Wavelength-dependent or spectral radiation effects are potentially significant for thermal protection materials. NASA atmospheric entry simulations include trajectories with significant levels of shock layer radiation which is concentrated in narrow spectral lines. Tests using two different high powered lasers, the 10.6 micron LHMEL I CO2 laser and the near-infrared 1.07 micron fiber laser, on low density ablative thermal protection materials offer a unique opportunity to evaluate spectral effects. Test results indicated that the laser wavelength can impact the thermal response of an ablative material, in terms of bond-line temperatures, penetration times, mass losses, and char layer thicknesses

White, Susan M.

NASA Technical Reports Server

National Aeronautics and Space Administration 
Effects of Laser Wavelength on Ablator Testing 
Susan White 
NASA Ames Research Center 
 
NASA Program support: 
Hypersonics Entry Descent and Landing (HEDL) 
Exploration Technology Development and Demonstration (ETDD)  
Entry Descent and Landing Technology Development Project (EDL TDP) 
 
 
2014 38th Annual Conference on Composites, Materials and Structures  
https://ntrs.nasa.gov/search.jsp?R=20140011553 2019-08-31T19:04:55+00:00Z
2 
• Background / Motivation 
• Approach 
• Test Results 
• Summary & Conclusions 
Outline 
3 
Motivation:  Need for Advanced TPS  
NASA conducted study in 2008 to establish entry system technologies 
required to put 40+ metric tons* on the surface of Mars    *state-of-the-art ~ 1 MT 
Heritage TPS / Entry Systems (5 m, 1 MT) do not meet requirements 
Concepts included ablative flexible thermal protection systems (TPS) 
time (sec) 
Fully Margined 23 meter diameter Deployable Heat Shield 
80 MT Aero Capture & Entry 
100 200 300 
20 
40 
60 
80 
100 
120 
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Aero Capture 
Entry 
Integrated Heat Load 
Aero Capture 
DK  KM 
4 
Flexible Ablator Technology Development, FY 10 - 14  
• Determine evaluation criteria to define successful development 
• Identify promising materials with flexible matrices / substrates  
  - carbon, silica, and polymer based felts / cloths 
  - organic / inorganic blended materials  
• Investigate resins, additives, solvents for flexible composites  
• Utilize lower cost screening tests to determine viability 
  - Aerothermal screening in NASA Ames X-jet plasma torch 
  - Thermal screening in radiant environment at Wright-Patterson AFB 
     Laser Hardened Materials Evaluation Laboratory (LHMEL) 
  - Aerothermal screening in NASA Johnson TP2 arc heater 
  - Fold testing for stowability effects  
• Downselect materials for further technology (TRL) maturation 
Flexible Ablator TPS Program  
5 
Shock Layer Radiation can significantly impact Spacecraft Heat Flux 
Robinson, J.S., and Wurster, K.E., and Mills, J. C., “Entry Trajectory and Aeroheating Environment Definition for Capsule-Shaped 
Vehicles”, JOURNAL OF SPACECRAFT AND ROCKETS, Vol. 46, No. 1, January–February 2009 
Radiative heating  depends on size and speed – 
• the larger the entry vehicle, the higher the radiative heating,  
• the higher the entry velocity, the higher the radiative heating 
        Moon-Earth nominal direct entry Mars-Earth nominal direct entry 
6 
Experiment vs. Predicted Air Shock Layer Radiation Spectrum 
Experimental 
data – 
measured in 
EAST facility 
 
 
 
Theoretical 
predictions - 
from NEQAIR 
simulations of 
shock layer in 
Air 
 
Shock layer radiation is concentrated in narrow spectral bands characteristic of 
atmospheric  chemistry.   
 
