Materials intended for use in aerospace oxygen systems are commonly screened for oxygen compatibility following NASA STD 6001. This standard allows qualification of materials based on results provided by only one test method. Potential issues related to this practice are reviewed and recommendations are proposed that would lead to improved aerospace oxygen systems safety

Beeson, Harold

Hirsch, David

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NASA Johnson Space Center 
David ~ i r s c h ,  ' Harold ~ e e s o n , ~  
Technical Note: Some Issues Related to the Selection of Polymers for Aerospace 
Oxygen Systems 
Abstract: 
Materials intended for use in aerospace oxygen systems are commonly screened for oxygen 
compatibility following NASA STD 6001. This standard allows qualification of materials based 
on results provided by only one test method. Potential issues related to this practice are reviewed 
and recommendations are proposed that would lead to improved aerospace oxygen systems 
safety. 
Keywords: oxygen systems, polymers, safety, ignition, flammability, aerospace materials 
' Aerospace Engineering Manager, Honeywell Technology Solutions Xnc., NASA Johnson Space Center 
Wlute Sands Test Facility, P.O. Box 20, Las Cruces, NM 88004. 
Special Projects Manager, NASA Johnson Space Center White Sands Test Facility, P.O. Box 20, 
Las Cruces, NM 88004. 
https://ntrs.nasa.gov/search.jsp?R=20070019871 2019-08-30T01:04:11+00:00Z
Introduction 
Aerospace materials intended for oxygen service are commonly screened for oxygen 
compatibility [I - 51. NASA STD 6001 establishes NASA program requirements for evaluation, 
testing, and selection of materials to preclude unsafe conditions related to oxygen compatibility 
161. Materials intended for use in liquid and gaseous oxygen systems of space vehicles and 
ground support equipment must meet the criteria of Test 1 (upward flame propagation) in NASA 
STD 6001 for nonmetallic materials in environments less than or equal to 375 kPa (50 psia), or 
Test 17 (upward flammability in GOX). At pressures higher than 375 kPa, materials must meet 
the criteria of Test 13A or Test 13B (mechanical impact), as applicable. 
Materials that do not meet the criteria of the required tests but remain candidates for use must 
be verified acceptable in the use configuration by analysis or testing and specifically approved by 
the responsible NASA Center materials organization. System evaluations are conducted only to 
demonstrate the acceptability of configurations that result from using flammable or otherwise 
incompatible materials. Polymeric materials that have passed Test 13 criteria are considered fully 
oxygen-compatible. The practice of selecting materials based only on Test 13 criteria 
requirements could subject aerospace oxygen systems to unnecessary risks. 
Discussion 
Tests 13A and 13B basically follow the ASTM G86 procedures [7]. For a material to be 
acceptable, 20 samples mas: no: react when impacted at 98 8 ('72 fi-IbS. If one sample out of 20 
reacts, 40 additional samples must be tested without any reactions. 
The Test 13 method allows testing samples with different surface areas; as a consequence, the 
energy density input into the sample (impact energylsurface area) is variable. The energy density 
could be larger, even considerably larger for samples with a lower surface area than the impact 
area of the striker pin, which could conversely affect materials reactivity. A higher impact energy 
density can lead to increased reactivity; on the other hand, a decreased impact area can reduce the 
probability of impacting a "hot spot," where ignition could be initiated. Consequently, specifying 
only the total impact energy of 98 J provides an incomplete picture that may be insufficient to 
draw definite conclusions about material reactivity. 
Qualifying materials based only on results from a single method that simulates only one 
ignition source could be risky. There may be other ignition sources in oxygen systems that could 
ignite materials with a different mechanism than that simulated by mechanical impact. These 
mechanisms could lead to ignitions even if the materials pass this particular test; consequently, 
not all materials passing Test 13 may be adequate. It has been indicated [8] that, under conditions 
of higher mechanical impact energies, nitrile rubber was less reactive than vitonQ' and silicone; 
yet most oxygen system designers know that nitrile rubber is not the best choice when oxygen 
conlpatibility is concerned. Furthermore, the mechanical impact test logic has been shown to be 
deficient [9], which increases the risk of relying solely on its results. Additionally, mechanical 
impact test methodology drawbacks were reported [lo, 1 I]. 
For the period 1993 to 2003, the NASA Lessons Learned database lists nine events involving 
oxygen systems, of which five resulted in fires and in four cases the potential system fires were 
prevented. All events occurred either on space systems, aerospace ground support facilities, or 
NASA test facilities. Of the five cases which resulted in oxygen fires, one resulted in major 
facility destruction; three were related to the materials used; one to an improperly mounted 
component; and one to the existence of particles. During 1993 to 2003, t h e  was at least one 
additional oxygen-system fire that resulted in major damage to an aerospace test facility; one 
contributor to this event was related to improper material usage. 
For oxygen system fires to occur, the right combination of factors has to exist. There are 
known instances of oxygen systems used safely for years; then, when a fire occurred, an 
investigation revealed design or material applications that made the forensic experts wonder how 
W o n @  is a registered trademark of E. I. Du Pont de Nernours & Co., Wilnlington, Delaware. 
the system had operated safely for so long. The obvious conclusion is that a past history of safe 
use should not preclude adapting better practices. 
Recommendations 
For existing systems, discontinue the practice of qualifying materials for oxygen service based 
on results from only one test method. More importantly, discontinue the practice of qualifying 
materials at energy levels lower than 98 J based on a summary systems analysis accounting only 
for mechanicaI impact events. Perform a systems hazards analysis according to NASA TM 
104823 [12] or an industry standard method, such as ASTTvf G63 [13] or ASTM G94 [14]. 
Evaluate potential ignition sources, their likeliness of occurrence, and their severity. If 
appropriate, the hazards analysis will identify the suitable tests to be performed and test 
conditions, while providing recommendations for a suitable safety factor. The safety factor is 
necessary to take into account configurational dependency of test results and test data uncertainty. 
Based on analysis and test results, mitigating actions can be considered. Recommendations for 
both material and component testing might be suitable. 
For new oxygen systems, a systems evaluation should be part of the design process, and it 
should be performed prior to systems buildup. A materials preselection during the initial phase of 
design should be followed by a final selection performed upon completion of a hazards analysis 
and the analysis of results obtained from the recommended testing. 
The mechanical impact test reports and databases should also provide reactivity information 
based on energy density. 
References 
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Technical Note: Some Issues Related to the Selection of Polymers for Aerospace Oxygen Systems

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