This presentation introduces a new sizing and margin methodology for dual-layer Thermal Protection Systems (TPS). The methodology has been tailored for application to a dual-layer 3D-woven TPS called Heat-shield for Extreme Entry Environments Technology (HEEET). Sizing is performed for a reference Saturn probe mission to show how uncertainties in trajectory, aerothermal modelling and TPS response impact the sizing of each layer

Mahzari, Milad

Milos, Frank S.

NASA Technical Reports Server

National Aeronautics and 
Space Administration
Entry Systems and Technology Division
Ames Research Center
Sizing and Margin Methodology 
for Dual-Layer Thermal 
Protection Systems
Milad Mahzari and Frank Milos
NASA Ames Research Center
Inputs from Don Ellerby, Peter Gage, Ethiraj Venkatapathy and Todd White
Thanks to Grant Palmer and Dinesh Prabhu for Aeroheating Simulations
15th International Planetary Probe Workshop
2018-06-12 1
https://ntrs.nasa.gov/search.jsp?R=20180004051 2019-08-31T15:59:26+00:00Z
Dual-Layer TPS Material (HEEET)
• Increased mass efficiency of dual-layer 
materials allows mission designers to select 
shallow entry trajectories
- Integrate a top layer with good recession 
performance with a bottom layer with good 
insulation performance
• NASA is maturing a dual-layer 3D-woven TPS 
called HEEET
- Top layer made of densely woven carbon fibers 
(Recession Layer, RL)
- Bottom layer made of carbon and phenolic yarns 
(Insulation Layer, IL)
• Need to develop a sizing process
- Project has developed and validated a one-
dimensional thermal response model based on 
material property and arcjet testing
- Adapt the conventional NASA ablator sizing process 
for application to dual-layer materials
• Weaving width limitation drives need for a 
tiled system
- This talk focuses on acreage material sizing
- Full sizing process has been developed and 
accounts for the gap filler thermal response 
uncertainties 2
Tile
Gap Filler
Insulation 
Layer (IL)
Recession 
Layer (IL)
TPS Sizing and Modelling Uncertainties
• TPS thickness is sized to satisfy certain mission-dependent design constraints
- Typically for single-layer materials, the constraint is a not-to-exceed bondline temperature driven 
by adhesive or structure temperature limits
• There are uncertainties associated with models used in TPS sizing process
- Trajectory dispersions
- Uncertainties in aerothermal environments (ground-to-flight traceability)
- Uncertainties in thermal response modelling (properties, models, initial conditions)
• TPS sizing process must include margins that protect against uncertainties in 
modelling
- Margins can be applied to initial conditions, boundary conditions, design constraints or sized 
thickness
- Margins are selected based on testing, uncertainty propagation or engineering judgement
3
Sizing 
constraint: 
TBL Limit
Aeroheating SimulationsTrajectory Dispersions
Max Heat 
Rate
Max Heat 
Load
TPS Sizing
TPS
Structure
Adhesive
Surface 
ConditionsMaterial 
Modelling
Conventional NASA Ablator Sizing Process
• Aerothermal environments are computed on the bounding trajectory (typically 
max heat load)
• At each sizing location, TPS thickness is sized along three branches and 
combined in a root-sum-square (RSS) process
- Zero margin: apply nominal environments and size thickness to bondline temperature limit
- Material margin: account for material modelling uncertainties (typically done by reducing 
bondline temperature by a margin informed through Monte Carlo analysis)
- Aerothermal margin: account for uncertainty in aerothermal environments (multiplying factors)
• Other considerations: manufacturing tolerance, factor of safety, recession margin
4
Aerothermal 
Environments on 
Bounding Trajectory
Trajectory & 
Atmospheric 
Dispersions
Aeroheating 
Predictions 
(CFD & Radiation)
Bounding 
Trajectory 
(Max Heat Load)
TPS Sizing
Thickness sized to 
maintain bondline 
temperature below 
specified limit
Aerothermal 
Margin Branch
Material Margin 
Branch
Zero Margin 
Branch
Margined 
Thickness 
(RSS)
Three-Branch Sizing (repeat for all sizing locations)
Max Thickness 
Across All Sizing 
Locations *
*Assumes uniform TPS 
thickness
Constraint A: 
TBL Limit
Dual-Layer Sizing Nuances
• New constraint at the interface 
between two layers
- HEEET insulation layer should not be 
exposed to flow 
- Arcjet testing scope limited to RL
• RL is sized to be equal to the 
predicted recession; IL is sized to 
bondline temperature limit
- Material margin must be considered for 
both interfaces
5
Single-Layer
TPS
Dual-Layer
TPS
Constraint B: 
Recession
• Current HEEET implementation requires uniform TPS thickness 
for both layers
- Need to find max required thickness for each layer across all body points 
and trajectories
• Max thickness for each layer may occur at different body points 
and trajectories
- Higher ablation leads to lower heat conduction into TPS
Dual-Layer Sizing Nuances
• Sizing RL and IL independently and then stacking max RL thickness from one location 
on max IL thickness from another location is not mass efficient
- Excess RL at some locations can serve as insulation
• More mass efficient to size IL after fixing RL to max sized thickness across all locations
6
Shoulder
Stag 
Pt.
Mid-Flank
Size RL and IL 
Independently
Size IL after Fixing 
RL to Max Thickness
Final 
Thickness
Final 
Thickness
