Julie Kleinhenz

Glenn Research Center, Cleveland, Ohio

Zeng-Guang Yuan

National Center for Space Exploration, Cleveland, Ohio

An Experimental Study of Upward Burning

Over Long Solid Fuels: Facility Development

and Comparison

NASA/TM—2011-217024

June 2011
NASA STI Program . . . in Profile

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA Scientific and Technical Information (STI) 

program plays a key part in helping NASA maintain 

this important role.

The NASA STI Program operates under the auspices 

of the Agency Chief Information Officer. It collects, 

organizes, provides for archiving, and disseminates 

NASA’s STI. The NASA STI program provides access 

to the NASA Aeronautics and Space Database and 

its public interface, the NASA Technical Reports 

Server, thus providing one of the largest collections 

of aeronautical and space science STI in the world. 

Results are published in both non-NASA channels 

and by NASA in the NASA STI Report Series, which 

includes the following report types:

• TECHNICAL PUBLICATION. Reports of 

completed research or a major significant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of significant 

scientific and technical data and information 

deemed to be of continuing reference value. 

NASA counterpart of peer-reviewed formal 

professional papers but has less stringent 

limitations on manuscript length and extent of 

graphic presentations.

• TECHNICAL MEMORANDUM. Scientific 

and technical findings that are preliminary or 

of specialized interest, e.g., quick release 

reports, working papers, and bibliographies that 

contain minimal annotation. Does not contain 

extensive analysis.

• CONTRACTOR REPORT. Scientific and 

technical findings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scientific and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by NASA.

• SPECIAL PUBLICATION. Scientific, 

technical, or historical information from 

NASA programs, projects, and missions, often 

concerned with subjects having substantial 

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific and 

technical material pertinent to NASA’s mission.

Specialized services also include creating custom 

thesauri, building customized databases, organizing 

and publishing research results.

For more information about the NASA STI 

program, see the following:

• Access the NASA STI program home page at 

http://www.sti.nasa.gov

• E-mail your question via the Internet to help@

sti.nasa.gov

• Fax your question to the NASA STI Help Desk 

at 443–757–5803

• Telephone the NASA STI Help Desk at

443–757–5802

• Write to:

 NASA Center for AeroSpace Information (CASI)

 7115 Standard Drive

 Hanover, MD 21076–1320
Julie Kleinhenz

Glenn Research Center, Cleveland, Ohio

Zeng-Guang Yuan

National Center for Space Exploration, Cleveland, Ohio

An Experimental Study of Upward Burning

Over Long Solid Fuels: Facility Development

and Comparison

NASA/TM—2011-217024

June 2011

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

This project was funded by the Fire Prevention, Detection and Supression (FPDS) project under the NASA Exploration Technology 

Development Program (ETDP). The authors would like to thank NASA GRC FPDS team: Gary Ruff, Paul Ferkul, Sandra Olson, 

Fletcher Miller, and James T’ien, design engineers Kenneth Gregg and Daniel Gotti, and the VF-13 facilities support team.

Co-operative education student Tavis Hall performed most of the stacked-chamber facility tests, John Brooker modeled the flow 

field of the stacked-chamber facility, and Jay Owens supported the imaging setup and data acquisition for the VF-13.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identification 

only. Their usage does not constitute an official endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2011-217024 iii

Contents

1.0 Introduction .......................................................................................................................................... 1

2.0 Hardware .............................................................................................................................................. 2

2.1 Vacuum Facility #13 (VF-13)..................................................................................................... 2

2.1.1 Vacuum Chamber ......................................................................................................... 3

2.1.2 Experimental Rack........................................................................................................ 4

2.1.3 Sample Holder............................................................................................................... 5

2.1.4 Video and Imaging........................................................................................................ 5

2.1.5 Reflecting Mirrors......................................................................................................... 6

2.1.6 Igniter and Igniter Actuator........................................................................................... 6

2.1.7 Gas System.................................................................................................................... 7

2.1.8 Electrical Power............................................................................................................ 7

2.2 Stacked Chamber Facility ........................................................................................................... 8

2.2.1 Combustion Chambers.................................................................................................. 8

2.2.2 Sample Holder............................................................................................................... 9

2.2.3 Gas System.................................................................................................................... 9

2.3 Video and Imaging...................................................................................................................... 9

3.0 Fuel Materials..................................................................................................................................... 11

3.1 Cellulose (Paper)....................................................................................................................... 11

3.2 PMMA ...................................................................................................................................... 12

3.3 Nomex....................................................................................................................................... 12

3.4 Oxygen Consumption ............................................................................................................... 13

3.4.1 VF-13 .......................................................................................................................... 13

3.4.2 Stacked Chamber Facility ........................................................................................... 14

3.5 Toxicity Analysis of Combustion Products.............................................................................. 15

3.6 Test Matrices............................................................................................................................. 15

3.6.1 VF-13 .......................................................................................................................... 15

3.6.2 Stacked-Chamber Facility........................................................................................... 16

4.0 Data Reduction................................................................................................................................... 16

4.1 VF-13 ........................................................................................................................................ 16

4.2 Stacked-Chamber Facility......................................................................................................... 17

5.0 Results................................................................................................................................................ 19

5.1 Nomex III Fabric Tests in VF-13.............................................................................................. 19

5.2 PMMA Plate Tests in VF-13 .................................................................................................... 19

5.3 Paper Fuel Tests........................................................................................................................ 21

5.3.1 Stacked-Chamber Facility—Adding Machine Paper.................................................. 21

5.3.2 Poster Paper—Both Facilities..................................................................................... 23

6.0 Closing Remarks................................................................................................................................ 26

References................................................................................................................................................... 27

Appendix A.—Test Matrix ......................................................................................................................... 29

A.1 Procedures................................................................................................................................. 29

A.1.1 VF-13 .......................................................................................................................... 29

A.1.2 Stacked-Chamber Facility........................................................................................... 30

A.2 Test Log .................................................................................................................................... 31

A.2.3 VF-13 .......................................................................................................................... 31

A.2.4 Stacked Chamber ........................................................................................................ 34
NASA/TM—2011-217024 iv

List of Tables

TABLE 1.—CALCULATIONS SHOWING THE FLOW RATE NEEDED TO COUNTER 

OXYGEN DEPLETION .................................................................................................................... 14

TABLE 2.—TOXICITY FOR THE POTENTIAL EXPERIMENTAL MATERIALS ............................. 15

TABLE 3.—A COMPARISON OF SPREAD RATE IN THE VF-13 FACILITY TO THOSE IN 

THE STACKED-CHAMBER FACILITY......................................................................................... 26

List of Figures

Figure 1.—The VF-13 chamber. Electrical and gas feed-throughs are located at the bottom part of 

the chamber. The white cylinder is the removable cap. ....................................................................... 3

Figure 2.—The experiment rack is mounted on the bottom part of the VF-13 chamber with all the 

components installed. ........................................................................................................................... 4

Figure 3.—Schematics of the experimental rack showing side (left) and top view (right). ......................... 4

Figure 4.—Top view of the arrangement of the aluminum angles and the mirrors. This made it 

possible to image the front view and two side views of the flame with the same set of 

cameras................................................................................................................................................. 5

Figure 5.—A video frame showing a flame in VF-13. The front view is in the center with the two 

mirrors reflecting the side views on either side.................................................................................... 6

Figure 6.—The Glo-stix silicone nitride igniter............................................................................................ 7

Figure 7.—The Stacked-chamber facility. A preexisting combustion facility with all the necessary 

support equipment is at the base with two chambers, identical to the pre-existing one, 

stacked on top to elongate the test section............................................................................................ 8

Figure 8.—A schematic of the stacked-chamber sample holder................................................................... 9

Figure 9.—The Glo-stix igniter on the stacked-chamber sample holder. The tip of the igniter 

contacted the fuel surface..................................................................................................................... 9

Figure 10.—Image of the camera mount looking down from the top of the stacked chamber. The 

LED light strip is shown beside the holes for the camera lenses. The sample card (not 

shown) would be located on the horizontal centerline. The inset image shows the back side 

of the camera mount........................................................................................................................... 10

Figure11.—This graphic shows the dimensions and calculations used to determine the stackedchamber facility imaging set up. Due to the size of the lens, there was significant distortion 

at either end of the image. The useable field of view was 17 cm assuming some overlap 

between the cameras........................................................................................................................... 10

Figure 12.—A schematic showing the video switching system. Three video recorders were used, 

so at any time three of the nine camera views were recorded. Once the flame propagated out 

of field of view, the recorder was switched to the next camera view. For example, the 

switchbox was initially set to cameras 9, 8, and 7 (ignition in camera 9). Once the flame 

exits the field of view in camera 9, the switch C was advanced to camera 6..................................... 11

Figure 13.—An image of a flame in the VF-13 facility. The cross hairs represent the flame 

positions which were tracked. ............................................................................................................ 17

Figure 14.—A schematic of the overlapping method used to mesh the three VF-13 camera views. 

The red and gray markers are visible in two cameras. ....................................................................... 17

Figure 15.—Images from a stacked chamber facility camera showing a flame (left) and scale 

(right). Note: the “fish-eye” distortion effect caused by the lens. ...................................................... 18

Figure 16.—The position dependant scale factor calculated for the image in Figure 14. Equations 

were determined for each of the 9 cameras and applied to the data................................................... 18

Figure 17.—The cross section view of the PMMA sample and the sample holder. Since the fuel 

was thick, two aluminum strips were installed to hold the sample in place and to quench the 

flame edges. The vertical distance between neighboring screws was 30 cm. .................................... 20
NASA/TM—2011-217024 v

Figure 18.—PMMA flame images at three different times after ignition. a) 10 min and 45 sec, 

flame was 60 cm long, b) 13 min and 50 sec, flame was 70 cm, with flame base still 

anchored to fuel bottom edge, and c) 20 min and 38 sec, the flame base advanced 15 cm. .............. 20

Figure 19.—The method of mounting the fuel to the sample holder is illustrated. (A) The original 

method with fuel on one side of the sample holder. (B) The revised method using a cross 

mounting method to better distribute the thermal mass of the sample holder.................................... 21

Figure 20.—Case 1: Position versus time plots for the VF-13 (top) and Stacked Chamber 

(bottom) comparison tests at 5.58 psia and 5 cm fuel width. The linear curve fit equations 

are shown for the steady region, which is highlighted in light gray................................................... 24

Figure 21.—Case 2: Position versus time plots for the VF-13 (top) and Stacked Chamber 

(bottom) comparison tests at 8 psia and 2 cm fuel width. The linear curve fit equations are 

shown for the steady region, which is highlighted in light gray. Note: the stacked chamber 

test at these conditions was the first to be analyzed. Only one flame base position was 

tracked (a visual average position). In all other tests the base position was an average of 

three tracked points. ........................................................................................................................... 25

NASA/TM—2011-217024 1

An Experimental Study of Upward Burning Over Long Solid Fuels: 

Facility Development and Comparison

Julie Kleinhenz

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Zeng-Guang Yuan

National Center for Space Exploration

Cleveland, Ohio 44135

Abstract

As NASA’s mission evolves, new spacecraft and habitat environments necessitate expanded study of 

materials flammability. Most of the upward burning tests to date, including the NASA standard material 

screening method NASA-STD-6001, have been conducted in small chambers where the flame often 

terminates before a steady state flame is established. In real environments, the same limitations may not 

be present. The use of long fuel samples would allow the flames to proceed in an unhindered manner. In 

order to explore sample size and chamber size effects, two large chambers were developed at NASA GRC 

under the Flame Prevention, Detection and Suppression (FPDS) project.

The first was an existing vacuum facility, VF-13, located at NASA John Glenn Research Center. This 

6350 liter chamber could accommodate fuels sample lengths up to 2 m. However, operational costs and 

restricted accessibility limited the test program, so a second laboratory scale facility was developed in 

parallel. By stacking additional two chambers on top of an existing combustion chamber facility, this

81 liter “Stacked-chamber” facility could accommodate a 1.5 m sample length. The larger volume, more 

ideal environment of VF-13 was used to obtain baseline data for comparison with the stacked chamber 

facility. In this way, the stacked chamber facility was intended for long term testing, with VF-13 as the 

proving ground.