Reference: “Analysis and Model Validation of Shock Layer Radiation in Air”, Bose, D., McCorkle,, E., Thompson, C., 
Bogdanoff, D.  , Prabhu, D.K. , Allen, G.,  And Grinstead, J., AIAA 2008-1246, 46th AIAA Aerospace Sciences Meeting and 
Exhibit., 7 - 10 January 2008, Reno, Nevada 
7 
• Material response to radiation can depend strongly on wavelength . For 
example, your car window traps heat (infrared) but transmits light (visible).  
• Unfortunately, existing convective arc jet test facilities are unable simulate 
shock layer radiation at the desired wavelengths and levels. 
• In addition, even for convective dominated heating environments, laser 
testing is less expensive per test and can be widely used for preliminary 
screening purposes. 
• High-powered spectral radiation sources needed to assess radiation 
transport effects on TPS materials. Lasers are the best radiation sources to 
provide high levels of energy at specific wavelengths. 
Example: Laser 1 radiant energy absorbed at or near surface (ideal). 
 Radiant energy from Laser 2 travels further in-depth. 
Vehicle 
Structure 
Thermal  
Protection 
Material 
Laser 1 
Laser 2 
Shocklayer Radiation and TPS Testing 
8 
• Background / Motivation 
• Approach 
• Test Results 
• Summary & Conclusions 
9 
Laser Hardened Materials Evaluation Laboratory (LHMEL)           
Reliable, calibrated, and economical laser test facility located at Wright-
Patterson AFB and operated by the U.S. Air Force Research Lab 
• CO2 Laser:  10.6 microns 
 LHMEL I 
 15 kW CO2 laser  
   (150kW LHMEL II not used) 
• Fiber Laser: 1.07 microns 
  IPG Photonics  
 10 kW Fiber Laser (new) 
 
 
AFRL Laser  Lab 
Test Facility used for Spectral Radiation Heating 
1 micron 
Workhorse 
CO2 Lasers 
10.6 microns 
(not to scale)  
10 
Test Set-Up 
Tests conducted in inert environment 
    - Nitrogen (N2) purged test box  
    - N2 gas crossflow to prevent beam blockage 
Burn plates used to verify exposed area, homogeneity 
Test Conditions: 115 W/cm2 30 seconds CW (non-pulsed) 
 
CO2 (LHMEL 1) & Fiber Laser Setup 
Nitrogen purged test box Laser alignment, burn plate, and model holder  
model holder  
burn plate 
alignment 
laser 
11 
Materials: a Subset of Flexible Ablators 
2011 Screening Test Matrix 
Fully Margined 23 meter diameter 
Deployable Heat Shield (80 MT Aero 
Capture & Entry per NASA 2008 study) 
time (sec) 
100 200 300
20 
40 
60 
80 
100 
120 
Entry 
Aero Capture 
Parameters of Interest 
• Mass Loss 
• Char Layer Thickness 
• Max Bondline Temperature 
• Time to Max Bondline Temp 
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Test Materials used to compare Results of Laser Tests 
• ) 
Carbon fiber felts (non-woven) 
impregnated with silicone resin 
Silica fiber felts impregnated with 
silicone resin  
SIRCA used as a reference.  
Comparable in : 
-    substrate felt commercial 
manufacturing 
-    processing 
- density 
Each pair of two samples for the 
CO2 and Fiber laser had similar 
thickness, but Refrasil was thickest 
and had highest areal weight 
compared to other materials. 
Test Materials 
Test Material
Areal Wt 
(g/cm2)
Density 
(g/cc)
Carbon Felt Silicone  
Morgan-S-T 0.34 0.18
Fiber Morgan-S-T 0.34 0.18
FMI-S-T 0.34 0.20
Fiber FMI-S-T 0.34 0.20
FMI-S-B 0.39 0.21
Fiber FMI-S-B 0.37 0.20
FMI-S-T-A2 0.43 0.22
Fiber FMI-S-T-A2 0.42 0.22
Glass Fiber w Silicone  
Refrasil-S-B 0.66 0.27
Fiber Refrasil-S-B 0.59 0.25
SIRCA 0.34 0.26
Fiber SIRCA 0.34 0.26
13 
• Background / Motivation 
• Approach 
• Test Results 
• Summary & Conclusions 
14 
Comparison of Silica, Glass Fiber-based Materials 
Refrasil was thicker, more insulative, than the other felts.  Mostly smooth appearance from the 
CO2 laser testing, but the Fiber laser testing of Refrasil resulted in a mottled, uneven surface. 
pre-test post CO2 laser post Fiber laser 
Refrasil felt-silicone resin 
Refrasil-S-B 
  Refrasil silica-felt 
  Silicone resin
  B processing 
 
Interference patterns visible in the photographs correspond to patterns seen in witness burn 
plates used to characterize the beam.  Concentric circles are characteristic of the LHMEL I 
CO2 laser, whereas the Fiber laser has a smaller scale mottled pattern of interference peaks. 
pre-test post CO2 laser post Fiber laser 
SIRCA 
Rigid Silica fiber matrix 
impregnated with silicone 
resin, used as the reference 
material for these tests. 
 
