Spacecraft materials selection is based on an upward flammability test conducted in a quiescent environment in the highest-expected oxygen-concentration environment. However, NASA s advanced space exploration program is anticipating using various habitable environments. Because limited data is available to support current program requirements, a different test logic is suggested to address these expanded atmospheric environments through the determination of materials self-extinguishment limits. This paper provides additional pressure effects data on oxygen concentration and partial pressure self-extinguishment limits under quiescent conditions. For the range of total pressures tested, the oxygen concentration and oxygen partial pressure flammability thresholds show a near linear function of total pressure. The oxygen concentration/oxygen partial pressure flammability thresholds depend on the total pressure and appear to increase with increasing oxygen concentration (and oxygen partial pressure). For the Constellation Program, the flammability threshold information will allow NASA to identify materials with increased flammability risk because of oxygen concentration and total pressure changes, minimize potential impacts, and allow for development of sound requirements for new spacecraft and extraterrestrial landers and habitats

Beeson, Harold

Hirsch, David

Williams, Jim

NASA Technical Reports Server

Journal of Testing and Evaluation, October 2006 
Paper ID:    
 
David Hirsch1, Jim Williams2, and Harold Beeson3
 
Pressure Effects on Oxygen Concentration Flammability Thresholds 
of Materials for Aerospace Applications 
 
 
ABSTRACT:  Spacecraft materials selection is based on an upward flammability test conducted in a quiescent 
environment in the highest-expected oxygen-concentration environment.  However, NASA’s advanced space 
exploration program is anticipating using various habitable environments.  Because limited data is available to 
support current program requirements, a different test logic is suggested to address these expanded 
atmospheric environments through the determination of materials self-extinguishment limits.  This paper 
provides additional pressure effects data on oxygen concentration and partial pressure self-extinguishment 
limits under quiescent conditions.  For the range of total pressures tested, the oxygen concentration and oxygen 
partial pressure flammability thresholds show a near linear function of total pressure.  The oxygen 
concentration/oxygen partial pressure flammability thresholds depend on the total pressure and appear to 
increase with increasing oxygen concentration (and oxygen partial pressure).  For the Constellation Program, 
the flammability threshold information will allow NASA to identify materials with increased flammability risk 
because of oxygen concentration and total pressure changes, minimize potential impacts, and allow for 
development of sound requirements for new spacecraft and extraterrestrial landers and habitats. 
 
KEYWORDS: test methods, flammability, aerospace materials, combustion, microgravity 
 
Introduction 
 
NASA’s goal of traveling to the moon, Mars, and beyond through the development of an 
advanced human space exploration program requires advanced technology and spacecraft 
hardware design.  Environment selection should be flexible to optimize complex mission 
operations.  The ambitious objectives of the Constellation Program limit the ability to select a 
firmly defined atmosphere.  Hypoxia, decompression sickness, and materials flammability are 
important factors that must be controlled when selecting spacecraft atmospheres [1].  Applicability 
of flammability test data needs to be broad to allow optimal design of multiple spacecraft, 
extraterrestrial landers and habitats, and surface vehicles.  Current test data on spacecraft 
materials flammability is limited to previously anticipated atmospheric environments.  Possible 
atmospheric conditions have expanded considerably.  A test logic is needed to obtain 
comprehensive materials flammability information to provide flexibility for spacecraft 
environment selection. 
 
Background 
 
Spacecraft fire safety emphasizes fire prevention, which is achieved primarily through the use of 
fire-resistant materials.  Materials selection for spacecraft is based on conventional flammability 
acceptance tests along with prescribed quantity limitations and configuration control for items 
                                                 
1 Aerospace Testing Branch Manager, NASA Test and Evaluation Contract, NASA Johnson Space Center 
White Sands Test Facility, Las  Cruces, NM 88004. 
2 Standard Testing Manager, NASA Laboratories Office, NASA Johnson Space Center White Sands Test Facility, 
Las Cruces, NM 88004. 
3 Chief, NASA Laboratories Office, NASA Johnson Space Center White Sands Test Facility, Las Cruces, NM 
 88004. 
https://ntrs.nasa.gov/search.jsp?R=20070005041 2019-08-30T00:17:42+00:00Z
2 
that are non-pass or questionable [2, 3].  The test required is NASA-STD-6001, Upward Flame 
Propagation (Test 1) [4]. 
 