Four different solid fuels (adding machine paper, poster paper, PMMA plates, and Nomex fabric) 

were tested with fuel sample lengths up to 2 m. For thin samples (papers) with widths up to 5 cm, the 

flame reached a steady state length, which demonstrates that flame length may be stabilized even when 

the edge effects are reduced. For the thick PMMA plates, flames reached lengths up to 70 cm but were 

highly energetic and restricted by oxygen depletion. Tests with the Nomex fabric confirmed that the 

cyclic flame phenomena, observed in small facility tests, continued over longer sample. New features 

were also observed at the higher oxygen/pressure conditions available in the large chamber. 

Comparison of flame behavior between the two facilities under identical conditions revealed 

disparities, both qualitative and quantitative. This suggests that, in certain ranges of controlling 

parameters, chamber size and shape could be one of the parameters that affect the material flammability. 

If this proves to be true, it may limit the applicability of existing flammability data.

1.0 Introduction

In order to select the best materials suitable for existing and new space vehicles, an understanding of 

material flammability is not only necessary, but crucial. The existing NASA standard, NASA-STD-6001

(Ref. 1), Test 1, has been successfully used for material screening since it was established. However, 

there are limitations to the test method that may cause it to be inadequate in realistic spacecraft scenarios.

In the standard test, an upward burning test apparatus is used to judge materials on a pass/fail basis 

using a fixed sample width and length, 2.5 and 12 in., respectively. If less than half of the total sample

length is burned, and fails to ignite surrounding materials, the material is approved for use. What is not 
NASA/TM—2011-217024 2

addressed is the role of the sample dimensions. Width effects have been studied previously and are known 

to play a role in the flammability and flame spread characteristics (Ref. 2). Flame lengths, even at these 

widths, can grown quite long and exceed a 1 ft sample length. The resulting flow field effects could 

influence propagation length.

Likewise, most of the past upward burning studies (Refs. 3 and 4) were performed in small,

laboratory-scale combustion chambers (~30 L). While 1 ft sample lengths were accommodated, the 

chamber size limitation necessitated narrow (< = 1 in.) samples. This was done to minimize the oxygen 

depletion effects in a closed chamber (without influencing the flame with a forced flow field). Narrower 

samples also mean that edge effects play a larger role, both in terms of heat loss to the sample holder and 

the three dimensionality of the flow field over the flame width. The result is that the flame reaches a 

stable length within the restricted chamber size. Upward burning in large chambers, those in the range of 

hundreds of cubic feet, using wider and thicker fuel has never been reported.

The goal of this study was to explore whether a flame would reach a stable length given ample sample 

length and chamber size. After ignition, the sustainability of an upward burning flame is dictated by the 

energy balance between the heat generated from the flame and that lost through various mechanisms 

including; radiative heat loss to far field or chamber wall, conductive/convective heat loss to surrounding 

gases (if the surrounding space is sufficiently large that the heated air does not flow back to the flame 

region), and heat loss into the fuel thickness. If the losses are too great, the flame may shrink or self 

extinguish. When in balance, the flame should maintain, or oscillate around, a stable length. This is similar 

to the well known case of a vertical gas jet flame. A gas jet flame will reach its maximum flame length at a 

certain fuel flow rate. Any further increase of that flow rate will lead to flickering and/or soot emission of 

the flame, indicating the flame tip is being quenched by heat loss. While there are more losses in the case of 

solid fuel combustion (heat loss to the fuel surface), the same fundamental principle should apply.

Another objective of this study was to understand whether flame self-extinguishing phenomenon is 

material dependent or is it also affected by sample size and chamber volume. This is related to the validity 

of the NASA Standard 6001 test, which implicitly assumes that flame self-extinguishing is a material 

property. The third objective of this study was to verify the cyclic propagation feature of Nomex flames

observed in earlier studies (Ref. 5). Limited by the sample size, the previous study showed no more than 

two cycles of Nomex flames. The last objective was to utilize the controllable oxygen/pressure 

environment to mimic a low gravity environment by reducing buoyancy effect. Flame spread rates in

reduced gravity environments can be estimated based on the pressure scaling method (Ref. 3) at various 

oxygen concentrations. This correlation was verified on-board an aircraft flying parabolic trajectories at 

partial gravity levels.

Two large scale facilities were used in this study; Vacuum Facility (VF) 13 at NASA GRC and a 

laboratory scale facility consisting three stacked combustion chambers (27 L each). The VF-13 chamber

provides a more ideal environment, but its operational costs are high, whereas the stacked chamber

facility is more accessible with a lower operational cost, but is more size-limited. Tests in the two 

facilities were run in parallel so the VF-13 results could be used to validate and define the limitations of 

the stacked chamber facility, where a large number of future studies would be performed.

This document describes these facilities and the tests conducted in them within the project’s time 

allotment. 

2.0 Hardware

2.1 Vacuum Facility #13 (VF-13)

The vacuum facility VF-13 is a large volume chamber designed for tests requiring a high vacuum 

environment. Here, it was utilized to provide a controlled gas environment at reduced pressures (less than 

atmospheric). An experimental rack was built to integrate the test specific components, such as cameras, 
NASA/TM—2011-217024 3

sample holder, igniter, mixing fan, etc. into the facility. Control hardware outside the chamber included a 

chamber gas filling panel, an oxygen bottle, video recorders, and a control panel. These systems will be 

described in detail below.

2.1.1 Vacuum Chamber

The vacuum chamber VF-13 (Fig. 1) is a vertical cylindrical chamber with a 149.9 cm inner diameter 

and 360 cm height. The total internal volume of the chamber is 6.35 m3

. The upper portion of the chamber 

is a removable cap (about 252.1 cm tall) with four viewing windows (~12.5 cm diameter each). The fixed 

base (107.9 cm deep) accommodates the electrical feed-throughs and gas line fittings. Since the chamber 

was originally designed for high vacuum applications, it is equipped with a roughing pump, which can

lower the chamber pressure to 0.03 torr, and a diffusion pump, which can further reduce the chamber 

pressure down to 4×10–7 torr. For this experiment the diffusion pump was not needed. 

Partial pressure gas mixing was used to achieve test conditions in lieu of premixed gas bottles. This 

was more cost effective given the large volume of gas, and offered more flexibility. Only nitrogen (from 

the facility reservoir) and oxygen (from K bottles) mixtures were used. An analog gas control panel was 

built to proportion the gases.

Each gas filling line consisted of two parallel paths; one was for fast filling controlled by a ball valve, 

the other was for slow filling through a metering valve and a ball valve, connected in series. A calibrated, 

precise pressure transducer (Setra, Model 270, 20 psia, ± 0.05 percent full scale accuracy) was used to 

monitor the chamber pressure during the filling process. A HIGHVAC* vacuum pump, model HVP 300 

was installed on the chamber to evacuate the combustion products after each experiment run. The unique 

feature of this vacuum pump was that it had no moving parts in it, thus use of this pump could prevent the 

roughing pump from possible corrosive damage from the combustion products after each test run.

Figure 1.—The VF-13 chamber. Electrical and 

gas feed-throughs are located at the bottom 

part of the chamber. The white cylinder is the 

removable cap.
NASA/TM—2011-217024 4

2.1.2 Experimental Rack

The frame of the experimental rack was made of aluminum BOSCH REXROTH* bars (45 Series 

Profiles with 10 mm T slots) with overall dimension of 229 cm high, 69 cm wide, and 125 cm long. The

experimental components mounted on the rack included a removable sample holder, two reflecting 

mirrors, an igniter with a pneumatic actuator, a mixing fan, three video cameras and a long green LED 

strip for illumination. Figure 2 shows the rack assembly in the VF-13 chamber with the lid removed and 

Figure 3 shows a schematic layout.

Figure 2.—The experiment rack is mounted on the 

bottom part of the VF-13 chamber with all the 

components installed. 

Figure 3.—Schematics of the experimental rack showing side (left) and top view (right).

LED Strip 

(on the back 

side of the bar)

Video

Cameras

Reflecting

Mirrors

Igniter 

w/ actuator

Mixing

Fan

Sample 

Holder LED Strip 

(on the back 

side of the bar)

Video

Cameras

Reflecting

Mirrors

Igniter 

w/ actuator

Mixing

Fan

Sample 

Holder
NASA/TM—2011-217024 5

Figure 4.—Top view of the arrangement of the aluminum angles 

and the mirrors. This made it possible to image the front view 

and two side views of the flame with the same set of cameras.

2.1.3 Sample Holder

The sample holder consisted of two parallel pieces of angled aluminum bars (37 by 50 by 1950 mm) 

connected by two cross plates at the top and bottom. Conical lugs on the cross plates mated to dovetail 

slots on the experimental rack for easy mounting. The gap between the parallel bars could be adjusted to 

distances of 1, 2, 5, and 10 cm using set screws. This gap governed the burnable width of the samples, 

which were attached to the bars using adhesive aluminum tape (for thin flexible materials). Thick, rigid 

materials were directly attached to the holder using screws. 

Figure 4 shows a top view schematic of the bars with the sample mounted. The vertical edges of the 

two aluminum angles faced opposite directions to accommodate the edge (side) views of the fuel sample. 

Two mirrors were positioned so that one displayed the side view of the rear flame and while the other 

displayed the side view of the front flame. In this manner, the same camera could image both edges as 

well as the front of the fuel sample.

Note: In several tests (indicated by and (*) in Section A.2.3) the sample card from the stacked chamber 

facility was used in VF-13. This was done to reduce the number of variables between the facilities.

2.1.4 Video and Imaging

Imaging the full 1.8 m viewable sample length posed some challenges. Cameras had to be mounted 

inside the vacuum vessel since the available window ports were not adequate to cover the sample length. 

The cameras were positioned as far from the sample as possible (850 mm) to avoid heat exposure. Three 

identical analog color CCD video cameras (Panasonic* VW-CP352) were used to record flame images. 

The cameras were mounted with the long side of the 6.4 by 4.8 mm CCD sensor in the vertical direction, 

to obtain a larger field of view. Using 12 mm lenses, each camera viewed 627 mm of the sample, which 

accounts for the full sample length with some overlap.

A time code generator was used to synchronize the three cameras and produce a time stamp in the video 

recordings. The signal from each camera went to a video amplifier which had two output ports. One port 

was connected to a DVCAM* tape recorder and the other to a designated computer that could perform realtime digitization of video image. The DVCAM* recording served as a backup for the digital system. 

Green LED lights were installed inside the chamber to illuminate the pyrolysis front of the fuel. The 

green light was found to illuminate the fuel without saturating the flame front. Six LED strips were 

mounted to the camera boom. Each strip was 1 ft long and 1/2 in. wide printed circuit board, which could 

be separated into four basic sections, of three LEDs each. Connectors located at the two ends of each strip 

allowed the individual strips to be connected together, and powered in parallel. Thus, they all shared a

12 VDC power source where each additional strip would increase current draw from the power source. 

Camera

Front view

Mirror

Mirror

Side view of rear flame

Side view of front flame

Rear flame

Front flame

Al. angle

Al. angle Fuel sample

Camera

Front view

Mirror

Mirror

Side view of rear flame

Side view of front flame

Rear flame

Front flame

Al. angle

Al. angle Fuel sample
NASA/TM—2011-217024 6

Figure 5.—A video frame showing a flame in VF-13. 

The front view is in the center with the two mirrors 

reflecting the side views on either side.

The wavelength of the green LED was 525 nm and the luminous intensity of each LED was

3000 millicandela (mcd) with 90° view angle. In this experiment, a total of 72 LEDs were used. They 

provided sufficient luminosity on the fuel example. 

2.1.5 Reflecting Mirrors

Mirrors were used so that front and side views of the sample could be obtained using a single camera. 

The angle bars used to construct the sample cards provided structural support, but occluded the view of 

one side of the sample, as shown in Figure 4. The mirrors were oriented parallel to the sample and 

positioned to minimize heat exposure while remaining within the camera field of view. However, this 

distance and the angle relative to the sample holder could be adjusted to ensure proper imaging. The 

width of the camera field of view was 460 mm, therefore the distance from the center of the mirror to the 

center of the fuel sample was 183 mm. Because the distance of the mirrors to the camera was offset, these 

side view images were slightly out of focus.