 
 
 
 
15 
Comparison of Carbon felt-based materials 
pre-test 
CO2 laser Fiber laser 
Carbon felt materials with silicone resin 
Morgan-S-T 
Morgan C-felt 
Silicone resin 
T processing 
FMI-S-T 
FMI C-felt 
Silicone resin 
T processing 
FMI-S-B 
FMI C-felt 
Silicone resin 
B processing 
FMI-S-T-A2 
FMI C-felt 
Silicone resin 
T processing 
A2 additive 
Material samples 
 - 2 different carbon felts 
 - impregnated with silicone resin 
 - processed with different 
methods and additives 
Laser tests 
Materials were tested with both a 
CO2 and Fiber laser @ 115 
W/cm2 for 30 sec 
Preliminary Results 
Post-test visual inspection 
showed no apparent differences 
between the carbon felt materials 
post-test 
16 
   Cross sections of one test material after laser testing 
post Fiber laser post CO2 laser 
• Char zone is created when enough energy is absorbed in depth to produce the 
temperature required to pyrolize the silicone resin.  
• Note that char zones resulting from laser tests are generally different thicknesses 
from arc jet test chars, even at equal heat fluxes (convective char  ≠ radiative char) 
• Photographs show thicker char and pyrolysis zones developed after exposure to the 
shorter wavelength Fiber laser 
• Photos visually demonstrate that the material absorbs energy deeper in depth at 1.07 
microns than at 10.6 microns 
Comparison of Representative cross sections 
17 
Test Results of Carbon & Silica Fiber-based Materials 
Char Layer Thickness 
Comparing char layer thickness results from the Fiber laser vs CO2 laser tests 
 - Carbon felt chars were on average 92% thicker  
 - Silica  fiber chars were thick & on average 19% thicker 
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CO2 Laser Fiber Laser KEY 
Carbon felt materials 
with silicone resin 
Silica fiber materials 
with silicone resin 
  Morgan S-T FMI S-T FMI S-B FMI S-T-A2 Refrasil S-B   SIRCA 
1 
2 
3 
4 
5 
20% 
thicker 
141% 
thicker 
18 
Test Results of Carbon & Silica Fiber-based Materials 
Mass Loss 
Comparing the mass loss of pyrolysis & vaporization from the Fiber laser vs CO2 laser tests 
 - Carbon felt materials lost on average 20% more mass from Fiber to CO2 laser test. 
 - Silica fiber materials lost on average 160% more ( but less overall: start at high reflectance) 
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CO2 Laser Fiber Laser KEY 
Carbon felt materials 
with silicone resin 
Silica fiber materials 
with silicone resin 
  Morgan S-T FMI S-T FMI S-B FMI S-T-A2 Refrasil S-B   SIRCA 
19 
Data ordered by increasing density of the carbon and silica (glass) felt materials 
Higher bondline temperatures for each material. 
Note Refrasil was thicker, insulative. 
  Morgan S-T FMI S-T FMI S-B FMI S-T-A2 Refrasil S-B   SIRCA 
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CO2 Laser* Fiber Laser* KEY 
Carbon felt materials 
with silicone resin 
Silica fiber materials 
with silicone resin 
Max Bondline Temperature 
* Materials subjected to 115 W/cm2 for 30 sec 
20 
Test Results of Carbon & Silica Fiber-based Materials 
Time to Max Bondline Temperature 
  Morgan S-T FMI S-T FMI S-B FMI S-T-A2 Refrasil S-B   SIRCA 
100 
200 
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400 
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8% 
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18% 
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19% 
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25% 
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Carbon felt materials 
with silicone resin 
Silica fiber materials 
with silicone resin 
CO2 Laser Fiber Laser KEY 
Comparing the time to max bondline temp for the Fiber laser vs CO2 laser tests 
 - Carbon felt materials peaked on average 15% faster 
 - Silica felt materials peaked ~22% faster 
21 
• Background / Motivation 
• Approach 
• Test Results 
• Summary & Conclusions 
22 
Summary and Conclusions 
• Overview: Experimental data was compared from tests in two non-pulsed 
lasers with widely separated wavelengths, at irradiances of 115 W/cm2 for 30 
sec. These low-density ablators incorporated silicone resin in commercial 
refractory felt substrates, and were comparable in processing and density.
• The carbon and silica substrate materials gave 
 
