National Aeronautics and Space Administration

A Researcher’s Guide to:

Combustion Science
2

This International Space Station (ISS) Researcher’s Guide is published by 

the NASA ISS Program Science Office.

Authors: 

Suleyman A. Gokoglu, NASA Glenn Research Center

Daniel L. Dietrich, NASA Glenn Research Center

Dennis P. Stocker, NASA Glenn Research Center

Paul V. Ferkul, Universities Space Research Association

Sandy L. Olson, NASA Glenn Research Center

Michael C. Hicks, NASA Glenn Research Center

Executive Editor: Amelia Rai

Technical Editor: Neesha Hosein

Designer: Cory Duke

Cover and back cover: 

a. Spherical flame in microgravity with ethylene flowing from a porous sphere into quiescent air. 

Glowing soot is trapped within the blue hydrocarbon flame, while a bright arc of large soot 

particles can be seen in the foreground. The test was conducted in NASA Glenn’s 2.2-Second 

Drop Tower as a prelude to the Flame Design experiment which is in development for conduct on 

the International Space Station as part of the Advanced Combustion via Microgravity Experiments 

(ACME) project. (Image credit: NASA, ACME project, Flame Design experiment)

b. Candle flames: burning in earth gravity (left) in comparison to one burning in 0-g (right). (Image 

credit: NASA, Candle Flames experiment)

c. A snapshot of 2-cm diameter acrylic sphere burning in microgravity at 17 percent oxygen and 

10 cm/s flow from the Burning and Suppression of Solids (BASS) experiment conducted on the 

International Space Station. Occasional yellowish-red streaks appear as vapor bubbles formed in the 

solid are ignited as they burst through the blue flame. (Image credit: NASA, BASS experiment)
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Astronaut holding a sample holder with a burned and bubbled acrylic sheet from the Burning and Suppression of Solids (BASS) 

experiment conducted in the Microgravity Science Glovebox on the International Space Station. (Image credit: NASA, BASS 

experiment)

Flying 250 miles above the Earth, the International Space Station (ISS) 

provides a platform for research to improve life on Earth, enable space 

exploration, and understand the universe. This Researcher’s Guide is 

intended to help potential ISS combustion science researchers plan 

experiments utilizing the microgravity environment in order to understand 

how heat and mass transport phenomena coupled with chemistry affect 

combustion behavior and fire safety. It covers the nature of the acceleration 

environment on ISS, available facilities for conducting combustion research, 

examples of previous microgravity investigations, and current combustion 

science and fire safety projects being developed for execution on the ISS.

The Lab is Open

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4
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1. Microgravity, or weightlessness, alters many observable phenomena 

within the physical and life sciences. Systems and processes affected by 

microgravity include surface wetting and interfacial tension, multiphase 

flow and heat transfer, multiphase system dynamics, solidification, and 

fire phenomena and combustion. Microgravity induces a vast array of 

changes in organisms ranging from bacteria to humans, including global 

alterations in gene expression and 3-D aggregation of cells into tissue-like 

architecture.

2. Extreme conditions in the ISS environment include exposure to extreme 

heat and cold cycling, ultra-vacuum, atomic oxygen, and high-energy 

radiation. Testing and qualification of materials exposed to these extreme 

conditions have provided data to enable the manufacturing of longlife, reliable components used on Earth as well as in the world’s most 

sophisticated satellite and spacecraft components.

3. Low-Earth orbit at 51 degrees inclination and at a 90-minute orbit 

affords ISS a unique vantage point with an altitude of approximately 240 

miles (400 kilometers) and an orbital path over 90 percent of the Earth’s 

population. This can provide improved spatial resolution and variable 

lighting conditions compared to the sun-synchronous orbits of typical 

Earth remote-sensing satellites.

Unique Features of the ISS 

Research Environment

1. Microgravity, or weightlessness, alters many observable phenomena 

within the physical and life sciences. Systems and processes affected by 

microgravity include surface wetting and interfacial tension, multiphase 

flow and heat transfer, multiphase system dynamics, solidification, and 

fire phenomena and combustion. Microgravity induces a vast array of 

changes in organisms ranging from bacteria to humans, including global 

alterations in gene expression and 3-D aggregation of cells into tissue-like 

architecture.

2. Extreme conditions in the ISS environment include exposure to extreme 

heat and cold cycling, ultra-vacuum, atomic oxygen, and high-energy 

radiation. Testing and qualification of materials exposed to these extreme 

conditions have provided data to enable the manufacturing of longlife, reliable components used on Earth as well as in the world’s most 

sophisticated satellite and spacecraft components.

3. Low-Earth orbit at 51 degrees inclination and at a 90-minute orbit 

affords ISS a unique vantage point with an altitude of approximately 240 

miles (400 kilometers) and an orbital path over 90 percent of the Earth’s 

population. This can provide improved spatial resolution and variable 

lighting conditions compared to the sun-synchronous orbits of typical 

Earth remote-sensing satellites.

Unique Features of the ISS 

Research Environment

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The Lab is Open 3

Unique Features of the ISS Research Environment 5

Why Conduct Combustion Research in Microgravity? 8

Results from Past and Recent Research 11

Droplet Combustion 11

Flame Extinguishment Experiment (FLEX) 12

Gaseous Diffusion Flames 13

Structure & Liftoff In Combustion Experiment (SLICE) 14

Burning Rate Emulator (BRE) 15

Coflow Laminar Diffusion Flame (CLD Flame) 16

Electric-Field Effects on Laminar Diffusion Flames (E-FIELD Flames) 16

Flame Design 16

Structure and Response of Spherical Diffusion Flames (s-Flame) 17

Solid-Fuel Combustion/Material Flammability 18

Burning and Suppression of Solids (BASS) 21

Correlation of Flammability between Normal and Microgravity 22

Fundamentals of Flame Growth and Extinction in Microgravity 23

Thermal and Species Field Effects on Material Flammability 23

External Heat Flux Effects on the Ignitability and Suppression of 

 Materials in Microgravity 24

Narrow Channel as a 1-g Flammability Test Method 24

Premixed Flames 25

Fire Safety 26

Super-Critical Processes 27

Highlights and Lessons Learned 30

Flame Existence 30

Low-Stretch (Low-Velocity) Flames 31

Jet Flames 32

Flame Stability 33

Cool Flames 33

Ancillary Hardware Developments 34

Spacecraft Fire Safety 34

Table of Contents
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Opportunities for Combustion Research on the ISS 36

Droplet Combustion 36

Gaseous Diffusion Flames 37

Solid-Fuel Combustion/Material Flammability 38

Premixed Flames 39

Super-Critical Processes 40

New Initiatives 41

Facilities and How to Choose Them 43

Acceleration Measurement and Environment Characterization 43

Combustion Integrated Rack (CIR) 43

Microgravity Science Glovebox (MSG) 44

Expedite the Processing of Experiments to Space Station (EXPRESS) Racks 45

Telescience Support Center 46

Developing and Flying Research on the ISS 47

What Should Principal Investigators Know About Conducting 

 Research on the ISS? 48

Funding Opportunities / Points of Contact 49

Citations 49

Appendix 52

Acronyms 53
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Combustion occurs when fuel and oxygen react to produce carbon dioxide, water 

and heat. For the foreseeable future, the overwhelming majority of delivered energy 

in terrestrial applications will be from combustion or other chemically reacting 

systems. These energy uses cover the range from electric power and transportation 

to processes directly tied to the delivered material (e.g., glass and steel manufacture). 

These processes produce some of the most important environmental hazards 

currently facing humanity (global climate change, acid gas pollution, mercury 

contamination from coal, and wild-land fires). 

Despite being the subject of active research for over 80 years, combustion processes 

remain one of the most poorly controlled phenomena that have a significant impact 

on human health, comfort and safety. This is because the simplest combustor 

(e.g., kitchen stove) remains beyond our detailed numerical modeling capabilities. 

Typically, the combustion process involves a large number of chemical species 

(hundreds) and reactions (even thousands). It is these species and reactions 

that determine flammability limits (combustor operating ranges) and pollutant 

emissions. Much of combustion research involves developing a comprehensive and 

predictive quantitative understanding of this complex process. This understanding 

usually occurs in smaller, bench-scale experiments that are amenable to detailed 

study and modeling. In these bench-scale experiments, however, buoyant forces, 

created by gravity and the large temperature differential between the flame and 

the ambient gas, dominate and frequently obscure the physical and chemical 

phenomena of interest, and hence, complicate analyses. In the absence of gravity, 

buoyancy can be suppressed, and the analyses can be reduced to much simpler onedimensional systems. 

To understand the strong influence of gravity on flames, one can consider the 

density gradients in flames. Typical flame temperatures are on the order of 2,300 

degrees Kelvin, whereas ambient temperatures are approximately 300 degrees 

Kelvin. This produces an eight-fold density change over the scale of a centimeter. 

The resultant density gradient induces a strong velocity field that dominates all but 

the highest flow fields. This flow field causes the flame to lift and point upward. 

The resultant flow instabilities cause the flicker typical of flames on Earth. Figure 1 

contains a gallery of 1-g to 0-g comparison flames.

By varying or eliminating the effects of gravity, we can extract fundamental data 

that are important for understanding combustion systems. This approach has 

been implemented to some extent in existing terrestrial reduced-gravity platforms, 

but the experimental time scales and sizes have been limited. Long-duration 

Why Conduct Combustion 

Research in Microgravity?
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experiments using realistic sizes are essential for a comprehensive understanding of 

the combustion phenomena and are possible only in the microgravity environments 

offered by space facilities. 

Besides terrestrial applications of combustion, gravity-related combustion research 

issues are also of crucial importance to NASA’s manned and unmanned missions. 

Fire safety is an important operational element of human space exploration 

missions to minimize the likelihood and impact of accidental fires in spacecraft 

and human habitats. The effects of these environmental changes on combustion 

in reduced gravity are unclear and need to be better understood for all missionrelevant scenarios. Every aspect of fire prevention, detection and suppression is 

Figure 1. 1-g and 0-g comparison flames. Left to right, top to bottom: ethane jet flames, ethylene jet flames, candle 

flames, burning heptane droplets and flame spread over a butanol pool. (Image credit: NASA)
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fundamentally different in space than on Earth. Thus, an improved knowledge of 

combustion in reduced gravity is essential to adapt fire safety concepts and systems 

to the more stringent conditions of the space environment. 

