Development of Durable Ceramic MatrixDevelopment of Durable Ceramic MatrixDevelopment of Durable Ceramic Matrix Composite Turbine Components for Advanced

Propulsion Engine Systems

Development of Durable Ceramic Matrix Composite Turbine Components for Advanced

Propulsion Engine SystemsPropulsion Engine Systems Propulsion Engine Systems

Dongming Zhu

Durability and Protective Coatings BranchStructures and Materials Division

NASA John H. Glenn Research CenterNASA John H. Glenn Research CenterCleveland, Ohio 44135, USA

1

8th Pacific Rim Conference on Ceramic and Glass TechnologyVancouver, May 31-June 5, 2009

Revolutionary Ceramic Coatings and Composites Greatly Impact Turbine Engine TechnologyImpact Turbine Engine Technology

— Ceramic thermal/environmental barrier coating (T/EBC) and ceramic matrix composite (CMC) development goals- Meet engine temperature and performance requirements- Ensure long-term durability- Develop design tools and lifing methodologies- Improve technology readiness

— Crucial for envisioned supersonic vehicles: reduced engine emission, improved efficiency and long-term supersonic cruise durability

2

Revolutionary Ceramic Coatings and Composites Greatly Impact Turbine Engine Technology - ContinuedImpact Turbine Engine Technology Continued

3000°F+ (1650°C+)

Temperature Capability (T/EBC) surface

Temperature Capability (T/EBC) surface

Step increase in the material’s temperature capability3100°F SiC/SiC

CMC combustor

2700°F SiC/SiC CMC2800ºF combustor

Increase in T across T/EBCIncrease in T across T/EBC

2700°F (1482C)2700 F SiC/SiC CMC

turbine systemcombustor TBC

2500ºF Turbine TBC

2400°F (1316°C)

Gen IV

Single Crystal Superalloy

Ceramic Matrix Composite

Gen IV

Single Crystal Superalloy

Ceramic Matrix Composite

2000°F (1093°C)

Gen I

Gen II – Current commercialGen III

Gen. IV

YearGen I

Gen II – Current commercialGen III

Gen. IV

Year

3

MOTIVATION— Thermal and environmental barrier coatings with advanced hot-section

substrate component materials help increase gas turbine operating temperatures, reduce cooling requirements, improve engine fuel efficiency and

li bilitreliability— Prime-reliant coating system is key to engine component durability

Combustor Vane Blade Ceramic nozzles

Temperature increaseincrease

Turbine

CMC combustor liner , vane/blade

4

(a) Current TEBCs (b) Advanced T/EBCs

Advanced Ceramic Matrix Composite Turbine Airfoil Systems Emphasized in the Current ProgramSystems Emphasized in the Current Program

— Advanced CMC turbine airfoil technology significantly improves engine performance by further increasing materials temperature capability, reducing engine weight and cooling requirementsg g g q

Gas TBC Bond coat

Metal blade

3200 F

Gas TBC Bond coat

Metal blade

3200 F

Gas EBC Bond coat

CMC blade

2200 F

Gas EBC Bond coat

CMC blade

2900 F(T41)

(T41) (T41)

Tsurface200 F

increase

Tsurface

2500 F TBCs 2700 F EBCs

Baseline metal temperature

300 FincreaseTsurface

increase

Thin turbine coating

development

Tsurface

2000-2200 F EBCs

Thin turbine coating

Current metal turbine airfoil system

State of the art metal turbine airfoil system

CMC HPT turbine airfoil system

CMC LPT turbine airfoil system

coating development

5

Outline

─ Research & development emphases and thrust areas

Hi h h fl i h d f CMC bi i f il─ High heat flux testing methods for CMC turbine airfoil development

─ Thermal and environmental barrier coatings systemsThermal and environmental barrier coatings systems development

─ Design tool and life prediction of coated CMC components

─ Summary and future directions

6

Research and Development Emphases and Thrust Areas (continued)(continued)

Advanced coating and CMC development and processingSimulated engine heat-flux testing for coated component design and process validation

Ad d i t l

High heat-flux thermal gradient mechanical testing and simulated engine testing of coated CMC specimens and subcomponents

