT FROM CYCLE ANALYSISFrom “Turbojet and Turbofan Engine Performance Increases Through Turbine Burners, by

Liu and Sirignano, JPP Vol. 17, No. 3, May-June 2001

19

Continuous burning in turbine

UNDERSTANDING BENEFIT FROM CYCLE ANALYSISFrom “Turbojet and Turbofan Engine Performance Increases Through Turbine Burners, by

Liu and Sirignano, JPP Vol. 17, No. 3, May-June 2001

20

MAGNETIC BEARINGS

• Magnetic bearings increase reliability• Reduce weight of engines by eliminating lubrication system• Increase DN2 limit on engine speed and allow active vibration cancellation systems to be used,

resulting in a more efficient, "more electric" engine• Actively control shaft vibration, damping, improve surge and stall characteristics• Integrated High Performance Turbine Engine Technology (IHPTET) program, identified a need for a

high-temperature (1200 °F) magnetic bearing • http://www.grc.nasa.gov/WWW/RT1997/5000/5930kascak.htm

21

EXOSKELETAL ENGINE CONCEPT

22

EXOSKELETAL ENGINE CONCEPT COMMENTS NASA/TM – 2004-212621

• An innovative approach to gas turbine design involves mounting compressor and turbine blades to an outer rotating shell → exoskeletal engine

• Compression (preferable to tension for high-temperature ceramic materials, generally) becomes dominant blade force

• Exoskeletal engine feasibility lies in structural and mechanical design (as opposed to cycle or aerothermodynamic design)

• Some Study Details:– Focused on development and assessment of a structural-mechanical exoskeletal

concept using Rolls-Royce AE3007 regional airliner all-axial turbofan as a baseline– Effort was further limited to definition of an exoskeletal high-pressure spool concept,

where major structural and thermal challenges are represented– Mass of high-pressure spool was calculated and compared with mass of AE3007

engine components• Exoskeletal engine rotating components can be significantly lighter than rotating

components of a conventional engine• However, bearing technology development is required, since mass of existing

bearing systems would exceed rotating machinery mass savings• Recommended that once bearing technology is sufficiently advanced, a "clean

sheet" preliminary design of an exoskeletal system be accomplished to better quantify potential for exoskeletal concept to deliver benefits in mass, structural efficiency, and cycle design flexibility.

23

NASA's X-43A Scramjet Breaks Speed Record

• “NASA's X-43A research vehicle screamed into the record books again Tuesday, demonstrating an air-breathing engine can fly at nearly 10 times the speed of sound. Preliminary data from the scramjet-powered research vehicle show its revolutionary engine worked successfully at nearly Mach 9.8, or 7,000 mph, as it flew at about 110,000 feet.”

• “NASA's X-43A scramjet program successfully smashed its own world speed record for aircraft by flying at nearly 10 times the speed of sound. The flight proves its radical, air-breathing engine can function at speeds of nearly 12,000 kilometers per hour.”

• “Aviation history was made today as NASA successfully flew its experimental X-43A research vehicle, a forerunner of craft that could well offer alternate access to space in the future.”

24

X-43 HIGHLIGHTS

25

HYPER-X FLIGHT TRAJECTORY

26

COMPARISON OF PROPULSION SYSTEMS

27

RAMJETS VS. SCRAMJETS

• Ramjets have a couple of important limits

1. Do not work until engine is moving at high speeds (also true for scramjets)

• Need a way to get plane moving at Mach speeds

• One way is combine ramjet and turbine engine: run turbine portion of engine until move through air fast enough for ramjet to work efficiently

• At speed shut turbines down and let ramjet work.

2. At high speeds all temperature rise is due to deceleration

• Dissociation is dominant

• Combustion can not add any more heat to flow

3. At speeds of ~ Mach 6, air flowing into inlet is moving so quickly that it creates a supersonic shock wave as it is compressed inside engine

• Shock wave may stop ignition of air-and-fuel mixture, shutting engine down

• Pressure and heat from shock wave tear engine to bits

• Solution is a supersonic combustion engine or scramjet

– Variation of a ramjet where combustion of fuel air mixture occurs at supersonic speeds

28

IMPACT OF HIGH SPEED FLIGHT AND DISSOCIATION

• At high Mach numbers affect of deceleration of flow is to produce higher temperature than combustion!

• Energy is ‘trapped’ in dissociated products (H, H2, OH, O, O2, NO, NO2, etc.)

• Combustion is actually cooling the flow

• How do we get around this?