Benefits of conducting combustion research in microgravity can thus be 

summarized as: 

• Removal of the complex interaction between the buoyant flow and the energy 

feedback to the flame.

• Realistic simulation of the fire risk seen in manned spacecraft.

• Practical simulation of the gravitational environment seen for reacting systems 

in future spacecraft. 
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Over the past two decades, microgravity combustion research has focused primarily 

on increasing our understanding of fundamental combustion processes (e.g., 

droplet combustion, soot production, flame spread, smoldering and gas jet flames). 

This research program has been highly successful and was aided by synergistic 

programs in Europe and in Japan. Overall improvements were made in the ability 

to model droplet combustion in spray combustors (e.g., jet engines), predict flame 

spread, predict soot production, and detection and prevention of spacecraft fires. 

Some of the findings from past microgravity combustion research are presented 

below. Current ongoing flight projects on the International Space Station (ISS) are 

given in each area. 

Droplet Combustion

The availability of a long-duration, high-quality microgravity environment allows 

researchers to study the textbook problem of a spherical droplet surrounded by a 

spherical flame in an infinite, quiescent, ambient atmosphere. A cadre of researchers 

from academia, industry and federal laboratories has successfully established this 

simple geometry as an excellent test bed to study fundamental combustion science 

problems and other topics of applied combustion research. Through a robust 

program, researchers gained insight into wide-ranging problems of interest in 

combustion science. These programs included ground-based experimental work in 

drop towers and aircraft flying parabolic trajectories (and normal gravity; i.e., 1-g, 

test laboratories), numerical and theoretical modeling, and ultimately experiments 

in space-based facilities. The programs provided the opportunity to investigate 

phenomena such as:

• Departures from quasi-steady combustion.

• Transition from diffusive to radiative extinction.

• Liquid-phase transport.

• The influence of convection on flame dynamics (refer to Figure 2).

• The dynamics of soot formation and destruction (refer to Figure 3).

• The behavior of multi-component fuels.

• Droplet-droplet interactions and flammability limits.

Results from Past and 

Recent Research 
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The droplet combustion community is fortunate to currently have the Combustion 

Integrated Rack (CIR) and a CIR insert called Multi-user Droplet Combustion 

Apparatus (MDCA) available to study droplet combustion aboard the ISS. This 

facility is currently operational and allows the dispensing, deployment and ignition 

of droplets ranging in size from 1 to 5 mm (either free floating or fiber supported) 

in a 0-3 atm ambient environment of up to 40 percent oxygen by volume (balance 

of a range of inert gases and inert gas mixtures).

Flame Extinguishment Experiment (FLEX)

The objective of the FLEX is to study heptane and methanol droplets in ambient 

mixtures of oxygen and nitrogen diluted with a second inert gas (nitrogen, carbon 

dioxide, helium or xenon). The goal of the research is to examine how the addition 

of an inert gas suppressant influences the flammability limit of the two fuels. The 

application is an improved quantitative understanding of how flammability limits 

change in reduced gravity as it relates to fire safety. The research to date has shown 

that the flammability limit is lower in reduced gravity. The implication is that 

terrestrial standards for inert gas suppressants in low gravity do not offer the same 

margin of safety as in normal gravity.

The FLEX is of interest for terrestrial applications as well. The physical and 

chemical kinetic models of heptane and methanol combustion are the building 

Figure 2. Flame image of n-heptane droplet 

burning in 70 percent carbon dioxide, 21 percent 

oxygen, balance nitrogen and translated to the right 

at 3 cm/s. (Image credit: NASA, FLEX experiment)

Figure 3. Comparison of sooting propensity in different 

diluents with ethanol burning in 30 percent oxygen at pressure of 

2.4 atm. (Image credit: NASA, FLEX-2 experiment)
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blocks for more complex models of practical liquid fuels such as diesel and jet fuel. 

The long-duration experiments on the ISS provide benchmark data that researchers 

can develop, improve and ultimately validate detailed theoretical and numerical 

models of liquid hydrocarbon fuel combustion. The FLEX experiments performed 

to date demonstrated radiative and diffusive extinction, combustion instabilities, 

lower flammability limits and recently explained cool flame phenomenon after 

visible flame extinction. Comparison with theoretical and numerical models 

validated aspects of the models while identifying areas that require improvement.

FLEX-2, the follow-on experiment, builds upon FLEX. It studies a range of fuels 

including mixtures of pure fuels, real fuel surrogates, soot formation, slow subbuoyant, convective flow effects and droplet-droplet interactions. International 

collaborations extend this to study flame propagation in droplet arrays and 

investigate the combustion of liquid, biologically derived fuels.

Gaseous Diffusion Flames

Past research involved four space experiments that focused on soot processes, flamevortex interaction and flame stability. The Laminar Soot Processes (LSP) experiment 

of G. Faeth (U. Michigan) et al. was conducted in the Combustion Module (CM) 

facility in 1997 on the STS-83 and STS-94 space shuttle missions and again in 

2003 on the STS-107 mission. The STS-87 mission in 1997 included both the 

Turbulent Gas-jet Diffusion Flame (TGDF) experiment of M. Y. Bahadori (SAIC) 

et al. and the Enclosed Laminar Flames (ELF) investigation of L. D. Chen (U. 

Iowa) et al.; TGDF was automated and conducted in a Get-Away-Special Canister 

(GAS Can) in the payload bay. ELF was crew operated and conducted in the 

Middeck Glovebox. Lastly, the Smoke Point In Coflow Experiment (SPICE) of D. 

L. Urban (NASA Glenn Research Center [GRC]) et al. was conducted in 2009 on 

the ISS in its Microgravity Science Glovebox (MSG). 

All four experiments were studies of laminar, axisymmetric, gas-jet diffusion flames. 

This is true of even the TGDF experiment, where the flame was not turbulent but 

where an iris mechanism around its base induced vortices in the entrained air. While 

TGDF and LSP flames were within sealed chambers, ELF and SPICE flames were 

within fan-driven flow ducts. The affiliated LSP and SPICE experiments studied 

soot processes in ambient and coflow configurations, respectively. The SPICE 

experiment investigated the transition from non-sooting to sooting with a major goal 

to provide simple predictions of soot emissions. The smoke-point is a result of the 

fuel chemistry and flow conditions and is an effective indicator of the propensity of 
14

a fuel to emit light and to produce 

soot (see Figure 4). Soot control 

remains one of the major unsolved 

problems in combustion and 

engine development. 

The current research program 

includes six experiments planned 

for the ISS. The Structure & 

Liftoff In Combustion Experiment 

(SLICE) of M.B. Long (Yale U.) 

et al. uses the SPICE hardware 

to study lifted and lifting flames 

in the space station glovebox. 

With the similarity in both 

research focus and hardware, SLICE is effectively a second generation of the ELF 

investigation. SLICE started and completed its operations in 2012. The other 

five planned experiments are part of the Advanced Combustion via Microgravity 

Experiments (ACME) project and are in joint development for the CIR, where 

they will each be conducted using a single modular set of hardware. The ACME are 

focused on soot processes, flame stability and extinction, materials flammability, 

and electric-field effects. In alphabetical order, the experiments include: Burning 

Rate Emulator (BRE) of J.G. Quintiere (U. Maryland) et al.; Coflow Laminar 

Diffusion Flame (CLD Flame) of M.B. Long (Yale U.) et al.; Electric-Field Effects 

on Laminar Diffusion Flames (E-FIELD Flames) of D. Dunn-Rankin (UC Irvine) 

et al.; Flame Design of R.L. Axelbaum (Washington U.) et al.; and Structure and 

Response of Spherical Diffusion Flames (s-Flame) of C.K. Law (Princeton U.) et 

al. The ACME are expected to begin operating in 2016. As with their predecessors, 

the ACME experiments are all laminar diffusion flames. Two experiments take 

advantage of the microgravity environment by using a one-dimensional, spherical 

flame enveloping a porous spherical burner. Another experiment uses a porous plate 

burner, whereas the remaining two use a gas-jet flame configuration, both with and 

without a coaxial air flow.

Structure & Liftoff in Combustion Experiment (SLICE)

The SLICE investigates the structure of lifting and lifted flames where flow 

conditions and the combustion chemistry cause the flame to detach from the 

Figure 4. Flames from the 2009 Smoke Point In Coflow experiment 

showing the transition, with increasing fuel flow, from non-smoking 

(left) to a fully open and smoking flame tip (right).
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burner and stabilize at a downstream position. The twin purposes for the SLICE 

research are increased fuel efficiency and reduced pollutant emission in practical 

combustion devices. More explicitly, the experiment is being conducted to advance 

combustion modeling capability, which “allows system designers to improve 

efficiency and reduce harmful pollutants in ways never before possible” (as stated 

by an external reviewer of the SLICE). SLICE will be carried out using the SPICE 

hardware in the MSG and is a precursor to the CLD Flame experiment that is now 

in development for the ISS CIR as part of the ACME project (Figure 5).

Burning Rate Emulator (BRE)

Unlike the other current ACME, the BRE is focused on fire prevention, especially 

in spacecraft. Specifically, BRE’s objective is to improve our fundamental 

understanding of materials flammability, such as ignition and extinction behavior, 

and assess the relevance of existing flammability test methods for low- and partialgravity environments. The burning of solid and liquid fuels will be simulated 

by using a flat porous burner fed with gaseous fuel. The fuel flow rate will be 

controlled based on the measured heat flux (at the burner) and surface temperature, 

mimicking the dependence of condensed-phase fuel vaporization on thermal 

feedback. A small number of gaseous fuels will be used to simulate the burning of 

fuels such as paper, plastic and alcohol by matching properties such as the surface 

temperature and smoke production. The significance of this work is that it has the 

potential to efficiently and completely measure burning rate over a wide range of 

fuel properties. Since this study controls the inherent fuel properties that affect 

various fire scenarios, it will ascertain the relevance of existing flammability test 

methods for low- and partial-gravity environments.