Test 1 is the primary method used to evaluate flammability of materials intended for use in 
habitable spacecraft environments.  This test exposes materials to a standard ignition source in a 
quiescent environment at the highest-expected spacecraft oxygen-concentration to identify 
materials that will self-extinguish and not transfer burning debris [4].  The method consists of an 
upward flammability test conducted with a well-defined igniter flame at the bottom of a 
vertically mounted sample.  If a material burns for less than 15.2-cm (6-in.), the material passes.  
Because of the current test logic, the existing materials flammability database contains mostly 
data in maximum oxygen concentrations of 30 percent by volume and 71.1 kPA (10.2 psia) total 
pressure.  Spacecraft environments with higher oxygen content at lower total pressures [5] would 
be more advantageous for important mission operations, such as extravehicular activities.   
 
NASA Johnson Space Center (JSC) White Sands Test Facility (WSTF) developed a method that 
will provide flammability threshold information [6].  The flammability threshold information 
will be valuable for new spacecraft and extraterrestrial habitation module designs.  For the 
Constellation Program, the flammability threshold information will allow NASA to identify 
materials with increased flammability risks from oxygen concentration and total pressure 
changes, minimize potential impacts, and allow for development of sound requirements for 
extraterrestrial landers and habitats [1, 7].  The following section provides additional 
experimental information related to pressure effects on the oxygen concentration flammability 
thresholds for common aerospace materials 
 
Experimental 
 
The test materials that were evaluated included plastics [Delrin®1, polyethylene (PE), Teflon®2 
TFE, Zytel® 42, and Kel-F] and elastomers [Silicone Rubber, Viton®3 A, Buna S, Buna N, 
Neoprene, and polyethylene/propylene diene (EPDM)].  Table 1 provides the tradename and 
chemical composition of the materials.  The samples were 6.4-cm (2.5-in.) wide and at least 
15.2-cm (6-in.) long; the sample lengths were selected to provide a burn length of at least 15.2 
cm, which is consistent with the failure criteria established in Test 1 [4].  All samples had a 
thickness of approximately 1.5 mm (0.06 in.). 
 
 
                                                 
1 Delrin® is a registered trademark of E. I. DuPont de Nemours & Co., Wilmington, Delaware. 
2 Teflon® is a registered trademark of E. I. DuPont de Nemours & Co., Wilmington, Delaware. 
3 Viton® is a registered trademark of DuPont Dow Elastomers, Wilmington, Delaware. 
 
3 
 
TABLE 1 – Materials Tested 
Generic or Trade Name Chemical Name or Composition 
 
Plastics 
Delrin®a polyoxymethylene (acetyl stabilized polyacetal) 
PE polyethylene 
Teflon®a TFE polytetrafluoroethylene (PTFE) 
Zytel®a 42 polyamide 6,6 (Nylon 6,6) 
Kel-F® 81 polychlorotrifluoroethylene (PCTFE) 
Elastomers 
Silicone Rubber polysiloxane 
Viton ®b A copolymer of vinylidene fluoride and hexafluoropropylene 
Buna S  polystyrene/butadiene 
Buna N polyacrylonitrile/butadiene 
Neoprene polychloroprene 
EPDM  polyethylene/propylene diene 
a Delrin®, Teflon®, and Zytel® are registered trademarks of E. I. DuPont de Nemours 
& Co., Wilmington, Delaware. 
b Viton® is a registered trademark of DuPont Dow Elastomers, Wilmington, Delaware. 
 
Tests in quiescent environments were conducted in a 1400-L flammability chamber connected to 
a vacuum pump with air, oxygen, and nitrogen supplies, which met the Test 1 requirements [4].  
Flammability burn/no-burn transition testing in quiescent environments was also conducted 
following Test 1 procedures.  The testing was conducted sequentially as recommended by ASTM 
D 2863 and used a step size of 1 percent oxygen by volume [8].  The upward flammability 
limiting oxygen indices (ULOIs) were calculated with the “up-and-down method for small 
samples” [9].  This method has been adopted by both ISO 4589 [10] and ASTM D 2863 for 
determining the “minimum oxygen concentration required to support combustion of plastics” [8].  
In this study, the maximum oxygen concentrations (MOCs), which consistently result in self-
extinguishment, were determined.   
 