29

RAMJETS VS. SCRAMJETS

Ramjet:Physical ThroatSub-Sonic Combustion

Scramjet:NO Physical Throat(Thermal Choking)Super-Sonic Combustion

30

SCRAMJET CHALLENGES

• Engine inlet has less compression than ramjet, allowing air to speed through engine at supersonic speeds

– Somewhat reduces shockwave problem

• When fuel is injected into onrushing air, small shock waves are created, so combustion chamber must be able to withstand pressure

• At supersonic speeds, fuel injection and combustion must be accomplished in milliseconds

• Making sure that fuel burns while still inside engine and not after it is ejected from it is one of main challenges for scramjet builders

• Launching a scramjet-powered plane into orbit has huge advantages over standard rocket-powered spacecraft

– Rockets must carry all their oxidizer with them

31

EXAMPLE OF SCRAMJET EXPERIMENTAL MODEL

Inlet

Cowl Isolator

Combustor

Nozzle

32

RAMJETS VS. SCRAMJETS

33

DUAL-MODE OPERATION

34

LAUNCH ASSISTMain Idea: Why waste fuel to get supersonic → launch at high speed from earth?

35

ROCKET-BASED COMBINED CYCLE (RBC2, RBC3)

• Combination of ROCKET, RAMJET and SCRAMJET

– Most efficient aspects of each form of propulsion

• Rocketdyne, Boeing Co., Pratt & Whitney, Aerojet and NASA

• Engines completely reusable, take off and land at airport runways, and ready to fly again within days

• By 2006 ground test version of an air-breathing rocket engine for a next-generation hypersonic flight vehicle

• Flight-test self-powered vehicle to more than M=6, demonstrating all modes of engine operation by end of decade

"Air-breathing propulsion is one of the most promising concepts we've

seen for reaching NASA's future-generation spaceflight goals"

36

RBCC: OPERATIONAL IDEAS

• KEY POINT– Use ambient air as an oxidizer– Compared with conventionally powered rocket vehicles significantly reduce

weight by eliminating a significant amount of its required on-board oxidizer

• PHASE 1: DUCTED ROCKET– Innovative air-breathing rocket engine for operational vehicle would get

initial power boost from rockets in a duct that captures air– Improves performance about 15 percent above conventional rockets

• PHASE 2: RAMJET / SCRAMJET MODES– Once vehicle has accelerated to more than M~2, rockets are turned off and

engine relies solely on oxygen in atmosphere to burn hydrogen fuel

• PHASE 3: TRADITIONAL ROCKET– When vehicle has accelerated to ~ 10 times the speed of sound, engine

converts to a conventional rocket-powered system to propel craft into orbit

37

PDE: PULSE DETONATION ENGINE

38

HOW DOES IT WORK: IN WORDS

• Tube, closed at one end and filled with a mixture of fuel and air

• A spark ignites fuel at closed end, and a combustion reaction propagates down the tube

• In deflagration, even in "fast flame" situations ordinarily called explosions—that reaction moves at tens of meters per second at most

• In detonation, a supersonic shock wave slams down the tube at thousands of meters per second, close to Mach 5, compressing and igniting fuel and air almost instantaneously in a narrow, high-pressure, heat-release zone

• The reaction to this detonation is used to propel the vehicle

39

PDE: HOW DOES IT WORK?

40

PDE: PULSE DETONATION ENGINES

• Main Idea: Detonates air/fuel mixture rather than just allowing it to simply combust

• Constant volume combustion is more efficient than constant pressure combustion, as demonstrated by a cycle analysis

41

PULSE JET: SUBSONIC V1

42

GAS TURBINE / PDE HYBRID

43

WHAT DID THIS?

44

PDE: AT HOME

45

PDE POTENTIAL

Mach Number

I sp

46

PDE SUMMARY

• Claimed Advantages – No moving parts – High thermodynamic efficiency – Operating in a potential large Mach number range (from 0 to 4-5) – Simplicity and flexibility of geometrical configuration – Easy integration to vehicle – Low cost

• Key Issues / Challenges– Detonation initiation (PDE/PDRE) – Air inlet design (PDE) – Fuel/air injection and mixing (PDE/PDRE) – Coupling with external flow (PDE) – Design optimization (PDE/PDRE)

• PDE looks attractive for special missions but is not yet totally mastered

47

MICRO PROPULSION

• Why Go So Small?

– T/W advantages

– Scalability

– Fine pointing

– Fine orbital corrections

– Constellation applications

• Traditional Manufacturing vs. MEMS (Micro-Electro-Mechanical Systems)

• Military Applications

48

MICRO TURBOMACHINERY

49

MICRO-ROCKET PROJECT• Design complete rocket system, tanks, valves, turbomachinery, thrust chamber

• Mass production, silicon-carbide material

• Launch and orbital applications, T~ 5-15 N, Isp ~ 300 sec

• Higher T/W than Space Shuttle