Dual-Layer Sizing Process
• Proposed sizing process takes advantage of the nonessential portion of 
RL thickness at locations that don’t drive RL sizing
- RL-alone calculation to determine recession for each sizing case; fix RL thickness to 
maximum RSSed recession across all cases (body points, bounding trajectories)
- IL is sized for all sizing cases to bondline temperature limit using the fixed RL 
thickness; Final IL thickness is the maximum thickness across all cases
7
RL Calculation
Calculate recession 
for each branch
Aerothermal 
Margin Branch
Material Margin 
Branch
Zero Margin 
Branch
RSSed 
Recession
Three-Branch Process (repeat for all sizing locations and trajectories)
RSSed IL 
Thickness
IL Sizing
IL sized to bondline 
temperature limit
Max RL 
Thickness 
Across All 
Sizing Cases
Max IL 
Thickness 
Across All 
Sizing Cases
Final 
Thickness 
(RL, IL)
Aerothermal 
Margin Branch
Material Margin 
Branch
Zero Margin 
Branch
Three-Branch Process (repeat for all sizing locations and trajectories)
3.
5 
m
Body Point 1
(Stag. point)
Body Point 9
(Shoulder)
Reference Missions
• Venus Lander
- 2010 NASA study VITaL (shallow)
- 45-deg spherecone
- D=3.5m, ME= 2750kg
- VE= 11.3 km/s, ƔE= -9 deg
- Aeroheating simulations by Grant 
Palmer
- 9 sizing cases (9 body points, 1 
trajectory)
- Highlights location impact on sizing
• Saturn Probe
- NF-4 proposal (SPRITE), PI: Amy 
Simon (GSFC), managed by JPL
- 45-deg Spherecone, 1.25m diameter, 
447kg entry mass
- VE= 26.9 km/s, ƔE= -14 deg
- Aeroheating simulations by Dinesh 
Prabhu
- Total of 8 sizing cases (4 body points for 
max heat rate and load trajectories)
- Highlights trajectory impact on sizing
8
Co
nv
ec
tiv
e 
He
at
 F
lu
x,
 W
/c
m
2
Co
nv
ec
tiv
e 
He
at
 F
lu
x,
 W
/c
m
2
50
0
10
00
15
00
20
00
Radial Coordinate, m
0.1 0.2 0.3 0.4 0.5 0.6
1.
25
 m
Body Point 1
(Stag. point)
Body Point 4
(shoulder)
Sizing for Venus Reference Mission
• Sizing done at 9 locations on the heatshield
- Figure on left: RL and IL sized independently
- Figure on right: RL sized first; then IL sized while for fixed RL thickness
• Taking advantage of the nonessential portion of RL thickness at locations that 
don’t drive RL sizing provides mass benefits  
- 62% reduction in IL thickness, 19% reduction in areal mass
9
Stag Sph1 Sph2 Junction Flank1 Flank2 Flank3 Flank4 Shoulder
0.6
0.4
0.2
0
0.2
0.4
0.6
R
L 
Th
ic
kn
es
s 
(in
)
IL
 T
hi
ck
ne
ss
 (i
n)
Max RL Thickness
Max IL Thickness
RL and IL Sized Independently 
(Not taking advantage of excess RL)
IL Sized after Fixing RL to Max Thickness
(Taking advantage of excess RL)
Heatshield Location Heatshield Location
Final 
Thickness
Stag Sph1 Sph2 Junction Flank1 Flank2 Flank3 Flank4 Shoulder
0.6
0.4
0.2
0
0.2
0.4
0.6
Required IL Thickness
Required RL Thickness
Excess RL Thickness
Required IL Thickness
Required RL Thickness
Excess RL Thickness
data2
data1
Sizing for Saturn Reference Mission
• Sizing done at four locations on the heatshield and for two bounding trajectories, Max Heat 
Rate (MHR) and Max Heat Load (MHL)
• Maximum RL thickness occurs at shoulder for max heat rate trajectory
• Maximum IL thickness occurs at stagnation point for max heat load trajectory
• Independent RL and IL sizing would have resulted in 21% increase in IL thickness and 9% 
increase in areal mass
10
R
L 
Th
ic
kn
es
s 
(in
)
IL
 T
hi
ck
ne
ss
 (i
n)
Heatshield Location
Stag SphCone Junc.Mid-Flank Shoulder
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
Required RL-MHL
Required RL-MHR
Required IL-MHL
Required IL-MHR
StagSphCone Junc.Mid-Flank Shoulder
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
 RL-MHL
Excess RL
Required IL-MHL
Required IL-MHR
Final 
Thickness
IL Sized after Fixing RL to Max Thickness
(Taking advantage of excess RL)
Stag SphCone Junc. Mid-Flank Shoulder
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
Stag SphCo e Junc. Mid-Flank Shoulder
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
Summary and Conclusions
• Sizing based on only stagnation point environments in early mission 
phases may not bound required thickness 
- Both for single-layer and dual-layer materials
- The size of impact is likely larger for dual-layer materials if each layer has to be 
constant thickness across the heatshield 
- In applications where off-stagnation environments are suspected to be higher, 
utilizing CFD simulations early in the design is highly recommended 
• Proposed sizing methodology takes advantage of the insulation 
properties of the excess recession layer at locations that don’t drive 
RL thickness
• Allowing the insulation layer to be exposed to flow will provide more 
flexibility in TPS sizing and design
- Requires arcjet testing of insulation layer to establish its max capability
- Sizing process needs to be modified for a different interface constraint (ex. limit 
on combined aerothermal environment experienced by insulation layer)
• Allowing varying TPS thickness across the heatshield will offer mass 
benefits
- Manufacturing challenges should not be underestimated
11
Backup
12
Complete HEEET Sizing Process (Including Gap Filler and 
Manufacturing Considerations)
13
National Aeronautics and 
Space Administration
Ames Research Center
Entry Systems and Technology Division


Sizing and Margin Methodology for Dual-Layer Thermal Protection Systems

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