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i

NASA/TP—2012–217459

Analysis and Derivation of Allocations

for Fiber Contaminants in Liquid

Bipropellant Systems

N.M. Lowrey

Jacobs Technology, Inc., Jacobs ESTS Group, Huntsville, Alabama

K.Y. Ibrahim

Marshall Space Flight Center, Huntsville, Alabama

June 2012

National Aeronautics and

Space Administration

Marshall Space Flight Center • Huntsville, Alabama 35812
ii

Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076 –1320

443–757–5802

This report is also available in electronic form at

<https://www2.sti.nasa.gov/login/wt/>

Acknowledgments

The author would like to thank the following members of ASTM International, Committee G04 on Flammability and Compatibility of Materials in Oxygen-Enriched Atmospheres, for their input regarding the list of potential 

risks of fiber contaminants in enriched oxygen systems: Eddie Davis (NASA Marshall Space Flight Center (MSFC)); 

Susana Harper, Stephen Peralta, Christine Pina-Arpin, and Joel Stotlzfus (NASA White Sands Test Facility (WSTF)); 

Kyle Sparks (WSTF/Jacobs Technology, Inc.), and Dr. Theodore A. Steinberg (Queensland University of Technology, 

Australia). The author would also like to thank Richard Joye and Jennifer Harrison McMillian (MSFC/InfoPro Corp.) 

for their input on typical fibers causing rejection of hardware in the MSFC Environmental Test Laboratory Valve Lab; 

Alan Brown, Vonnie Douglas, Heather Miller, and Margarite Sylvia of the Pratt & Whitney Rocketdyne Materials and 

Processes Engineering organization for historical background on particle cleanliness requirements for the space shuttle 

main engine; and Eddie Davis (MSFC) and Randy Raley (MSFC/Jacobs Technology, Inc.) for internal peer review of 

the analysis. 

TRADEMARKS

Trade names and trademarks are used in this report for identification only. This usage does not constitute an official 

endorsement, either expressed or implied, by the National Aeronautics and Space Administration.
iii

TABLE OF CONTENTS

 1. INTRODUCTION ............................................................................................................ 1

 2. BACKGROUND AND ANALYTICAL APPROACH ..................................................... 2

 3. LIMITATIONS OF THIS ANALYSIS ............................................................................. 4

 4. PRECEDENT FOR FIBER ALLOCATIONS ................................................................. 5

 5. DERIVATION OF RATIONALE FOR THE CURRENT PARTICULATE LIMITS .... 7

 6. FIBER LIMITS BASED ON CRITICAL ORIFICE PLUGGING ................................. 10

6.1 Scenario 1: Fibers Pass Through the Orifice Lengthwise ............................................ 10

6.2 Scenario 2: Fibers Passing Through the Orifice are Bent Double 

or Broken in Half ....................................................................................................... 11

6.3 Scenario 3: Fibers Passing Through the Orifice are Rigid and Bent or Twisted .......... 12

6.4 Scenario 4: Fibers and Particles Combine to Restrict the Orifice ................................ 13

6.5 Conclusions Based on Critical Orifice Plugging or Restriction .................................... 16

 7. OTHER POTENTIAL FAILURE MECHANISMS DUE TO FIBERS .......................... 17

7.1 Filter Face Obscuration Leading to Flow Restriction .................................................. 17

7.2 Valve Seal Leakage ...................................................................................................... 19

7.3 Fire Hazards in Oxygen Systems ................................................................................. 21

 8. RESULTS ......................................................................................................................... 32

8.1 Fiber Size and Quantity Limits ................................................................................... 32

8.2 Fiber Composition ...................................................................................................... 33

8.3 Measurement of Fibers ............................................................................................... 34

 9. CONCLUSIONS ............................................................................................................... 36

10. RECOMMENDATIONS .................................................................................................. 38

10.1 MSFC-SPEC-164 Particle Limits .............................................................................. 38

REFERENCES ....................................................................................................................... 40
iv

LIST OF FIGURES

1. Packing of doubled fibers in a square versus a circle of the same width ....................... 12

2. Twisted fiber lodged in an orifice ................................................................................. 13

3. Orifice obstruction by two particles and one fiber ........................................................ 14

4. Obstruction ratios of three round particles versus three parallel fibers ........................ 14

5. Obstruction ratios resulting from overlapped fibers ..................................................... 15

6. Alternate contaminant orientations and their maximum obstruction ratios ................. 15

7. A fiber of aspect ratio 10:1 and a particle of equivalent obscured area ........................ 18
v

LIST OF TABLES

1. Classification of particulate cleanliness in MSFC-SPEC-164C ................................... 7

2. Number of fibers that can pass lengthwise through an orifice simultaneously ............. 11

3. Number of doubled fibers that can pass lengthwise through an orifice 

simultaneously ............................................................................................................ 11

4. MSFC-SPEC-164C classes with equivalent fiber lengths based on obscuration .......... 18

5. Heat of combustion of contaminants ......................................................................... 26

6. Derived fiber contaminant size limits for identified risk scenarios ............................... 32

7. Recommended MSFC-SPEC-164 requirements for fiber contaminants ...................... 38
vi

LIST OF ACRONYMS AND SYMBOLS

ASRDI Aerospace Safety Research and Data Institute

ASTM American Society for Testing and Materials

ET external tank

FOD foreign object debris

GOX gaseous oxygen

GRC Glenn Research Center

GSE ground support equipment

KSC Kennedy Space Center

LH2 liquid hydrogen

LOX liquid oxygen

MSFC Marshall Space Flight Center

NVR nonvolatile residue

PAR Problem Action Record

PVC polyvinyl chloride

PWR Pratt & Whitney Rocketdyne

SSME space shuttle main engine

UV ultraviolet
vii

NOMENCLATURE

D larger circle diameter

d smaller circle diameter

L length

x number of circles to fit
viii
1

TECHNICAL PUBLICATION

ANALYSIS AND DERIVATION OF ALLOCATIONS FOR FIBER CONTAMINANTS

IN LIQUID BIPROPELLANT SYSTEMS

1. INTRODUCTION

MSFC-SPEC-164, Cleanliness of Components for Use in Oxygen, Fuel, and Pneumatic Systems, has established quantitative limits for particulate and nonvolatile residue (NVR) contamination in liquid bipropellant systems manufactured or refurbished at or for NASA’s George C. Marshall 

Space Flight Center (MSFC).1 Originating in 1962, this specification has never included specific 

allocations or size limits for fiber contaminants as a distinct particle type. Fibers, however, are often 

more difficult to control than other types of particles during precision cleaning and assembly of large 

components. Fibers are readily transported in air currents and deposited on hardware. Fibers also 

can slip through screens in cleaning equipment, defeating the filtration systems intended to keep the 

cleaning agent free of particulate.

The presence of a fiber in a component cleanliness verification sample may result in a rejection, requiring the shop to reclean the part. This has been a recurring challenge at MSFC when 

processing large components for launch vehicles and their corresponding propulsion test systems, 

driving up processing costs. Some difficult to clean parts, such as liquid oxygen (LOX) system pressure regulators, frequently require multiple cleaning cycles to eliminate fibers.

At the request of the MSFC Engineering Test Valve and Components Lab, an analysis was 

performed to determine whether the internal contamination limits for liquid propulsion systems 

could be revised to provide an allocation for fibers beyond the established particulate limits. 
2

2. BACKGROUND AND ANALYTICAL APPROACH

A particle is any minute quantity of matter, metallic or nonmetallic, with observable length, 

width, and thickness (see ref. 1, app. C—Definitions). Particulate contaminants within liquid bipropellant systems can have a number of deleterious effects on system performance and safety. These 

contaminants can plug or restrict flow through filters and orifices and can create leak paths or 

mechanical interference in pumps, valves, and regulators. In LOX systems, particles can, under certain conditions, cause impact ignition. Both particulate and film residue contaminants can be ignited 

in LOX systems by other mechanisms.

A fiber is defined as a particle having a length-to-width ratio (aspect ratio) of 10:1 or greater.1

Most fiber contaminants, such as cotton clothing fibers, have a length-to-width ratio many times 

greater than this. Some cleaning specifications used within the aerospace industry also state that a 

fiber must have a length of 100 μm or greater,2–4 must have a diameter of <40 μm,5 or must be nonmetallic and flexible.6 MSFC and many aerospace companies, however, have avoided requirements 

stating that certain particle contaminants must be nonmetallic or flexible as these characteristics are 

difficult to determine conclusively by simple visual or microscopic evaluation and cannot be determined by automated image analysis or light-scattering methods.

The baseline version of MSFC-SPEC-164, originally released in 1962, contained detailed 

instructions for cleaning the components of liquid bipropellant systems for both flight systems and 

ground support systems. This document specified one quantified limit for particulate contamination 

that was applied to all components. MSFC-SPEC-164 underwent a major rewrite in 1994, released 

as revision B. In revision B, three additional classes of particulate cleanliness were added, with the 

original particulate cleanliness requirement retained as class I and identified as the default cleanliness level to be used when no level is specified by drawing. MSFC-SPEC-164B contained an appendix, “Space Shuttle Cleanliness Requirements,” which showed that class I was only applied to flight 

pneumatic systems in the external tank (ET).7 The limits for NVR were established in the baseline 

release and have never been modified.* Written rationales for the baseline contamination limits in 

MSFC-SPEC-164 have not been located. 

The first task when challenging an existing requirement is to understand, and if necessary, 

reconstruct, the rationale for that requirement. Once the rationale for a requirement is sufficiently 

understood, changes to the requirement can be analyzed using the same rationale to determine the 

impact of the change. A risk analysis must also be performed to determine whether the change could 

have other consequences that would render it unacceptable.

____________

*In MSFC-SPEC-164, revision B, to eliminate hybrid English/metric units, the contamination limits were changed 

from maximum number (particulate) or mg (NVR) per ft2 to per 0.1 m2. This change is only slightly more conservative and not considered significant. A note in both revisions B and C states that “for the purpose of this specification, 

0.1 square meter = 1 square foot.”
3

The steps used to analyze the system tolerance for fibers were as follows:

(1) Understand the technical rationale for the current particulate limits via a search for 

published historical rationale and by deriving them from historical system design and performance 

criteria. 

(2) Based on this rationale, identify and analyze the potential consequences of allowing fibers 

larger than the current particulate limits.

(3) Identify other potential risks, apart from those directly related to the derived technical 

rationale, that must be considered and determine whether those risks are of significance.

(4) Compare the fiber size and quantity limits identified for each risk scenario to determine 

the risk drivers and associated maximum fiber size and quantity to establish allocations. 

(5) Determine whether there is a precedent in published data for other liquid hydrogen (LH2) 

and LOX systems for the size and quantity allocations derived in step (4).
4

3. LIMITATIONS OF THIS ANALYSIS

This analysis addresses particulate contaminants in LOX/LH2 bipropellant systems and associated helium and nitrogen pressurization systems. Assumptions are based on published NASA and 

industry data for LOX and LH2 propellant systems from the Saturn program to the present. The 

results of this analysis may not be applicable to high-pressure gaseous oxygen (GOX) systems such as 

those used for life support systems, fuel cells, extravehicular activity suits, and orbital maneuvering 

units.

The results of this analysis will apply to the corresponding helium and nitrogen pneumatic and 

ullage systems that interface with the LOX and LH2 systems, as a minimum. Some valves and regulators for these systems require cleaner particulate levels. Particulate and fiber cleanliness requirements 

for the systems upstream of these components must be derived to prevent damage to the components 

or excessive restriction of the filters protecting them.

New engines and propulsion system designs for future launch vehicles may require different 

particle contamination limits and associated fiber contamination limits. The rationale and assumptions used to derive the contamination limits in MSFC-SPEC-164 must be reevaluated for application to a new propulsion system design.
5

4. PRECEDENT FOR FIBER ALLOCATIONS

A literature search was performed to determine whether there is current or historical precedent for fiber allocations in LOX/LH2 propellant fluid systems. Several significant sources were 

identified where fiber limits were established for LOX/LH2 propulsion systems as well as highpressure GOX systems that exceeded the particle size limits for those systems.

The Compressed Gas Association standard CGA G-4.1-2009, Cleaning Equipment for 

Oxygen Service, clearly allows for the presence of lint fibers as long as there are no accumulations.8

The section on visual ultraviolet (UV) inspection states that “some materials such as cotton lint that 

fluoresce are acceptable unless present in excessive amounts.” This document further states that when 

quantified limits for particulate are specified, typical specifications limit particles to no more than 

1,000 μm, with no more than 20 particles/ft2 (0.1 m2) between 500 and 1,000 μm, and isolated fibers 

of lint to no longer than 2,000 μm with no accumulation. The repeated use of the terms “cotton lint” 

and “lint fibers” implies that these fibers are assumed to be nonmetallic, as opposed to metal “filings” 

which are noted elsewhere in the document as another form of contamination. 