2.1.6 Igniter and Igniter Actuator

The igniter used in VF-13 was a 24 VDC, 50 W Glo-stix* igniter, which had a 1 cm long silicon 

nitride heating tip and an insulated mounting plate (Fig. 6). Since silicon nitride does not react with 

oxygen when heated, it can be used repeatedly without deteriorating. 
NASA/TM—2011-217024 7

Figure 6.—The Glo-stix silicone nitride igniter.

The igniter reached its stable temperature (1200 °C) in 10 to 15 sec using the existing power supply. 

A pneumatic piston actuator controlled the position of the igniter so that it only contacted the sample after 

reaching its full temperature. If advanced prior to this, the slow heat up would create a pyrolysis hole in 

the fuel and there would be insufficient fuel in the igniter region to trigger an ignition. The actuator was 

driven by low pressure nitrogen gas. During the warm up period, the igniter remained about 3 cm from 

the fuel surface. When actuated, the igniter contacted the fuel surface, with slight over drive to ensure 

firm contact between the igniter and the fuel surface.

2.1.7 Gas System

Partial pressure mixing was used to obtain the desired gas concentration in the chamber. Because the 

oxygen and nitrogen are filled separately, there is potential for a non-uniform gas mixture. Pure molecular 

diffusion is a very slow process. Thus, a 550 cfm, 12 to 28 VDC fan with brushless motor was installed in 

the chamber to promote the mixing process. The capacity of the fan was large enough to circulate the 

entire chamber air 2.5 times per minute under standard conditions. The fan was run for 10 min after gas 

fill, followed by 2 hr of settling time before the test was initiated.

The gas filling procedure was also designed to promote gas mixing. The gas feed port was located at 

the bottom of the chamber. Thus, by filling the lighter gas (nitrogen) after the oxygen, the upward 

buoyant flow would encourage mixing. Inside the chamber, the gas feeding tube was installed tangentially 

to the chamber wall and slightly tilted upward. Thus the gas flow into the chamber would follow a spiral 

path, which would help mixing. 

2.1.8 Electrical Power

Three Tektronix* PS280 DC Dual power supplies were used for a total of six power supply units. The 

maximum capacity of each unit was 2 A and 30 VDC. All power units were operated independently to 

power three cameras, the igniter, the mixing fan, the LEDs, and the pressure transducer. 

A switch box was built to control the electric devices inside the chamber; each of the three cameras 

had its own designated switch, as well as the LED strip, the mixing fan and the igniter. The input ports 

were connected to the DC power supplies, while the output cable connected to the devices inside the 

chamber via an electric feed-through.

Metal mounting

plate

Ceramic insulation

heating tip

Electric wires
NASA/TM—2011-217024 8

2.2 Stacked Chamber Facility

Though the VF-13 facility was well suited to the project’s test objectives, it is expensive to operate

and has limited availability. Therefore, a smaller volume, laboratory scale facility was developed for 

longer term test objectives. However, size restrictions of this smaller facility limited the range of test 

conditions in terms of sample width and environmental conditions. (Higher oxygen and/pressure 

conditions would lead to more energetic and larger flames. Heat loss to hardware and local oxygen 

depletion may result from the restricted volume). Data from the larger, less restricted, VF-13 were used to 

define the test range. If the results from the smaller facility compared well to VF-13, then an acceptable 

test condition in the stacked chamber facility was achieved.

2.2.1 Combustion Chambers

The combustion chamber for this facility (Fig. 7) consisted of three identical chambers stacked 

vertically. Each chamber was 53.7 cm tall and 25.4 cm in diameter (27 L), resulting in a stacked height of 

161 cm (81 L). The bottom chamber was contained in a pre-existing combustion facility known as the 

Spacecraft Fire Safety Facility (SFSF), which was frequently used for low gravity testing aboard the 

NASA reduced gravity aircraft. This facility was already setup with an ignition system, instrumentation 

(pressure transducers, thermocouples, etc.), video recording, gas flow, control and data acquisition 

devices, and a designated computer with operational software. Thus the addition of the chambers to this 

facility was a simple, low cost, solution to achieve the test objectives.

Figure 7.—The Stacked-chamber facility. A 

preexisting combustion facility with all the 

necessary support equipment is at the base 

with two chambers, identical to the preexisting one, stacked on top to elongate the 

test section.
NASA/TM—2011-217024 9

 

Figure 8.—A schematic

of the stacked-chamber

sample holder.

2.2.2 Sample Holder

The fuel sample was held on a thin metal card set in the middle of the chambers. This holder had two 

parallel aluminum rails with a 1 by 1/8 in. cross section area, connected by two crossbars at ends, and is 

shown schematically in Figure 8. The sample was taped to the rails such that the exposed area between 

was the burnable sample. The sample width could be adjusted to 1, 2, or 5 cm, and the maximum length 

was 1.5 m. Metal tape was used to affix the sample and prevent edge flames. A reusable glow plug 

resistance heater was used for ignition (Fig. 9) and placed such that its tip was in direct contact with the 

sample near its lower edge. An identical igniter was used in the VF-13 (Fig. 6), though the stackedchamber facility igniter could not be retracted.

2.2.3 Gas System

Unlike the VF-13 facility, the stacked-chamber facility had a relatively narrow diameter, and thus a 

limited volume where oxygen depletion could be significant. To mitigate this effect, fresh oxidizer was 

flowed through the chambers at low speed. These gas flows were less than 10 cm/s, which was high 

enough to counter depletion (see Section 3.4.2 for the calculations) but well below the estimated buoyant 

flow velocity. The inlet flow was supplied by a gas cylinder (type K) containing the desired premixed 

oxygen/nitrogen concentrations, and regulated by a mass flow controller. Screens and honeycombs in the 

base of the bottommost chamber assured a uniform flow entering the chambers. A vacuum pump on the 

exit line drew the flow through a port at the center of the lid at the top of the stack. A backpressure valve 

which regulated the chamber pressure (3 to 14 psia) was autonomously controlled based on feedback 

from the chamber’s pressure transducer.

2.3 Video and Imaging

As with the VF-13, the primary data acquisition was visual flame images. Due to the large aspect 

ratio of the chamber, video imaging was particularly difficult. While the individual chambers had 

windows, they were not spaced to allow for viewing of the entire sample. Therefore, the cameras had to 

be mounted inside the chamber (Fig. 10). The sample was mounted in the center plane of the chamber, 

whose radius was 12 cm. Allowing room for a camera mounting bracket, the distance between the lens 

and the fuel was 8.3 cm. Thus, nine cameras, each with a 2.5 mm lens, were needed to cover the total 

sample length with some field of view overlap (Fig. 11). Green LED lighting strips, identical to those in 

the VF-13 facility were installed to illuminate the sample.

Figure 9.—The Glo-stix igniter on the stacked-chamber 

sample holder. The tip of the igniter contacted the fuel 

surface.
NASA/TM—2011-217024 10

Figure 10.—Image of the camera mount looking down 

from the top of the stacked chamber. The LED light 

strip is shown beside the holes for the camera lenses. 

The sample card (not shown) would be located on the 

horizontal centerline. The inset image shows the back 

side of the camera mount.

Figure11.—This graphic shows the dimensions and calculations used to determine the 

stacked-chamber facility imaging set up. Due to the size of the lens, there was 

significant distortion at either end of the image. The useable field of view was 17 cm 

assuming some overlap between the cameras.

Chip f

D H •













 =

window

12.5cm

camera

4.2cm

D=8.3cm

H

cm Cameras 153.7 # =

H

f= focal length (lens)

Chip = Sensor dimension

D=8.3cm

Microboard Camera: Polaris* USA, MB-1850, 

Chip: 1/3 “ => 4.8mm width

So for a 2.5mm microboard lens

(smallest f available):

Hcalc = 16cm

Hmeasured= 17cm

#camera = 9 (no overlap)
NASA/TM—2011-217024 11

Figure 12.—A schematic showing the video switching system. Three video recorders were 

used, so at any time three of the nine camera views were recorded. Once the flame 

propagated out of field of view, the recorder was switched to the next camera view. For 

example, the switchbox was initially set to cameras 9, 8, and 7 (ignition in camera 9). Once 

the flame exits the field of view in camera 9, the switch C was advanced to camera 6. 

The nine camera signals were sent to three digital video recorders. A manual switchbox was used to

toggle between camera views as the flame advanced. Figure 12 shows the arrangement of the cameras and 

the recorders. Ignition occurred in the field of view of camera 9, so the switch box was initially set: C:9, 

B:8, and A:7 (where the letter designates the recorder and the number designates the camera). Once the 

flame base propagated out view, switch box C was set to camera 6, and so on. The assumption was that 

the flame length would not exceed the field of view three cameras. This arrangement saved cost (fewer 

recorders and time synchronization signals).

3.0 Fuel Materials

Three types of fuel materials were tested in this project, all are hydrocarbons so the products were

primarily water and CO2, with trace amounts of CO. These materials all have some prior testing in an 

upward flame spread configuration.

3.1 Cellulose (Paper)

Paper fuels have been traditionally used in combustion experiments because they are widely 

available, used in many applications, and they typically burn out (little to no material remaining after 

flame spread). The primary products of combustion are H2O and CO2, with trace amounts of CO.

C H O 6O 6CO 5H O 6 10 5 2 2 2 +⇒ + (1)

They are also convenient for low gravity experimentation because they ignite and spread quickly. 

This is conducive to the short test durations available in the ground based microgravity facilities (drop 
NASA/TM—2011-217024 12

towers and reduced gravity aircraft). Paper fuels were chosen in this application to make use of the 

existing knowledge and data. Many of the previous studies (Refs. 3 and 4) utilized Kimwipe paper (thin 

tissue paper used as laboratory wipes). However this material is only fabricated in 42 by 38 cm sheets. A 

continuous 2 m long, seamless sample was needed to make full use of the facilities.

The first set of tests were performed using adding machine paper which has an area density of

6.3 mg/cm2

. Adding machine paper was a convenient choice since it is fabricated in long strips and was 

used in the study by Chu (Ref. 2). However, some unexpected flame behaviors were observed with this 

material, particularly in the stacked-chamber facility (Section 5.3.1). Since the main goal of testing was 

facility comparison, as opposed to fuel property characterization, a better performing material was found.

The new paper material was a 7 mg/cm2 poster paper. This material is manufactured by Bienfang,

part #321-128, is trade named “Brushmaster White Poster paper.” It has a basis weight of 18 lb and is 

manufactured in a long, seamless lengths.

3.2 PMMA

Polymethyl methacrylate (PMMA) is a plastic material that can be formed into a variety of 

thicknesses. Therefore, this material was used to study sample thickness effects in VF-13. Clear cast 

PMMA plates from Professional Plastics 5.59 mm thick were used for this experiment. The primary 

products of combustion are H2O and CO2, with trace amounts of CO and MMA.

C H O 6O 5CO 4H O 582 2 2 2 +⇒ + (2)

3.3 Nomex

Nomex fabric is widely used in industries, including aviation and spacecraft, as a fire protection 

material, because it is not flammable at standard atmospheric conditions. However it is advantageous to 

employ higher oxygen, lower pressure conditions in spacecraft. To ensure health and comfort of the crew, 

the conditions are typically normoxic, where the oxygen partial pressure is the same as at sea level. At 

these conditions Nomex can become flammable (Ref. 5). Nomex is not completely consumed by flame;

the remaining charred material is brittle, but retains structure. The primary products of combustion are

H2O and CO2, with trace amounts of CO and N2O.

14 10 2 2 2 2 2 2 2C H O N 31O 28CO 10H O 2N +⇒ + + (3)

Different blends and weaves of Nomex fabric are available. The blend used in this study is called 

Nomex III, which contains 92 percent Nomex, 5 percent Kevlar, and 3 percent p-140 (carbon based antistatic material). This is the blend commonly used in flight suits. The weave was a 0.18 mm thick fabric 

with an area density of 11 mg/cm2 and a thread count of 30×28 threads per centimeter.