Advanced environmental barrier coating development Embedded

TC Temperature

3D Orthogonal

2D Five-harness Satin

SiC/BN nanotube () synthesis for

Angle Interlock

Braid

7

SiC/BN nanotube () synthesis for nano-composite coating

applications

Embedded thermocouple and heat flux sensor developmentCMC fiber architecture design

and property modeling

Research and Development Emphases and Thrust Areas (continued)(continued)

Development of physics-based design tools and computational life models

FEA S lMaterials and

systemsAdvanced testing-

simulatingFEA Solver

Atomistic and i h i

Physical process d l d l

systemsDatabase

Mechanisms-i

simulating module

Continuum and lmicro-mechanics

models modulemodels module environment

interaction module

structural mechanics

module

Design Tool and Life Prediction

Test rigTC

g

Design tool development for coated CMCSimulated high pressure and high

8

Design tool development for coated CMC turbine components

Simulated high pressure and high velocity combustion flow and CMC

turbine airfoil for heat transfer modeling

High-Heat-Flux Test Approaches– Turbine level high-heat-flux tests crucial for CMC coating system developments

• High power CO2 laser high-heat-flux rig (up to 315 W/cm2)

Heat flux

Turbine: 450°F across 100 microns Combustor:1250°F across 400 microns

T

rom

sur

face

Dis

tanc

e fr

Test rig

cooling

Test rig

9

Thermal Conductivity Measurement by a Laser High-Heat-Flux ApproachFlux Approach

Where )(/)( tTlqtk ceramicceramicthruceramic radiatedreflecteddeliveredthru qqqq

substratebond l

substrate

thrul

bond

thrubackmetalsurafceceramicceramic Tk

dlqTkdlqTTtT

00 )()()(and

8 m pyrometer for Tceramic-surface

radiatedqreflectedq

deliveredq

ceramic coatingbond coat

TT

thruq

b t t

bondsubstrate

measuredceramic

TTTT

Ttc

Two-color and 8 m pyrometers for

Optional miniature thermocouple for

thruq

substratetc

10

Tsubstrate-back additional heat-flux calibration

thruq

Laser Heat Flux Testing in Water Vapor Environments for High Temperature SiC/SiC Ceramic Matrix CompositesHigh Temperature SiC/SiC Ceramic Matrix Composites

─ High temperature and high-heat-flux testing capabilities─ “Micro-steam environment” allowing high water vapor pressure, relatively high

velocity under very high temperature conditionvelocity under very high temperature condition

- Steam injected at up to 5m/sec- Testing temperature >1700°C

11

High Pressure Burner Rig for Thermal and Environmental Barrier Coating DevelopmentBarrier Coating Development

─ Realistic engine combustion environments for specimen and component testing

High Pressure Burner rig (6 to 12 atm)V l it 900 / C t d i t bi

12

Velocity 900 m/s Coated ceramic turbine vane test fixtures

Thermal Conductivity of EBC Material Systems

– Thermal conductivity of plasma-sprayed HfO2-(Y,Gd/Nd,Yb)2O3 systems determined using laser heat flux approaches

1.5 2.51.5

1.0

cond

uctiv

ity, W

/m-K

Tsurface=~3000°F1.0

1.5

2.0

con

duct

ivity

, W/m

-K

1.0

cond

uctiv

ity, W

/m-K

~k0

0.0

0.5

0.0 5.0 10.0 15.0 20.0

Ther

mal

c

Time, hours

Tsurface=~3000 FTinterface=~2012°F

0.0

0.5

1000 1100 1200 1300 1400 1500

k20k0k20 predicted

Ther

mal

Average temperature, °C

0.0

0.5

1000 1100 1200 1300 1400 1500 1600 1700 1800

<k20><k0>k(T) predicted

Ther

mal

cTemperature, °C

Time, hours

1 2

1.4

1.6

k0k20

y, W

/m-K

0.8

1.0

1.2

Ther

mal

con

duct

ivity

Possible optimum regionLow

durability Higher conductivity

13

0.6

10 12 14 16 18 20 22

T

Total dopant concentration, mol%

Possible optimum regionyregion conductivity

region

High-Heat-Flux Thermal Gradient Cyclic Testing of EBC Material SystemsMaterial Systems