Pratt & Whitney Rocketdyne (PWR) specification RA0110-004, Cleaning and Handling Parts 

for Propellant, Pneumatic, and Hydraulic Service, details cleaning practice for some of their rocket 

engines.5 This document specifies allocations for fibers up to 6,000 μm in oxidizer, fuel, pneumatic, 

and hydraulic systems. A fiber is defined as solid matter having a diameter of <40 μm. The particle 

and fiber limits are the same for all systems. This specification, however, is one of PWR’s older specifications, used on expendable engines, and does not reflect current practice on other engines (V.M. 

Douglas, Nonmetallic Materials Technology, PWR, Private Communication, August 31, 2011).

The NASA Glenn Research Center (GRC) Safety Manual GLM-QS-1700.1, chapter 5 on 

oxygen systems,9 specifies contamination limits for GOX/LOX service in GRC test facility systems. 

The cleanliness limits include allocations for particles and fibers in defined size ranges, with allowed 

particles no larger than 300 μm in maximum dimension and fibers no larger than 1,875 μm in length 

and 25 μm in diameter per square foot of sampled area. This specification also establishes a limit for 

total solids and fibers of 25 mg/0.1 m2. Pressure gauges and transducers certified oxygen-clean, while 

requiring the same fiber limits, are permitted to contain particles up to 500 μm. The Lewis Safety 

Manual (NASA Lewis Research Center is now GRC), dated November 1992,10 is evidence that these 

particle and fiber limits have been in use since at least 1992. 

A detailed survey, published as NASA SP-3072, was performed in 1972 by the Aerospace 

Safety Research and Data Institute, NASA Lewis Research Center, to document current practice 

in the aerospace industry for cleaning oxygen systems.11 Twenty oxygen system cleanliness specifications were referenced in the survey report, including specifications for spacecraft and aircraft 

breathing oxygen systems, manned launch vehicle and missile propulsion systems, and commercial 

systems. Fourteen of these specifications contained allocations for fiber contaminants that exceeded 

the maximum dimension of the corresponding particle limit. The fiber length limits exceed the 
6

particle size limits by a factor of 2 to a factor of 10. The maximum allowed fiber length was 

6,000 μ; fiber length limits around 2,000 μm were more common. Apparently, the contamination 

limits for oxygen systems now in use at GRC were derived from this survey.

In addition to the above sources documenting common use of fiber allocations in the aerospace industry since the 1960’s, SD 70-557-5, a sustaining engineering report from the Saturn S-II 

program,12 recommended specific fiber allocations for future S-II production. This document, starting from MSFC-SPEC-164A (now MSFC-SPEC-164C, class I) as the basis, recommended that the 

allocation for one particle 700 to 2,500 μm per square foot be restricted to nonmetallic fibers only. The 

recommendations from this report were not captured in subsequent revisions of MSFC-SPEC-164. 

No more S-II stages were built. 
7

5. DERIVATION OF RATIONALE FOR THE CURRENT PARTICULATE LIMITS

The analyses here are based on the main propulsion system designs for the space shuttle and 

the Ares I launch vehicles. These systems were required to be cleaned in accordance with MSFCSPEC-164, revisions B and C, respectively. Both designs used LH2 as the fuel and LOX as the oxidizer. Both systems also used PWR engines that required propellants compliant with a common 

specification, RA0615-072, Use Limits for SSME Propellants and Propellant Pressurizing Agents.

13

The upper stage engine for the Ares I vehicle, the J-2X, was a derivative of the J-2 engine used on 

the second and third stages of the Saturn V rocket, which is assumed to have also required the same 

propellant contamination limits. The PWR propellant specification for the J2-X dates back to at 

least 1972. Therefore, it is assumed that the basis for the original particulate contamination limits in 

MSFC-SPEC-164, released early in the Apollo program, is the same as for the Ares I. 

The particulate levels in MSFC-SPEC-164C are shown in table 1. Particle size is measured by 

the longest dimension. Class I, which permits one particle up to 2,500 μm but severely restricts the 

presence of particles down to 175 μm, dates back to the origin of MSFC-SPEC-164, April 16, 1962. 

The “no silting” restriction was added in revision B. All fuel, oxidizer, and associated pneumatic systems were cleaned to this level with the exception of nonmetallic miscellaneous components. Nonmetallic components that could be degraded by the trichloroethylene solvent originally specified for 

quantitative verification were permitted to be verified by visual inspection alone.

Table 1. Classification of particulate cleanliness in MSFC-SPEC-164C.

Class

Particle Size

(μm)

Maximum Number

(per 0.1 m2)

I >2,500

700 < x ≤2,500

175 < x ≤700

No silting

0

1

5

II >1,000

700 < x ≤1,000

175 < x ≤700

No silting

0

40

150

III >800

No silting

0

III X >800

175 < x ≤800

No silting

0

5

IV >400

No silting

0

IV X >400

175 < x ≤400

No silting

0

5

V Visually clean/no silting
8

The PWR propellant specification imposes size limits of 400 and 800 μm, respectively, for 

allowable particulate in LH2 and LOX. These are reported by PWR to have been derived from the 

smallest system orifice diameters using the conservative three-ball approach.14 This approach uses 

the three-ball equation to establish the maximum particle diameter that will permit three balls (contaminant particles) to pass through the smallest system orifice simultaneously. Limiting all particulate contamination to below this size will minimize the risk that particulate will lodge in a critical 

orifice, blocking or restricting propellant flow. Such blockage or restriction could inhibit engine performance during launch and result in a loss of mission.

The particulate cleanliness limits established for the space shuttle main engine (SSME), also 

known as the RS-25, in the 1970’s were also based on critical orifice sizes. However, the three-ball 

equation was not used and the particle limits were less conservative. In the SSME Contamination 

Control Plan,15 for orifice sizes of 450 μm (a slot) in the LH2 system and 900 μm in the LOX system, 

particulate size limits of 400 and 800, respectively, were applied. 

The formula for calculating the diameters of a specified number of circles of equal size that 

will fit side by side within a larger circle is:

d = D * sin(π / x) / (1+ sin(π / x)) , (1)

where d=the smaller circle diameter, D=the larger circle diameter, and x=the number of circles to 

fit ring-like (x=2,3,...).

For the three-ball calculation, the smaller circle diameter (d) corresponds to the diameter of 

the maximum allowable particle size, the large circle diameter (D) is the orifice size, and the number 

of circles to fit (x) is 3. Therefore:

d = D * sin(π / 3) / (1+ sin(π / 3)) = D * sin(1.0472) / (1+ sin(1.0472)

= D * (0.86603) / (1.86603) = D * (0.4641) . (2)

Seven/sixteenths of the orifice diameter (D*(0.4375)) may be used as a simple, conservative 

approximation.

Calculating with the three-ball equation, particle size limits of 400 μm for the LH2 system 

and 800 μm for the LOX system indicate that the smallest downstream orifices to be protected (in 

the engine) are ≥862 μm (0.034 in) and 1,724 μm (0.068 in), respectively. PWR, however, has reported 

that establishing the particulate limits based on the three-ball calculation was a “goal” and that the 

downstream orifices in the J2-X engine are actually smaller than would be indicated by this calculation (M.A. Sylvia, Materials & Processes, Contamination Control, PWR, Private Communication, 

September 8, 2011). Actual critical orifice sizes in the J2-X engine were not available for this analysis. 

To be conservative, where critical orifice size was a decisive factor for this analysis, the published 

orifice sizes for the SSME were used.
9

MSFC-SPEC-164, revision B established classes III and IV for oxygen system components 

and hydrogen system components, respectively, that feed the engine downstream of the propellant 

tank filter screens.7 Given the difficulty of meeting, these particulate limits on the entire surface area 

of the main propellant tanks, a less stringent limit, class II, was established for the tanks. The orifice 

sizes for the tank feed filters are the same as the maximum allowed particle size for the propellants; 

i.e., 400 and 800 μm. Propulsion test stands are also required to meet these cleanliness limits to prevent contamination of engines and integrated stages during green runs and development tests.

The smallest particle size with limits in MSFC-SPEC-164, 175 μm, can be derived as the maximum allowable particle size for a 400-μm orifice using the simple approximation (7/16) of the threeball equation. The “no silting” requirement, added in revision B, was likely established to assure 

accurate particle counts and to avoid the presence of excessive quantities of finer contaminants that 

could agglomerate and act as larger particles or as combustible matter. At least one industry study 

indicated that, under some circumstances, an excessive quantity of particles smaller than 40 μm 

could cause an ignition due to particle impact.16 Silting may also be a warning sign that hardware 

has corroded or the cleaning system has become contaminated. Silting is defined as an accumulation 

of minute particles in the size range not normally counted but of sufficient quantity to interfere with 

sample analysis.1

The original particle limits, now shown as class I, permitted five particles 175 to 700 μm per 

square foot and one particle per square foot up to 2,500 μm. These limits applied to all components 

throughout the system. The 700 and 2,500 μm limits do not correlate, directly or via the three-ball 

equation, to any known system orifice size. It has been reported that these larger particle allocations 

were derived from actual “field tests” at the Kennedy Space Center (KSC) complexes and represent 

the maximum cleanliness results obtainable in the field during the 1960–1964 time period (ref. 11, 

p. 4).

It should be noted that current designs for ambient and cryogenic helium regulator sets and 

other pneumatic system components integral to bipropellant propulsion systems require protection 

from particulate smaller than 175 μm. These components are cleaned to levels more stringent than 

those specified in MSFC-SPEC-164, table I, and are generally protected by in-line filters on the order 

of 25 to 100 μm. Systems upstream of the components and filters must be cleaned accordingly to prevent plugging or restriction of these in-line filters during system operation. Cleaning of these components is specified to meet a more stringent cleanliness level, usually per IEST-STD-CC1246, Product 

Cleanliness Levels and Contamination Control Program,

17 or a modification thereof.18 Cleanliness 

levels not specified in MSFC-SPEC-164, table I, are not addressed in this analysis. However, the 

same rationale for establishing limits for fibers versus particulate based on prevention of filter plugging or restriction may be applicable to these systems as well.
10

6. FIBER LIMITS BASED ON CRITICAL ORIFICE PLUGGING

Analysis of the tolerance of a system to fiber contamination based on orifice plugging must 

consider contaminant locations upstream and downstream of the propellant tank feed filters. Fiber 

contaminants downstream of the tank feed filters must not present a greater risk of plugging the critical engine orifices than the baseline 400- or 800-μm particle size limits. Fiber contaminants upstream 

of the tank filters, which are 400- or 800-μm absolute rated screens, must not present a greater risk of 

restricting flow through these screens. Fibers from the tanks that pass through the tank filter screens 

lengthwise during operation also must not present a greater risk to the critical engine orifices.

Using the 10:1 aspect ratio for fiber width to length as worst case, several scenarios are considered for the potential to plug or restrict 400- or 800-μm orifices. These orifice sizes reflect the pore 

sizes of the tank filter screens. The critical engine orifices downstream are significantly larger, further 

reducing the probability that fibers, permitted based on the tank filter pore size, will restrict flow. 

6.1 Scenario 1: Fibers Pass Through the Orifice Lengthwise

Assuming that all fibers entrained in flowing fluid will orient lengthwise, passing any orifice 

larger than the width of the fiber, the maximum allowable particle length is theoretically unlimited. 

The allowable particle width would be the same as the allowed particle diameter. However, fibers 

pose additional risks for orifice plugging beyond particulate. The increased length of a fiber increases 

the time required for that fiber to pass through an orifice. This greatly increases the probability of 

orifice plugging due to multiple fibers or particles encountering an orifice simultaneously. Assuming 

that the ratio of the downstream critical orifice to the upstream filter screen adheres to the three-ball 

calculation, limiting the width of all fibers in the system, downstream or upstream of the tank filter 

screens, to that based on the three-ball calculation for the screen pore size rather than the downstream orifice size will yield an allowed fiber width of (0.4641×0.4641) or 21.5% of the downstream 

orifice size. The downstream orifice will pass as many as 16 fibers of this width without plugging.**

Allowing fibers in each system with a maximum length of 10 times the approximated (7/16) 

three ball calculation for the tank filter screen pore size, 1,750 μm (LH2) and 3,500 μm (LOX), and 

quantities limited by the “no silting” requirement, would appear to be conservative. It should be 

noted that such fibers at the maximum aspect ratio of 10:1 would be clearly visible to the unaided 

eye and would not pass the visual inspection requirement. Fibers of these lengths with widths 

<50 μm are more typical of environmental fibers such as clothing fibers. These would pose an insignificant risk of critical orifice plugging. Table 2 shows the number of fibers that could theoretically 

pass through an orifice simultaneously, based on best known packing calculations of equal circles 

within a larger circle.