Previous studies of this material (Ref. 5)showed unique cyclic flame phenomena, whereby 

continually elongating flames would break into two smaller propagating flames. Oxygen shadowing and 

heat loss mechanisms would drive one flame to extinction, leaving the remaining flame to elongate and 

repeat the cycle. This phenomenon was attributed to a two stage pyrolysis resulting from the material 

blend (the break off occurs when the lower temperature pyrolysis is triggered by the flame from the 

higher temperature reaction). The smaller chamber used in the previous study limited the test conditions 

and number of cycles that could be observed.
NASA/TM—2011-217024 13

3.4 Oxygen Consumption

3.4.1 VF-13

Determination of flame spread rates requires well defined chamber conditions, i.e., the chamber 

pressure, temperature and oxygen concentration. However, VF-13 is a sealed chamber with no flow into 

or out of the system during combustion, so the oxygen is depleted as the flame propagates. In order to 

have a meaningful value of the flame spread rate, the oxygen depletion should be less than 10 percent of 

its initial amount. This requirement was maintained by setting a limit to the fuel mass that could be 

burned. The maximum allowable fuel mass that was calculated using the following equation.

i oi

fa

VPC M

m

RT

η = ε

(4)

mfa maximum amount of allowable mass of fuel consumed in a test before the oxygen depletion in the 

chamber reaches the allowable limit, kg,

η maximum amount of O2 allowed to be consumed in a test divided by the initial amount of O2 in 

the chamber, dimensionless,

V The total volume of the chamber, 6.35 m3

,

Pi The initial total pressure in the chamber, Pa,

Coi The initial mole concentration of O2 in the chamber, dimensionless,

M Molecular weight of the fuel, kg/kMole, 162.1 for cellulose, 100 for PMMA and 238 for Nomex,

ε Number of moles of O2 divided by number of moles of the fuel reacting with the O2 in a complete 

combustion condition, dimensionless, 6 for celluloses (paper) and PMMA, 14 for Nomex fabric,

R Universal gas constant, 8.314 kJ/kMol/K,

T Initial temperature of the gas in the chamber, 300 K.

The initial chamber pressure and the volumetric O2 concentration were the two independent variables in 

the equation.

m r PC fa f i oi = (5)

where rf is a constant depending on fuel type with the unit to be kg/Pa.

6.878 6 for Cellulose

4.243 6 for PMMA

4.328 6 for Nomex

f

E

r E

E

 −  = − 



 −

(6)

For example, in the case of a Nomex sheet in the initial chamber conditions of 9 psia (or 62053 Pa) total 

pressure and 30 percent O2 concentration, the maximum mass of Nomex that could be burnt without 

consuming more than 10 percent of original amount of O2 in the chamber would be

mfa = 4.328E-6*62053*0.3 = 80.57E-3 kg = 80.6 g (7)

In most tests with thin fuel sheets, the oxygen consumption was less than 10 percent of the initial amount. 

The above equation could also be used to check the validity of the test result, i.e., to see if the flame 

spread rate observed in test was within the oxygen variation limit.

Note: Since the chamber was closed, during combustion, the solid fuel that was consumed changed

from solid phase to gas phase, resulting in an increase of total mole number in the gas phase. Thus

10 percent oxygen consumption should not be confused with 10 percent reduction of oxygen 

concentration.
NASA/TM—2011-217024 14

3.4.2 Stacked Chamber Facility

In the smaller volume of the stacked-chamber facility a fresh oxidizer flow was needed to counter 

oxygen depletion. In order to maintain a buoyancy dominant flow field, the added oxidizer flow was kept 

low, under 10 cm/s. The following simple calculations were used to verify that the flow needed to counter 

depletion was below this value. The oxygen depletion rate was calculated assuming stoichiometric 

reaction

molesox_deplete =γ∗molesfuel_consumed (8)

where γ is the mole ratio of oxygen to fuel.

The rate of fuel consumption is found using the flame spread rate and sample dimensions. The flame 

spread rates were approximated using prior test data (Refs. 3 and 5).

m = ρ v wτ fuel_consumed fuel f  (9)

Where ρfuel is the fuel density, vf is the flame spread rate, w and τ are the width and thickness of the fuel 

sample. 

The flow of oxidizer into the facility is calculated

( ) ox air chamber mox _ in = f ρ vA •

(10)

where fox is the oxygen mole fraction fill gas, ρair and v are the density and velocity of the fill gas, and 

Achamber is the cross-sectional area of the chamber. Combining these equations and solving for velocity:

( )

fule air chamber ox

ox fuel

MW A f

MW v w

v f

ρ

ρ τ γ = (11)

Table 1 shows example calculations for a typical test conditions. For most tests a flow of 5 cm/s was used.

TABLE 1.—CALCULATIONS SHOWING THE FLOW RATE NEEDED

TO COUNTER OXYGEN DEPLETION

Cellulose Nomex PMMA

Spread rate 0.427 cm/s 1.12 cm/s 0.5 cm/s

fuel area density 0.00185 g/cm2 0.011 g/cm2 0.119 g/cm2

Thickness 0.031 cm 0.018 cm 0.1 cm

fuel width 2 cm 2 cm 2 cm 

Oxygen fraction 0.21 0.21 0.21

MW fuel 162.14 g/mol 238.16 g/mol 101.1 g/mol

MW O2 32 g/mol 32 g/mol 32 g/mol

Pressure 10 psia 10 psia 10 psia

0.680 atm 0.680 atm 0.680 atm

density fill air 0.000802 g/cm3 0.0008023 g/cm3 0.00080229 g/cm3

Area of chamber cross se 506.71 cm2 506.71 cm2 506.71 cm2

mol Ox/ mol fuel 6 15.5 6

mass flow ox needed 0.00 g/s 0.05 g/s 0.23 g/s

velocity needed to 0.02 cm/s 0.60 cm/s 2.65 cm/s

counter ox consumption
NASA/TM—2011-217024 15

3.5 Toxicity Analysis of Combustion Products

A toxicity analysis was performed to meet safety regulations. The primary toxic byproduct of all three 

fuels was CO2. Table 2 shows the CO2 generation for the “worst case” experimental scenarios, those in

which the maximum possible sample dimensions were used and in which complete consumption of either 

the material or oxygen (40 percent) (whichever occurs first) was assumed. These were very conservative 

measurements since the materials were not completely consumed, and the flame would self-extinguish 

long before all of the oxygen was consumed. The actual tests conducted never reached these extreme 

conditions. The results show that only PMMA exceeds the OSHA PEL for CO2 of 5000 ppm. In all tests, 

the combustion products were vented outside the building prior to opening the chamber, so the 

concentration of CO2 in the room would never reach OSHA’s limit.

TABLE 2.—TOXICITY FOR THE POTENTIAL EXPERIMENTAL MATERIALS

Cellulose PMMA Nomex

Sample size (cm) 185 by 5 by 0.031 185 by 5 by 0.56 185 by 5 by 0.018

Density (g/cm3) 0.597 1.19 0.611

CO2 Produced

(ppm in chamber)

2637 115110 2500

CO2 Produced

(grams released)

28 1312 27

3.6 Test Matrices

3.6.1 VF-13

The tests encompassed three different goals, listed below. Refer to Section A.2 for the log of all tests 

performed in the VF-13 chamber.

1. Obtain baseline data to use for comparison with a smaller volume chamber known as the stackedchamber facility. The geometry of the stacked-chamber facility poses some limitations of the 

flame size (including oxygen depletion concerns, and sample width restrictions). These 

limitations could be explored by comparing the results from the two facilities, where the more 

ideal VF-13 environment was used as the baseline. Paper fuel samples were used for this 

comparison.

a. Early tests were done in a small (27 L) chamber of the GIFFTS (Gravitational Influences 

on Flammability and Flame spread Test System) facility. A significant amount of reduced 

gravity and ground based tests were done using this facility. Due to the small size, most 

fuel samples were 2 cm in width or less. Several tests were done in VF-13 were done at 

these conditions for comparison.

b. Previous work (Ref. 3) showed that spread rates of reduced gravity flames could be 

demonstrated in normal gravity by reducing the pressure environment. This 

pressure/gravity correlation was used in several tests in VF-13 for comparison.

2. Examine flame spread over long samples of Nomex III fabric. Earlier testing performed in small 

(27 L) chambers indicated a cyclic flame phenomena (Ref. 5). Using the large VF-13 facility, the 

full extent of this phenomena could be explored; including longer sample lengths and higher 

oxygen mole fractions.

3. Explore the steady state flame lengths of thick fuel samples. With the large chamber volume, the 

flames were free to reach their full lengths without hindrance. Samples of 0.5 cm thick PMMA 

were used.

This report primarily covers objective #1, the comparison of the two facilities. The results from the 

Nomex III fabric tests (objective #2) will be briefly mentioned in this report for completeness. A later 

publication will provide detailed description of the cyclic flame phenomena observed in VF-13 facility. 

Only a limited number of thick fuels tests (objective #3) were performed and will be addressed here. This 

objective cannot be met using the current configurations, hardware modifications would be required.
NASA/TM—2011-217024 16

3.6.2 Stacked-Chamber Facility

The test matrix for the stacked-chamber facility was more free-form than that of the VF-13 facility. 

This laboratory facility was easily accessible, inexpensive, and could be operated quickly on an as needed 

basis. Section A.2.4 shows the log of all tests performed in the facility during this program.

The goal was to obtain spread rate data of paper samples for comparison with VF-13 tests. However, 

several phenomena were noticed that could not be readily explained (covered in more detail in

Section 5.3.1). Many different hardware configurations and fuel samples were used to explore these 

phenomena.

4.0 Data Reduction

The primary metric for these tests was flame spread rate. This was determined by tracking the flame 

position in the video frame by frame using an image processing software called “spotlight” (Ref. 8). The 

output of the software was a pixel location per time, which would then be converted to distance using a 

scale factor. A scale in the video field of view was used to obtain these factors. This process is well 

documented (Ref. 9).

Unlike previous studies, multiple cameras were needed to capture the entire sample length. These 

flame positions had to be synchronized between the camera views, taking into account field of view 

overlap and (especially in the case of the stacked-chamber facility) lens distortion. The following sections 

describe the procedure for these data reductions. All the post processing was performed using Microsoft 

Excel (Microsoft Corporation), with customized macros to perform the data merging.

4.1 VF-13

Due to cracking in the paper fuel, especially over the 5 cm width, it was difficult to identify a single 

flame base location. To obtain a representative value, the average of three locations was used; the leading 

edge of the crack, the flame along the right edge of the sample, and the flame along the left edge.

Figure 13 shows an image identifying these locations as well as the pyrolysis front.

In order to merge the three camera views, a visual marker was placed near the junction of the camera 

views. This marker, which could be seen in two cameras, defined the overlap in the field of views

(Fig. 14). The length of each camera field of view was 640 pixels, where the origin is at the top. (Recall 

that the cameras were positioned sideways to mazimize the field of view). The gray colored marker in 

Figure 14 was located at pixel position 500 in camera 1 and at pixel position 70 in camera 2. Therefore 

there the first 70 pixels of camera 2 are the same image as the last 140 pixels in camera 1. Accounting for 

the total overlap, there are a total of 1645 unique pixels in the system reference frame.

To merge the camera views, the pixel position of each individual camera had to be converted to the 

system coordinates. Equations were developed that could be applied to the tracking file of each camera. In 

the following equations “pix” is the location (pixels) of the flame at a given time step, in a given camera 

reference frame. The result of the equation is the flame location (centimeters) in the system frame of 

reference (where the origin is the bottom of camera 3). The terms “OL” are the overlap positions (pixels) 

as defined in Figure 14. Finally, “SF” are the scale factor (pixels/cm) for each camera.

Camera 3: position (cm) 640

3

3 = −

SF

OL

Camera 2: position (cm) 640

2

2 3

3

3 = − + − −

SF

OL pix

SF

OL (12)

Camera 1: position (cm) 640

1

1

2

2 3 2 1

3

3 = − + − + − − −

SF

OL pix

SF

OL OL

SF

OL
NASA/TM—2011-217024 17

Figure 13.—An image of a 

flame in the VF-13 facility. 

The cross hairs represent 

the flame positions which 

were tracked.

Figure 14.—A schematic of 

the overlapping method 

used to mesh the three

VF-13 camera views. The 

red and gray markers are 

visible in two cameras.

In addition to the flame position, the time stamp from the tracking files also had to be merged 

appropriately. The video recorded at 30 Hz and since the tracking file logged the video frame number for 

each pixel position, the elapsed time could be calculated. The system time was available as a 

synchronized time stamp in all three camera views. The time stamp in the first tracked video frame was 

used as the reference point to convert elapsed time to system time.