Figure 5. Effect of increasing coflow velocity (from left to right, where the image scale is not consistent) on lifted flame 

behavior from exploratory tests for the Structure & Liftoff In Combustion experiment, which were conducted in 2009 on 

the ISS as part of the Smoke Point In Coflow experiment.
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Coflow Laminar Diffusion Flame (CLD Flame)

Research, especially including that already conducted in microgravity, has revealed 

that our current predictive ability is significantly lacking for flames at the extremes 

of fuel dilution, namely for sooty, pure-fuel flames and dilute flames that are 

near extinction. The general goal of the CLD Flame experiment is to extend the 

range of flame conditions that can be accurately predicted by developing and 

experimentally verifying chemical kinetic and soot formation submodels. The 

dependence of normal coflow flames on injection velocity and fuel dilution will be 

carefully examined for flames at both very dilute and highly sooting conditions. 

Measurements will be made of the structure of diluted methane and ethylene flames 

in an air coflow. Lifted flames will be used as the basis for the research to avoid 

flame dependence on heat loss to the burner. The results of this experiment will be 

directly applicable to practical combustion issues such as turbulent combustion, 

ignition, flame stability and more.

Electric-Field Effects on Laminar Diffusion Flames (E-FIELD Flames)

Electric fields can strongly influence flames because of their effect on the ions 

present as a result of the combustion reactions. The direct ion transport and the 

induced ion wind can modify the flame shape, alter the soot or flammability limits, 

direct heat transfer, and reduce pollutant emission. The purpose of the E-FIELD 

Flames experiment is to gain an improved understanding of flame-ion production 

and investigate how the ions can be used to control non-premixed flames. Outside 

reviewers recently concluded that the experiment “... will contribute to our critical 

understanding to our knowledge of combustion processes in the presence of electric 

fields.” The experiment will be conducted with a normal coflow flame (as in the 

CLD Flame experiment) or perhaps with a simple gas-jet flame where there is no 

surrounding coflow. An electric field will be generated by creating a high-voltage 

(up to 10 kV) differential between the burner and a flat, circular mesh suspended 

above (i.e., downstream of) the burner. Measurements, as a function of field 

strength and fuel dilution, will be made of the ion current through the flame and 

the flame’s response time to electric forcing.

Flame Design

The primary goal of the Flame Design experiment is to improve our understanding 

of soot inception and control in order to enable the optimization of oxygenenriched combustion and the “design” of non-premixed flames that are both robust 
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and soot free. An outside review panel recently 

declared that Flame Design “... could lead to greatly 

improved burner designs that are efficient and less 

polluting than current designs.” Flame Design will 

investigate the soot inception and extinction limits 

of spherical microgravity flames, created in the 

same manner as for the s-Flame experiment. Tests 

will be conducted with various concentrations of 

both the injected fuel (i.e., ethylene or methane) 

and the oxygen-enriched atmosphere in order to 

determine the role of the flame structure on soot 

inception (Figure 6). The effect of the flow direction 

on soot formation will be assessed with an inverse, 

spherical flame unless such testing is not approved 

by the Payload Safety Review Panel. If inverse 

spherical flame testing is not allowed, the plan is to 

use a coflow burner, conducting both normal and 

inverse flame tests. In the case of the inverse flames, 

the oxygen/inert mixture is injected from a central 

tube while the fuel is ejected from a surrounding 

annulus. The Flame Design experiment will explore 

whether the stoichiometric mixture fraction can 

characterize soot and flammability limits for nonpremixed flames like the equivalence ratio serves as 

an indicator of those limits for premixed flames.

Structure and Response of Spherical Diffusion 

Flames (s-Flame)

The purpose of the s-Flame experiment is to 

advance our ability to predict the structure and 

dynamics, including extinction, of both sootfree and sooty flames. The spherical flame, which 

is only possible in microgravity, will be created 

through use of a porous spherical burner from which a fuel/inert gas mixture 

will issue into the CIR chamber (Figure 7). Flames will be ignited at non-steady 

conditions and allowed to transition naturally toward extinction. Tests will be 

conducted with various inert diluents in both the fuel and chamber atmosphere. 

Figure 6. Image of a spherical diffusion 

flame on a porous burner (which is also 

visible) at the end of a test conducted 

in NASA’s 2.2-second Drop Tower. 

The gas from the burner was 1.51 mg/s 

of 100 percent ethylene issuing into air 

at atmospheric pressure. (Image credit: 

NASA, ACME project, Flame Design 

experiment)

Figure 7. Image of a partially premixed 

spherical flame on a porous burner (which 

is also visible). The microgravity test was 

conducted in a NASA’s 2.2-second Drop 

Tower. The gas issuing from the burner 

was 25 percent propane, 2 percent oxygen, 

49 percent argon, and 24 percent nitrogen. 

(Image credit: NASA, ACME project, 

s-Flame experiment)
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The fuel gases include hydrogen and methane 

for soot-free flames, and ethylene for sooty 

flames. One experiment objective is to identify 

the extinction limits for both radiative and 

convective extinction (i.e., at high and low 

system Damkohler numbers, respectively). 

Another objective is to determine the existence, 

onset and nature of pulsating instabilities that 

have been theoretically predicted to occur in 

such flames with fuel/diluent mixtures that are 

above a critical Lewis number.

Solid-Fuel Combustion/Material 

Flammability

Space-based and ground-based, low-gravity 

experiments in material flammability have 

revealed substantial differences in the behavior 

of flames over solid materials in reduced 

gravity. Many of these observations have 

specific impact on the hazard of spacecraft 

fires in addition to being of fundamental solid 

fuel combustion importance. Microgravity 

experiments demonstrated that the assumed 

margin of safety of reduced flammability in microgravity, based on quiescent test 

results, may not be valid for most spacecraft environments where ventilation flow 

strongly enhances flames. In the low-flow conditions of microgravity or partial 

gravity, some materials are flammable at lower oxygen concentrations than in 

normal gravity (Figure 8). Materials are easier to ignite in low gravity because of 

reduced natural convective cooling from the fuel surface. Once established, flames 

in low-speed airflows may preferentially spread into the oncoming airflow (Figure 

9) depending on the flow speed. This is opposite to the effect that occurs in normal 

gravity. Radiant heat transfer was shown to substantially affect the material’s 

flammability. Surface radiative heat loss reduces the flammability, but an external 

radiative heat flux can cause materials to ignite and spread faster. 

Currently, solid fuel combustion flight research is limited to the Burning and 

Suppression of Solids (BASS) experiment aboard the ISS. BASS is a rapid turnFigure 8. Flammability map shows region of 

enhanced flammability in the low-convective flow 

range.

Figure 9. Flame spreading preferentially upstream 

in slow convective flow. (Image credit: NASA, 

Radiative Ignition and Transition to Spread 

Investigation experiment)
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around flight experiment that started its operations in March 2012. It is a precursor 

for the Solid Fuel Ignition and Extinction (SoFIE) flight experiments that are 

currently planned for launch in 2019. The BASS approach is to use an existing 

flow duct, SPICE, to carry out microgravity tests in the ISS Microgravity Science 

Glovebox. Several types of solid materials were burned including flat samples, 

polymethylmethacrylate (PMMA) spheres, porous wax slabs and wax candles. 

The glovebox is normally limited to ISS “air,” nominally 21 percent oxygen by 

volume and one atmospheric pressure. However, more recently nitrogen has been 

used to perform tests as low as 17 percent oxygen. The tests have yielded useful results 

on the flow-velocity effect and flame suppression (by nitrogen), and the quick return 

of the test results has helped to guide the next, more complete flight experiments.

The SoFIE flight hardware (Figure 10) is currently under development and 

includes an insert for the CIR rack. In addition, some of the investigations will 

make use of a larger and improved flow duct along the lines of the BASS concept. 

The considerations for the solid fuels include material, preparation and purity, 

geometry, external heating, pre-ignition conditions, and ignition. The most likely 

candidates include plastics like PMMA or Delrin. There is a tremendous advantage 

to be gained if the sample can be burned, extinguished and then re-ignited multiple 

times. This minimizes the time-consuming process of opening the chamber and 

replacing the fuel specimens. The chamber environment will consist of oxygen 

from 10 percent to 40 percent by volume at a pressure between 0.2 and 1.5 atm. 

Figure 10. Solid Fuel Ignition and Extinction insert concept under development. A recirculating flow provides a region of well 

characterized jet flow within which solid samples will be burned.
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Figure 12. Digital still camera images showing a flame burning a 2-cm wide cotton-fiberglass fabric in opposed flow. 

Images are taken every 1.25 seconds (starting at top and moving from left to right). The flow is decreased in discrete 

steps from 10 cm/s all the way down to less than 1 cm/s. The flame response to flow changes is very rapid, and the 

flow effects on the flame and its spread rate are dramatic. Total burn time is 90 seconds. Flow changes are indicated 

by numbers.

Figure 11. Flame sequence of a burning 2-cm diameter acrylic sphere at 17 percent oxygen and 12 cm/s flow from the 

Burning and Suppression of Solids experiment. The images are about 1.5 sec. apart. With the reduction in oxygen from 

21 percent (air) to 17 percent, the flame stays bluer much longer (soot formation is delayed) facilitating comparison with 

model. The high-resolution images allow model comparison of flame growth rate, flame-to-fuel distance, and the solid 

regression rate.
21

Space Exploration Atmospheres (SEA) relevant to NASA can be investigated. 

Airflows in the spacecraft ventilation range of 0 to 30 cm/s will be supplied to the 

flame. The flow field must be well-characterized so the models will have accurate 

boundary conditions.

Burning and Suppression of Solids (BASS)

The BASS investigation is currently being conducted in the MSG to examine 

the burning and extinction characteristics of a wide variety of fuel samples in 

microgravity (Figures 11-13). The BASS experiment will improve 1-g material fire 

safety rating tests and help to guide strategies for extinguishing accidental fires in 

microgravity. BASS results contribute to the limited database of solid combustion 

in microgravity. In addition, BASS will guide combustion computational models 

used in the design of fire detection and suppression systems in microgravity 

and on Earth. Detailed combustion models are validated using the simpler flow 

environment afforded by tests in microgravity. Once validated, they can be used to 

build more complex combustion models needed to capture the important details 

of flames burning in normal gravity. These models have wide applicability to the 

general understanding of many terrestrial combustion problems.