____________

**Calculation of the packing of equal circles beyond seven within a larger circle becomes complicated, as 

there is extra space. Several internet sources on circle packing point to this article for packing of 16 equal 

circles into a larger circle.19
11

Table 2. Number of fibers that can pass lengthwise through 

an orifice simultaneously.20

Orifice Size

Fiber Diameter

(μm) 400 μm 800 μm 862 μm* 1,724 μm*

100 11 50 59 249

70 24 106 124 519

50 50 213 249 1,032

*Calculated downstream orifice sizes assuming that the filter screen pore size 

 was selected based on the three-ball calculation from the downstream orifice size.

6.2 Scenario 2: Fibers Passing Through the Orifice are Bent Double or Broken in Half

Scenario 1 assumes that each fiber will align with the fluid flow and pass through the orifice 

end-on. An accumulation of fibers could be deposited on the surface of a filter, either during fill and 

drain of a system prior to launch or by settling during shipment while the stage is filled with purge air 

rather than fluid. If the fibers are deposited crosswise on the filter screen prior to initiation of flow, 

they may either remain in place, be swept through lengthwise, or pass through the orifice doubled 

should they either fold in two or break under the force of the flow, thus acting as two fibers. Assuming that these fibers pass through doubled, the number of fibers that could theoretically pass the 

orifice is one-half the numbers shown in table 2, to the next lower even number of fibers. The result 

is shown in table 3.

Table 3. Number of doubled fibers that can pass lengthwise through an orifice simultaneously.

Orifice Size

Fiber Diameter

(μm) 400 μm 800 μm 862 μm 1,724 μm

100 5 25 29 124

70 12 53 62 259

50 25 106 124 516

Given that fibers are much longer than particles and therefore take longer to pass through 

an orifice, the opportunity for fibers to pass through an orifice simultaneously is much greater than 

particles. There is, therefore, some question whether 100-μm-diameter fibers should be permitted in 

a system with a 400-μm critical orifice, where six folded or broken fibers could block an orifice. Narrowing the allowable fiber diameter to 70 μm would add conservatism and still accommodate the 

most common and troublesome environmental fibers. However, 400 μm is the orifice diameter of the 

filter used to protect a larger downstream critical orifice. Even if the critical orifice is only slightly 

larger than the filter pore size, the number of fibers that may pass lengthwise, even doubled over, 

increases substantially. For example, a 450-μm-diameter critical orifice will simultaneously pass up 

to seven doubled 100-μm-wide fibers and up to 32 doubled 50-μm-wide fibers.
12

This analysis assumes either broken or perfectly folded fibers with a zero bend radius. The 

idealized case of fibers folded with a zero bend radius is unlikely. Restricting the fibers to <50 μm 

diameter yields a generous margin for imperfectly folded fibers. Fibers that are bent but not tightly 

folded are addressed in scenario 3. 

It should be noted that the filter screens used in liquid bipropellant system tanks are constructed of woven wire mesh, with pore size defined by an absolute glass bead rating. The orifices are 

actually square, with ≈22% more flow area than a round orifice of the same width. Whereas a circular 

orifice will pass three particles <0.4641 times its diameter, a square of the same width will pass three 

particles <0.5087 times its width. The number of fibers of smaller diameter that can pass lengthwise 

simultaneously through a square orifice also rises significantly. The worst case scenario shown in 

table 3, where up to five doubled 100-μm fibers will pass through a 400-μm round orifice, compares 

to eight doubled 100-μm fibers that may pass through a 400-μm square orifice. This is illustrated in 

figure 1.

Figure 1. Packing of doubled fibers in a square versus a circle of the same width.

6.3 Scenario 3: Fibers Passing Through the Orifice are Rigid and Bent or Twisted

It is assumed in scenarios 1 and 2 that all of the fibers are either straight or are sufficiently 

flexible to pass the orifice lengthwise without interference. Natural clothing fibers, however, are not 

typically straight. Cotton, linen, and wool are twisted when observed under magnification. If these 

fibers also become inflexible at cryogenic temperatures, they will not pass an orifice in a parallel orientation.

Fibers shorter than the width of the critical orifice will pass unhindered regardless of their 

shape. Long, twisted, rigid fibers will most likely be captured by filters as the probability of the fibers 

lining up in an orderly fashion with the flow is significantly reduced. Should these rigid, twisted fibers 

be present downstream, where they could lodge at the entrance to a critical orifice, they will still present a significantly lower risk of restricting flow than particles of the maximum allowed size. When 

the width of fibers is restricted to <50–100 μm, even very long fibers pose little obstacle to fluid flow 

or to the passage of particles restricted to 7/16 of the orifice size (illustrated in fig. 2).
13

Unless the fiber is both

very long and tightly

twisted or balled, it will

provide minimal

obstruction to fluid flow

if lodged in an orifice.

50-μm-Diameter Fiber

400-μm Orifice

175-μm Particle

Fluid Flow

Figure 2. Twisted fiber lodged in an orifice.

Fibers would need to be tightly bent and 2.5 or more times the diameter of the critical orifice 

in length to begin to create an obstruction sufficient to collect particles within the orifice. 

6.4 Scenario 4: Fibers and Particles Combine to Restrict the Orifice

If rigid fibers, whether straight or twisted, land across the critical orifice rather than passing 

through, and are not simply swept aside by the fluid flow, then the flow restriction caused by either 

multiple fibers or combinations of fibers and particles must be considered. Assuming that not more 

than three particles of any type will encounter an orifice simultaneously, the worst case scenario 

involving a fiber is for two particles and one fiber to encounter the orifice simultaneously, with the 

orifice fiber landing crosswise. Using the three-ball calculation to establish the particle size limit, fiber 

widths of up to 0.0718 times the diameter of the circular orifice will not cause an obstruction and can 

be permitted without limit:

D = (D * (0.4641) + D * (0.4641)) + D * (x) ≥ x = 0.0718 . (3)

Using the approximation of 7/16 times the orifice diameter to set the maximum particle size, 

the maximum fiber width becomes 1/8 times the orifice diameter. Thus, for a 400-μm orifice and 

a maximum particle size of 175 μm, fibers up to 50 μm in diameter can be allowed without limit 

(illustrated in fig. 3). 

The relative level of restriction of the orifice can be calculated for each combination of up 

to three particles or fibers using the two-dimensional shadow of each contaminant as the worst 

case. The area of the obscuring particles versus the area of the orifice yields an obstruction ratio. 

Figures 4–6 show the various permutations of 50-μm-diameter fibers and 186-μm-diameter particles 

(the actual three-ball limit) obscuring 400-μm round and square orifices and their resulting obscuration ratios.
14

100-μm-diameter

fiber creates

obstruction if the

fiber is not pushed

aside by the particles

during flow

400-μm Orifice

400-μm Orifice

175-μm

Particles

175-μm

Particles

≤50-μm-diameter

fiber. No obstruction–

particles will pass

Figure 3. Orifice obstruction by two particles and one fiber. 

400-μm

Orifice

400-μm

Orifice

Obstruction ratio of

three particles at

three-ball limit = 65%

for a round orifice

For a square orifice,

the obstruction ratio

of these same

particles = 51%

(cannot create 

an obstruction)

≤50-μm-diameter fiber.

Worst case obscuration

for three fibers is

<48% and nearly 

identical for a round or

square orifice

50-μm-Diameters Fibers, 186-μm-Diameter Particles (Three-Ball Limit)

Figure 4. Obstruction ratios of three round particles versus three

parallel fibers.
15

Obstruction of

16%–44% when three 

are overlapped

400-μm Orifice

50-μm-Diameter Fibers

Obstruction of

13%–43% when three are

overlapped

On the tank filter screens, fibers

may cross-multiple pores.

A large quantity of fibers must

be present for this to create

substational obstruction.

Figure 5. Obstruction ratios resulting from overlapped fibers.

For a square orifice,

the obstruction ratio

of these same

particles = 46%.

However, any other

alignment of the fiber

cannot cause obstruction.

Two fibers and one

particle are more likely

to create an obstruction

than three particles, but

result in less obstruction.

Obstruction

ratio <48%

Obstruction

ratio

<52%

Obstruction ratio of

two particles and one

fiber = 59% for a 

round orifice

400-μm Orifice

400-μm Orifice

50-μm Diameter Fibers, 186-μm-Diameter Particles (Three-Ball Limit)

Figure 6. Alternate contaminant orientations and their maximum

obstruction ratios.

It can be seen in figure 6 that one 50-μm fiber and two 186-μm particles can cause obstruction 

of a square 400-μm orifice if the fiber lands just right. Likewise, two fibers and one particle of these 

sizes, oriented just right, can cause obstruction of a circular 400-μm orifice. The overlap is slight 

and, when multiplied with the low probability that the three contaminants will encounter the orifice 

simultaneously, the required orientation is highly improbable. Furthermore, the resulting obscuration ratio is lower in each case than obstruction by three particles. 
16

It is assumed for the calculations shown in figures 4–6 that the fibers are straight. Fibers that 

are not straight will not line up side by side as shown in figure 4 to obstruct flow. Twisted fibers would 

be unlikely to cause more flow restriction than shown in figures 5 or 6. 

6.5 Conclusions Based on Critical Orifice Plugging or Restriction

Based on the above analyses, an unlimited number of fibers of a length less than or equal 

to the diameter of the critical downstream orifice size can be tolerated without reservation. Longer 

fibers, when limited to a maximum width of 50 μm or less should be acceptable in the LH2 system 

without risk. Given that the downstream critical orifices are larger than the 400-μm orifice used in 

these calculations, and that some restriction of the filter screens is acceptable, fibers up to 70 μm or 

even 100 μm in diameter should be acceptable. The LOX system, with orifices assumed to be 900 μm 

or wider, should be able to tolerate fibers as wide as 100 μm without obstruction. If the smallest critical orifice is a slot rather than circular, as was the case in the SSME LH2 system, then the probability 

of plugging by these larger fibers is even less likely. 

These calculations do not point to a hard upper limit for length of fibers or quantity. To minimize the probability that rigid twisted fibers will create an obstruction within an orifice, the maximum length of fibers should be limited to <2.5 times the diameter of the downstream critical orifice. 

Based on the SSME orifice sizes, conservative maximum length for fibers should be 1,000 μm in the 

LH2 system and 2,000 μm in the LOX system. 

Without detailed knowledge of fluid system surface areas, filter designs, and filter locations 

internal to the engine(s), a maximum allocation for quantity of fibers cannot be determined. However, an allocation of two fibers per sample smaller than 50 μm diameter × 1,000 μm long provides 

less surface area restriction than one particle 400 μm in maximum dimension. Two fibers per sample 

smaller than 100 μm diameter × 2,000 μm long provides less surface area restriction than one particle 

800 μm in maximum dimension. This would seem to be a conservative allocation. 

In the propellant tanks, long, twisted fibers capable of being lodged in a critical orifice will be 

prevented from reaching the orifice by the tank filter screens. Straight fibers, however, when aligned 

with the direction of fluid flow, will bypass the tank filter. Fibers present in the tanks must also be 

restricted to the maximum diameter required for the downstream system. 

None of the above scenarios require that the fibers be nonmetallic. Restrictions on fiber composition may be based on other potential failure mechanisms for a system. 
17

7. OTHER POTENTIAL FAILURE MECHANISMS DUE TO FIBERS

Other potential failure mechanisms from fiber contamination that must be considered include 

excessive restriction of system filters, valve seal leakage, and potential ignition of contaminants in 

LOX systems. As a baseline for these analyses, it is assumed that the allocations for size and quantity 

of particulate established for the space shuttle ET, defined by MSFC-SPEC-164, are acceptable for 

future liquid bipropellant launch systems with the same or larger downstream orifice sizes.

7.1 Filter Face Obscuration Leading to Flow Restriction

Scenarios 1–3 above assume that particles will align with the flow and encounter the orifice 

lengthwise, generally passing though. Scenario 4 considered the potential for fibers or a combination 

of fibers and particles to block a critical orifice. If, as noted in scenario 4, the fibers become rigid at 

cryogenic temperatures and do not align with the flow, then the potential also exists for a quantity 

of fibers to accumulate on the filter face upstream of a critical orifice and restrict flow. To manage 

this risk, an allocation must be established for fibers to limit the potential restriction to an acceptable 

level. 

To fully analyze the potential for fibers to obscure a filter face to an unacceptable degree, the 

area of the filter face and the allowable restriction must be known. The total internal surface area 

upstream of the filter also determines the total fiber load that is potentially available to travel downstream to the filter. This can be approximated using the shuttle ET system as a worst case. The system 

performance details that determine allowable restriction were not available for this analysis. However, it is possible to evaluate the relative restriction potential of fibers versus particles accumulated 

on a filter surface. Assuming that the current allocations for particles upstream of system filters are 

acceptable, equivalent allocations for fibers can be calculated. 

Using the two-dimensional area, or “shadow” of the particle as the measure of restriction 

potential for particles larger than the orifice size of the filter, relative restriction of particles versus 

fibers can be calculated. This is a simplification since particles and fibers are three-dimensional and 

rarely appear as disks or ribbons. This simplification, however, is conservative because fluids will 

more easily flow around round, cylindrical, or irregular particles. 