4.2 Stacked-Chamber Facility

A similar procedure was used to reduce the stacked-chamber facility data, but with an added 

complication in the scale factors. The 2.5 mm lens permitted a wide field of view, but also distorted the 

image severely (Fig. 15). This meant that the scale factor was dependant on the pixel position, instead of 

being uniform over the entire field of view. 
NASA/TM—2011-217024 18

Figure 15.—Images from a stacked chamber facility camera showing 

a flame (left) and scale (right). Note: the “fish-eye” distortion effect 

caused by the lens.

Figure 16.—The position dependant scale factor calculated for the image in Figure 14. 

Equations were determined for each of the 9 cameras and applied to the data.

A tape measure was placed along the entire length the sample card. Images were taken with each 

camera, like Figure 15, which were used to determine the scale factor for each camera. The pixel 

positions of the scale marks were recorded (for example the 66 in. mark in Figure 15 corresponds to pixel 

location of 360). These pixel positions were converted to system wide coordinates using the same method 

used for VF-13. In this case the overlap markers were the tape measure marks. Equations (12) were used 

without the scale factor (neglect the “SF” terms in the denominator) so that the result is a system pixel 

position instead of centimeters. This pixel position was plotted against its corresponding tape measure 

position (cm) and curve fit to obtain the position dependant scale factor equation. Figure 16 shows an 

example of this. The resulting scale factor equations can be applied directly to the tracking data to give a 

flame distance in system wide coordinates.

Temporally, the data was merged using the same time stamp method as with VF-13.
NASA/TM—2011-217024 19

5.0 Results

5.1 Nomex III Fabric Tests in VF-13

As discussed in Section 3.3, Nomex III fabric is a fire protection material, that not only becomes 

flammable at elevated oxygen conditions, but also demonstrates a unique cyclic flame phenomena during 

upward burning. The flame elongates, separates into two flames. One flame ultimately blows off, while 

the other continues, and repeats the cycle. Limited by sample length, only one or two cycles were 

observed in those earlier tests.

VF-13 chamber provided an ideal environment to investigate this phenomenon further. A total of

8 tests with Nomex III fabric were performed in VF-13 with various oxygen concentrations and chamber 

pressures. The results of those tests confirmed that the cyclic dual-flame behavior continued over the long 

samples. Additionally the cyclic behavior demonstrated two different modes: continuous-lower-flame 

mode and continuous-upper-flame mode. In the former, the lower flame consistently survived after the 

upper flame blew off. In the later, the lower flame consistantly blew off while the upper flame stabilized. 

A test condition of 9 psia and 30 percent oxygen concentration seemed to be the turning point, below 

which the flame tended toward the continuous-lower-flame mode and above was the continuous-upperflame mode. 

Details of the Nomex III tests will not be presented here. The main purpose of this report is to 

compare the tests performed in VF-13 and stacked-chamber facilities. Nomex was not used in the 

stacked-chamber facility. A detailed description and analysis of the Nomex III tests in the VF-13 chamber 

will be documented in a separate publication.

5.2 PMMA Plate Tests in VF-13

Cast PMMA, i.e., poly(methyl methacrylate), plates have been used widely in solid fuel combustion 

research because of several unique features the material possesses. It can be made clear or opaque and can 

be burned completely without charring and with little to no dripping. PMMA was used in the VF-13

chamber to see if the flame length of an upward burning flame over a thermally thick fuel could reach its 

steady state flame length. A flame that reaches a stable flame length can be treated as a steady state 

combustion problem in the fixed flame coordinates. The flame front, the pyrolysis front, the flame base 

and all the temperature or concentration profiles would travel at the same speed along the fuel, thus 

simplifying modeling efforts.

Two identical PMMA fuel samples were burned in the VF-13 chamber. The sample dimensions were 

5.59 mm thick, 70 mm wide (50 mm burnable width) and 1850 mm long. The sample length consisted of 

two PMMA sheet butted together; the bottom sheet was 1219 mm long, whereas the top was 631 mm. 

Figure 17 shows the arrangement of the fuel sample and the sample holder. The sample holder width 

was set to 50 mm, with extra fuel material (10 mm on each side) used to mount the fuel to the sample 

holder with screws. The chamber conditions for the two tests were 21 percent O2 concentration and

5.58 psia (i.e., the lunar equivalent condition), and 30 percent O2 concentration and 10.29 psia

(the normoxic condition of 30 percent O2).

In the first run, the lunar equivalent condition, the flame grew continually after ignition. It took 

almost 15 min for the fuel to burn out at bottom edge. Figure 18(a) was taken at 10 min and 45 sec after 

ignition at which time the flame was 60 cm long. Figure 8(b) was taken at 13 min and 50 sec where the 

flame length was over 70 cm, with the flame base still anchored at the bottom edge of the fuel. In

Figure 8(c), 20 min and 38 sec after ignition, the flame base had advanced 15 cm from the bottom of the 

fuel. Significant oxygen depletion caused the flame to weaken and became very blue, thus preventing it 

from achieving a steady state length. Then the flame extinguished soon afterward.
NASA/TM—2011-217024 20

Figure 17.—The cross section view of the PMMA sample and the 

sample holder. Since the fuel was thick, two aluminum strips were 

installed to hold the sample in place and to quench the flame edges. 

The vertical distance between neighboring screws was 30 cm.

Figure 18.—PMMA flame images at three different times after ignition. a) 10 min and 45 sec, 

flame was 60 cm long, b) 13 min and 50 sec, flame was 70 cm, with flame base still anchored

to fuel bottom edge, and c) 20 min and 38 sec, the flame base advanced 15 cm.

Post test stoichiometric calculations show that 136.64 g or 1.366 g-moles PMMA fuel were

consumed, with a corresponding O2 consumption of 8.2 moles (1.366*6). The original oxygen in the 

chamber was 20.6 gr-mole, so about 40 percent of the oxygen was consumed leaving 12.6 percent mole 

fraction at the end of test. The actual oxygen concentration near the fuel surface would have been even 

lower. With such low oxygen concentration, flame extinction was inevitable.

The second PMMA test was performed at 30 percent oxygen and 10.29 psia, a point on the normoxic 

curve. Since the oxygen mole number was much higher than the first run, the flame quickly became very 

energetic and turbulent. In about 10 min, the chamber pressure increased from 10.29 to 13.52 psia, and 

the temperature at the bottom of the chamber rose from 23.4 to 36.6 °C, causing two of the three video 

cameras to shutdown. The flame length exceeded 1 m long. For the safety considerations, the flame was 

extinguished by evacuating the chamber. A total of 481.2 g of PMMA was consumed, about of 55 percent

of the original fuel mass. Post test examination indicates that the sample holder was not able to quench 

the flame at fuel edges. The fuel material that was sandwiched between the sample holder and the 

aluminum bars was found softened and melted, indicating that the full 7 cm sample width had reacted. 

Thus, the chamber condition was too energetic for the study of the steady state flame length.

7 cm

5 cm

3.81 cm

1.9 cm PMMA Sample

Aluminum strips

Aluminum angle

0.56 cm

Screws

7 cm

5 cm

3.81 cm

1.9 cm PMMA Sample

Aluminum strips

Aluminum angle

0.56 cm

Screws
NASA/TM—2011-217024 21

The two tests mentioned above suggested that the current configuration of VF-13 was not suitable to 

study thick fuel upward burning at these conditions. To do so, continuous oxygen supply and product gas 

removal should be maintained during the test to avoid excessive oxygen depletion. 

5.3 Paper Fuel Tests

All tests with the paper material were done in the presence of 21 percent oxygen/nitrogen gas 

mixture. This was chosen because 1) a similar test condition could be easily achieved in the stackedchamber facility, and 2) previous studies regarding pressure scaling to simulate low gravity (Ref. 3) were 

performed at this mixture.

The first material chosen for testing was Adding Machine paper. This thin paper is available in long 

rolls and seemed well suited for the tall chambers. However some unusual, and ultimately unresolved, 

flame behavior prompted the use of poster paper instead. This material was used in both facilities for 

comparison. Below, the difficulties with the adding machine paper will be discussed as well as the results 

of the poster paper tests in both facilities.

5.3.1 Stacked-Chamber Facility—Adding Machine Paper

During 13 tests of the adding machine paper in the stacked-chambers facility, 11 experienced onesided burning, i.e., flames extinguished on one side of the material during propagation. This phenomenon

is typically reserved for thicker, denser materials, so a hardware issue was suspected. Despite several 

theories, summarized below, and extensive testing, no definitive cause could be found.

1. The thickness of the sample holder could have been acting as a heat sink on one side of the 

material. The sample was originally mounted to the sample holder as shown in Figure 19A. The rails of 

the sample holder were 1/8 in. thick, and mounting the sample in this manner biased the full thickness of 

this metal to one side of the material. To test if this caused a heat sink effect, the sample was instead 

mounted across the rails as shown in Figure 19B (the rails themselves were shifted so that the sample 

surface remained fully perpendicular to the cameras). In this way, the sample was mounted symmetrically 

with respect to the sample holder thickness. The tests performed in this manner failed to show any 

significant affect on the one-sided burning. However, this mounting method was maintained through all 

subsequent testing.

Figure 19.—The method of 

mounting the fuel to the sample 

holder is illustrated. (A) The 

original method with fuel on one 

side of the sample holder. (B) 

The revised method using a 

cross mounting method to better 

distribute the thermal mass of 

the sample holder. 
NASA/TM—2011-217024 22

The chamber was also temporarily configured to accommodate a thinner (1/32 in.) sample holder that 

has typically been used in a single 27 L combustion chamber. Despite a decrease in the thermal mass of 

the sample holder, one sided burning persisted.

2. The camera mount could have been acting as a heat sink on one side of the material. In the

majority of tests demonstrating one-sided burning, the flame blew out on the side of the fuel facing the 

cameras. Due to the chamber size, the camera mount was only 8.3 cm from the fuel surface. While this 

distance was considerably greater than the flame stand-off distance, some heat loss effect could be 

present.

To test this, an identical piece of metal channel stock was placed on the opposite side of the chamber.

In this way, the internal profile of the chamber became symmetric. However, the effect on the flame was 

not significant, and the one-sided burning effect persisted. 

3. The igniter only contacts one side of the fuel. The one sided burning could have been an 

ignition effect. The configuration of the chamber only allowed video viewing of one side of the sample.

When the flame extinguished on the camera side, a slowly propagating pyrolysis front, and reflection of 

the flame through the sample, indicated a flame on the opposite side. The chamber windows, typically 

covered to improve imaging, were used by the test operator to manually confirm the one sided flame. 

However, the ignition event could not be manually monitored in this way due to the proximity of the 

control interface.

To explore this theory, disposable hot wire igniters were used in place of the glow plug. These 2.7 Ω 

Kanthol wires were bent into a saw-tooth shape and interleaved around the edge of the fuel sample. In this 

way, the hot igniter prongs contacted both sides of the material to ensure two sided ignition. Additionally, 

a low quality video camera was added to a chamber window to confirm the ignition. The ignition was 

always two-sided, with the flame blowing out on one side further downstream.

4. Flow effects inside the chamber could have been causing the blow off on one side. A low 

speed (5 cm/s) flow was used during the tests to counter O2 depletion. It entered through a series of 

screens and honeycombs to ensure a uniform flow. The flow exited at the top of the chamber through a

1 in. orifice. If this orifice was too restrictive, combustion products could build up at the create 

recirculation cells.

To test this, the flow was deactivated in several tests. The tests were run at the same reduced pressure 

condition used in the previous tests (the flames were better behaved at reduced pressure). One sided 

burning still occurred, verifying that the flow velocity was not the cause.

Tests were also performed with the lid removed to check if the outlet orifice was too restrictive. Since 

the chamber was open to atmosphere, it could not be performed at a reduced pressure. The resulting flame 

was far more energetic, which likely overcame any other factors that may have played a role in one sided 

burning. While the flame did not blow out on one side, it was observed to slow at intervals, with 

flickering in the flame base. This would indicate that the cause of the one-sided burning was still present.