– Coating degradation modes can be monitored in real time

1.5

2.0

1200

1600Normalized kcera

ctiv

ity, W

/m-K

C

1.5

2.0

1500

2000Normalized k

ctiv

ity, W

/m-K

C

0 5

1.0800

1200

Tsurfaced th

erm

al c

ondu

c

Tem

pera

ture

, °

0 5

1.0

500

1000

d th

erm

al c

ondu

c

Tem

pera

ture

, °

0.0

0.5

0

400

0 5 10 15 20 25

TinterfaceTback

Nor

mal

ized

Ti h

0.0

0.5

0

500

0.0 2.0 4.0 6.0 8.0 10.0

TsurfaceTinterfaceTback

Nor

mal

ized

Ti hTime, hours Time, hours

14

Interface Reactions of Baseline Coating in Heat Flux and Water Vapor Cyclic EnvironmentsWater Vapor Cyclic Environments

— Significant interfacial pore and eutectic phase formation due to the water vapor attack and Si diffusion under the thermal gradient cycling conditions at interface temperature 1300°C

ZrO2-8wt%Y2O3ZrO2-8wt%Y2O32 % 2 3

Mullite+BSAS

Si

2 % 2 3

Mullite+BSAS

SiSiSiC/SiC

SiSiC/SiC

BSAS MulliteBSAS Mullite MulliteBSAS

SiSi

15

Si

Sintering and Creep Induced Failure of TEBCs

― Models used to predict long-term sintering behavior from sintering data― Variable sintering rates observed

- Initially very fast sinteringy y g- Reduced sintering rates with increasing time

― Sintering-creep can induce surface cracking and delamination

1200°C-e%1300°C-e%1400°C-e%

1200°C-de/dt1300°C-de/dt1400°C-de/dt 0.0 200

ZrO2-8wt%Y2O3/Mullite+BSAS/Si SystemPlasma-sprayed ZrO2-8wt%Y2O3

0 0

0.5

1.0

1.5

-5.0 10-9

0.01500°C-e% 1500°C-de/dt

in e

=DL/

L, %

t, 1/

sec -0.4

-0.2150

train

s, %

e ra

te, J

/m2

thruC

G 1500mindelaC

G 1500

-1.5

-1.0

-0.5

0.0

-1.5 10-8

-1.0 10-8

ring

shrin

kage

stra

i

Stra

in ra

te-d

e/dt

-1.0

-0.8

-0.6

50

100

Shrik

age

st

Ener

gy re

leas

G

ETBC

~60GPathru

CG 1400

mindelaG

16

-2.5

-2.0

-2.0 10-8

0 100 200 300 400 500 600

Sint

e

Time, hours

-1.2 00 5 10 15 20

Time, hours

GC

mindelaC

G 1400

Advanced Coating System Development

— Mutli-component zirconia/hafnia-, perovskite and pyrochlore-oxide-based systems as high stability top coats for ceramic components: defect cluster coating concept demonstrated

• Rare earth dopants for improved thermal stability• Transition metal dopants for phase stability and temperature capability of EBCs• Thin coating configurations emphasized for turbine applicationThin coating configurations emphasized for turbine application

— Low stress, strain tolerant interlayer and high strength bond coat concepts• Advanced modified HfO2-aluminosilicates show high performance

– Nano composite structures– Controlled thermal conductivity and thermal expansion– Controllable Si activity suitable for improved bonding and stability

• Novel compositional and architectural design to achieve maximum energy dissipationNovel compositional and architectural design to achieve maximum energy dissipation and durability – Alternating composition layered coating (ACLC) concept demonstrated– Temperature Actuation Coating (TAC) systems

17

Advanced Thermal and Environmental Barrier Coatings for Si-based Ceramic Components

– Advanced TEBC System (US Patent Appl. No. 11/510 573)for Si based Ceramic Components

• High stability HfO2 layer with graded interlayer environmental barrier andHigh stability HfO2 layer with graded interlayer, environmental barrier and advanced bond coats

• Alternating composition layered and nano-composite coating interlayer• BSAS, alloyed mullite and rare earth silicate EBCs

O id Si i b d• Oxide-Si composite bond coats

Interlayer: Compositional layer graded systemLow expansion HfO2

Doped mullite-HfO2, with and rare earth silicate EBCsCeramic composite bond coat

Interlayer: Compositional layer graded system

SiC/SiC CMC Si NHfO2 and HfO2 composites

SiC/SiC CMC or Si3N4 Doped mullitewith ACLC (Hf rich bands) Doped mullite/Si (SiC/Si3N4) composite bond coat(Hi h t t bl ith lf h li )