For the lowest fiber aspect ratio (by definition) of 10:1, the fiber length that equates to the 

shadow area of a round particle is calculated as follows: 

 

L ×W = L × L / 10 = L2 / 10...L2 / 10 = πr

2

↓

L = π(D / 2)

2 ( ×10) , (4)

where L = the length of a fiber with aspect ratio 10:1 and D = the diameter of the particle.
18

A 10:1 fiber with a length 2.8 times the diameter of the particle yields equivalent obscuration 

(shadow area). This relationship is shown in figure 7. 

2.8

1 = 0.28

Figure 7. A fiber of aspect ratio 10:1 and a particle of equivalent obscured area.

Based on equivalent filter face obscuration, the lengths and quantities of fibers permitted for 

each class of particulate cleanliness in MSFC-SPEC-164C are shown in table 4. It should be noted 

that the larger fibers, if present at the 10:1 aspect ratio, would be clearly visible as debris. 

Table 4. MSFC-SPEC-164C classes with equivalent fiber lengths based on obscuration.

Class

Particle Size

(μm)

Maximum No.

(per 0.1 m2)

Length of Equivalent 

Fiber at 10:1 Aspect 

Ratio (μm)

Length of Equivalent 

Fiber With 100 μm 

Diameter (μm)

Length of Equivalent 

Fiber With 50 μm 

Diameter (μm)

I >2,500

700 < x ≤ 2,500

175 < x ≤ 700

No silting

0

1

5

>7,000

1,960 < x ≤ 7,000

490 < x ≤ 1,960

>49,063

3,847 < x ≤ 49,062

240* < x ≤ 3,847

>98,125

7,693 < x ≤ 98,125

481* < x ≤ 7,693

II >1,000

700 < x ≤ 1,000

175 < x ≤ 700

No silting

0

40

150

>2,800

1,960 < x ≤ 2,800

490 < x ≤ 1,960

>7,850

3,847 < x ≤ 7,850

240*< x ≤ 3,847

>15,700

7,693< x ≤ 15,700

481*< x ≤ 7,693

III >800

No silting

0 >2,240 >5,024 >10,048

III X >800

175 < x ≤ 800

No silting

0

5

>2,240

490 < x ≤ 2,240

>5,024

240* < x ≤ 5,024

>10,048

481* < x ≤ 10,048

IV >400

No silting

0 >1,120 >1,256 >2,512

IV X >400

175 < x ≤ 400

No silting

0

5

>1,120

490 < x ≤ 1,120

>1,256

240* < x ≤ 1,256

>2,512

481* < x ≤ 2,512

V Visually clean/no silting

* Does not meet the minimum 10:1 aspect ratio for a fiber.

If fibers are limited to a diameter of 100 or 50 μm due to concerns over plugging of individual 

orifices downstream, then the fiber length that will create an equivalent level of obstruction versus 

a round particle becomes much longer for the larger particle sizes. This is shown in the two right 

columns of table 4. These thin fibers become absurdly long when equated to particles larger than 800 

μm, with lengths better measured in centimeters than micrometers. 
19

As shown in table 4, the allocations for two fibers per square foot, at no larger than 50 μm 

diameter by 1,000 μm long in the fuel system and no larger than 100 μm diameter by 2,000 μm long 

in the oxygen system, yield less obscuration than one maximum size particle for those systems. 

Allocations for fibers in the propellant tanks must consider both restriction of the tank filters and restriction of downstream filters within the engine by straight fibers that bypass the tank 

filter. While it could be assumed that 50- or 100-μm-diameter fibers that are able to bypass the tank 

filter will also bypass engine filters and be ingested, a conservative approach would assume that at 

least some of these fibers will land on the engine filters. In addition to the level II particle limits established for the space shuttle main propulsion tanks, one fiber per square foot may be permitted, up to 

50 μm in diameter and 1,000–2,500 μm long in the fuel tank and up to 100 μm in diameter and 

1,000–5,000 μm long in the LOX tank. 

7.2 Valve Seal Leakage

A single particle of contamination, if sufficiently large and positioned on a sealing surface, 

can prevent a valve from fully closing, creating a leak path for propellants or pressurants. Abrasive 

particles in excessive quantities can also, over time, erode seats and seals, resulting in leakage. All 

propulsion system valves and seals have allocations for allowable leakage.

It is difficult to calculate or predict the minimum size particle that will cause a valve to leak. 

Valve seals are designed to deform around minor surface variations and small contaminants to 

assure a good seal. This is true even for metal sealing surfaces in oxygen systems. These surfaces are 

composed of relatively soft metal alloys or metals coated with oxygen-compatible, nonmetal coatings 

that are slightly compliant.

The tolerance of valve seat coatings to leakage from hard particle contamination has been 

studied. A study performed by McDonnell Douglas Astronautics Co. in 1975 for Lewis Research 

Center tested the contamination tolerance of various nonmetallic coatings on valve seats.21 This 

study concluded that particles will be embedded in coatings of relatively hard plastic materials such 

as Xylan® 1010, Kynar® 202, and Teflon® S, without resulting in leakage. While some particle 

deformation generally occurs during embedment, a coating thickness of 20%–30% more than the 

largest anticipated particle diameter was recommended. Particles up to 420 μm were tested to verify 

leakage resistance. Assuming that seals will crush very thin, twisted fibers down to their diameter and 

deform around them, even metallic fibers limited to 50–100 μm in diameter should be of minimal 

concern.

North American Rockwell report SD 70-557-5,12 dated 1970, documented several instances 

of excessive valve leakage in the Saturn S-II stage LH2 systems where particulate contamination may 

have contributed to the failure. Forty-one Problem Action Records (PARs) related to contamination were reported during the processing of 15 S-II units at Seal Beach, the Mississippi Test Facility, and KSC. The failures due to contamination were primarily seal leakage above specification 

requirements, none of which would have resulted in a mission failure. Four specific incidents were 

reported of excessive leakage in LH2 prevalve solenoids where teardown and analysis indicated that 

metallic contamination may have contributed to the failures. This report recommended that critical 
20

systems undergo a pneumatic purge (blowdown) to remove particles entrapped or generated during the assembly and test processes with collection of blowdown products on a 5-μm filter to verify 

completion. This report also recommended that particle contamination allocations be limited to 

only nonmetallic particles and fibers above 175 μm, and only nonmetallic fibers above 700 μm. These 

two limits, considered together, imply a maximum width for fibers of 70 μm. The rationale for distinguishing between metallic and nonmetallic particles was not explained. While five specific instances 

were noted of leakage in LH2 prevalve solenoids due to metallic particles, the PAR history showed 

that approximately half of the contaminants identified as the cause of leakage were nonmetallic. 

The report and attached detailed particle analyses did not distinguish between metallic particles and 

metallic fibers. Based on this report, it appears that if all fibers are restricted to a maximum diameter 

of 70 μm, then it is not necessary to distinguish between metallic and nonmetallic fibers.

It is conceivable that a large quantity of hard metallic fibers, even if restricted to below 70 μm 

in diameter, could abrade the surfaces of seals and pumps. Contamination by a large quantity of any 

type of small, hard particles, such as sand, could also cause such abrasion. The requirement for “no 

silting,” which was added to all MSFC-SPEC-164 particle classes in 1994 with revision B, is expected 

to avoid this scenario as well as ensure a valid inspection.

A problem report on “Engine LOX Pump Contamination”22 (pp. 3-42 to 3-43) on Saturn 

S-II-12 identified metallic and nonmetallic fibers and shavings that were found during an engine 

modification not related to any failure. These particles included a stainless steel bristle with a diameter of 0.003 in and a length of 0.5 in (76 μm by 12,700 μm), a Kel-F® shaving 0.001 × 0.002 × 4 in 

(25 μm × 51 μm × 10 cm), a stainless steel fragment 0.001 × 0.005 × 0.030 in (25 μm × 127 μm 

× 762 μm), and a crescent of PCV (sic) such as Tygon® 0.005 × 0.25 in (127 μm × 6,350 μm). During a 

subsequent inspection of the LOX pumps on Saturn S-II-13, a “51/2-in-long strand” (14 cm) of Kel-F 

with “irregular surfaces” was found in a LOX pump. Four of these five particles meet the definition 

of fiber. It was concluded in this report that none of these contaminants would be detrimental to 

stage performance and, beyond a source analysis to determine whether a component was chafing, 

no further action was recommended. No rationale was stated to support the conclusion that none of 

these contaminants would be detrimental to stage performance.

Other reports on space shuttle valve leakage noted the size of particle that appeared to be the 

cause of the leakage. One particle noted to cause leakage was a nitrile rubber particle 0.146 × 0.046 

× 0.018 in (3,700 × 1,168 × 457 μm) located on a poppet face.23 This particle far exceeded the maximum allowed size of 800 μm for the oxygen system.

Significant advances have been made in the understanding of seal design for cryogenic systems since the Saturn days. Given the known capabilities to design valves and sealing surfaces for 

these systems that will tolerate isolated particulate contaminants up to 400 μm in LH2 systems and 

800 μm in LOX systems, the presence of occasional fibers that are limited in these systems to a maximum diameter of 50–100 μm should be of little consequence. Based on the Saturn S-II experience, 

a maximum fiber diameter of 70 μm for all systems seems reasonable. No rationale has been

identified to support different allocations for nonmetallic versus metallic fibers when such a diameter 

restriction is imposed. 
21

7.3 Fire Hazards in Oxygen Systems

Most materials ignite more readily in enriched oxygen environments. Both organic and inorganic contaminants may present an ignition hazard in these systems. Extensive documentation is 

available on material selection and design principles for oxygen systems. American Society for Testing and Materials (ASTM) committee G04 on Flammability and Compatibility of Materials in 

Oxygen-Enriched Atmospheres, formed in 1975, has numerous publications on this topic including 

ASTM Manual 36, Safe Use of Oxygen and Oxygen Systems.

24 The NASA Materials and Processes 

Technical Information System contains an extensive database of test results for oxygen compatibility 

of materials.

Particle contaminants entrained in flowing oxygen may ignite on impact, presenting an ignition hazard in high pressure or high velocity oxygen systems, particularly in poorly designed systems. 

Accumulations of organic matter such as oil films can be ignited by particle impact, rapid pressurization, or other mechanisms. Acoustic resonance has also been shown to be capable of igniting an 

accumulation of particulate. Fibers must not present a greater ignition hazard than either an impacting particle or an accumulation of ignitable organic residue or particulate.

7.3.1 Particle Impact Ignition and the Kindling Chain

When a particle impact occurs within an oxygen system, the localized heat of ignition must 

be sufficient to ignite the impacted material in order to cause a system hazard. This cascade of events 

is referred to as a kindling chain. The maximum allowable particle mass to prevent impact ignition is highly dependent on the design characteristics of the system and its operating pressures and 

velocities. 

Particle impact has been studied for high-pressure GOX systems at NASA’s Lyndon B. Johnson White Sands Test Facility. Similar studies of particle impact in lower pressure LOX systems have 

not been identified for this analysis. Studies of the conditions under which particle impact ignition 

of various target materials will occur in GOX indicate that such ignition will not occur at LOX temperatures or at the relatively low pressures and velocities found in LOX propellant tanks and feed 

systems. However, higher transient temperatures and velocities may occur with the presence of GOX 

during system fill and cooldown preflight where particulate impact ignition must be considered. 

Hundreds of successful tanking operations of the space shuttle and its associated ground support 

equipment (GSE) demonstrate that hazards are effectively controlled by particle limits of up to 1,000 

μm in LOX propellant tanks and 800 μm in LOX feed and drain systems and GOX vent systems.

Studies of the ignition of metals by impact of low velocity16 and high velocity25 particles 

have shown that ignition depends on both the kinetic energy of the particle at impact and the heat of 

combustion of the particle. The localized heat generated by this impact must be sufficient to ignite 

the material impacted in order to initiate a kindling chain. With the possible exception of aluminum 

targets at high particle velocities, it appeared that ignition of the impacting particle was required in 

order to ignite the target. Higher system pressures, flow velocities, and oxygen temperatures increased 

the probability of ignition of the particle and target. It was also noted that although small particles 
22

were easier to ignite, a single large particle was more likely to ignite the target than a large quantity 

of smaller particles(ref. 16, p. 75).

An impacting fiber of the same composition and mass as an impacting particle should yield 

the same or lower kinetic energy and energy flux density. Benz et al. (pp. 32–33) noted that deformation of the target or particle on impact will reduce the heat energy of the impact.25 A small diameter 

metal fiber, that could be expected to deform on impact, may be less likely to ignite a target than 

a metal particle of comparable composition and mass. Based on the formulas for the volume of 

a sphere versus the volume of a cylinder, a fiber with the worst case aspect ratio of 10:1 will have 

a length of 3.75 times the diameter of a particle of the same material to yield the same impact mass. 