BASS tests the hypothesis that materials in microgravity, with adequate ventilation, 

burn as well if not better than the same material in normal gravity with other 

conditions being identical (pressure, oxygen concentration, temperature, etc.). 

Figure 13. Drop tower test burning PMMA sphere showing transition to 0-g and flame response to the application of a jet of 

nitrogen for extinguishment.
22

The fuel is ignited and the airflow speed is the main variable. After some time, 

a nitrogen gas suppressant was applied to selected flames. There are important 

differences in the suppression of fires in space compared to on Earth. On Earth 

it is understood that the best results are generally obtained when the extinguisher 

“attacks” the base of the flame, which is both the stabilization point and the point 

where fresh air first enters the flame. For a fire burning in microgravity, the best 

point of application of suppressant may not be immediately apparent, especially for 

a partially obstructed flame or a wake-stabilized flame. Depending on the geometry 

of the flame and the characteristics of the extinguisher (distance from flame, 

dispersion angle), it is possible that the suppressant stream will be ineffective or 

might actually make the flame worse through the entrainment of oxygen. 

Early results have demonstrated that nitrogen applied to the flame can actually 

strengthen it because of this effect. Using nitrogen as a flame suppressant in 

microgravity provides a direct link to current and planned extinguishment 

techniques.

The SoFIE project is in the Science Definition phase with the plan of developing 

a CIR insert and large flow duct within the MSG to serve up to five separate 

investigations. These investigations all support the overall objective of the SoFIE 

project, which is to study and characterize ignition and flammability of solid 

spacecraft materials in practical geometries and realistic atmospheric conditions. 

There are currently five investigations described on the following pages.

Correlation of Flammability between Normal and Microgravity

The goal is to obtain the microgravity flammability map for select materials to 

provide information on the sensitivity of the material’s flammability to ventilation 

flow, ambient oxygen concentration and possibly pressure, to find the fundamental 

limiting oxygen concentration in microgravity. This will be defined as the worst 

case in microgravity for comparison with the NASA-STD-6001 Test 1 worst case 

upward flame spread in normal gravity. The comparison will enable evaluation of 

the oxygen margins of safety associated with normal gravity testing.

The microgravity burning behavior of some actual engineering materials planned 

for spacecraft can be obtained as a function of oxygen, pressure and flow speed for 

comparison to normal gravity results.

The significance of this portion of the work is that it will provide a better 

understanding of how the NASA Test 1 flammability limits relate to the actual 
23

material flammability limits in microgravity. It will also provide an increased margin 

of safety for materials flammability based upon a slightly modified NASA Test 1 

protocol, since recent drop-tower results suggest that the current Test 1 might not 

be conservative.

Fundamentals of Flame Growth and Extinction in Microgravity

The goal is to understand the fundamental processes of flame growth, spread and 

decay over solid materials, and their dependence on gravity, flow velocity, oxygen 

percentage, pressure, sample size and preheating. Solid spheres of plastic fuel will be 

burned in long-duration tests so that the entire flame history can be obtained.

Using the experimental results, a robust numerical model will be developed to a) 

simulate the transient flame development in the reduced-gravity experiment; b) 

relate material flammability performance between normal and reduced gravity; 

and c) examine the relevance of NASA-STD-6001 (Flammability, Offgassing, and 

Compatibility Requirements and Test Procedures) Test 1.

The practical limitations of ground-based microgravity facilities are well 

understood. While future ISS solid combustion experiments can be conducted 

for long durations, even these will have some limitations. The significance of this 

portion of the work is that by careful selection of ISS experiments together with the 

model, many of these limitations can be overcome to a degree sufficient to lead to 

a significant improvement in our understanding of how material flammability tests 

conducted on Earth apply to space environments.

Thermal and Species Field Effects on Material Flammability

Theory and modeling results suggest that an important difference between normal 

gravity and microgravity material flammability is caused by the influence of the 

flow field on the relative overlap of the thermal and species fields in the flame zone. 

The goal is to determine flame spread rate over thin fuels in quiescent microgravity 

and compare that with a closed-form expression including the pertinent solid 

properties and environmental parameters. Gas phase temperature and concentration 

fields of carbon dioxide and oxygen will be measured at certain key locations 

with the objective of delineating the propagation of the thermal field from the 

propagation of the species field; the separation between the two is postulated to be 

the mechanism for radiative flame extinguishment. The separation between the two 

fields will be controlled by changing the fuel thickness and ambient pressure.
24

The significance of this portion of the work is that it will establish quantitative 

criteria, based on fundamentals, which determine the flammability of materials 

in microgravity where radiative heat transfer dominates because of high-residence 

time created by a lack of buoyancy-induced or forced flow. This can lead to the 

specification of a critical fuel thickness, a critical forced velocity, or a critical oxygen 

level that makes a material inherently fire safe.

External Heat Flux Effects on the Ignitability and Suppression of 

Materials in Microgravity

The goal is to conduct microgravity ignition and suppression experiments for materials 

burning in low-speed, forced flow with externally applied heat flux in SEA. Time to 

ignition and the time between introduction of suppression agent and extinction are 

measured as a function of environmental conditions. The experiment will be supported 

by a comprehensive model of piloted ignition and extinction of materials in SEA.

Reduced-gravity aircraft tests have been conducted. However, because of the limited, 

reduced gravity time, ignition times could be obtained only at high, radiant heat 

fluxes. At airflow velocities below those induced by buoyancy, ignition times reach 

values that are significantly lower than those observed in normal-gravity buoyant 

flows. The reduction of the ignition times indicates that, under these low-flow 

conditions, the ignition process is affected more by the reduction in surface heat loss 

than by the corresponding oxygen transport reduction. By developing a theoretical 

model for the ignition time, it was also shown that the critical heat flux for ignition 

in microgravity can be as short as half the value measured in normal gravity.

Microgravity experiments will be used to validate the model, which in turn will 

be used to predict the fire behavior of different combustible materials in SEA. 

Currently, there exists no testing methodology specifically designed to determine 

the fire hazard of materials under those conditions. The significance of this portion 

of the work is that it will fill that void with a new test methodology and provide 

additional information about the effect of SEA on the flammability and suppression 

of materials. It may also help to interpret and extend current NASA testing 

procedures to SEA.

Narrow Channel as a 1-g Flammability Test Method

The goal is to measure flame spread rate across a representative, thick, solid fuel 

as a function of forced opposed flow velocity, oxygen concentration, and pressure, 
25

corresponding in particular to NASA‘s SEA in microgravity. Opposed flow 

extinction limits for the thick, solid fuel will be found by lowering the velocity 

until the flame extinguishes at a given oxygen concentration and pressure.

The data will be used to determine if an optimum gap height in the corresponding 

Narrow Channel device can be found to reproduce these results in normal gravity, 

or how the gap height needs to be changed for different conditions. The Narrow 

Channel is a 1-g material flammability test, which minimizes buoyant flow by 

constraining flame height. By restricting the buoyant plume, the flame size is 

controlled and in many ways mimics microgravity flames.

The significance of this portion of the work is that experiments can be structured 

using the Narrow Channel in normal gravity to predict aspects of microgravity 

flammability for a wide range of candidate materials. The flight experiment will 

provide a direct comparison of the Narrow Channel tests and the numerical 

model results.

Premixed Flames

Prior work in premixed flames, 

flown on two separate space 

shuttle missions, was limited to 

very fuel-lean conditions in which 

the mass diffusivity dominated 

the thermal diffusivity (i.e., very 

low Lewis number). Under these 

conditions, steady flame balls were 

shown to be possible (Figure 14). 

This remarkable result validated 

prior theoretical predictions, 

improved our understanding of combustion reactions near extinction limits, and 

provided pure test beds for numerical modeling. The Structure of Flame Balls at 

Low Lewis-number (SOFBALL) experiment of P. Ronney (USC) was conducted 

in the CM facility in 1997 on the STS-83 and STS-94 space shuttle missions and 

again in 2003 on the STS-107 mission.

Another noteworthy microgravity work related to premixed flames was the invention 

of a burner with ultra-low pollutant emissions (Figure 15). This has applications in 

domestic water heaters and furnaces.

Figure 14. Flame balls, a classical combustion phenomenon that is 

unique to microgravity. (Image credit: NASA, SOFBALL experiment)
26

Fire Safety

Past research involved a space experiment 

that focused on measuring the 

extinguishing capability of water mist on 

a premixed flame propagating along a 

tube to gain a better understanding of the 

water mist fire suppression phenomenon 

and help design more effective mist firesuppression systems to replace banned 

Halon systems. The Water Mist Fire 

Suppression experiment (Mist) of T. 

McKinnon (Colorado School of Mines) 

et al. was conducted in the CM facility 

in the space shuttle mission STS-107 in 

2003. The purpose of the Mist experiment 

was to study how water droplet diameter 

and water concentration affect the speed, 

strength and shape of flames created 

in various fuel and oxygen mixtures. 

After years of research, testing, flight 

experiments, and advocating the benefits 

of water-mist technology, non-toxic 

water-mist portable fire extinguishers 

were designed, tested, fabricated and 

certified for spaceflight. The first two of the planned nine water-mist portable fire 

extinguishers were sent to the ISS in December 2015 to replace CO2

 units that are 

currently onboard. The rest of the units will be sent in three more missions.

Spacecraft fire detection provides significant challenges compared to terrestrial 

conditions because the properties of the smoke emitted from fires is affected 

by buoyancy. The conditions on a spacecraft require rapid detection of fires 

with limited false alarms. Two recent experiments examined the issues related 

to spacecraft fire detection. The Dust Aerosol measurement Feasibility Test 

examined the background aerosol particulate levels on the ISS. The Smoke Aerosol 

Measurement Experiment (SAME) examined the particle-size distribution of the 

smoke from spacecraft materials. These instruments can provide useful guidance for 

the designers of future spacecraft smoke detectors.

Figure 15. Low-polluting burner remains cool to touch. 