The MOC was the level of oxygen concentration where at least five samples passed the burning 
criteria [4] in the ULOI vicinity and where at least one sample failed in the environment that 
contained 1 percent more oxygen by volume.  To evaluate pressure effects, the oxygen 
concentration flammability thresholds were determined at 48.2 kPa (7 psia), 85.3 kPa (12.4 psia) 
and 101.3 kPa (14.7 psia).  The lowest total test pressures were limited to 48.2 kPa (7 psia) based 
on a previous study.  This study indicated that the power penalty to the spacecraft air cooling 
subsystem due to reduced pressure puts a practical minimum cabin design pressure of around 
50.5 kPa (7.35 psia) [5].  Data obtained in 85.3 kPa (12.4 psia) and 101.3 kPa (14.7 psia) were 
initially reported in 2002 [6].  The maximum oxygen partial pressures that resulted in self-
extinguishment were calculated at various total pressures based on the MOC.   
 
Results and Discussion 
 
For the 48.2 to 101.3 kPa (7 to 14.7 psia) total pressure range, the oxygen concentration and 
partial pressure flammability thresholds show a near linear function of total pressure.  The 
dependency of oxygen concentration/oxygen partial pressure flammability thresholds on total 
4 
pressure appears to increase with increasing oxygen concentration (and oxygen partial pressure) 
thresholds.  The experimental test results are summarized in Table 2 and Figures 1 and 2. 
 
Kel F-81, PTFE, Zytel,®1 Silicone, and Viton have higher oxygen concentration and oxygen 
partial pressure flammability thresholds than the rest of the polymers tested.  Interestingly, these 
materials are also considered more resistant to ignition in many of the various ignition 
susceptibility tests in 100 percent oxygen.   
 
Oxygen concentration or oxygen partial pressure flammability threshold dependency on total 
pressures needs to be evaluated using additional materials to establish general correlations for 
flammability parameters.  The availability of data on dependency of oxygen concentration or 
partial pressure self-extinguishment limits on total pressure will allow identification of 
flammability risks associated with oxygen concentration or total pressure changes and minimize 
their potential impacts on spacecraft safety.  Furthermore, these data will allow closer evaluation 
of fire risks and increased flexibility in choosing spacecraft internal atmospheres, which could 
lead to increased efficiency of mission operations.  Current technological advances and 
emergency operations that may involve spacecraft depressurization would benefit from 
flammability threshold studies at total pressures outside the 48.2 to 101.3 kPa (7 to 14.7 psia) 
range.    
 
 
TABLE 2 — Oxygen concentration, oxygen partial pressure and total pressure flammability 
limits for selected materials at ambient initial temperatures 
  48.2 kPa (7 psia)  
total pressure 
85.3 kPa (12.4 psia) 
total pressure 
101.3 kPa (14.7 psia) 
total pressure 
Material MOC 
(vol %) 
MOP 
(psia)  
MOC 
(vol %) 
MOP 
(psia) 
MOC 
(vol %) 
MOP 
(psia) 
PTFE 53 3.7 46 5.7 42 6.2 
Kel-F 81 76 5.3 56 6.9 53 7.8 
Silicone 28 2.0 23 2.8 21 3.1 
Zytel 42 25 1.8 23 2.8 23 3.4 
Viton A 29 2.0 21 2.6 21 3.1 
Buna S 18 1.3 17 2.1 16 2.4 
Neoprene 18 1.3 17 2.1 16 2.4 
Buna N 18 1.3 16 2.0 15 2.2 
EPDM 
Rubber  
18 1.3 16 2.0 16 2.4 
Polyethylene 18 1.3 18 2.2 17 2.5 
Delrin 13 0.9 12 1.5 11 1.6 
MOC = Maximum oxygen concentration which consistently results in material self-extinguishment. 
MOP = Maximum oxygen partial pressure when extinguishment occurs (based on MOC). 
 