Based purely on the impact mass equivalent to a 1,000- or 800-μm metallic particle, the maximum 

allowable metallic fiber length would be 3,750 μm in the tank and 3,000 μm in downstream oxygen 

systems. At the 10:1 aspect ratio, these fibers would be clearly visible as chips or shavings.

Most laboratory particle impact tests have used metal or oxide particles, impacting metal 

targets. In 1995, a report was published by Dees et al.26 on an evaluation of polymer particles as 

an ignition source in oxygen. Particles of Teflon (PTFE), Vespel® SP-21, Kel-F 81, and Viton® A, 

materials commonly used in oxygen systems, were impacted against stainless steel targets. Both large 

(up to 2,000 μm) and small (250 μm) particles were tested at subsonic and supersonic velocities and 

a range of temperatures. No target ignitions were observed under any of the test conditions, including those known to ignite the target when the particle is metallic. In several cases, it was evident that 

the polymeric particle ignited but no ignition of the target occurred. It was theorized that the lower 

densities, lower yield strengths, and different burning characteristics of polymers result in lower 

kinetic energy and energy flux density transferred to the target than comparably sized metal particles. 

It has been generally accepted by the oxygen systems design community that “nonmetal particulate 

is not an effective igniter” (ref. 24, p. 25).

By comparing the volume and specific gravity of fibers versus metallic particles, maximum 

fiber sizes can be calculated that yield an equivalent impact mass. Using a round steel particle of 

approximately 8 g/cm3 as a worst case particle, the most massive particle permitted in the tank is 

4.2 mg, 2.1 mg in all other LOX system components.

The specific gravity of nonmetallic fibers ranges from 0.3 g/cm3 for lightweight paper 

to 2.6 g/cm3 for fiberglass. Carbon composite fibers fall around 2 g/cm3. Clothing fibers range from 

1.1 g/cm3 for nylon to 1.5 g/cm3 for cotton. Even using the specific gravity of fiberglass, which is 

inert and will not ignite, calculations based on equivalent particle mass yield absurdly large fibers of 

4,700 μm (versus an 800-μm particle) and 9,000 μm (versus a 1,000-μm particle) at the 10:1 aspect 

ratio. If the fibers are restricted to a diameter <100 μm, as indicated by the calculations for critical 

orifice plugging, the risk of a fiber of any composition causing an impact ignition based on mass 

becomes insignificant.

It should be noted that particle impact ignition studies published in ASTM G04 committee 

Standard Technical Publications have focused on GOX systems, generally under elevated temperatures and pressures and at high velocities. LOX system components, which operate at much lower 
23

temperatures, pressures, and flow velocities than the test conditions, are considered to be far less 

susceptible, if at all, to particle impact ignition.

7.3.2 Ignition of Accumulated Fiber Mass 

The potential for ignition of an accumulated mass of contaminants must also be considered 

when establishing an allocation for fibers. Fibers present within assembled fluid systems prior to 

system fill, if entrained in the fluid flow, will tend to accumulate on system filters if the fibers are 

longer than the filter orifice diameters. Wire mesh, used as filter media in oxygen systems, propagates 

combustion more readily than rods or plates of the same material. Testing reported by Stoltzfus 

et al.27 has shown that, while combustion rates increase with pressure, stainless steel, low carbon 

steel, and Monel® 400 will propagate combustion in oxygen at atmospheric pressures. Stainless steel 

mesh has historically been used for flight oxygen filters in NASA propulsion systems.

Due to significant differences in system internal surface area and in operating pressures, temperatures, and flow velocities, accumulation of fibers in the LOX tank, LOX feed systems, and GOX 

systems (such as a GOX ullage repressurization loop) must be considered separately. 

Test methods have been developed to evaluate the suitability of filter designs and materials 

of construction for use in high-pressure oxygen service. Recognizing that filters must continue to 

perform safely in the presence of contaminants, these tests used particles and oils as ignition promoters with adiabatic compression or particle impact as the ignition source. Barthelemy et al.28 used 

adiabatic compression to ignite oil, which then ignited 1 g of 44 μm, 97% iron powder (hydrogen 

reduced), which was then ingested into the filter. Odom et al.29 adapted ASTM Test Method G175, 

Standard Test Method for Evaluating the Ignition Sensitivity and Fault Tolerance of Oxygen Regulators Used for Medical and Emergency Applications, to test the ignition resistance of contaminated 

stainless steel and brass filters in high-pressure oxygen. The test contaminant was a mixture of aluminum powder and iron particles with a perfluorinated lubricant. Neither of these investigations 

tested fibers or particles alone, without an oil or lubricant, nor was sensitivity testing performed to 

determine the influence of different levels of contaminant on the combustion of the filter. 

Sensitivity studies on the hazards of ignition of accumulated surface contaminants in oxygen 

systems have focused on NVR such as oils and greases in high-pressure GOX systems. These contaminants, if present in sufficient quantity at high oxygen concentrations, can be ignited by adiabatic 

compression or particle impact and promote ignition of the substrate.

Limits established in the industry for NVR in oxygen systems range from 1 mg/0.1 m2 in 

NASA LOX and GOX systems1,30 to 47.5 mg/0.1 m2 in commercial oxygen systems.2,8,31 MSFCSPEC-164 permits up to 1 mg/0.1 m2 of NVR in flight oxygen systems except for tanks. Up to 

5 mg/0.1 m2 of NVR is permitted in flight LOX tanks and in ground test systems operating below 

5,000 psig. The original technical rationale used to establish these NVR limits in the 1962 baseline 

release of MSFC-SPEC-164 has not been identified. The NVR limits for flight vehicles have never 

been revised and are still applicable in MSFC-SPEC-164C. Since 1994 (rev. B), MSFC-SPEC-164 

has permitted NVR as high as 20 mg/0.1 m2, with approval of the Director of Safety, in ground test 

systems operating below 5,000 psig. 
24

7.3.2.1 Ignition of Accumulated Fiber in the Liquid Oxygen Tank. If an allocation for fibers 

is established with a maximum size and a maximum quantity per square foot of surface area, then it 

is possible for this allowed background level of fiber to migrate and accumulate. The largest surface 

area where particulate could accumulate prior to being collected on a filter or ingested is within the 

LOX propellant tank of the main propulsion system. Assuming that fibers are present at the maximum size and quantity on all interior surfaces of the LOX tank and all of these fibers are swept to 

and collected on the tank sump filter face during system operation, then a worst case total accumulation on the filter can be calculated. 

It has been suggested that the allocation for one particle, up to 2,500 μm, per square foot in 

the baseline release of MSFC-SPEC-164 may have been intended to accommodate the occasional 

fiber. This cleanliness level is now shown in MSFC-SPEC-164C as class I and does not distinguish 

between particles and fibers. Using this limit as the proposed allocation for fibers in the LOX tank, 

a worst case total accumulation of fibers 2,500 μm long by 50 μm diameter is <20 mg/0.1 m2, based 

on the following assumptions:

(1) A 0.43-m- (17-in-) diameter filter screen (the space shuttle ET LOX screen) (0.185 m2

(1.58 ft2)).

(2) Particles are evenly distributed on a tank internal surface area of 372 m2 (4,000 ft2) 

(estimated space shuttle LOX tank internal surface area including baffles and orthogrid).

(3) Average fiber density of 1.6 g/cm3 (the high end of common environmental fibers).

(4) Maximum fiber volume is 4.9 × 10–6 cm3 (2,500 μm long × 50 μm diameter).

(5) All fibers from the tank surfaces become entrained in the flowing LOX during system 

operation and accumulate on the tank filter screen.

(6) Due to mixing during flow, the distribution of accumulated fibers on the filter screen is 

uniform.

(7) All fibers shorter than 800 μm and most fibers shorter than 1,000 μm, the diagonal measure of an 800-μm square mesh orifice minus 10% to account for the width of the fiber, will pass 

through the filter screen unhindered. Assuming that the internal engine filters are no more restrictive 

than the tank screen, these fibers will be ingested by the engine and burned with the propellants. No 

accumulation of these fibers will occur.

(8) Most or all of these fibers are nonmetallic. Metallic particles do not typically occur 

as fibers, and when they do, metal fibers <50 μm in diameter are not typically found in such 

long lengths. Reports of cleanliness inspection failures due to fibers during propulsion systems 

cleaning have been reported to be from clothing fibers or something similar (M. Campbell, 

ET Project, Lockheed Martin, MSFC-Michoud Assembly Facility, Private Communication, 

March 13, 2008, and J.H. McMillian, Chemist, InfoPro Corp, Propulsion Test Valve Lab, MSFC, 

Private Communication, March, 17, 2011) (see ref. 12, table 2-5).
25

Limiting the fiber length to 2,000 μm reduces this accumulation to <16 mg/0.1 m2. Further 

limiting the maximum fiber size by limiting the maximum diameter to 40 μm reduces this accumulation to <10.3 mg/0.1 m2.

If the fibers are metallic, with a density of approximately 8 g/cm2, then accumulation on the 

filter of 2,500 μm × 50 μm fibers could reach 100 mg/0.1 m2.

All of these scenarios exceed the current MSFC-SPEC-164 NVR limit of 5 mg/0.1 m2 for 

LOX tanks. However, there are several mitigating factors to indicate that this level of accumulation 

on the filter is both highly unlikely to occur and, if it did, unlikely to ignite and initiate a kindling 

chain. 

7.3.2.2 Mitigating Factors, Fiber Floating. Tests of the response of fibers of cotton, silk, 

linen, jute, wool, paper, polyester, nylon, and miscellaneous dryer lint in both LN2 and LOX showed 

that all of these fibers float to the surface of the cryogen during fill of the vessel and tend to form 

clumps there.32,33 It can be assumed that nonmetallic fibers present within the LOX tank and not 

adhered to the walls by another mechanism will float to the LOX surface during fill.

Fibers floating at the LOX/GOX interface are not in contact with a combustible substrate. 

Even if an ignition of these fibers was possible, the initiation of a kindling chain in this location is 

not credible. Furthermore, if the tank is drained and refilled prior to flight, these fibers will most 

likely be swept out of the tank and captured in the 175-μm GSE filters. Such LOX drain and refill 

cycles are not uncommon on the launch pad. Even if they are not removed in this manner, floating 

fibers can be expected to reach the tank screen only near the end of the stage burn, if at all, leaving 

almost no opportunity for a metallic particle entrained in the flow to impact and ignite the accumulated fiber in a location where a kindling chain could be initiated.

Metallic fibers are 5 times denser than nonmetallic environmental fibers. These fibers are not 

expected to float in the cryogen and therefore could potentially collect on the tank filter.

7.3.2.3 Mitigating Factors, Heat of Combustion. Heat of combustion data are used to evaluate the potential for a material, once ignited, to ignite surrounding materials. Materials with lower 

heats of combustion are preferred for oxygen service (ref. 24, pp. 27–37). Tests of promoted combustion by ignition of contaminants on metal surfaces have been performed using hydrocarbon-based 

oil films known to ignite readily in high-pressure oxygen systems.34 These oils, such as mineral oil 

and hydraulic oil, have high heats of combustion as compared to common environmental fibers. The 

heats of combustion of various materials that constitute fibers, oils, and other types of potential 

oxygen system contaminants are shown in table 5.
26

Table 5. Heat of combustion of contaminants.

Material

Heat of Combustion

(cal/g)

Stainless steel 1,850–2,000*

Cotton fiber 3,900**; 4,000***

Polyester resin 4,300***

Wood, pine 4,700***

Titanium 4,711*

Silicone grease, Dow Corning 7 4,800***

Wool fiber 4,900**; 6,350***

Silicone 5,400†

Polyester fiber 5,700**

Aluminum 7,425*

Carbon 7,835***

Nylon fiber, nylon 6/6 7,400–7,900**, ***

Esters 8,500–9,600†

Hydraulic oil, MIL-H-83282 9,800†

Hydraulic oil, MIL-H-5606 10,10†

Mineral oil 10,700–11,000***, †

 * ASTM Manual 36, table 3-1124

 ** CSIRO fact sheet35

*** Lowrie, R., contains a compilation of data from other sources36

 † Totten, G.37

The heats of combustion of common environmental fibers are 20% to 60% lower than the oils 

used to evaluate the risk of contaminant ignition in oxygen systems. Heats of combustion of metals found in oxygen systems are 25% to 80% less than these oils. This reduces the energy available to 

initiate a kindling chain.

Metallic fibers have comparable heats of combustion to nonmetallic fibers but are denser. 

If ignited, an accumulation of metallic fibers, particularly aluminum fibers, in a high-pressure system theoretically could release sufficient energy to initiate a kindling chain. The threshold fiber 

mass required to create such a hazard is unknown. Ignition studies of multiple particles have 

been performed by injecting 2–3 g of fine particles as small as 10 μm into flowing GOX upstream 

of a target.16,25 Lower masses were insufficient to ignite the target. This is a much greater mass 

than the predicted 100 mg/0.1 m2 of metallic particles. The study by Odom et al.29 of promoted 

ignition of metallic filters used 350 mg to 3.5 g of mixed aluminum and iron in a thin layer of 

lubricant as the contaminant promoter. These ignition studies tested different scenarios than 

the ignition of fibers alone, accumulated on the target by another source. 