Any other flow effects caused by chamber obstructions or the aspect ratio itself would be difficult to 

observe experimentally. Since the flow field was dominated by the flame itself, flow visualization by 

most techniques was impossible. A numerical model of the chamber was explored examine potential flow 

field effects, but the project ceased before conclusive results could be obtained.

5. The effect could have been caused by the fuel itself. One sided burning could be caused by 

either the material composition itself or a surface coating (though not apparent on the adding machine 

samples). Several other paper materials were tested at the same conditions to see if the one sided burning 

was fuel dependant or hardware configuration dependant. Three additional paper samples used were; 

• Kimwipes: 2 mg/cm2 (laboratory tissue paper wipes)

• Masking paper: 5 mg/cm2 (used in painting, etc.)

• Poster paper: 7 mg/cm2

Note: that the adding machine paper has an area density of 6.3 mg/cm2 which was in this range. All three 

new materials demonstrated full two-sided flames through the entire propagation length with no signs of 

reduced spread rate or flickering in the flame base. 
NASA/TM—2011-217024 23

While the cause of one-sided blow off is still unresolved, time restraints prohibited further 

exploration. Since the other materials did not demonstrate the phenomenon, the poster paper was chosen 

for continued testing in both the VF-13 facility and stacked-chamber facility.

5.3.2 Poster Paper—Both Facilities

A total of 4 tests, two in each facility, were performed using poster paper for comparison purposes. 

The two test conditions were: 21 percent oxygen, 5.58 psia, 5 cm width (case 1) and 21 percent oxygen,

8 psia, 2 cm width (case 2).

All the tests were planned to be at the same pressure, 5.58 psia, which mimics flame spread in lunar 

gravity according to the pressure scaling correlation. However, the 2 cm wide material failed to burn at

5.58 psia (Test #8, 9) in VF-13. While the extinction limit has not been explicitly defined for this material, 

the pressure limit for similar materials is well below 5.58 psia. The same conditions in the stacked-chamber 

facility resulted in a strong flame that consumed the entire 1.2 m sample (Stacked chamber test 5/19/2008 

#1). The sample holder from the stacked-chamber facility was used in the VF-13 to reduce facility 

differences, but failed to solve the problem. Due to the limited number of tests available in the VF-13

facility, this phenomenon could not be fully explored or tested for reproducibility. Therefore, the higher 

pressure, 8 psia, was used to ensure successful flame propagation at a 2 cm sample width. The stackedchamber sample card was maintained in an effort to reduce the disparities between the two facilities. 

Results for the two successful VF-13 poster paper tests and two tests in the stacked-chamber facility

are presented in Figures 20 and 21. The pyrolysis spread rates, displayed on the graphs, were obtained by 

curve fitting the “steady” portion of the flame spread. The curve fit does not include the transient ignition 

region, which ends when the flame length ceases to grow and approaches a nearly constant value. 

It should also be noted that the data reduction is quite complex, particularly for the stacked-chamber 

facility. In both facilities there are multiple cameras, and therefore multiple videos. To analyze flame 

spread during a test, data from these videos must be merged and synchronized temporally and spatially, as 

described in Section 4.0. Multiple iterations of this data merging processes were performed. Errors found 

in the early iterations significantly affected the results. 

Since the project ended prior to completion, a full uncertainty analysis has not yet been performed. A 

meaningful uncertainty analysis would require a multiple repeated tests under the same test conditions, 

which was unfeasible under the various constraints of this project. However, the standard deviation in the 

curve fit equations indicates it can predicted flame position within ±3 cm. Missing data points in the 

stacked chamber data was caused by camera switching. Likewise, some sudden slope changes may be 

artifacts of data merging.

Table 3 shows a direct comparison of the spread rates. A steady flame should imply that the spread 

rate of the base and pyrolysis front are the same, but both spread rates fluctuate. They can never be 

exactly “steady.” Thus, the table shows some variation between the two. The highest of these is the VF13 

Case 1 test which shows a 0.58 cm/s difference between the pyrolysis and base spread rates. The plot in 

Figure 20 indicates a stable flame length, supporting the steady state claim. The larger flame base spread 

rate is due to the crack propagation.
NASA/TM—2011-217024 24

Figure 20.—Case 1: Position versus time plots for the VF-13 (top) and Stacked Chamber (bottom) 

comparison tests at 5.58 psia and 5 cm fuel width. The linear curve fit equations are shown for the 

steady region, which is highlighted in light gray.
NASA/TM—2011-217024 25

Figure 21.—Case 2: Position versus time plots for the VF-13 (top) and Stacked Chamber (bottom) 

comparison tests at 8 psia and 2 cm fuel width. The linear curve fit equations are shown for the steady 

region, which is highlighted in light gray. Note: the stacked chamber test at these conditions was the first 

to be analyzed. Only one flame base position was tracked (a visual average position). In all other tests 

the base position was an average of three tracked points.
NASA/TM—2011-217024 26

TABLE 3.—A COMPARISON OF SPREAD RATE IN THE VF-13 FACILITY TO

THOSE IN THE STACKED-CHAMBER FACILITY

[All VF-13 tests were done in 21 percent oxygen/nitrogen environment

while the stacked-chamber used filtered room air.]

VF-13 Stacked-chamber

Base,

cm/s

Pyrolysis 

front,

cm/s

Base,

cm/s

Pyrolysis 

front,

cm/s

Case 1 5 cm width 

5.58 psia

6.88 6.30 4.12 4.34

Case 2 2 cm width

8 psia

4.02 4.26 5.33 5.20

The spread rates are different between the two facilities. In Case 1 the flame spreads faster in the

VF-13. The opposite is true of Case 2, the 2 cm wide sample. Also note that the difference in spread rate 

between Case 1 and 2 in the VF-13 facility is greater than that in the Stacked-chamber. Possible causes 

could not be explored in detail, but flow or edge effects caused by the small diameter of the Stackedchamber facility may be muting effects of the other variables.

6.0 Closing Remarks

The two large chamber facilities have great potential for materials flammability testing. The types of 

materials and the variety of test conditions possible in these facilities are more extensive than the smaller 

facilities traditionally used. 

For thin samples (papers) the flame was able reach a steady state length using these long samples. In 

previous small chamber tests, sample widths were restricted to 1 or 2 cm where flame lengths were 

largely determined by edge effects, including heat loss to the sample holder and the 3D nature of the 

entraining air. The use of 5 cm sample widths in the large chambers demonstrated that flame length may 

be stabilized even when the edge effects are reduced.

The thick sample tests using PMMA resulted in highly energetic, long flames. It took over 10 min for 

the flame base to propagate off the lower edge of the fuel. Even in the large volume of VF-13, oxygen 

depletion began to play a role. As such, a flame spread and steady state flame length could not be 

determined. However the flame length reached 70 cm, which would not have been possible in other 

facilities.

Tests of both thin and thick fuels suggest that an upward burning flame can reach a steady flame 

length given sufficient fuel, oxidizer, and an unrestricted environment.

Tests with the Nomex fabric in VF-13 verified that the cyclic flame phenomena was not a chamber 

dependant or transient phenomena. The cycles continued over the entire 2 m length. Additionally, the 

ability to tests at more energetic conditions revealed new cyclic modes dependant on environmental 

conditions. Details of this will be published separately. It is believed that this cyclic phenomena is due to 

the composition of the Nomex fabric which may contain materials with distinct pyrolysis temperatures. 

No existing flame model currently addresses this cyclic flame phenomenon.

Limited time and numerous technical issues hindered taking full advantage of either facility. Several 

open questions remain, which would need to be resolved to make either facility viable.

• Ignition difficulties in the VF-13 facility

o There were difficulties igniting 2 cm wide paper samples. While an ignition flash did 

occur, the flame failed to sustain. This occurred in tests #3 and #9. In test #17, a higher 

pressure was used to create a more flammable condition. While the flame did sustain, 

there was a brief blow-out on one side several seconds into the test. The flame recovered 

and continued to propagate. Tests done in the same conditions in the stacked-chamber 
NASA/TM—2011-217024 27

facility ignited and burned the entire sample length without interruption, implying that 

this is a configuration issue.

• One sided burning in the stacked-chamber facility

o In many of the early 2 cm sample width tests, the flame blew out on one side of the 

material, while continuing to propagate on the other. This is an unexpected phenomenon 

for a thin homogenous material. While considerable effort was expended to trace the 

cause, it remains unresolved. However, the phenomenon was only observed using adding 

machine paper and did not occur for other types of paper materials. 

• Data reduction schemes

o The merging of data from up to 9 different cameras adds significant complexity and 

error to the data reduction process. The process underwent several iterations, which 

significantly impacted the results. While the current method seems rigorous, a full 

uncertainty analysis has not been performed.

Despite the open questions, it was clear from both the qualitative and quantitative results that the 

flames in the two facilities behaved differently. Despite using the same fuels size, sample holder, oxygen 

and pressure conditions, the spread rates were significantly different. This raises significant question 

regarding the impact of the sample size and test facility on the flame behavior. Traditionally, flame 

properties for a material have been studied in relatively small or high aspect ratios chambers, including

the NASA test standard STD 6001. But it is not clear how applicable those results are to a real world fire 

scenario, which may occur in a variety of different enclosures (an electronics box or storage compartment 

on a spacecraft) or in a larger open space. The shape of the enclosure or the objects in the environment 

impacts the flow field, which directly affects the transport of oxygen to the flame region. Likewise, the 

way the sample is held and the proximity of other objects may hinder or even encourage the flame spread. 

While more study is needed, this indicates the need for improved test standards.

References

1 NASA, 1998, Flammability, Odor, Offgasing, and Compatibility Requirments and Test Procedures 

for Materials in Environments that Support Combustion, NASA Technical Standard, NASA-STD6001.

2 Honda, L.K and Ronney, P.D., 2000, Mechanisms of Concurrent-Flow Flame Spread over Solid Fuel 

Beds, Proceedings of the Combustion Institute, Volume 28, pp. 2793–2801.

3 Feier, I., Shih, H.Y., Sacksteader, K.R., and Tien, J.S., 2002, Upward Flame Spread Over Thin Solids 

in Partial Gravity, Proceeding of the Combustion Institute, Volume 29, pp. 2569–2577.

4 Kleinhenz J., Feier, I., Tien, J.S., Ferkul, P.V., and Sacksteder, K.R. 2008. Pressure Modeling of 

Upward Flame Spread and Burning Rates in Partial Gravity. Combustion and Flame, Vol. 154,

pp. 637–643.

5 Kleinhenz, J. and T’ien, J.S. 2007. Combustion of Nomex® III Fabric in Potential Space Habitat 

Atmospheres: Cyclic Flame Spread Phenomena. Combustion Science and Technology. Vol. 179 

Issue 10. pp. 2153–5169.

6 Chu, L., Chen, C.H., and T’ien, J.S., 1981, Upward Spreading Flames Over Paper Samples,

ASME 81-WA/HT-42.

7 Ferkul, P.V. and T’ien, J.S., 1994, A Model of Low-speed Concurrent Flow Flame Spread Over a 

Thin Fuel, Combustion Science and Technology, Volume 99, pp. 345–370.

8 Klimek R.B. and Wright, T.W., 2004, Spotlight-16 Image Analysis Software, Software Manual 

version 2004.4.27.

9 Kleinhenz, J.E. 2006. Flammability and Flame Spread of Nomex and Cellulose in Space Habitat 

Environments. PhD dissertation, Case Western Reserve University.

10 Kleinhenz, J., Pettegrew, R., Sacksteder, K., and T’ien, J.S. 2005. One-sided Spread Phenomena of a 

Thermally Thin Composite Cotton/Fiberglass Fabric. Fire and Materials, 29:27-37.

NASA/TM—2011-217024 29

Appendix A.—Test Matrix

A.1 Procedures

A.1.1 VF-13

The experimental procedures for the VF-13 facility are listed below:

1. Adjust the position of the aluminum angles of the sample holder to have the desired lateral 

distance between the two angles (aka: set the fuel width). 

2. Install the fuel sample on the sample holder. If the fuel is a thin sheet of paper or Nomex, use 

adhesive aluminum tape to attach the sample on the aluminum angles. Make sure the sample is 

flat and evenly stretched. For thick fuel of PMMA, use screws to mount the PMMA plate on the 

sample holder.