Increased Si activity

Increased dopant RE/Transition metal concentrations & increased Al/Si ratio

18

(High temperature capable with self-healing)

Advanced Thermal and Environmental Barrier Coatings for Si-based Ceramic Components

– Advanced HfO2-Rare Earth Aluminosilicate composite coatings synthesized for improved long-term durability

for Si based Ceramic Components

19

Advanced TEBCs Integrated CMCs

– Ceramic-nanotube composite bond coats with optimized CMC systems for high performance complex engine components

warp

fillL= length

warp

fillL= length

warp

fillL= length

X1, Z2 X2, Z2X1, Z2 X2, Z2

TGA Data in Air

90100110

Improved SiCNT yield during conversion reaction from 50% to 65 %

SiCNT retained nanotube morphologyfollowing heat in air to 1200°C

Improvement

W = width

H = height

t =thickness

W = width

H = height

t =thickness

W = width

H = height

t =thickness

1020304050607080

Mas

s C

hang

e (%

) SiCNT

MWCNT

SiCNT Improvedconversion

Improvement in conversion

10 1600

X1, Z1 X2, Z1X1, Z1 X2, Z1

Fiber architecture modeling to optimize fiber contents and structures and improve the complex-shaped blade strength

010

0 200 400 600 800 1000 1200

Temperature (C)

MWCNT startingmaterial

4

6

8

800

1200

TsurfaceTinterfaceTback

cond

uctv

ity, W

/m-K

empe

ratu

re, °

C

0

20

400

0 20 40 60 80 100 120

kcera

Ther

mal

Te

Time, hours

1 hr cycles, in air

20

After testingThermal gradient cyclic testing of Mullite-

SiCNT composite coating on SiC/SiC

Laser High-Heat-Flux Thermal Fatigue Testing of CMC under Simulate Turbine Blade Temperature Gradients and

St E i tStress Environments

200

300

coating

1.6 in

0

100

200

ess,

MPa

coating

-200

-100

Stre

Specimen testing

-3000 0.5 1 1.5 2 2.5

Distance from surface, mm

Stress rupture and low cycle fatigue life i ti t d f t d d t d

21

Coated CMC temperature modeling

investigated for coated and uncoated CMC systems

Laser High-Heat-Flux Thermal Fatigue Testing of CMC under Simulate Turbine Blade Temperature Gradients and

St E i t C ti dStress Environments- Continued- Laser high heat flux tensile fatigue test rig allows very high temperature, high heat flux thermal gradient testing under turbine blade stress conditions

22

Advanced Low Pressure Plasma Spray Thin Film and Physical Vapor Deposition (LPPS-TF and LPPS-PVD) for

CMC Ai f il C ti P iCMC Airfoil Coating Processing─ Advanced hybrid plasma-vapor deposition thin film coating technologies are

currently being pursued for CMC turbine blade coating processing

Conventional air plasma-spray (APS) processing of environmental barrier coatings

Low pressure plasma-spray (LPPS)

Hybrid LPPS-TF and LPPS-PVD coater under

construction

23

construction

Environmental Barrier Coating and Embedded Sensor Development for Component Health MonitoringDevelopment for Component Health Monitoring

― High temperature harsh environment embedded temperature/heat flux sensors for coated SiC/SiC ceramic matrix composite (CMC) turbine engine components are being developedcomponents are being developed

Laser high heat-flux rig High pressure burner rig

Mach 0.3 Burner RigA heat flux sensor on an environmental barrier coating (EBC)-CMC specimen

24

Embedded sensor testing and validation in simulated engine environments

The 3100°F SiC/SiC CMC Turbine Vane Coating Systems

— The coating system successfully completed total 100, 1 hr cycle laser heat flux 3100°F test (60min hot, 3min cool)

Top layerTsurfaceTinterface

Interlayer

Top layer

EBC3 0

3.5

4.0

1600

kcera Tback

W/m

-K

3100°F coatings3100°F coatingsEBCSi bond coat

1 5

2.0

2.5

3.0

800

1200

ondu

ctiv

ity, W

mpe

ratu

re, °

C

0 0

0.5

1.0

1.5

0

400

Ther

mal

c Tem

25

After testing0.0 0

0 20 40 60 80 100 120Time, hours

The Long-Term Durable CMC Turbine Blade Coating System Testing under High Heat Flux Conditions