7.3.2.4 Mitigating Factors, Low Temperature and Heat Removal. Within the LOX tank and 

feed system of a bipropellant propulsion system, the temperature and pressure conditions are considerably lower than those used to evaluate the risk of ignition of contaminants in oxygen systems. 

While LOX is at a concentration equivalent to GOX at a very high pressure, the temperature is much 
27

lower than found in GOX systems. Assuming that fibers distributed within a LOX system will accumulate on a filter during fluid flow rather than float to the LOX surface, the cryogenic LOX flow 

will rapidly remove heat generated by a possible ignition of these fibers, preventing the initiation of 

a kindling chain. This risk mitigation in LOX systems is discussed by Lowrie36 (p. 7), and is supported by other industry tests referenced there.

The hardware that must be ignited by burning fiber has also been cooled by the LOX to a very 

low temperature. The effect of target temperature on target ignition by an impacting particle was 

studied by Benz et al.25 (figs. 7–9). In those studies, no ignition of the target was observed below 

about 390 K. At lower system pressures, no ignition of the target was observed below about 500 K. 

The scenario where a particle might impact an accumulation of fibers on a filter is only credible during LOX flow when the filter (target) is at a very low temperature.

7.3.2.5 Mitigating Factors, Contaminant Geometry. Lowrie also mentions the influence 

of geometric conditions on heat transfer of an ignited promoter to the other material. To initiate 

a kindling chain, the ignited contaminant must transfer sufficient heat flux to ignite the surface on 

which it resides. Most fiber materials have densities, discussed above, which are higher than mineral 

oils or hydraulic oils. The density of these oils is typically 0.7 to 0.8 g/cm3. However, fibers do not 

accumulate in a continuous layer. The fibers that have the potential to accumulate on filter screens 

are very long and often twisted. Accumulations of fibers contain a considerable amount of free space 

that is expected to both dissipate heat to the LOX and reduce the physical contact between the fibers 

and the substrate, reducing heat transfer to the substrate material.

Furthermore, as discussed in the section above on particle impact ignition, nonmetallic particles have been shown to be incapable of transferring sufficient heat flux to substrates during impact 

ignition to initiate a kindling chain. The lower yield strengths and the burning characteristics of 

polymers that are theorized to contribute to lower kinetic energy and energy flux density transfer to 

the target during impact can also be expected to limit the transfer of heat flux from the ignition of 

an accumulation of nonmetallic fibers to the filter screen substrate.

7.3.2.6 Mitigating Factors, Conservative Nature of the Existing Nonvolatile Residue Limit.

The technical basis for the 5 mg/0.1 m2 limit for NVR in LOX tanks, established in MSFC-SPEC-164 

in 1962, has not been identified. It has been suggested that this NVR limit was the best that could be 

achieved and verified at that time on such a large surface area. More recent testing (see ref. 34) has 

shown that hydrocarbon-based oils could be ignited in GOX by rapid pressurization at levels as low 

as 6 mg/0.1 m2 (65 mg/m2). However, this reference also reported that none of the ignitions observed 

during testing, even at the very high NVR levels of 300 mg/0.1 m2 (3,200 mg/m2), led to an ignition 

of the stainless steel tube used as the test article. The author cautioned that other substrate materials, 

such as nonmetals, may ignite under these conditions.

Werley, in the paper “Oil Film Hazards in Oxygen Systems,” summarized oil film ignition 

data from a number of earlier sources.38 One of the quoted NASA sources (C.J. Bryan, “Final 

Report on the Effect of Surface Contamination on LOX Sensitivity,” KSC Letter Report MTB 306-

71, 1971 (a copy of this secondary source has not been located)) stated surface contamination limits 

for Dow Corning 704 silicone oil (a highly reactive oil) that resulted in reaction of a substrate during 
28

LOX mechanical impact tests at 98 N-m of energy. This source reported no reactions on aluminum 

6061-T6 below 43.2 mg/0.1 m2 and no reactions on Teflon below 21.6 mg/0.1 m2. NASA launch 

vehicle LOX tank screens have used aluminum alloys and 300-series stainless steel for the frames and 

300-series stainless steel for the filter mesh.

These reports indicate that the general NVR limit of 5 mg/0.1 m2 in LOX tanks is very conservative. If fiber accumulation can be compared to NVR as an ignition promoter, then ignition of 

the aluminum and stainless steel tank filter screens by fibers at accumulated levels equal to or below 

20 mg/0.1 m2 is highly unlikely.

7.3.2.7 Mitigating Factors, No Credible Ignition Mechanism. For a fire to occur in the LOX 

tank there must be a credible ignition mechanism. ASTM Manual 36 lists the following 12 ignition 

mechanisms that have been known to cause fires in oxygen systems: particle impact, heat of compression from rapid pressurization, flow friction, mechanical impact, friction, fresh metal exposure, static 

discharge, electrical arc, chemical reaction, thermal runaway, resonance, and external heat. Some of 

these mechanisms are poorly understood and this list of mechanisms may not be complete (ref. 24, 

pp. 9–15).

As noted above, particle impact does not appear to be a credible ignition source in LOX. 

GOX will be present at the expected fiber accumulation location, the tank filter screen, during the 

beginning of the fill of the LOX tank, as boil-off from the LOX. Fluid velocity is insufficient at this 

time to support a particle impact risk. The LOX tank is purged with nitrogen prior to LOX fill.

Rapid pressurization and resonance sufficient to ignite contaminants are not credible within 

the large volume of the LOX tank. No materials are present that react chemically or exothermally in 

LOX or ambient temperature GOX. The LOX tank filter screen assembly contains no moving parts 

to generate friction heat. Static discharge, electrical arc, and external heat are precluded by design 

and operational precautions as a general safety measure.

Current theory indicates that flow friction can occur above 3.4 MPa (500 psi), which is about 

4 times the maximum design pressure of the LOX tank manhole seal and LOX feed line on the Ares I 

upper stage and more than 10 times the ambient service pressure. (Refer to MSFC’s flange seals, 

specification control drawing 97M28929 and detailed specification for LOX feed line, ARES-USODE-25178, rev. A.) Also, heat generated by friction during LOX flow will immediately be dissipated 

by the flowing cryogenic fluid.

Mechanical impact ignition with fresh metal exposure could potentially occur from a large 

foreign object falling onto the LOX tank skin or filter screen aluminum frame. This would need to 

occur at the beginning of LOX tank fill before sufficient LOX is present to buoy the falling object and 

dampen the impact energy. Foreign object debris (FOD) prevention measures, including tank FOD 

rolls—rotations of the tank during manufacturing to listen for and release loose objects—are used 

throughout vehicle processing to eliminate FOD. A LH2/LOX vehicle stage also undergoes several 

rotations and moves between horizontal and vertical prior to a LOX fill. A loose item of sufficient 

mass to cause ignition escaping notice prior to LOX fill and dropping at just the right time is highly 

unlikely.
29

7.3.2.8 Conclusions on the Ignition Risk of Accumulated Fiber in the Liquid Oxygen Tank. The 

potential exists for fibers, present at the maximum allocation per square foot of surface area throughout a LOX tank, to accumulate on the LOX tank filter screen during system operation to a level that 

exceeds the 5 mg/0.1 m2 NVR limit in the tank. If one fiber up to 50 μm in diameter and 2,500 μm in 

length is permitted per square foot of tank surface area, nonmetallic fiber accumulation could be as 

high as 20 mg/0.1 m2 in the filter screen. Metallic fiber accumulation could be as high as 100 mg/0.1 

m2. This is not a credible fire risk, however, for the following reasons:

(1) Laboratory tests indicate that nonmetallic fibers will float and accumulate at the LOX/

GOX interface during ground fill rather than accumulating in on the tank filter screen during operation. Fibers accumulated on the LOX/GOX interface do not pose a credible fire risk because there is 

no ignition source in this location or opportunity to initiate a kindling chain.

(2) Should fibers accumulate on the filter screen during system operation, several mitigating 

factors indicate that an ignition of these fibers is highly unlikely due to the lack of a credible ignition 

source. 

(3) Should an ignition occur, the ignited fiber mass is unlikely to transfer sufficient heat flux 

to ignite the filter material. Due to the number of variables and the lack of test data on the ignition of fiber accumulations in LOX or GOX, an acceptable threshold of accumulation is difficult 

to determine. Test data on the ignition of oil films in high-pressure oxygen systems indicate that the 

threshold to initiate a kindling chain reaction in a LOX tank is well above 20 mg/0.1 m2. Other factors, such as the heat sink of flowing LOX and cryogenically cooled metal surfaces, and poor fiber 

contact with the substrate to transfer heat, are also expected to reduce the probability of transferring 

sufficient heat flux to ignite the filter material.

It should be noted that this analysis assumes an even initial distribution of fibers throughout 

the tank. Given that environmental fibers have the tendency to entangle and form clumps that may 

lead to measurement error on large components, accumulations of fibers that are visible during 

inspections should not be permitted. 

It should also be noted that nonmetallic fiber contamination originating from clothing or 

from the outdoor environment are both more likely to occur and more difficult to control than 

metallic fiber contamination. Detection of more than an occasional metallic particle or fiber within 

a flight system is a potential indication of hardware damage or defect and should trigger a source 

investigation. 

7.3.2.9 Ignition of Accumulated Fiber in Other Liquid Oxygen System Components. It is 

assumed that most fiber contaminants that pass through the LOX tank filter screen will also pass 

through downstream filters and orifices to be eventually ingested and burned by the engine. The fiber 

mass that is available to be accumulated on a filter is dependent on the internal surface area between 

the upstream filter and the downstream collection point.

Concept architecture for the future Space Launch System indicates that the largest downstream surface area in the LOX system is driven by the surface area of the oxidizer feed line. With 
30

a LOX tank mounted above the LH2 tank, the feed system could be as long as 46 m (150 ft). Multiplied by the circumference of a feed line diameter of 0.3–0.5 m2 (1–1.5 ft), the estimated internal 

surface area is 46–70 m2 (500–750 ft2); one-fifth to one-eighth the surface area of the tank. Potential fiber accumulation is reduced proportionally to roughly 4 mg/0.1 m2 of nonmetallic or 20 mg/

0.1 m2 of metallic fiber. This assumes a single feed line with end point distribution to the planned 

five SSME-derived engines. Multiple feed lines will further reduce the potential accumulated mass. 

Operating parameters for the SLS LOX feed system components are expected to be similar 

to the Ares I upper stage LOX parameters. These systems had maximum design pressures under 

125 psia with nominal operating pressures under 50 psia and flow velocities <15 lbm/s. Given these 

operating conditions and a lower potential accumulation, the fiber limits allocated for the LOX 

tank should be acceptable for downstream LOX components as well. However, a system analysis 

should be performed to verify that transient high pressure or high temperature GOX will not occur 

in regions of the LOX components where fiber accumulation may occur.

7.3.2.10 Ignition of Accumulated Fiber in Gaseous Oxygen System Components. The potential risk of promoted combustion from accumulated fiber contamination in locations where GOX 

may be present is less clear. While substantial test data are available regarding the risks of particle 

impact ignition of metal and nonmetal substrates, no test data on accumulated particulate or fibers 

as an ignition promoter has been identified. Colson et al. have reported fires in oxygen system filters 

under conditions of high velocity, particle impact, and excessive contamination.39 Monitoring of 

pressure drop and maintenance at regular intervals is recommended for all industrial oxygen systems. While this publication notes that particles, dust, and debris trapped on filters can ignite when 

ignition conditions are met, very little data are available to establish safe thresholds for maximum 

accumulation, dependant on particle or fiber size and system parameters such as pressure, velocity, 

temperature, and configuration. 

A related earlier publication by Fano et al. noted that the auto-ignition temperatures of metals in powder form are much lower than in bulk materials, and that smaller particles may ignite larger 

particles which may in turn ignite bulk materials.40 However, this and other sources have indicated 

that to initiate a kindling chain, the particle(s) must be ignited prior to impacting the target.16,40

Particle ignition in tests reported by Williams et al. did not occur at velocities <45 m/s, independent 

of pressure between 2 and 30 MPa.16 These conditions are much higher than typically found in LOX 

propulsion systems upstream of the engine and are not found in the tanks. 

Within propulsion system components where GOX is present at temperature, pressure, and 

velocity conditions known to ignite particulate, accumulated metallic fiber could potentially serve as 

an ignition promoter. It is unlikely that nonmetallic fibers limited to less than one fiber per square 

foot, with a maximum dimension of 50 μm diameter by 2,500 μm long, could provide a sufficient 

accumulated combustion mass to create a hazard. 