 
 
 
• Numerical modelling (not shown) shows lower extinction ( i.e. greater 
penetration and forward scattering of energy) at 1 micron than at 10 microns. 
Test results are consistent with greater in-depth absorption from 1.07 versus 
10.6 micron radiation ( i.e. more efficient surface absorption at the 10.6 versus 
1.07 micron radiation). 
• Even for carbon-fiber-dominated porous composite materials, wavelength-
dependent (i.e. spectral) radiation effects can have an impact on the 
material’s response to intense shock layer radiation!  
23 
Acknowledgements 
• This work was supported by the EDL TDP of the Exploration Technology 
Development and Demonstration (ETDD) Program, managed at NASA-
Glenn Research Center.   
• Robin Beck and Matt Gasch, project managers 
• The entire NASA Ames EDL/TDP materials team (Parul Agrawal, Jim 
Arnold, Al Covington, Wendy Fan, Matt Gasch, Howard Goldstein, Bernie 
Laub, Joe Mach, Frank Milos, Steve Sepka, Mairead Stackpoole, Jeremy 
Thornton )  
• Test materials described herein were from NASA Ames. The vendors 
Boeing, Lockheed Martin and Textron supplied materials for related testing 
not described here. 
• John Bagford and Dan Seibert from the LHMEL facility.  
• S-C Lee of Applied Sciences Laboratory for fiber radiation scattering 
theoretical modelling & consultation. 
 
24 
Effects of Laser Wavelength on Ablator Testing 
• Background / Motivation 
• Approach 
• Test Results 
• Summary & Conclusions 
• Appendix 
25 
Key Results Table 
This table shows the areal weight and density before testing, the peak bondline temperatures and the time taken to reach the peak bondline temperature 
beneath the test specimen, the mass loss, the char thickness and the combined char plus pyrolysis zone thickness. In the table, the following naming 
convention is used to describe the materials: the samples used for the Fiber laser test include the word Fiber in the test material name, the carbon felts 
were procured from FMI or Morgan, the S stands for silicone, and the designations after the S refer to different chemical alterations and processing 
methods. Each test material gave higher bondline temperatures from the Fiber laser test than the CO2 10.6 micron laser test.  The absorption of energy in 
the material leads to mass loss, due to resin being pyrolyzed, and water and residual solvent being vaporized, whereas spallation and vaporization were 
minimal in these tests. All the test material listed gave shorter times to peak temperature, greater mass loss, and thicker zones heated to pyrolysis or char 
temperatures, when irradiated at the 1.07 micron fiber laser wavelength than at the CO2 10.6 micron laser. The glass-silicone materials started with higher 
reflectance when they were virgin materials, which would lead to lower energy absorption rather than higher energy absorption if a significant fraction of 
energy were reflected away from the surface, however, the test materials quickly charred during testing, reducing differences in reflectance. 
Test Material
Areal Wt 
(g/cm2)
Density 
(g/cc)
Peak 
Bondline 
Temp (C)
Time to 
Peak Temp 
(sec)
Mass loss 
(g)
Virgin 
(mm) Char (mm)
Char+Pyro 
(mm)
Recession 
(mm)
Carbon Felt Silicone  
Morgan-S-T 0.34 0.18 110 195 0.34 13.18 1.81 4.03 0.2
Fiber Morgan-S-T 0.34 0.18 139 157 0.43 11.24 2.81 7.70 0.0
FMI-S-T 0.34 0.20 142 154 0.75 8.77 1.47 8.20 0.1
Fiber FMI-S-T 0.34 0.20 158 131 0.78 7.68 2.94 9.89 -0.8
FMI-S-B 0.39 0.21 138 171 0.86 10.07 1.54 8.52 0.5
Fiber FMI-S-B 0.37 0.20 166 158 1.22 7.06 3.72 12.17 -0.2
FMI-S-T-A2 0.43 0.22 124 187 1.09 10.79 1.78 8.38 -1.0
Fiber FMI-S-T-A2 0.42 0.22 154 154 1.14 8.08 3.01 10.90 2.5
Glass Fiber w Silicone  
AFRSI-S-B 0.54 0.23 80 408 0.43 14.17 4.16 8.39 1.0
Fiber AFRSI-S-B 0.54 0.23 80 346 0.55 14.06 5.30 9.40 -0.7
Refrasil-S-B 0.66 0.27 72 438 0.34 17.27 4.29 5.85 0.0
Fiber Refrasil-S-B 0.59 0.25 95 353 0.69 15.32 5.06 6.55 0.0
SIRCA 0.34 0.26 198 120 0.24 3.88 4.40 8.75 0.3
Fiber SIRCA 0.34 0.26 227 90 0.77 4.34 5.27 9.04 2.0
26 
Comparison of Fiber and CO2 laser tests 
• For each test material shown, all the non-layered carbon and silica materials gave: 
– higher peak bondline temperatures, 
– decreased penetration times,  
– greater mass loss,  
– and thicker zones heated to pyrolysis or char temperatures. 
Test 
Material
Bondline 
Temperature 
Increase (C)
Time to Peak 
Temperature 
Change  (%)
Mass Loss 
Change  
(%)
Char 
Thickness 
Change  
(%)
Char + 
Pyrolysis 
Zone 
Change  (%)
 Morgan-S-T 28 -22 26 56 43
 FMI-S-T 16 -17 3 100 38
 FMI-S-B 28 -9 41 141 75
 FMI-S-T-A2 30 -20 5 69 626
 Refrasil-S-B 23 -20 106 18 18
 SIRCA 29 -30 220 20 20
27 
Why? Carbon fiber extinction depends on wavelength
• Theoretical Modelling or 
low-powered laboratory 
transmission 
measurements can provide 
insight into material 
behavior at different 
wavelengths. 
 