— Coating successfully tested at Tsurface 2700°F and Tinterface 2400°F for 250 hrs (60min heating, 5min cooling cycles)

System Testing under High Heat Flux Conditions

Thin coating turbine CMC demonstrated high-heat-flux

26

gcyclic durability at 2700˚F

Radiative Diffusion Models for Non-Gray Materials

- The diffusion conduction models established for non-gray coating materials to understand complex thermal conductivity behavior

scond

scondtotal dx

dTa

Tnk

dxdT

aTn

dxdTkq

aveave

316

316 3232

- The diffusion conduction equations

Gray model

radconds

condeffective kka

Tnkk

ave

3

16 32condk Intrinsic lattice conductive thermal conductivity

8 2 4

radk radiation thermal conductivity

totalq Total heat flux

effectivek effective thermal conductivity T(d)T(0)FFTn16σkq3ave

2

Gray model

n

a

s

T

Stefan-Boltzman constant 5.6704x10-8 W/(m2-K4)

Refractive index, 2.2Absorption coefficient, cm-1

Scattering coefficient, cm-1

Average temperature of the material, K

d

FF3κ

kqavestaveop Tλ0Tλ0total

1n32

23

nς

4λT0 n6

n6ς

n3ςς

ne

π15F

Non-Graymodel

1rq 2rq

opaqueRadiativediffusion

approximationtransparent

0

opaqueRadiativediffusion

approximationtransparentopaque

Radiativediffusion

approximationtransparent

0

1rq

1sT1gT

2gTD

27

Regions of optical thicknessc10 c2

Regions of optical thicknessc10 c2

Radiative Diffusion Model Validation for Coating Systems

2 5

3.0

m-K 0.8

1.00.0 0.50 1.0 1.5 2.0

ratio

n I/I

0

scattering-

absorption

Coating thickness, mm

2.0

2.5

vity

, W/m

0.2

0.4

0.6

B likbod

y ra

diat

ion

pene

tr scatteringbaseline coatings

scattering-advanced coatings

1.5

cond

uctiv

Radiation component0.0

0 500 1000 1500 2000

BaselinesAdvanced coatingsB

lack

Coating thickness, microns

0 5

1.0

Ther

mal

c

0.0

0.5

200 400 600 800 1000 1200 1400 1600 1800

T

28

200 400 600 800 1000 1200 1400 1600 1800Surface temperature, °C

High Pressure Burner Rig Durability Evaluations

― High velocity nozzle designed to improve heat flux under high pressure condition― Optimum heat flux regime determined

200 1000

160

180800Heat flux

Chamber pressureGas velocity

ty, m

/s

140

160

600flu

x, W

/cm

2

re; p

si; g

as v

eloc

it

100

120

200

400Hea

t

Cha

mbe

r pre

ssu r

60

80

010 15 20 25 30 35 40

29

Pressure difference in combustor and test chamber, psi

High Pressure Burner Rig Durability Evaluations(Continued)(Continued)

― High Pressure Burner Rig Stability being evaluated for TEBCs on SiC/SiC― High stability coatings being downselected

1AS800SN2822 -h

1300Temperature, °C

14001500 12001600

SiC/SiC under high velocity

0.1

SiC/SIC CMCLa

2Hf

2O

7

HfO2 (doped)

RE-Hf-luminosilicatesBSASan

ge, m

g/cm

2

0.01

c w

eigh

t cha Rare earth silicates

BSAS Baseline

0.0010 0005 0 00055 0 0006 0 00065 0 0007 0 00075 0 0008

Spec

ific

Future EBC stability development goal

30

0.0005 0.00055 0.0006 0.00065 0.0007 0.00075 0.00081/T, K-1

Physics-Based Life Prediction Models and Design Tools for Advanced EBC-Coated CMC Material Systemsfor Advanced EBC Coated CMC Material Systems

– Heat transfer and stress models developed for various EBC coated specimen configurations

– Fracture-mechanics based component life prediction approaches emphasized for blade applications

0 .5 8 4 2 m m

E B C C ra c k

C ra c k d e p th , a

M

M M M

O2+H2O(g)

M

M M M

O2+H2O(g)