7.3.3 Conclusions on Fibers as Fire Hazards in Oxygen Systems

When fiber size is limited to below 50 μm in diameter, and quantities are limited by the “no 

silting” rule to prevent gross presence in the flowing oxygen, fibers of any composition do not present 
31

an impact ignition hazard. When quantities and size are further restricted to limit accumulation on 

filters, the credibility of fibers creating a fire hazard by impact ignition is even lower.

An allocation for fibers in LOX tanks and downstream LOX components, when limited to 

one fiber/0.1 m2 with maximum dimensions of 50 μm diameter and 2,500 μm long, does not appear to 

present a credible fire risk. There does not appear to be any rationale to require a distinction between 

metallic and nonmetallic fibers. However, the LOX system design should be analyzed to verify that 

transient GOX will not occur under temperature, pressure, and velocity conditions sufficient to ignite 

a stray particle contaminant that might ignite an accumulation of metallic fiber. 

Insufficient data are available to fully assess the potential fire risk from the ignition of an 

accumulation of metallic fibers within a GOX system, such as an oxidizer tank repressurization 

system, under high temperature, pressure, or velocity conditions. Limited testing by Odom et al. has 

shown that accumulated metallic particulate may contribute to ignition of stainless steel filter media 

in GOX even under standard atmospheric pressures.29
32

8. RESULTS

8.1 Fiber Size and Quantity Limits

Based on the analyzed risk scenarios, fiber length limits larger than the established particle 

size limits should be permissible in LH2 and LOX propulsion systems and associated GSE and test 

systems. Several of these scenarios require that a maximum allocation be established for fiber size 

and quantity. Fiber size limits derived from the identified risk scenarios are shown in table 6.

Table 6. Derived fiber contaminant size limits for identified risk scenarios.

Maximum Fiber Length Limit (μm)

(Worst case fiber is a cylinder with length = 10 x diameter unless otherwise specified)

Particle Class

Risk Scenario

Class IV

 (400-μm Particle Limit)

Class III

 (800-μm Particle Limit)

Class II (Tanks)

 (700- and 1,000-μm Particle 

Limits)

Critical orifice plugging 

or restriction 

1,000

(50-μm maximum

diameter)

2,000

(100-μm maximum 

diameter)

Same as downstream

Equivalent surface obscuration 1,120 2,240* 1,960*/2,800*

Equivalent surface obscuration

100-μm maximum diameter

1,256 5,024 3,847/7,850

Equivalent surface obscuration

50-μm maximum diameter

2,512 10,048 7,693/15,700

Downstream filter obscuration from 

fibers bypassing the tank filter

NA NA 50-μm diameter x 2,500 (fuel)

100-μm diameter x 5,000 (LOX) 

Valve seal leakage 70-μm maximum diameter

No clear length limit

70-μm maximum diameter

No clear length limit

70-μm maximum diameter

No clear length limit

Equivalent particle impact ignition 

mass

NA 3,000* 3,750* (LOX only)

Particle impact ignition 

by nonmetallic fibers 

NA No limit No limit

Accumulated mass of 

50-μm-diameter metal fibers 

limited to 100 mg/0.1 m2 in tank/20 

mg/0.1 m2 downstream**

NA 2,000 

(two fibers/0.1 m2) 

LOX only

GOX limits TBD

2,500

(one fiber/0.1 m2 in tank)

 *At the maximum 10:1 aspect ratio, these particles would be clearly visible during visual inspection.

**Yields an accumulated mass of nonmetallic fibers of 20 mg/0.1 m2 in tank/4 mg/0.1 m2 downstream.

Several of the above scenarios indicate that a width restriction on fibers is required. To minimize the risk of orifice plugging in LH2 systems, prevent leakage of valve seals, and minimize the 

risk of accumulated combustible mass in LOX tanks, a maximum fiber width of 50 μm is advisable in 
33

both systems. With this width restriction imposed, it is not necessary to distinguish between metallic 

and nonmetallic fibers in the fuel system or in the LOX tanks. In regions of the oxygen system where 

GOX may be present, an analysis of pressure, temperature, and flow velocity conditions, including 

transients, should be performed at potential accumulation points to determine whether fibers can be 

tolerated.

Assuming a maximum fiber diameter of 50 μm, the maximum fiber length in the LH2 downstream systems should be 1,000 μm. This is based on plugging of a critical orifice in the engine by 

a combination of a twisted fiber and particles. The maximum fiber length may be increased if the 

critical orifice is determined to be larger than assumed for this analysis.

While these calculations do not point to a quantity limit for fibers between 400 and 1,000 μm 

in length, some limit would seem prudent to limit the risk of restriction of a critical orifice or internal 

engine filter from an accumulation of these smaller fibers from the feed system. The limits established 

in MSFC-SPEC-164 classes I, III X, and IV X of not more than five particles between 175 μm and 

the higher limit, applied to these intermediate fibers, should provide an equivalent level of protection 

from flow restriction.

The maximum fiber size and quantity in the downstream oxygen systems is driven by both 

oxygen system safety concerns and plugging of a critical orifice in the engine by a combination of 

a twisted fiber and particles. Limiting all fibers to a maximum diameter of 50 μm and a maximum 

length of 2,000 μm will minimize the risk of critical orifice plugging. Limiting the quantity of fibers 

to not more than five fibers between 800 and 2,000 μm per 0.1 m2 of sampled surface in these systems 

will limit the potential for accumulation of these fibers on a filter face or orifice to cause restriction. 

Given the limited surface area in the downstream systems versus the tanks, the potential for accumulation of fibers on a downstream system filter is about an order of magnitude lower. Limiting the 

quantity of these fibers to less than two per 0.1 m2 will limit downstream accumulation on a system 

filter to <4 mg/0.1 m2, a level considered by Pedley et al.34 to be below the NVR contaminant promoted ignition threshold for even very high pressure GOX systems. This level of contamination can 

only be applied with confidence to LOX systems and to low pressure GOX systems where there is no 

credible ignition source at potential accumulation points.

The original MSFC-SPEC-164 upper limit of one particle up to 2,500 μm, when restricted to 

fibers as recommended by Rodebücher,12 appears to be a reasonable limit for both the LH2 and LOX 

tanks. In the LH2 tanks, this limit is driven by filter screen restriction. In the LOX tanks, this is driven 

by a prudent approach to limit the opportunity for accumulation of potentially combustible matter 

on the filter screen. This will maintain the modified class II as a common requirement for both tanks.

8.2 Fiber Composition

As stated in section 2, cleanliness specifications that require the inspector to distinguish 

between metallic and nonmetallic fibers, or between flexible and nonflexible fibers as a proxy for 

metallic versus nonmetallic, create difficulties in the inspection process. Trained microscopists 

can determine whether a fiber appears to be metallic based on apparent reflectivity or a silvery
34

appearance, or flexible based on an apparent twisting of the fiber, but these distinctions are somewhat 

subjective. Oxidized metallic whiskers may not be reflective. Some fibers, such as silk, jute, wood, or 

grass, are often straight and may not appear to be flexible. Definitive identification of metallic fibers 

versus nonmetallic fibers requires chemical analysis using expensive analytical methods such as scanning electron microscopy with energy-dispersive x-ray spectroscopy. Cleanliness specifications that 

require identification of contaminant composition should be avoided if possible.

The analyses for fiber size and allocation based on orifice plugging, filter restriction, and valve 

seal leakage do not require a distinction between metallic and nonmetallic fibers when fiber diameter 

is limited to 50 μm. These are the only considerations that apply to the fuel system; therefore, there is 

no need to distinguish between metallic and nonmetallic fibers in LH2 systems and associated pressurization or pneumatic systems that do not interface with oxygen systems.

Industry test data have shown that metallic particles pose an impact ignition hazard in high 

pressure, high velocity oxygen systems whereas nonmetallic particles do not. Limitation of all fibers 

to a maximum diameter of 50 μm and maximum length of 2,000 to 2,500 μm effectively limits the 

mass such that individual metallic fibers cannot cause an impact ignition. Ignition of a limited accumulation of metallic or nonmetallic fibers inside a LOX tank appears highly unlikely due to the very 

low temperature, low tank pressure, relatively low velocities of potential impinging particles, and 

general absence of credible ignition sources. The relative risk of ignition in GOX of an accumulation 

of metallic versus nonmetallic fibers, where a credible ignition source may exist, is not clear. Allocations for any type of fiber that may permit accumulation in GOX systems must be approached with 

caution.

8.3 Measurement of Fibers

A specification that limits fiber size and quantity begs the question of how to measure the 

fiber. A circular cross section is assumed; therefore, two measurements are required for each fiber—

the apparent diameter and the length of the fiber at full extension. Both manual and automated 

methods of sizing and counting particles have limitations when applied to fibers. Methods currently 

used to size and count particulate include automated measurement by light scattering, automated 

image analysis, and microscopic analysis.

8.3.1 Automated Measurement by Light Scattering

Automated devices that detect light scattering from particles are available for measurement in 

air, in liquids, and on surfaces. These devices can accurately count large quantities of particles within 

defined size bins but do not discern morphology or composition. These devices do not distinguish 

fibers from particles. Fiber size limits larger than the allowed particle size limits should not be permitted when using these devices as the sole measurement technique.

8.3.2 Automated Image Analysis

Image analysis systems are commonly used for sizing and counting of particles collected on 

a filter or a reference surface such as a fallout plate. Some sophisticated image analysis systems are 
35

capable of distinguishing between particles and fibers and sizing both types provided that the fibers 

are not overlapping. Automated image analysis may be used when combined with human observation of the counted images to verify that fibers in the field of view have been accurately interpreted 

by the automated system.

Automated image analysis systems are not generally able to distinguish between contaminant 

materials although some may be capable of distinguishing between particles of different colors.

8.3.3 Microscopic Analysis

A microscopist trained in the counting and sizing of particles can readily distinguish between 

particles and fibers and can easily measure the apparent diameter of fibers and the length of straight 

fibers. Measurement of twisted fibers is more complex. Length can be approximated by measuring 

the fiber in segments or, more accurately, by using an opisometer on a calibrated image of the fiber 

displayed on a computer screen. Either method should be sufficiently accurate for verification.

Fibers that are electrostatically charged or very twisted may stand away from the measurement surface making them difficult to measure. Wetting of the background surface may help to flatten the fiber to the surface for easier measurement, neutralize the static charge, and prevent loss of 

the fiber sample to local air currents.
36

9. CONCLUSIONS

System contamination by environmental fibers is a recurring challenge when manufacturing and testing large liquid bipropellant launch systems. An analysis was performed to determine 

whether fibers could be permitted in these systems beyond the established limits for particulate, to 

derive allocations for fibers if allowable, and determine whether there is precedent for such allocations. This analysis concluded that:

(1) There is technical justification to permit fibers longer than the maximum dimension for 

particles in both oxygen and fuel systems, with the restriction that these fibers be <50 μm in diameter.

(2) Maximum fiber length limits and quantity allocations can be derived to prevent plugging 

or unacceptable flow restriction in critical orifices and filters.

(3) There are numerous precedents, both historical and current, in NASA and commercial 

documents and specifications for the allocation of fiber limits as a discrete particle type in oxygen/

fuel bipropellant systems. Fiber limits specified elsewhere are consistent with the limits derived by 

this analysis.

(4) It is not necessary to distinguish between metallic and nonmetallic fibers in fuel systems 

when the fiber diameter is limited to 50 μm.

(5) Fuel tanks require fiber limits by size and quantity to prevent excessive restriction of the 

tank feed filters and to prevent fibers that pass the tank filters from restricting downstream filters or 

creating a leak in a valve seal.

(6) Individual fibers limited to 50 μm in diameter, metallic or nonmetallic, do not pose a particle impact ignition hazard in LOX or GOX systems and are not expected to create a leak path in 

valve seals. 

(7) In very large oxygen system components, such as main propulsion tanks and feed lines, 

there is a potential for even a low level of background fiber contamination to migrate and accumulate on a system filter to a potentially combustible level.

(8) The accumulation of a background level of either metallic or nonmetallic fibers, within 

limits to prevent filter restriction, does not pose an ignition risk in LOX tanks or downstream LOX 

components due to the lack of a credible ignition source and, if ignited, insufficient heat flux under 

low LOX temperature, pressure, and flow velocity conditions to initiate a kindling chain.

(9) An assessment of ignition mechanism and kindling chain is required for regions in the 

propulsion system where GOX may be present at elevated temperatures, pressures, or flow velocities 
37

to evaluate the risk of ignition of an accumulation of fibers. However, insufficient data exist to evaluate the potential for an ignition source, such as a particle impact, to ignite an accumulation of fibers 

that in turn ignites a filter. This kindling chain scenario has not been tested. Threshold sensitivity of 

filter media to promoted ignition by an accumulation of fiber also has not been tested.