(Image credit: NASA-Lawrence Berkeley Lab, Ultraclean 

Low Swirl Combustion project)
27

Super-Critical Processes

Because of its unique and significant advantages, Super-Critical Water Oxidation 

(SCWO) was identified by NASA as a potential technology for use in closed-loop, 

life-support systems addressing waste management and resource reclamation needs. 

Initial efforts in applying SCWO to solid waste management were conducted at 

NASA’s Ames Research Center in the 1990s in ground-based laboratories. These 

early tests were designed to look at oxidation rates and destruction efficiencies for 

cellulosic material as a function of particle size, mixing, temperature and pressure. 

Results from these tests provided empirical correlations for use in reactor design.

In 2004, an investigation to assess the impact of buoyancy on SCWO processes 

was initiated as part of GRC’s microgravity combustion program. The SCWO 

Test Facility, shown in Figure 16, with a 500-ml reactor along with all necessary 

supporting hardware and diagnostics was designed and built for use in the Zero 

Gravity Facility (ZGF, the 5.2-second drop tower). A limited number of ZGF 

tests were conducted for the oxidation of methanol in water (methanol 10 weight 

percent) using air, injected into the super-critical mixture, as the oxidizer.

Figure 16. (a) Super-Critical Water Oxidation (SCWO) Test Facility designed for use in the Zero Gravity Facility shown loaded 

in the drop bus just prior to dropping; (b) 500 ml SCWO reactor vessel installed in the SCWO Test Facility; (c) schematic 

showing axial location of thermocouples inside the SCWO reactor. (Image credit: NASA, SCWO experiment)
28

The first experimental objective was to determine the extent to which buoyancy 

affects the developing temperature profile. Buoyancy effects were expected because 

the reaction is exothermic; however, because of the limited microgravity time in the 

drop tower, it was not entirely clear these would be observable. Comparison of the 

temperature traces, as shown in Figure 17 for both normal gravity and microgravity, 

confirmed this expectation. In normal gravity, temperatures were found to be 

more uniform in the bulk fluid, which is due to buoyant mixing; whereas, nonuniformities resulting in thermal stratifications (evidenced by a “hot zone” between 

two cooler regions) were observed under microgravity conditions. A second 

experimental objective, to measure the reaction rate and product concentrations 

with Raman spectroscopy, was not met because of budgetary constraints.

This earlier work, which was initiated in 2008, at GRC was instrumental in 

development of a collaborative effort between NASA and the French space agency, 

CNES. The objective of this ISS flight investigation, referred to as the SuperCritical Water Mixture experiment (SCWM), is to investigate the behavior of 

salt precipitate formation and transport in super-critical water in the presence of 

temperature gradients. This will provide practical insights into the mechanisms of 

salt agglomeration, transport and deposition that often plague SCWO reactors.

Figure 17. Comparison between 1-g and 0-g axial temperature immediately following axial injection of oxidizer; (a) in 

1-g temperature profiles show high degree of buoyant-driven thermal mixing for locations above injector and (b) in 0-g 

temperature profiles show significant thermal stratification with a hot zone (TC-B) established between two cooler regions 

suggesting poor thermal mixing in 0-g. (Image credit: NASA, SCWO experiment)
29

The investigation will utilize a refurbished High Temperature Insert (HTI-R), built 

by CNES, with a fluid cell (approximately 0.3 cm3) filled with a water-salt mixture 

using Na2SO4 (0.5 percent w/v) at its critical density. During an experiment, the 

salt-water mixture will be slowly raised to a temperature and pressure above its 

critical point enabling observation of homogenous salt precipitation, measurement 

of the critical end-point (i.e., the shift in critical temperature that is due to the 

presence of the salt), and the transport mechanisms both with and without the 

presence of a temperature gradient. A baseline test was conducted using pure water 

in the HTI during a test sequence on the ISS performed in July 2010. Images from 

this test showing the location of the liquid phase and meniscus geometry at subcritical and super-critical conditions is shown in Figure 18. This test sequence was 

designed to investigate the formation and stability of a temperature gradient in the 

fluid in its super-critical state. The follow-on SCWM experiment will employ a 

similar test sequence and results from the pure-water test sequence will establish a 

useful baseline for comparison.

Figure 18. Sub-critical and super-critical states of water mixtures imaged aboard the International Space Station showing 

(a) the geometry of the fluid meniscus at sub-critical conditions and the monophasic super-critical fluid with no meniscus at 

super-critical conditions. (Image credit: NASA-CNES, SCWM experiment)
30

Well over 200 principal investigators have utilized reduced-gravity facilities to 

study fundamental combustion phenomena in the areas described above. Most of 

the research was conducted in drop towers and aircraft, and the most promising 

experiments, after a thorough scientific and engineering review process, were 

selected for conduct in space-based facilities. The investigations led to numerous 

discoveries, highlighted the importance of frequently neglected phenomena, 

improved our understanding and predictive capabilities for all combustion systems, 

and enhanced our ability to detect and prevent spacecraft fires. Some of the most 

pertinent highlights and lessons learned are presented below.

Flame Existence

One of the most seemingly simple, but long-standing, questions addressed by 

microgravity testing involved whether flames could even exist without buoyancy. 

Many researchers, including notable experts, argued openly in the scientific 

literature that, in the absence of convection, a steady flame could not exist; 

diffusive transport alone could not sustain a flame. Two spaceflight experiments 

demonstrated conclusively the fallacy of such arguments.

The first involved the behavior of a simple candle flame in a long-duration 

microgravity environment (Dietrich et al., 2000). The candle flames in Figure 1 show 

images of a familiar shape seen in normal gravity and a flame during a microgravity 

test aboard the Russian Mir orbiting station. Contrary to published speculation, the 

microgravity flame burned in some tests for over 45 minutes before extinguishing 

from a lack of fuel. Despite having a much lower burning rate (the microgravity flame 

in Figure 1 was nearly invisible to the naked eye and a very long shutter time was 

needed to record the image shown), the microgravity flame burned to a lower ambient 

oxygen mole fraction than the identical candle flame would have on Earth.

The second example involves the existence of stationary, spherical premixed flames 

or flame balls. These flames were predicted theoretically to exist if (among other 

conditions) the thermal diffusivity of the bulk mixture was much lower than the 

mass diffusion of the limiting reactant: that is, a low Lewis number. Their existence, 

however, was thought to be a strictly mathematical exercise. The SOFBALL studied 

premixed fuel/oxidizer/diluent mixtures in a large combustion chamber and showed 

conclusively that the flames could exist (Figure 14), producing the weakest flames 

ever burned (approximately 1 Watt) and some burning for over 40 minutes before 

the experiment timed out (e.g. Ronney, et al., 1998).

Highlights and Lessons 

Learned
31

Low-Stretch (Low-Velocity) Flames

Reduced-gravity facilities offer the opportunity to study flames where the 

characteristic velocities are much lower than typical buoyancy-induced gas 

velocities, approximately 1 m/s in the candle flame in Figure 1. These low-speed 

flows result in flame stretch or strain rates that are unachievable in terrestrial, 

bench-scale experiments. Conventional theories of flame limits all predicted that, 

as the flame stretch characteristic velocity decreased, the oxygen concentration 

that would support steady or quasi-steady-state combustion would monotonically 

decrease.

This was not the case, however, as several classes of experiments demonstrated. At 

low-characteristic velocities (low-flame stretch), radiative loss begins to dominate, 

eventually leading to flame extinction at sufficiently low velocities or stretch rates 

(e.g., Olson et al., 1988). Figure 8 shows this schematically. For flame spread across 

solid materials, the ordinate in Figure 8 is the ambient oxygen mole fraction and the 

abscissa the gas velocity across the solid surface. The boundary represents limit of 

steady flame spread. Outside of this boundary, steady flame spread does not occur; 

inside, it does. The curve identifies a much lower flammability limit than originally 

predicted. For flame spread across solid materials, the characteristic gas velocity 

of the minimum in the curve is on the order of 10 cm/s and also corresponds 

to a maximum in the flame spread rate. To give perspective, this is the velocity 

typical of that of spacecraft ventilation systems. This observed minimum has large 

implications for material flammability in reduced gravity and, thus, implications for 

spacecraft fire safety. 

Another related phenomenon with spacecraft fire safety implications is the 

observation of flame spread direction at low-flow velocities in the microgravity 

environment of spaceflight when a thin fuel is ignited at the middle of the sample. 

Completely contrary to normal-gravity behavior, the flame spreads upstream, into 

the oncoming airflow, in space when the forced-flow velocities are low (Figure 9); 

whereas, the flame always spreads downstream, in the buoyant flow direction, in 

normal gravity, even in the absence of forced flow. 

Recent results from the BASS experiment currently running on the ISS indicate 

that NASA-STD-6001 Test 1 is not conservative, and materials can burn at lower 

oxygen levels in microgravity than on Earth. Also, current findings show that local 

application of suppressant is not adequate for flame extinguishment. The local jet 

entrains air and sustains the flame even when the ambient airflow is turned off. 
32

The unique microgravity environment also allows researchers to study the classical 

problem of a spherical flame surrounding a spherical droplet in a quiescent ambient 

(e.g., Dietrich et al., 1996; Nayagam et al., 1998). Virtually every combustion 

textbook contains an analysis of this problem. For droplet combustion, the ordinate 

in Figure 8 is the ambient oxygen mole fraction and the abscissa is the droplet 

diameter, with the boundary defining again the flammability boundary. Note also 

that for droplet combustion, the curve in Figure 8 is actually reversed, with larger 

droplets having lower stretch rates or larger Damkohler numbers. Both spacebased and, to a limited extent, drop-tower testing demonstrated the existence 

of this limit. Identification of this curve is important not only for identifying 

flammability limits for spacecraft fire safety applications but for practical reasons 

as well. Droplet combustion experiments involve liquid fuels, and the curve in 

Figure 8 is determined by the detailed interaction of the heat and mass transport 

and chemical kinetics. The chemical kinetic mechanisms responsible for the curve 

are the building blocks of mechanisms for practical fuels. These droplet combustion 

experiments, therefore, provide benchmark data for the validation of physical and 

chemical kinetic sub-models of practical fuels.