• Carbon fiber modeling in 
the figure shows lower 
extinction and scattering, 
higher absorption at ~ 1 
(Fiber Laser Wavelength) 
vs. 10 microns (CO2 Laser 
Wavelength) 
 
• Silica  (glass) fibers are 
already known to respond  
like this.  
 
• But Carbon… it’s 
unexpected! Carbon fiber matrix radiative properties based on theoretical absorbing  fiber scattering  model by S-C. Lee ( Applied Sciences Laboratory ) 
28 
Radiative heat transfer with the diffusion approximation 
(Ref:  Theoretical model by S-C Lee, Applied Sciences Laboratory) 
• For optically thick materials, radiative heat transfer can be 
modeled using the diffusion approximation in the radiative 
transfer equation – with the effective radiative conductivity 
for scattering and absorbing fibers. 
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29 
0
1000
2000
3000
4000
5000
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Wavelength (m)
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Solar (6000 K source) (GREEN)
Xenon lamp (5800 K source) (BLACK)
Nd:YAG (line doubled/pulsed)
Nd:YAG
COIL
CN radiation at Titan (102) (RED)
HF
LASERS (PURPLE)
Excimer lasers (pulsed)
Mercury-Xenon lamp radiation (102) (BLUE)
High speed Earth atmospheric entry includes significant energy in the UV/ 
Spectra of Available Radiation Sources 
30 
Char Zone
Pyrolysis 
Zone 
Virgin TPS 
Material 
Recessed material - vaporization, sublimation, spallation or shrinkage 
Highest temperature outer mold line (OML) region.   
Coking, other reactions may occur. Organics have burned out, leaving 
substrate material or a refractory compound (e.g. C, SiO2, Si-C).   
Sufficient temperature for chemical decomposition of pyrolyzing 
components, which vary by type (e.g. phenolic,silicone) and processing 
Original material, thermally and chemically unchanged, typically 
consisting of a substrate and resin  
Vehicle 
Structure 
Accurate thermal prediction and analysis requires proper modeling 
of the char and pyrolysis zones – including reaction kinetics  
Temperature and heat load limits of the vehicle structure drive TPS 
requirements and sizing with margins 
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Inner mold line (IML) interface between TPS and vehicle structure 
Background: In-depth Response of TPS Ablators 


Effects of Laser Wavelength on Ablator Testing

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