(10) The analyses for maximum fiber size and quantity are based on available design data 

from the Saturn V, space shuttle, and Ares I launch vehicles. Different engines may have different 

critical orifice sizes and filter designs that may require different size limits and quantity allocations 

for particles and fibers.
38

10. RECOMMENDATIONS

10.1 MSFC-SPEC-164 Particle Limits

To reduce manufacturing costs and schedule impacts driven by recleaning due to fiber contamination, the particulate cleanliness requirements in MSFC-SPEC-164 for propulsion systems 

may be relaxed to permit fibers up to 50 μm in diameter, with limits on maximum length and quantity. Recommended fiber allocations for MSFC-SPEC-164 particulate classes are shown in table 7. 

Also noted in table 7 are the propellant systems to which each class of requirement is applied.

Table 7. Recommended MSFC-SPEC-164 requirements for fiber contaminants.

Class

Particle Size

(μm)

Maximum No.

(per 0.1 m2)

Fiber Length*

(μm)

Maximum No.

(per 0.1 m2)

I >2,500 0

700 < x ≤ 2,500 1

175 < x ≤ 700 5

No silting

II (tanks) >2,000 0

>1,000 0 1,000 < x ≤ 2,000 1

700 < x ≤ 1,000 40

175 < x ≤ 700 150

No silting

III (LOX) >2,000 0

>800 0 800 < x ≤ 2,000 2

No silting

III X (LOX) >2,000 0

>800 0 800 < x ≤ 2,000 2

175 < x ≤ 800 5

No silting

IV (fuel) >1,000 0

>400 0 400 < x ≤ 1,000 5

No silting

IV X (fuel) >1,000 0

>400 0 400 < x ≤ 1,000 5

175 < x ≤ 400 5

No silting

V Visually clean/no silting

* Maximum fiber diameter 50 μm; fibers allowed only in liquid propellant tanks, and fuel and LOX components. 

 No fibers allowed in components that may be exposed to GOX without a risk analysis and approval of the procuring authority.
39

Table 7 includes no recommendation for class I. The current allocation for one particle up to 

2,500 μm will accommodate the occasional fiber. Class I is no longer used for LH2/LOX propulsion 

systems. Class III X with the added fiber allocation appears to be very similar to class I.

No change is recommended for class V, the visually clean inspection used for some nonmetallic components that cannot be flushed with a solvent for verification. Fibers limited to 50 μm 

diameter are very difficult to detect visually but may be detected under UV illumination. Ultraviolet 

is permitted in MSFC-SPEC-164 as an inspection aid. When UV is used, an occasional single, small 

fiber may be permitted. No visible accumulation of fibers is permitted.

10.1.1 Application to Future Vehicles

The assumptions used in this analysis to derive recommended fiber size and quantity limits 

should be evaluated for applicability to future liquid bipropellant propulsion systems. Engines other 

than the SSME and the J-2X require analysis for critical orifice sizes to determine the suitability of 

the MSFC-SPEC-164 particulate and fiber limits. Other parameters including system internal volumes and the presence of GOX subsystems or transients at high pressures, temperatures, or velocities 

may require adjustment to the allowable size or quantity of fibers.

10.1.2 Recommendations for Further Study

To understand the potential for accumulated fiber to act as an ignition promoter, testing is 

recommended. Both metallic and nonmetallic fibers in the range of 20 to 50 μm in diameter should 

be studied in concentrations of 20 mg/0.1 m2 to 300 mg/0.1 m2 on a test filter. Metal fibers tested 

should be composed of alloys used in the construction of propulsion system components. Nonmetallic fibers tested should be composed of common facility contaminants such as clothing fibers, 

paper and cardboard fibers, and carbon fibers used in the production of composites. Because NVR 

contamination is a known ignition promoter in oxygen systems, the potential synergistic effects of 

a combination of accumulated fibers and credible NVR levels should also be studied.

Tests should be performed to determine whether accumulated fiber lowers the threshold at 

which an understood ignition mechanism will ignite a target. Flight oxygen system filter materials 

should be the primary focus of this investigation. The test equipment and procedures used by Pedley 

et al.34 to evaluate NVR contaminants as an ignition promoter or used by Odom et al.29 to study 

filter design may be adapted to study this phenomenon.
40

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and Data Institute, Lewis Research Center, Cleveland, OH, 1972.
41

12. Rodenbücher, P.G.: SD 70-557-5, Saturn S-II Quality Assurance Techniques, Critical Process 

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14. RD07-051, Revision B, J-2X Rocket Engine Development Project Contamination Control 

and Implementation Plan, Pratt & Whitney Rocketdyne, Canoga Park, CA, June 6, 2007. 

15. SSME Contamination Control Plan, Rockwell International, Rocketdyne Division, 

September 15, 1976.

16. Williams, R.E.; Benz, F.J.; and McIlroy, K.: Ignition of Steel Alloys by Impact of Low-Velocity 

Iron/Inert Particles in Gaseous Oxygen, Flammability and Sensitivity of Materials in OxygenEnriched Atmospheres, ASTM STP 986, D.W. Schroll (ed.), American Society for Testing and 

Materials, Philadelphia, PA, pp. 72–84, 1988.

17. IEST-STD-1246, Revision D, Product Cleanliness Levels and Contamination Control Program, 

Institute of Environmental Sciences and Technology, <publicationsales@iest.org>, December 

2002.

18. SN-C-0005, Revision D, Space Shuttle Contamination Control Requirements, NASA Lyndon B. 

Johnson Space Center, Table A.1 Precision Particulate Levels, July 20, 1998. 

19. Goldberg, M., “Packing of 14, 16, 17, and 20 Circles in a Circle,” Math. Mag., Vol. 44, No. 3, 

pp. 134–139, 1971.

20. Specht, E.: “The Best Known Packings of Equal Circles in a Circle (Complete up to N=1,500),” 

Copyright E, <http://hydra.nat.uni-magdeburg.de/packing/cci/cci.html>, June 17, 2010.

21. Endicott, D.L.: “Valve Contamination Avoidance Devices Final Report,” NASA CR–134877, 

McDonnell Douglas Astronautics Co., May 1974–July 1975. 

22. SD 72-SA-0158, Saturn S-II Stage Pre-Flight Areas of Concern Review Final Report, 

Space Division of North American Rockwell, Seal Beach, CA, 1972.

23. Wood, B.K.: RSS-8898, STS-55 Pad Abort Engine 2011 Oxidizer Preburner Augmented Spark 

Igniter Check Valve Leak, Rockwell International, March 22, 1993.

24. ASTM Manual 36, Safe Use of Oxygen and Oxygen Systems, 2nd Ed., H.D. Beeson, S.R. Smith, 

and W.F. Stewart (eds)., American Society for Testing and Materials, West Conshohocken, PA, 

ISBN 978-0-8031-4470-5, 2007. 
42

25. Benz, F.J.; Williams, R.E.; and Armstrong, D.: “Ignition of Metals and Alloys by HighVelocity Particles,” Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres, 

ASTM STP 910, M.A. Benning (ed.), American Society for Testing and Materials, Philadelphia, PA, pp. 16–37, 1986.

26. Dees, J.; Forsyth, E.; Gunaji, M.; and Stoltzfus, J.M.: “An Evaluation of Polymers as Ignition Sources during Particle Impact in Oxygen,” Flammability and Sensitivity of Materials in 

Oxygen-Enriched Atmospheres, ASTM STP 1267, D. Janoff et al. (eds.), American Society for 

Testing and Materials, Philadelphia, PA, pp. 143–151, 1995. 

27. Stoltzfus, J.M.; Lowrie, R.; and Gunaji, M.V.: “Burn Propagation Behavior of Wire Mesh Made 

from Several Alloys,” Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres: Fifth Volume, ASTM STP 1111, J.M. Stoltzfus and K. McIlroy (eds.), American Society 

for Testing and Materials, Philadelphia, PA, pp. 326–337, 1991.

28. Barthelemy, H.; Roy, D.; and Schaaf, J.-P.: “Filters for Use in High Pressure Oxygen Service – 

Qualification and Test Methods,” Flammability and Sensitivity of Materials in Oxygen-Enriched 

Atmospheres: Tenth Volume, ASTM STP 1454, T.A. Steinberg, H.D. Beeson, and B.E. Newton 

(eds.), American Society for Testing and Materials, Philadelphia, PA, pp. 69–74, 2003. 

29. Odom, G.A.; Chiffoleau, G.J.A.; Newton, B.E.; and Fielding, J.R.: “Promoted Ignition Testing of Metallic Filters in High-Pressure Oxygen,” J. ASTM Intl., Vol. 6, No. 8, doi: 10.1520/

JAI102300, 2009.

30. SE-S-0073, Revision H, Space Shuttle Fluid Procurement and Use Control Specification, NASA 

Johnson Space Center, June 18, 2007, and CxP 70156, Revision B, Constellation Program Fluid 

Procurement and Use Control Specification, NASA, May 21, 2010.

31. NFPA 53, Recommended Practice on Materials, Equipment, and Systems Used in Oxygen 

Enriched Atmospheres, National Fire Protection Association, Quincy, MA, 2011 Edition.

32. Lowrey, N.: ESTSG-FY11-01112, An Evaluation of the Response of Nonmetallic Fibers to Cryogenic Exposure, Jacobs ESTS Group, Marshall Space Flight Center, Science and Engineering 

Contract NNM-05AB50C, August 2011.

33. Lowrey, N.: ESTSG-FY12-00079, An Evaluation of the Response of Nonmetallic Fibers 

to Cryogenic Exposure Phase Two: Liquid Oxygen Exposure, Jacobs ESTS Group, Marshall 

Space Flight Center, Science and Engineering Contract NNM-05AB50C, November 2011. 

34. Pedley, M.D.; Pao, J.; Bamford, L.; et al.: “Ignition of Contaminants by Impact of High 

Pressure Oxygen,” Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres, 

ASTM STP 986, D.W. Schroll (ed.), American Society for Testing and Materials, Philadelphia, 

PA, pp. 305–317, 1987.
43

35. Fact Sheet – Flame Resistance of Wool, Commonwealth Scientific and Industrial Research 

Organization (CSIRO), Australia, September 17, 2008. 

36. Lowrie, R.: “Heat of Combustion and Oxygen Compatibility,” Flammability and Sensitivity of 

Materials in Oxygen-Enriched Atmospheres, ASTM STP 812, B.L. Werley (ed.), American Society for Testing and Materials, Philadelphia, PA, pp. 84–96, 1983.

37. Totten, G. (ed.), Handbook of Hydraulic Fluid Technology, Marcel Dekker, Inc., New York, NY, 

ISBN: 0-8247-6022-0, 2000. 

38. Werley, B.L.: Oil Film Hazards in Oxygen Systems, Flammability and Sensitivity of Materials in 

Oxygen-Enriched Atmospheres, ASTM STP 812, B.L. Werley (ed.), American Society for Testing and Materials, p. 114, 1983. 

39. Colson, A.; and Fano, E.: Filtration of Gaseous or Liquid Oxygen in Industrial Applications 

and Associated Fire Risks, Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres, ASTM STP 1479, D. Hirsch, R. Zawiercha, T. Steinberg, and H. Barthelemy (eds.), 

American Society for Testing and Materials, Philadelphia, PA, 2006. 

40. Fano, E.; Faupin, A.; and Barthelemny, H.: Selection of Metal for Oxygen Valves, Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres, ASTM STP 1395, T.A. 

Steinberg, B.E. Newton, and H.D. Beeson (eds.), American Society for Testing and Materials, 

Philadelphia, PA, 2000.
44

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Analysis and Derivation of Allocations for Fiber Contaminants

in Liquid Bipropellant Systems

N.M. Lowrey* and K.Y. Ibrahim

George C. Marshall Space Flight Center

Huntsville, AL 35812

National Aeronautics and Space Administration

Washington, DC 20546–0001

Unclassified-Unlimited

Subject Category 20 

Availability: NASA CASI (443–757–5802)

Prepared by the Materials and Processes Laboratory, Engineering Directorate

*Jacobs Technology, Inc., ESTS Group, Marshall Space Flight Center, Huntsville, AL

M–1335

Technical Publication

NASA/TP—2012–217459

contamination, fuel contamination, particulates, liquid oxygen, liquid hydrogen, cryogenic rocket 

propellants

01–06–2012

UU 56

NASA

U U U

An analysis was performed to identify the engineering rationale for the existing particulate limits in MSFCSPEC-164, Cleanliness of Components for Use in Oxygen, Fuel, and Pneumatic Systems, determine the applicability 

of this rationale to fibers, identify potential risks that may result from fiber contamination in liquid oxygen/fuel 

bipropellant systems, and bound each of these risks. The objective of this analysis was to determine whether fiber 

contamination exceeding the established quantitative limits for particulate can be tolerated in these systems and, 

if so, to derive and recommend quantitative allocations for fibers beyond the limits established for other particulate. Knowledge gaps were identified that limit a complete understanding of the risk of promoted ignition from an 

accumulation of fibers in a gaseous oxygen system. 

STI Help Desk at email: help@sti.nasa.gov

STI Help Desk at: 443–757–5802

National Aeronautics and

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George C. Marshall Space Flight Center

Huntsville, Alabama 35812