Jet Flames

Another common class of flames in combustion research is gaseous jet flames (a 

fuel gas issuing from a nozzle into an initially quiescent oxidizing atmosphere). In 

terrestrial laboratories, gas-jet diffusion flames are used to elucidate the chemical 

kinetic mechanisms of simple fuels, to understand flame stability, and to study 

the details of soot production and destruction. For bench-scale laminar gas-jet 

experiments in normal gravity, it is difficult to suppress the effects of buoyancy, 

making the problem significantly less tractable. Microgravity studies in drop towers 

and space-based facilities show striking differences (in comparison to normalgravity studies) in the observed flame heights of jet flames at low and moderate 

jet Reynolds numbers (e.g., Hegde et al., 1994; Urban et al., 1998). The onset of 

smoke production found in low gravity is substantially different than in normal 

gravity and contrary to predictions in prior literature. The experiments also show 

significant differences in the size of the primary soot particles and in the larger 

fractal dimension of the soot agglomerates (e.g., Greenberg et. al., 1997). The data 

from these experiments serve as benchmark data for the development and validation 

of detailed chemical kinetic and soot formation and destruction models.
33

Flame Stability

In normal gravity, flames that approach their flammability limit are frequently 

“blown off” by the large buoyancy-induced flow. These same near-limit flames in 

microgravity exhibit very different and unique behaviors in many cases. The simple 

microgravity candle flame in Figure 1, as it depleted the ambient oxygen and 

neared extinction, exhibited flame oscillations in most tests (Dietrich et al., 2000). 

These oscillations increased in magnitude until the flame finally extinguished, but 

they could exist for prolonged periods of time (over one minute in extreme cases). 

Similar flame oscillations occur during spherically symmetric droplet combustion 

(Dietrich et al., 1996).

Instabilities also occur during flame spread over solid and liquid surfaces. Over 

solids, a complex fingerlike smoldering front occurs during near-limit flame spread 

under certain conditions (Olson et al., 1998). A pulsating gas-phase flame spread 

occurs as a flame spreads across a liquid pool under selected ambient conditions (e.g., 

Miller et al., 2000). Finally, microgravity experiments involving premixed flames 

demonstrate instabilities in both low- and high-Lewis-number gas mixtures (e.g., 

Pearlman and Ronney, 1994). 

These instabilities, obscured in normal gravity by the strong buoyant flow, highlight 

the ability of microgravity experiments to provide insights into fundamental flame 

behavior. Highly exothermic reactions coupled with nonlinear heat loss through 

radiative and diffusive transport of both heat and mass provide combustion 

theoreticians and modelers a rich canvas on which to develop, test and validate 

simplified and detailed flame models.

Cool Flames

Recent spaceflight experiments (FLEX) involving single alkane droplets burning in 

a quiescent ambient uncovered unexplained and unexpected behavior. For the large 

alkane droplets, after ignition the droplet burns for a short time with a typical hot 

flame. As expected, the hot flame extinguished when the radiative loss reduced the 

flame temperature below a critical value. Inexplicably after hot flame extinction, 

however, the droplet continued to vaporize at the same rate for an extended period 

and then suddenly stopped vaporizing at a finite droplet size. The plateau in 

vaporization occurred coincident with the formation of a large vapor cloud. This 

vaporization, we now know, is supported by a lower-temperature cool flame that 

forms after visible flame extinction. The vaporization stops when the cool flame 
34

extinguishes and the vapor cloud forms because the cool flame only burns a fraction 

of the fuel vaporized from the droplet surface.

This cool flame supported droplet combustion was not only unexpected, but also 

thought to be impossible by many in the peer community (it took a year for the 

team to convince themselves and then another year to convince the skeptical peer 

community). Cool flames form as an intermediate step between unreacted fuel and 

oxidizer, but should not form after a hot flame extinguishes. The FLEX tests showed 

that the cool flame can burn for a prolonged period of time. The microgravity 

droplet combustion tests offer a unique platform to study these low temperature 

chemical reactions. Cool flame chemistry is very important in internal combustion 

engines and a predictive understanding of their chemistry is critical in developing the 

next generation engines with higher efficiency and lower emissions.

Ancillary Hardware Developments

Although experiments in microgravity offer enticing rewards to combustion 

researchers, the harsh realities of experiments in reduced-gravity facilities present 

significant hardware and diagnostic challenges to scientists and engineers. Drop 

tower and space-based experiments have severe space, weight, volume, safety and 

power limitations. These limitations have driven technology developments with 

applications far beyond the microgravity combustion science program.

Some noteworthy accomplishments include the development of a system for 

measuring full-field, quantitative soot volume fraction via laser-light extinction 

(Greenberg et. al., 1997a). This novel system allows the precise measurement 

of soot volume fraction and is the enabling technology for experiments that continue 

to provide benchmark data on soot formation and destruction. Rainbow Schlieren 

Deflectometry system developed at GRC provided the first full-field quantitative 

measurements of gas-phase temperature in microgravity combustion experiments 

(Greenberg, et al., 1995). The most significant developments have 

been technologies to both measure and characterize small aerosol particles (e.g., Qi 

et. al., 2008).

Spacecraft Fire Safety

The 1967 tragic accident of Apollo 1, in which three NASA astronauts lost their 

lives when a cabin fire broke out during a launch pad test, and the near-tragedy 

of Apollo 13 in 1970, when an oxygen tank exploded during transit to the moon, 
35

highlight the hazard posed by accidental fires on spacecraft. A fire or explosion 

requires all elements of the “fire triangle” to be present: fuel, oxygen and a source of 

ignition. Crewed spacecraft, by necessity, contain all three. In addition, spacecraft 

frequently operate at an ambient oxygen mole fraction higher than that of air, have 

limited options for escape, and have almost no tolerance for damage to critical life 

support systems. As a result, NASA maintains a very disciplined safety culture with 

respect to fire through strict material controls, rigid design standards for electronic 

equipment (and all possible ignition sources), and mandatory suppression systems on 

all spacecraft. 

Although NASA has been vigilant in its approach to accidental fires, the approach 

to fire safety aboard spacecraft was dominated by a general lack of knowledge about 

how fire behaves in microgravity. This resulted in several critical misconceptions 

about potential fires in spacecraft:

1. A steady fire cannot exist in a quiescent microgravity environment.

2. A fire will self-extinguish if ventilation is shut off.

3. Because the reduced-gravity environment is inherently safer, terrestrial fire 

standards and tests represent a very conservative approach to fire safety.

Research at NASA GRC in both ground-based laboratories and microgravity 

facilities and space-based microgravity experiments (summarized by Friedman and 

Ross, 2001 and Ruff et al., 2009) demonstrates the fallacy of these assumptions and 

demonstrates GRC’s contribution to spacecraft fire safety science with respect to fire 

prevention through material flammability and screening. 

GRC also maintains active research programs in both fire detection and suppression. 

Fundamental research in microgravity highlighted the vastly different particulate size 

associated with flames in microgravity. Since many fire detectors operate by sensing 

fire-generated particulates, they would need to use different signatures to detect 

fires in microgravity. If terrestrial fire detectors were used in space, they would be 

“looking” for something different than they should. One of the achievements in this 

area is a new device that monitors particulates and chemical species simultaneously 

to better detect the onset of a fire. In joint testing with the Federal Aviation 

Administration, this device outperformed current aircraft smoke detectors in falsealarm rejection and real-event detection. The new device can be “tuned” to examine 

particulates and species characteristics associated with fires in microgravity.
36

Combustion research remains an active field of research with direct application to 

terrestrial issues, fundamental science, and NASA‘s exploration goals. The New 

Initiative program is based upon continued use of the in-orbit resources (CIR 

inserts) with the addition of another CIR insert to the on-orbit resources. These 

topics were selected based upon the data sources considered by the Decadal survey. 

All of these topics have direct application to either terrestrial or exploration issues 

and are among the most challenging questions faced in the discipline. 

Droplet Combustion

Currently operating inside the CIR is the FLEX, which uses the MDCA to study 

droplet combustion. FLEX and the follow-on FLEX–2 experiments are exploiting 

the microgravity environment to study the spherically symmetric combustion 

of liquid droplets. FLEX is motivated by the need to better identify and predict 

limiting oxygen indices (LOIs) and inert-gas suppressant efficacy. The FLEX–2 

experiments are motivated by the need to better understand droplet combustion as 

it applies to the fundamental problems of energy conversion and utilization where 

liquid hydrocarbon fuels are a predominant source of energy in the world.

To date, these experiments have identified both the radiative and diffusive 

extinction limits for heptane and methanol droplets and identified an LOI that is 

both significantly lower than in normal gravity and lower than previously predicted 

and anticipated. The data from these experiments are helping researchers to develop, 

refine and validate chemical kinetic models important to terrestrial combustion 

systems. The most significant observation from these experiments, however, is the 

identification of stable, quasi-steady cool flames that form after radiative extinction 

of the visible flame for heptane droplet combustion (Nayagam et al., 2012). This 

low-temperature chemistry is important in automobile engines because it is one of 

the major phenomena limiting engine efficiency.

While the droplet combustion research community is fortunate to have access to 

the CIR and MDCA, funding limitations have severely cut down on the number of 

researchers, the number of research projects, and the overall extent of the program. 

The space-based research in the CIR/MDCA should be expanded to a wider range 

of fuels, specifically focusing on practical fuels and practical fuel surrogates. Ideally, 

this would be closely aligned with similar efforts funded by other federal agencies 

(e.g., Department of Energy and Department of Defense). Virtually absent from 

the current program is also critical research on high-pressure droplet combustion 

Opportunities for Combustion 

Research on the ISS
37

and spray combustion. These programs have immense practical applications 

to the nation’s energy future as the vast majority of energy consumption in the 

transportation sector is from liquid fuels. Finally, a critical missing element in the 

current program is the lack of a comprehensive, ground-based research program 

that includes experiments in drop towers and the companion theoretical and 

numerical modeling efforts. These programs serve two purposes. First, they provide 

valuable and relevant insights into the physics of liquid fuel combustion, while 

providing unparalleled educational opportunities to graduate and undergraduate 

students. Second, they form the basis of the next generation of space-based 

microgravity experiments.