                                                 
1 Zytel® is a registered trademark of E. I. DuPont de Nemours & Co., Wilmington, Delaware. 
5 
0 
10 
20 
30 
40 
50 
60 
70 
80 
6 7 8 9 10 11 12 13 14 15 16
Total Pressure, psia
Fl
am
m
ab
ili
ty
 T
hr
es
ho
ld
 O
xy
ge
n 
C
on
ce
nt
ra
tio
n,
 %
 
KelF-81 
PTFE 
Zytel 
Silicone 
PE 
Buna-S, Neoprene
Delrin 
Viton A 
Buna-N, EPDM 
12.35 14.77.0 
 
Figure 1 — Oxygen concentration and total pressure  
flammability limits at ambient initial temperatures  
 
0 
1 
2 
3 
4 
5 
6 
7 
8 
9 
6 7 8 9 10 11 12 13 14 15 16
Total pressure, psia
Fl
am
m
ab
ili
ty
 T
hr
es
ho
ld
 O
xy
ge
n 
Pa
rt
ia
l P
re
ss
ur
e 
ps
ia
   
   
 
KelF-81 
PTFE 
Zytel 
Silicone 
PE 
Buna-S, Neoprene
Delrin 
Viton A 
Buna-N, EPDM 
12.3 14.77.0 
 
Figure 2 — Oxygen partial pressure and total pressure  
flammability limits at ambient initial temperatures  
6 
Conclusions and Re
 
en concentration flammability thresholds is greater for 
aterials with higher thresholds.  To determine if this relationship holds outside the 48.2 to 
ell, P.  Recommendations for Exploration Spacecraft Internal Atmospheres.  
JSC-63309.  NASA Johnson Space Center, Houston, Texas, January 2006. 
[2] SAE Technical 
[3] Spacecraft Applications, JSC 29353, 
August 2002. 
[4] 
 
[5] 
on Space Center, 
 
[6] 
Materials.” Journal of Testing and Evaluation. JTEVA, Vol. 30, No. 2, pp. 156-159.  
 
[7] . and P. Campbell.  Briefing to Constellation CxCB on Exploration Working 
Group Recommendations.  NASA Johnson Space Center, Houston, Texas, July 6, 2006. 
[8] ort 
Candle-Like Combustion of Plastics (Oxygen Index).”  , 
 
[9] 
for Standardization, Geneva, 
Switzerland, 1996. 
commendations 
The effect of total pressure on oxyg
m
101.3 kPa (7 to 14.7 psia) range further testing is recommended.  Testing additional materials 
will provide general flammability parameter correlations and predict flammability behavior for 
other materials.  The flammability threshold information will be valuable to the development of 
new spacecraft and extraterrestrial habitation module designs.  The data will provide a 
knowledge base for material selection to support the needs of advanced aerospace programs and 
mitigate spacecraft fire risks.  
 
References 
 
[1] Campb
 
Friedman, R.  Fire Safety in the Low-Gravity Spacecraft Environment. 
Paper 1999-01-1937 (also NASA/TM-209285), 1999. 
 
Pedley, M.D. Flammability Configuration Analysis for 
NASA Johnson Space Center, Houston, Texas, 
 
Flammability, Odor, Offgassing, and Compatibility Requirements and Test Procedures for 
Materials in Environments that Support Combustion.  NASA STD 6001. Test 1, Upward 
Flame Propagation, February 9, 1998. 
Lange, K., A. Perka, B. Duffield, and F. Jeng.  Bounding the Spacecraft Design Space for 
Future Exploration Missions, NASA/CR-2005-213689, NASA Johns
Houston, Texas, June 2005. 
Hirsch, D., and H. Beeson. “Test Method to Determine Flammability of Aerospace 
March 2002. 
Henninger, D
 
“Standard Test Method for Measuring the Minimum Oxygen Concentration to Supp
Annual Book of ASTM Standards
Vol. 08-02.  D 2863-06.  American Society for Testing and Materials, West Conshohocken, 
Pennsylvania, 2006. 
Dixon, W. Up-and-Down Method for Small Samples. American Statistical Association 
Journal, pp. 967-970, 1965. 
 
[10] ISO 4589-2.  Plastics – Determination of Burning Behavior by Oxygen Index – Part 2: 
Ambient-temperature Test.  International Organization 


Pressure Effects on Oxygen Concentration Flammability Thresholds of Materials for Aerospace Applications

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