3. Install the sample holder w/ fuel sample on the experimental rack.

4. Manually move the igniter to verify that the igniter tip is centered on the sample.

5. Operate the switches on the panel to activate the igniter actuator to verify that the igniter can 

reach the fuel sample with adequate pressure of nitrogen gas.

6. Check the mixing fan, cameras and the led strips via the switch box to make sure they are all 

functional.

7. Put the upper portion of the chamber on top of the lower portion and secure it with C clamps.

8. Evacuate the chamber with the roughing pump to the pressure lower that 0.15 torr.

9. Turn on the excitation voltage of the pressure transducer and the power of the readout volt meter. 

10. Check the oxygen line to the chamber to make sure the check valve on the valve panel is closed. 

11. Open the valve on the oxygen bottle and adjust the pressure regulator to get 50~60 psig at the 

outlet of the regulator.

12. Fill the chamber with oxygen via the ball valve on the valve panel, while monitoring the chamber 

pressure via the pressure transducer voltmeter. Adjust the filling speed as needed to prevent 

overfilling.

13. Switch to the metering valve for more finite control when the chamber is close to the desired 

oxygen partial pressure. 

14. Close the oxygen line with ball valves on the valve panel.

15. Fill the chamber with nitrogen via the ball valve on the nitrogen line on the valve panel while 

monitoring the chamber pressure via the voltmeter. 

16. Switch to the metering valve on the nitrogen line when the chamber pressure is close to desired 

total chamber pressure. Slowly adjust the metering valve as the chamber pressure approaches the 

desired total pressure. 

17. Close the nitrogen line when the chamber pressure reaches the desired total pressure. 

18. Turn on the mixing fan. And monitor the current the fan consumes which should be below 1 A

during stable operation. 

19. Turn the mixing fan off after running about 10 min.

20. Turn the LED chamber lights on.

21. Turn each camera on at separate time to see if the fuel sample can be seen. 

22. Turn all cameras and LED chamber lights off. 

23. Wait for 2 hr for chamber gas to be well mixed and residual flow to diminish. 

24. Connect the DV cam recorders.

25. Load the video tape to the recorders.

26. Connect the video signal to the real-time image digitizing computer.

27. Turn the LED chamber lights on.

28. Power on all three cameras.

29. Start the time code generator and make sure all the video recording devices are receiving good 

signals. 
NASA/TM—2011-217024 30

30. Turn the igniter power on and record the time in the test notebook.

31. Monitor the electrical current of the igniter which should start with a high value and decrease to a 

stable value around 2 A. And monitor the image on the camera #3. A very bright igniter should be 

seen after about 20 sec it was turned on.

32. Turn the igniter control switch to “in” position to actuate the igniter actuator to move the igniter 

to the fuel. Record the time in the test notebook.

33. When the ignition is confirmed, turn the control switch to “retract” position to retract the igniter. 

34. Monitor the flame spread during the burning. If the flame extinguishes because of either selfextinguishing or exhausting all the fuel, turn off video devices. If the flame becomes too violent 

and causes significant chamber pressure and temperature increases, inject nitrogen to the chamber 

to put the flame off. Then turn the video device off. 

35. Evacuate the chamber with the HIGHVAC Venturi pump. 

36. Recharge the chamber with room air.

37. Open the chamber after the chamber pressure reaches equilibrium with room pressure. 

38. Inspect the burned sample and remove the chamber holder from the rack for the next run. 

A.1.2 Stacked-Chamber Facility

The following list outlines the test procedures for operation of the stacked -chamber facility:

Sample Preparation

1. Remove the lid and undo the wire connections to the sample holder. 

2. Remove the sample holder from the chamber.

3. Remove the igniter and spent sample from the holder.

4. Adjust the sample holder to the appropriate width for the next test.

5. Cut the new sample to the desired width and tape the sample to the holder using Scotch tape.

6. Use adhesive aluminum tape along the inner edge of the sample holder, to act as an inert

(non flammable) edge. 

7. Position the igniter, secure it to the sample holder, and plug it into the wiring on the sample 

holder.

8. Insert the sample holder into the chamber and make the electrical connections.

9. Verify that all wiring is away from the sample. If any wire is too close to the fuel sample (at risk 

of flame exposure), tape it to the side of the chamber.

10. Verify that all cameras have clear view of the sample (again, tape wiring out of the way if 

needed)

11. Clamp the lid onto the chamber

Power Up and Run Test

1. Turn on all the power supplies.

2. Activate the appropriate systems on the SFSF and start the control software (LabVIEW)

3. Set the 3 camera selection switches to the highest numbers. (A:7, B:8, C:9) to accommodate the

three lowest cameras in the chamber (ignition is at the bottom)

4. Load and queue the Digital Video tapes 

5. Set the source gas bottle regulator and turn on the vacuum pump.

6. Purge the chamber of room gases by activating the “purge” sequence in the control software.

a. This sequence will vacuum the chamber

7. Enter the desired Fill pressure and execute the “fill sequence”

a. This sequence will fill the chamber with source gas to the set-point pressure.

8. Enter the desired test pressure and the flow velocity, and begin the “Flow oxidizer” sequence

a. This sequence will begin flow of source gas through the chamber and regulate the 

chamber to the desired pressure using the mass flow controller and back pressure valve.
NASA/TM—2011-217024 31

9. Monitor pressure and wait for it to level off at the set pressure

10. Verify the camera selection switches are set to the appropriate camera feeds and start recording

11. Start the time code generators.

a. The time code will appear in all video recordings and is needed to synchronize the 

camera views during data analysis.

12. Activate the igniter and monitor video

13. Once the flame base has moved off the igniter, deactivate the igniter

14. As soon as the flame propagates out of the field of view (FOV) of a camera, switch to the next 

camera on the switchbox. For example: once the flame moves out of camera 8’s FOV, switch 

selector B to “5”, then monitor camera 7. Once the flame is out of its FOV, switch selector A to 

“4”, and so on until the flame extinguishes

15. Once the flame has extinguished activate the “Vent” sequence

a. This sequence will vacuum the chamber to vent combustion by-products.

16. Stop the video recorders

17. Verify the chamber is equilibrated with atmospheric pressure and open the chamber lid.

18. Prepare sample for the next test or power down the system.

A.2 Test Log

A.2.3 VF-13

The test matrix of the VF-13 facility experiments. The (*) next to the test number indicates that the 

sample card from the stacked chamber facility was used. Fuel abbreviation AMP is “Adding Machine 

Paper,” while PMMA is Poly(methyl methacrylate). Oxygen percentages are molar fraction balanced with 

Nitrogen. The chamber pressures listed are the setpoint pressures, actual pressures are within ±0.012 psi 

of this amount.

Date Test Fuel Width,

cm

Ox,

percent

Pressure,

psia

Purpose Results

11/21/07 1 AMP 5 21 5.58 Lunar equivalent 

spread rate

5.78 cm/s flame

5.94 cm/s pyrolysis

11/27/07 2 AMP 5 30 10.6 Normoxic 

condition 

considered by 

future space craft

~15.88 cm/s flame

Energetic, difficult to 

track, pyrolysis not visible

11/30/07 3 AMP 2 21 5.58 Lunar equivalent, 

smaller width for 

previous small 

chamber 

(GIFFTS) 

compare

NO PROPAGATION

12/5/07 4 AMP 2 30 10.6 Normoxic, width 

for previous small 

chamber 

(GIFFTS) 

compare

6.84 cm/s flame

12/12/07 5 PMMA 5 21 5.58 To see if thick 

fuel can reach 

steady state flame 

length in upward 

burning

Fuel thickness is

0.56 cm and flame 

extinguished after 

consuming 136.6 g of fuel

12/18/07 6 PMMA 5 30 10.6 To see if flame 

can continue at 

higher O2 

concentration

HIGHLY energetic
NASA/TM—2011-217024 32

Date Test Fuel Width,

cm

Ox,

percent

Pressure,

psia

Purpose Results

1/10/08 7 Nomex 2 27 9 Comparable to 

small chamber 

(GIFFTS) tests, 

but with longer 

length to see if 

cyclic continues

2 cycles then extinguish

4/17/08 8 Poster 

paper

2 21 5.58 Lunar equivalent 

spread rate

Extinguished shortly after 

ignition. Blue flame

5/2/08 9 Poster 

paper

2 21 5.58 Serve as baseline 

test for stacked 

chamber. 2 

existing tests in 

stacked chamber, 

both burned the 

whole sample, 

flame length of 

~20 cm 

No propagation

5/8/08 10* Nomex 5 30 6 Look for cyclic 

phenomena. This 

is a consistently 

flammable point 

at 2 cm width, so 

should be away 

from 5 cm limit 

(undefined). If the 

flame is too 

energetic, it may 

self extinguish 

during a blow off 

cycle (previous 

data).

3 cycles, then 

extinguished (secondary 

flame stabilized in last 

cycle). Blue flame.

5/12/08 11* Nomex 5 27

(20 actual)

9 Examine cyclic 

phenomena. The 

cycles should be 

shorter with less 

chance of 

secondary flame 

survival at this 

condition.

No propagation. Error in 

oxygen fill pressure. 

Actual oxygen of test was 

20 percent, below 

flammability limit.

5/13/08 12* Nomex 5 27 9 Repeat of test 11 

at correct oxygen

Full propagation! Flame 

continued to cycle through 

entire test.

5/14/08 13* Nomex 5 30 9 Higher oxygen, 

but same pressure 

as test 12.

Examine how 

oxygen effects 

cycles. 

3 cycles, then 

extinguished (secondary 

flame stabilized in last 

cycle). Orange flame.

5/20/08 14* Nomex 5 35 9 Do cycles 

continue at higher 

ox? Does 2nd

flame survival 

cause extinction 

like in test 13? Or 

do we overcome 2 

stage pyrolysis?

Continuous cycling, 

multiple breakups. 

Full propagation.

5/23/08 15* Nomex 5 32 9 Provide another

points in the curve 

of test

12 to 14 

Full propagation. 

Continuous cycling. 

Secondary flame always 

survives.
NASA/TM—2011-217024 33

Date Test Fuel Width,

cm

Ox,

percent

Pressure,

psia

Purpose Results

5/28/08 16* Poster 

paper

5 21 5.58 Look at flame 

shape and

compare to 

stacked chamber 

(edge effect 

indicator). 

5/30/08 17* Poster 

paper

2 21 8 Better behaved 

tests (more 

“trackable”) for 

stacked chamber 

compare

Ignited, one side blew out 

briefly. Re-ignition and

full propagation

6/5/08 18* Nomex 5 30 9 Repeat for 

comparison

Full propagation. Varied 

between primary and 

secondary flame survival. 

Almost extinguished after 

1st breakoff.
NASA/TM—2011-217024 34

A.2.4 Stacked Chamber

The test record from the stacked chamber experiments. Fuel type abbreviations include: AMP for 

Adding machine paper, Poster, Kimwipe, and Masking all identify types of paper samples. Whereas 

SIBAL is a specialized fabric used for a previous project (Ref. 10). Some test objective reference the 

“GIFFTS” facility which was a single 27 L chamber used in previous testing.