Gaseous Diffusion Flames

The gaseous diffusion flame research is generally focused on improved combustion 

technology for Earth applications. Practical combustion often involves turbulent 

flames at elevated pressures with at least partial premixing. These aspects suggest 

promising opportunities for future research. Turbulent combustion is exceedingly 

complex, but microgravity testing provides the opportunity to study momentumdominated flames with increased scales and residence times making the research 

more tractable. Although turbulent flames can present significant challenges for 

microgravity research, innovative experiments studying pulsed laminar flames, 

and the interaction of vortices with laminar flames (e.g., TGDF) can advance 

fundamental understanding of turbulent combustion. As previously discussed, 

high-pressure combustion is strongly influenced by buoyancy and challenging to 

investigate in normal gravity. Partially premixed combustion is found in a wide 

variety of applications and has promising potential for high fuel efficiency and low 

pollutant emission, for example in modern gas turbine combustion. 

The ACME hardware that is now in development will accommodate experiments 

with burner-stabilized, partially premixed flames. The ACME investigations, 

planned to follow the FLEX experiments, examine gas-jet diffusion flames. These 

fundamental experiments again exploit the simplified flame geometry afforded 

by microgravity to develop, validate and refine chemical kinetic models relevant 

to nearly all terrestrial combustion systems. Simplified precursor experiments are 

underway in the MSG to provide interim data to the ACME investigators that 

complement the CIR experiments. 
38

Solid Fuel Combustion/Material Flammability

New initiatives are needed to allow ongoing, long-duration, solid fuel combustion 

testing in microgravity. A robust, ground-based solid fuel combustion program 

needs to be restarted to hone and augment the flight research portfolio. Many of 

the prior research results were obtained in ground-based tests, so this investment 

is well worth the modest price. The SoFIE insert in the CIR could continue to be 

used by additional NASA Research Announcement-selected principal investigators 

who wish to study solid fuel flammability and flame spread with different geometry 

fuels in a variety of oxygen, pressure, flow and external heat flux conditions. To 

further augment the solid fuel in-orbit test capability, a Microgravity Wind Tunnel 

for fire research is being proposed for the ISS MSG. This facility would enable large 

samples to be used to study fire growth patterns, materials flammability compared 

to NASA’s STD-6001 Test 1, fire suppression testing, and post-fire cleanup 

applied research. These two in-orbit facilities complement each other and provide 

different capabilities for principal investigator use—SoFIE can accommodate 

testing in exploration atmospheres with reduced pressure and elevated oxygen, and 

the Wind Tunnel can accommodate large samples and easy sample change out. 

This investment in future research will improve our ability to evaluate material 

flammability and fire risk in low gravity by increasing our understanding of the 

actual material flammability risk for practical materials and flow configurations 

expected on future spacecraft.

Figure 19. A chaotic mixture of flamelets in an intense turbulent flame obtained from sophisticated 3-D computations 

conducted at the Naval Research Labs.
39

Premixed Flames

Combustion experts agree that 

understanding premixed flames, the basic 

unit of combustion, is absolutely key to 

understanding turbulent combustion in 

general. Burners and engines based on 

fuel-lean mixtures promise higher thermal 

efficiencies and lower pollutant emissions 

because of reduced fuel consumption 

and more complete combustion of fuel 

in oxygen-rich environments. Real-life 

accidental explosions can originate from tiny 

kernels of barely flammable mixtures (Figure 

19). Conclusive settlements of the extinction 

limits, if they exist, are of paramount 

importance. Therefore, the resolution 

of issues related to the structure and 

behavior of premixed-gas flames has great 

significance for and can have a major impact 

on many fields ranging from automobiles 

to propulsion to fire safety, and even to 

our understanding into what might spark 

colossal explosions of Type Ia supernovae.

Although premixed flames are in the current 

baseline, no funding is allocated in the ISS 

budget for a specific experiment or insert 

on the ISS. It is anticipated that future 

studies of premixed flames will allow new 

exploration of the true limits of reactivity 

under limiting conditions and have 

application to both terrestrial combustion systems and the astrophysical modeling of 

stellar ignition and explosions. Perhaps the most important parameter in fundamental 

and practical premixed-gas combustion is the laminar burning velocity, defined as the 

propagation rate of a flame relative to the background gas, as it indicates the mixture‘s 

burning intensity as well as its propensity to exhibit such critical responses as blowout, 

flashback, and combustion instability. The fidelity of computational fluid dynamics 

Figure 20. Hydrothermal flame existing in a super-critical 

core region surrounded by a sub-critical annular region 

(Wellig, B., Swiss Federal Institute of Technology, Zurich).
40

(CFD) codes for combustor design and analysis is inherently tied to the reliability of 

laminar flame velocities, either built directly into the codes or, more frequently, used 

to calibrate the chemical models employed in the codes. 

Super-Critical Processes

Hydrothermal flames (e.g., Figure 20) are defined as regions where oxidation 

reactions occur in a super-critical water medium with sufficient intensity and rates 

as to produce luminous flames. They may be utilized to solve both of the major 

issues impacting SCWO mentioned earlier; i.e., surface fouling and flow blockage 

that is due to the precipitation of salts and corrosion of internal reactor surfaces. 

This may be accomplished by confining the flame to the interior of the reactor 

within a co-flowing region of sub-critical water. Any precipitated salts formed in 

the super-critical flame region will dissolve in the sub-critical fluid and the acidity 

is diluted, thereby eliminating both precipitate build-up on and corrosion of the 

reactor walls.

Ground-based studies of hydrothermal flames have focused primarily on laminar and 

turbulent diffusion flames. Major challenges remain in the understanding of heat and 

mass transport, chemical kinetics, the mixture equation-of-state and hydrodynamics 

associated with laminar and turbulent flows. Eliminating buoyancy will lead to an 

improved understanding of laminar and turbulent flames by controlling the flow rate 

independent of buoyant acceleration. For turbulent diffusion flames, eliminating 

buoyancy will eliminate its influence on the energetic large scales of turbulence 

allowing for the development of improved turbulent sub-models.

A SCWO research facility for the ISS to investigate the effects of buoyancy in 

SCWO reactors and to enable the progression of SCWO technology for NASA 

life support application to higher technology readiness levels (TRLs) has been 

proposed. It will have a reactor with an internal volume of approximately one liter, 

two orthogonal optical views and will be designed with a removable, principalinvestigator-specific reactor base to accommodate different injection and/or 

burner configurations. It is envisioned that the facility will be placed in an Express 

Rack and will require the build-up of new diagnostic hardware, comprising two 

cameras, a collimated backlight, a Raman spectrometer and an array of temperature, 

pressure and flow sensors. It will have a wide range of science objectives including 

studying the hydrodynamics of sub-critical and super-critical mixing, reaction zone 

characterization, heat transfer in super-critical fluids and precipitate transport in 

reacting super-critical water.
41

An alternative option with considerably less flexibility and scope has been proposed. 

It is termed the SCWO-D (for DEvice for the study of Critical LIquids and 

Crystallization [DECLIC]) and envisions build-up of an insert to be installed in the 

DECLIC facility. This option, because of the constraints imposed by fitting into an 

existing facility, will be limited in capability and will only be able to accomplish a 

subset of the science scope described above. The primary limiting features are: (i) 

the reactor volume (approximately 0.5 percent of Option 1), which limits the range 

of hydrodynamic studies (e.g., no bulk fluid motion); (ii) the diagnostic limitations 

that are due to the reliance on existing DECLIC hardware (e.g., no Raman 

spectroscopic diagnostic will eliminate chemical kinetic studies); (iii) a single optical 

axis, making simultaneous reaction zone and precipitate transport observations 

difficult if not impossible; and (iv) experiments may be limited to “quasi-steady” 

processes since constant pressure conditions will be difficult to maintain for any 

significant time because of the volume limitations.

New Initiatives

Besides the SoFIE and ACME experiments planned to be conducted in CIR on the 

ISS, there are a number of new combustion initiatives that are envisioned for future 

ISS research. Some of these may be undertaken more readily in the near term as spinoffs of existing research since they require relatively smaller resources and hence may 

be faster to develop. New initiatives such as Cool Flames in CIR, NASA Flammability 

Test Campaign and Schlieren Diagnostic in MSG may fall into this category. 

Cool Flames would be an example of how one of the unprecedented findings of CIRFLEX studies can set the stage for a new research direction on combustion utilizing 

existing hardware available on the ISS. Cool Flames objectives include: (i) use a range 

of fuel types and environmental conditions (e.g., pressure, diluent concentrations, 

etc.) in order to determine the necessary pre-conditions for hot-flame/cool-flame 

transitions, cool flame stability and spontaneous re-ignition and (ii) development 

of a robust numerical modeling capability with detailed chemistry to complement 

analytical development of Cool Flame Theory. The initiative has great relevance to 

the next-generation engine designs such as Homogeneous Charged Compression 

Ignition and Reactivity Controlled Compression Ignition since these engines rely on 

low-temperature combustion processes, such as Cool Flames, to control emissions 

and reduce fuel consumption. This research also has significant impact on spacecraft 

fire safety concerns; reduced-gravity environments have been shown to be particularly 

conducive to a low-temperature combustion regime marked by very weak but 
42

persistent flames, invisible to the unaided eye, that will spontaneously re-ignite 

long after the visible flame has been extinguished. The necessary pre-conditions, the 

combustion chemistry, and the mechanisms for spontaneous re-ignition are poorly 

understood. The Cool Flames development effort would utilize existing FLEX-2 CIR/

MDCA hardware augmented with some additional diagnostic capabilities.

Other new initiatives currently envisioned include: 

• Clean-airLab: series of investigations directed toward carbon sequestration and 

pollution reduction.

• NanopropLab: self-assembling propellants and catalysts.

• CombustionLab: focusing on high pressures as related to (i) engines and (ii) 

supernova formation.

• CyberLab: focusing on developing a Fire Database Reference containing 

information on such things as transport measurements at flame conditions, 

flame speeds and chemical kinetics.