M

M M M

O2+H2O(g)

M

M M M

O2+H2O(g)

S i

E B CT B C

W

2 m m

0 .1 2 7 m m0 -0 .5 0 8 m m

0 .6 3 5 -0 .1 2 7 m m

0 .0 7 8 7 4 in

5 m ils0 -2 0 m ils

2 5 -5 m ils

C M C

2 C

Quarter Section of a coated CMC Tubular specimen12.7 mm outer diameter2.0 mm tube thickness

0.127 mm coating thickness

Laser heat flux Heating

Pores Cracks

Pores Cracks

W

EBC coated CMC crack configuration

2.5

3.0

3.5

a m

0.5 tTB C=0 .381m m

tEBC =0.254m mE = 10GPa

T BC Crack

Mesh Steady-State Temperature profileMulti-mechanisms Interactions

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0 1 2 3 4

C rack Length a , m m

Stre

ss In

tens

ity F

acto

r K, M

P

M od e IM od e IIM ixed M o de

ET BC= 10GPa

Gradient

Co nstant

T em perature

8 000 RPM

Subcomponent modeling

31

2sin

2cos3

2cos 23 mm

IIm

Ieff KKK

Max. Principal Stress Criterion

Failure driving force as a surface crack length Hollow CMC Blade 2mm wall thickness

8,000 RPM

Physics-Based Life Prediction Models and Design Tools for Advanced EBC-Coated CMC Material Systems-

C ti dContinued – Environmental interactions and accelerated crack propagation incorporated in the fracture mechanics based lifing models

Physics-Based Life Prediction Models for Thermal and Environmental Barrier Coatings-CMCs

DATA INPUTComponent/Coating Geometry; Operating Conditions

Coating Crack Initiation with size , ti i

Physics-/Mechanism-Based Life Prediction Main Module

Processing

Microstructure

Mechanical

Physicalia

Test/CharacterizationCoating and CMC Data Base

Coating FailureDriving Force (dynamic)

GK

Coating Failure Resistance (dynamic)

GcKc

Coating Physical Processes

Coating Crack Initiation with size , tiniMicrostructure

Properties

Failure Modes

Chemical

Simulated Engine Test

Engine Test

i

PhenomenologicalGmax Gc

Damage Accumulations

Coating Physical Processes Modeling Module

Coating/Interface Degradation

Ceramic Sintering

Ceramic Coat/CMC Creep i

inm MKKA

dtda

max

yesno

K(t) K (t) Critical Functions Determined)(ta

Constitutive Equations

ProbabilityStrain EnergyDamage Laws

gLife Models Module

Stress/modulus Evolution

datMtKtKA

tta

ai

inmprop

c

i

)(

max )()()(1

Ceramic Fatigue K(t), Kc(t), Critical Functions Determined)(tac

Additional Coating Failure Criteria

P di ti C ti F il M dThermal Conductivity

Erosion, Impact and Recession

In-situ Methods/NDE

Interactions Mi, etc.Life Model Validation

Materials Property Change

Viscoelasticity/Viscoplasticity

32

Interactions Functions Mi

Convective/Radiative Heat TransferCoating Life tf=tini+tporp

Predicting Coating Failure Modes

Crack Growth

StressesViscoelasticity/Viscoplasticity

Summary

• Advanced ceramic turbine component testing capabilities established

• High stability thermal and environmental barrier coatings developed

• Advanced hybrid plasma-vapor deposition being developed for complex-h d t d ith thi ti fi tishaped components and with thin coating configurations

• Coated CMC stability demonstrated in high velocity-high pressure burner rig simulated engine environmentsrig simulated engine environments

• Coated CMC systems demonstrated initial low cycle fatigue and thermomechanical fatigue durabilitythermomechanical fatigue durability

• Heat transfer, fracture mechanics and stochastic approaches being established to develop coated CMC life prediction models

33

established to develop coated CMC life prediction models

Future Directions• High stability thin coatings system development

– Emphasize advanced processing and composites

• Coatings with significantly improved thermal and mechanical load capability

• Materials and component system integration– Enhanced functionality with embedded sensors and self-healing capability

• Laboratory simulated high heat flux stress testing and life prediction methodology development

34

AcknowledgementsThe work was supported by NASA Fundamental Aeronautics

Program.

35