Date Test # Fuel Oxygen,

percent

Pressure, 

psia

Flow,

cm/s

Width,

cm

Sample 

Length,

cm

Test purpose Flame status

2/14/08 1 AMP 21 8 5 2 120 Turn off 

lights and 

watch for one 

or two sided 

flame through 

window

Ignition and 

flame one sided 

entire time

Extinguished:

camera 6

2/14/08 2 AMP 21 8 5 2 120 Add a second 

dummy 

camera 

mounts to see 

if cam mount 

is a heat sink

Two sided, 

blowout on cam 

side, 

Extinguished:

camera 6

2/14/08 3 AMP 21 8 5 2 120 Two sided 

ignition blow 

out on camera 

side, 

Extinguished: 

camera 6

2/21/08 1 AMP 21 8 5 2 120 Tape sample 

only on 

camera side to 

see if sample 

holder as heat 

sink

Two sided 

ignition blow 

out on camera 

side, 

Extinguished: 

camera 7

2/21/08 2 AMP 21 8 5 2 120 Tape sample 

only on 

camera side to 

see if sample 

holder as heat 

sink

Two sided 

ignition blow 

out on back side, 

Extinguished:

camera 5

2/21/08 3 AMP 21 8 5 2 120 Tape sample 

only on back 

side to see if 

sample holder 

as heat sink

Two sided 

ignition, became 

one sided flame 

and blow out on 

camera side, 

Extinguished:

camera 6

2/25/08 1 AMP Shop 8 5 2 120 Extended 

sample card 

holder, taped 

to “thin” side, 

“thick” side to 

camera check 

sample holder 

as heat sink

No ignition

2/25/08 2 AMP Shop 8 5 2 120 Repeat One sided 

flame, blowout 

on cam side
NASA/TM—2011-217024 35

Date Test # Fuel Oxygen,

percent

Pressure, 

psia

Flow,

cm/s

Width,

cm

Sample 

Length,

cm

Test purpose Flame status

2/25/08 3 AMP Shop 8 5 2 120 “Thin” side to 

camera 

2/26/08 1 AMP Shop 11 5 2 120 Extended, 

“thick” side to 

cam, higher 

pressure 

check if near 

limit

Two sided 

ignition then one 

sided flame, 

blowout on cam 

side, 

Extinguished:

camera 6

2/26/08 2 AMP Shop 11 5 2 120 Extended, 

“thin” side to 

camera, 

higher 

pressure 

check if near 

limit

Two sided 

ignition then one 

sided flame 

blowout on cam 

side, 

Extinguished:

camera 5

2/26/08 3 AMP Shop 11 0 2 120 Extended, no 

flow check 

flow problem

Two sided 

ignition then one 

sided flame, 

blowout on cam 

side, 

Extinguished:

camera 6

2/28/08 1 AMP Shop 11 5 1 80 Small sample 

card holder 

check sample 

holder 

Two sided 

ignition then one 

sided flame 

blowout on cam 

side, 

Extinguished:

camera 7

3/3/08 1 AMP Shop 11 5 5 120 Wider 

sample, 

symmetric 

holder check 

near limit

Two sided,

Full propagation

3/3/08 2 AMP Shop 11 5 2 120 Same as 

previous but 

thinner check 

width factor

Two sided 

ignition then one 

sided flame, 

blow out on 

camera side, 

Extinguished: 

camera 5

3/3/08 3 AMP Shop 8 8 2 120 Upped flow to 

highest 

possible try to 

“blow out the 

blow out 

effect”, rig 

can't go too 

high without 

blowing 

pressure 

window

Two sided 

ignition but not 

sure of flame, 

two sided or one 

sided cam side, 

slow flame so 

probably one 

sided, 

Extinguished:

camera 6
NASA/TM—2011-217024 36

Date Test # Fuel Oxygen,

percent

Pressure, 

psia

Flow,

cm/s

Width,

cm

Sample 

Length,

cm

Test purpose Flame status

3/3/08 4 AMP Room Room 0 2 120 Lid off 

checking lid 

effect

Two sided, 

Extinguished:

camera 6

3/3/08 5 AMP Room Room 0 2 120 Repeat of 

previous

Two sided, one 

blowout on cam 

side but with 

restart,

Full propagation

3/3/08 6 Masking Room Room 0 2 120 Repeat Two sided,

Full propagation

3/3/08 7 AMP Room Room 0 2 120 Repeat Two sided, one 

near extinction,

Full propagation

3/4/08 1 AMP Shop ~14 0 2 135 Lid on, no 

flow replicate 

room pressure 

w/ lid 

checking lid 

effect

Two sided, one 

near extinction,

Full propagation

3/4/08 2 AMP Shop 11 5 2 103 Light at top of 

cam 7, 47 cm 

from bottom 

check “cam 6 

problematic 

area”

Two sided, two 

near blowouts, 

one at cam 6 and 

5 (was 

struggling), one 

at camera 1 

(skipped paper), 

close call in 

camera 2,

Full propagation

3/5/08 1 Masking Shop 8 5 2 120 Green paper, 

lower 

pressure back 

to 8 try to get 

new paper to 

blow out

Two sided,

Full propagation

3/6/08 1 AMP Shop 8 5 5 120 Wider sample 

check “near 

limitness”

Two sided,

Full propagation

3/6/08 2 AMP Shop 6 5 5 120 Lower 

pressure 

check “near 

limitness”

Two sided,

Full propagation
NASA/TM—2011-217024 37

Date Test # Fuel Oxygen,

percent

Pressure, 

psia

Flow,

cm/s

Width,

cm

Sample 

Length,

cm

Test purpose Flame status

3/10/08 1 AMP Shop Room 5 5 135 Lid off with 

flow check to 

see if flow 

helps

Two sided,

Full propagation

3/10/08 2 Masking Shop 6 5 2 120 Change paper, 

lower 

pressure try to 

make blow 

outs occur by 

dropping 

pressure

Two sided,

Full propagation

3/10/08 3 Masking Shop 4.35 5 2 120 Try to make 

blow outs 

occur by 

dropping 

pressure

Two sided,

Full propagation

3/12/08 1 Kimwipes Shop 4.35 5 1 ? Compare with 

gifts small 

chamber 

facility

Two sided,

Full propagation

3/17/08 1 Kimwipes Shop 4.35 5 1 ? Repeat of 

3/12/2008 

Test 1 

Two sided,

Full propagation

3/17/08 2 AMP Shop 8 5 2 120 Flipped paper 

check to see if 

paper burns 

different (may 

have a 

coating)

Two sided 

ignition then one 

sided flame cam 

side, flame 

propagation: 42 

cm

3/17/08 3 AMP Shop 8 5 2 120 Repeat of 

3/17/2008 

Test 2

Two sided 

ignition then one 

sided flame back 

side, flame 

propagation: 60 

cm

3/18/08 1 AMP Shop Room 5 2 120 Lid off with 

flow check to 

see effect of 

flow 

Two sided, near 

blow out on

camera 7,

Full propagation

3/18/08 2 Kimwipes Shop 4 5 1 ? Compare with 

gifts small 

chamber 

facility

Two sided,

Full propagation

3/18/08 3 Kimwipes Shop 4 5 1 ? Repeat of 

3/18/2008 

Test 2

Two sided,

Full propagation
NASA/TM—2011-217024 38

Date Test # Fuel Oxygen,

percent

Pressure, 

psia

Flow,

cm/s

Width,

cm

Sample 

Length,

cm

Test purpose Flame status

3/18/08 4 AMP Shop 8 5 2 120 Repeat Two sided 

ignition then one 

sided flame

3/24/08 1 Poster Shop 8 5 2 120 Denser paper 

see if blow 

out occurs 

with denser 

paper

Two sided,

Full propagation

3/24/08 2 Poster Shop 6 5 2 120 Lower 

pressure 

looking for 

blowout

Two sided,

Full propagation

3/26/08 1 SIBAL Shop 11 5 2 120 Looking for 

blowout with 

new fuel

Two sided 

ignition then one 

sided flame, 

blow out in 

chamber 2, 

flame continued 

on front side, 

full propagation

3/26/08 2 SIBAL Shop 11 5 2 120 Repeat of 

3/26/2008

test 1

One sided entire 

time: ignition to 

end, Full 

propagation

3/31/08 1 AMP Shop 5.58 5 5 120 Compare to 

test done in 

VF 13

Two sided, mini 

blow out in 

camera 1,

Full propagation

3/31/08 2 AMP Shop 5.58 5 5 120 Repeat of 

3/31/2008 

Test 1

Two sided, blow 

out cam side, 

Extinguished in 

camera 2

4/1/08 1 Masking Shop 6 5 2 120 Test to see if 

the slow 

down effect 

happens with 

masking 

paper

Two sided,

Full propagation

4/1/08 2 Masking Shop 6 5 2 120 Epeat of 

4/1/2008

test 1

Two sided,

Full propagation

4/3/08 1 Poster Shop 6 5 2 120 Test to see if 

the slow 

down effect 

happens with 

poster paper

Two sided,

Full propagation
NASA/TM—2011-217024 39

Date Test # Fuel Oxygen,

percent

Pressure, 

psia

Flow,

cm/s

Width,

cm

Sample 

Length,

cm

Test purpose Flame status

4/3/08 2 Masking Shop 4 5 2 120 lower 

pressure to

4 psia test to 

see if the slow 

down effect is 

caused by test 

being near 

limit

Two sided,

Full propagation

4/9/08 1 Denim Shop 11 5 2 120 Testing denim Two sided full 

propagation

5/19/08 1 Poster Shop 5.58 5 2 100 Glo plug 

ignitor, ignite 

in cam 7,

35 cm from 

bottom 

compare to 

test done in 

VF 13

Two sided full 

propagation

5/19/08 2 Poster Shop 5.58 5 5 120 Glo plug 

ignitor 

compare to 

test done in 

VF 13

Two sided full 

propagation

7/8/09 1 Poster Shop 5.58 no 5 120 Glo plug 

ignitor, ignite 

at 8 compare 

to test done in 

VF 13

Two sided full 

propagation

7/9/08 2 Poster Shop 5.58 no 5 120 Glo plug 

ignitor,

ignite at 8 

compare to 

test done in 

VF 13

Two sided full 

propagation
REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188 

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. 

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB 

control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 

1. REPORT DATE (DD-MM-YYYY)

01-06-2011 

2. REPORT TYPE

Technical Memorandum 

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

An Experimental Study of Upward Burning Over Long Solid Fuels: Facility Development 

and Comparison 

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Kleinhenz, Julie; Yuan, Zeng-Guang 

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

WBS 253225.01.03 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

8. PERFORMING ORGANIZATION

 REPORT NUMBER

E-17689 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

10. SPONSORING/MONITOR'S

 ACRONYM(S)

NASA 

11. SPONSORING/MONITORING

 REPORT NUMBER

NASA/TM-2011-217024 

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited 

Subject Category: 34 

Available electronically at http://www.sti.nasa.gov 

This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT

As NASA’s mission evolves, new spacecraft and habitat environments necessitate expanded study of materials flammability. Most of the upward burning tests to date, 

including the NASA standard material screening method NASA-STD-6001, have been conducted in small chambers where the flame often terminates before a steady state 

flame is established. In real environments, the same limitations may not be present. The use of long fuel samples would allow the flames to proceed in an unhindered manner. 

In order to explore sample size and chamber size effects, two large chambers were developed at NASA GRC under the Flame Prevention, Detection and Suppression (FPDS) 

project. The first was an existing vacuum facility, VF-13, located at NASA John Glenn Research Center. This 6350 liter chamber could accommodate fuels sample lengths up 

to 2 m. However, operational costs and restricted accessibility limited the test program, so a second laboratory scale facility was developed in parallel. By stacking additional 

two chambers on top of an existing combustion chamber facility, this 81 liter “Stacked-chamber” facility could accommodate a 1.5 m sample length. The larger volume, more 

ideal environment of VF-13 was used to obtain baseline data for comparison with the stacked chamber facility. In this way, the stacked chamber facility was intended for long 

term testing, with VF-13 as the proving ground. Four different solid fuels (adding machine paper, poster paper, PMMA plates, and Nomex fabric) were tested with fuel sample 

lengths up to 2 m. For thin samples (papers) with widths up to 5 cm, the flame reached a steady state length, which demonstrates that flame length may be stabilized even when 

the edge effects are reduced. For the thick PMMA plates, flames reached lengths up to 70 cm but were highly energetic and restricted by oxygen depletion. Tests with the 

Nomex fabric confirmed that the cyclic flame phenomena, observed in small facility tests, continued over longer sample. New features were also observed at the higher 

oxygen/pressure conditions available in the large chamber. Comparison of flame behavior between the two facilities under identical conditions revealed disparities, both 

qualitative and quantitative. This suggests that, in certain ranges of controlling parameters, chamber size and shape could be one of the parameters that affect the material 

flammability. If this proves to be true, it may limit the applicability of existing flammability data.

15. SUBJECT TERMS

Flammability; Flame propagation; Spacecraft environments; Fire prevention 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF

 ABSTRACT

UU 

18. NUMBER

 OF 

 PAGES

47 

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email:help@sti.nasa.gov) 

a. REPORT

U 

b. ABSTRACT

U 

c. THIS 

PAGE

U 

19b. TELEPHONE NUMBER (include area code)

443-757-5802 

Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39-18