Microgravity combustion research has had significant impact on the combustion 

discipline and can be expected to continue to make strong contributions. Research 

directions in spacecraft fire safety have promise to significantly improve vehicle 

safety. The microgravity environment continues to be a critical resource for 

improved understanding of combustion phenomena. The combustion discipline has 

been very successful in utilizing low gravity to advance the field and is anxious to 

seek further opportunities.
43

Facilities and How 

to Choose Them

Acceleration Measurement and Environment Characterization

The terms microgravity, reduced gravity, low gravity, zero gravity and weightlessness 

are often used interchangeably to describe an environment where the gravitational 

body force is significantly less than that on Earth. This ambiguous description 

points to a question of how to measure and characterize a reduced-gravity 

environment and any disturbances. Disturbances in a microgravity environment 

are created by crew activities such as exercising, push-offs and landings, and simply 

opening and closing drawers and doors in addition to equipment operation, vehicle 

docking and undocking, thruster firings, and other movements. These disturbances 

must be measured to understand their impact on the data from scientific and 

engineering experiments.

The Space Acceleration Measurement System (SAMS) and the Microgravity 

Acceleration Measurement System (MAMS) provide continuous measurement of 

the ISS vibratory and quasi-steady acceleration environment, respectively. SAMS 

sensors measure acceleration levels using tri-axis accelerometers. SAMS flew on 

Mir from 1994 to 1998, continued on 20 shuttle missions and currently flies 

aboard the ISS. To complement SAMS, MAMS was developed to measure the 

very low-frequency disturbances (less than 0.01 hertz) that the SAMS sensors are 

unable to detect. Both SAMS and MAMS sensors have measured acceleration levels 

aboard the ISS since its establishment in 2001 and currently record acceleration 

disturbances in 21 unique locations aboard the ISS. SAMS measurement capability 

extends to all three laboratories, while MAMS data can be mathematically mapped 

to any arbitrary location using rigid-body assumptions.

SAMS and MAMS support NASA‘s Physical Sciences Research Program. These 

instruments, along with the Principal Investigator Microgravity Services (PIMS) 

project, serve a critical, ongoing role in support of vehicle loads and dynamics 

monitoring and assist technology developers and principal investigators in various 

disciplines. The goal is to characterize and understand the acceleration environment 

as related to a wide array of disturbances and events that routinely or uniquely take 

place on the ISS. 

Combustion Integrated Rack (CIR)

The CIR was designed to test experiments that focus on fire prevention, detection 

and suppression of fires in space. The hardware was delivered on STS-126 

(November 2008) to the ISS and installed in the U.S. Laboratory. The CIR 
44

provides a 100-liter chamber with eight optical windows with easily reconfigurable 

diagnostics, digital cameras and lighting with large data storage capability, gas 

distribution/cleanup, passive vibration isolation, and vacuum resources to support a 

wide range of gravity-dependent gaseous, liquid and solid combustion experiments. 

The current research program on gaseous fuels includes five experiments planned for 

the CIR on the ISS. These experiments are part of the ACME project where each 

will be conducted using a single modular set of hardware. The ACME experiments 

are focused on soot processes, flame stability and extinction, materials flammability 

and electric-field effects. In alphabetical order, the experiments, described in more 

detail above, include BRE, CLD Flame, E-FIELD Flames, Flame Design and 

Structure and Response of Spherical Diffusion Flames (s-Flame). The ACME set of 

experiments are expected to begin operating in 2016. As with their predecessors, 

the ACME experiments are all laminar diffusion flames. Two experiments take 

advantage of the microgravity environment by using a one-dimensional, spherical 

flame enveloping a porous spherical burner. Another experiment uses a porous plate 

burner, while the remaining two use a gas-jet flame configuration, both with and 

without a coaxial air flow.

The SoFIE flight hardware (Figure 10), involving combustion research on solid 

fuels, is currently under development as a CIR insert and planned for launch in 

2019. The SoFIE project is in the Science Definition phase to serve up to five 

separate investigations. These investigations all support the overall objective of the 

SoFIE project, which is to study and characterize ignition and flammability of solid 

spacecraft materials in practical geometries and realistic atmospheric conditions. 

The SoFIE insert in the CIR could continue to be used by additional NRA-selected 

private investigators who wish to study solid fuel flammability and flame spread 

with different geometry fuels in a variety of oxygen, pressure, flow and external heat 

flux conditions.

A brief summary of available mini-facilities/CIR inserts is tabulated in the Appendix. 

Microgravity Science Glovebox (MSG)

The MSG is a rack facility aboard the ISS in which fundamental and applied 

scientific research is conducted. The MSG has been operating on the ISS since July 

2002 and is located in the U.S. Laboratory. The unique design of the facility allows 

it to accommodate science and technology investigations in a workbench-type 

environment. The facility has an enclosed working volume that is held at a negative 
45

pressure with respect to the ISS pressurized volume. This allows the facility to 

provide two levels of containment for small parts, particulates, fluids and gases. This 

containment approach protects the crew from possible hazardous operations that 

take place inside the MSG work volume.

Research investigations operating inside the MSG are provided a large, 255-liter, 

enclosed work space, 1,000 watts of dc power via a versatile supply interface (120, 

28, + 12, and 5 Vdc), 1,000 watts of cooling capability, video and data recording 

and real-time downlink, ground commanding capabilities, access to ISS Vacuum 

Exhaust and Vacuum Resource Systems, and gaseous nitrogen supply. These 

capabilities make the MSG one of the most utilized science facilities on ISS. In 

fact, the MSG has been used for over 10,000 hours of scientific payload operations. 

MSG investigations involve research in cryogenic fluid management, fluid 

physics, spacecraft fire safety, materials science, combustion, plant growth, human 

health and life support technologies. The MSG facility is ideal for advancing our 

understanding of the role of gravity upon science investigations and research and to 

utilize the ISS as a technology platform for space exploration. 

The microgravity combustion research program has conducted the SAME project 

in 2007 and 2010, SPICE project in 2009, SLICE project in 2012, and has been 

running the BASS experiments since 2012 in the MSG facility. To further augment 

the solid-fuel, in-orbit test capabilities, a Microgravity Wind Tunnel for fire research 

is being proposed for the MSG in conjunction with the SoFIE project planned to 

be conducted in the CIR.

EXpedite the PRocessing of Experiments to Space Station 

(EXPRESS) Racks

The eight EXPRESS racks are multi-use facilities, which provide standard 

interfaces and resources for Middeck Locker and International Subrack Interface 

Standard Drawer Payloads. Payloads using single-, double-, or quad-locker 

configurations can be accommodated by these racks. The racks provide a number 

of services for payloads including nitrogen, vacuum, RS422, ethernet, video, and 

air- and water-cooling.

The proposed SCWO research facility to investigate the effects of buoyancy in 

SCWO reactors and to enable the progression of SCWO technology for NASA life 

support applications is envisioned to be placed in an EXPRESS Rack.
46

Telescience Support Center

The Telescience Support Center (TSC) was established in 1993 to provide ground 

support for shuttle experiments. With the development of the ISS in 2001, 

additional capabilities were needed to support new ISS experiments. During this 

time, the TSC was reconstructed and doubled in size. Currently, the TSC consists 

of 33 workstations that provide real-time audio and video downlinks allowing 

researchers to observe and control experimental operations aboard the ISS from 

the ground. The ISS video downlink provides a real-time view of the astronauts 

aboard the ISS interacting with the experiment. The TSC also allows researchers 

to operate and control experiments remotely with little astronaut interaction after 

the installation and setup of experimental hardware. The TSC supports projects 24 

hours a day, seven days a week; and since 2001, the TSC has logged over 26,000 

hours of continuous support of microgravity experiments aboard the ISS. The TSC 

continues to provide ground support to the ISS experiments today.
47

Complexity of the experiment should be minimized if at all possible. Science 

requirements that add complexity not only add additional cost to the development, 

but often lengthen the development time, making turnover an issue for both the 

project team and, in particular, for the principal investigator and his/her science 

team when the experiment timeline extends beyond the typical tenure of a Ph.D. 

student. It is strongly recommended to design the experiment to be flexible and 

repairable. Unlike the space shuttle experiment days, the International Space Station 

is very much like a laboratory on Earth and crew members gain good experimental 

skills while operating a particular experiment over months rather than just a week 

or so, and they can become quite adept at experiment repair if this capability is 

designed into the experiment and clear straightforward procedures are provided.

Although experiments in microgravity offer enticing rewards to combustion 

researchers, the harsh realities of experiments in reduced-gravity facilities present 

significant hardware and diagnostic challenges to scientists and engineers. Drop 

tower and space-based experiments have severe space, weight, volume, safety, 

and power limitations. The hidden benefit of these limitations have been the 

developments of technology with applications far beyond the microgravity 

combustion science program. Future capability needs for conducting combustion 

science in space certainly include advanced diagnostics, and supporting such needs 

requires more space and power. 

One of the most severe limitations of future space-based combustion science 

experiments is crew time. Astronaut availability is prioritized on the basis of many 

factors and the realistic assumption should be that it will remain as limited as it has 

been in the past. It is therefore extremely important that the space experiments be 

built with a capability for autonomous start, operation, and finish, and with as little 

maintenance as possible. The test matrices should be designed such that minimal 

crew interaction is necessary for fuel and oxygen supplies, sample and holder 

change-out, diagnostic adjustments, etc. 

Given these additional limitations beyond the challenges of the past, it is 

anticipated that upcoming, near-term NRA soliciting proposals for space 

experiments will be based on using existing hardware. NRA soliciting proposals to 

use larger, new hardware builds to enhance capability will be part of a long-term 

plan to follow in the future.

Developing and Flying 

Research on the ISS 
48

Supporting research in science and technology is an important part of NASA’s 

overall mission. NASA solicits research through the release of NASA Research 

Announcements (NRA), which cover a wide range of scientific disciplines. All NRA 

solicitations are facilitated through the web-based NASA Solicitation and Proposal 

Integrated Review and Evaluation System (NSPIRES) http://nspires.nasaprs.com/

external/. Registering with NSPIRES allows investigators to stay informed of newly 

released NRAs and enables submission of proposals. NSPIRES supports the entire 

lifecycle of NASA research solicitations and awards, from the release of new research 

calls through the peer review and selection process. 

In planning the scope of their proposal, investigators should be aware of available 

resources and the general direction guiding NASA research selection. NASA places 

high priority on recommendations from the 2011 National Research Council’s 

NRC Decadal Survey, which placed emphasis on hypothesis-driven spaceflight 

research. In addition, principal investigators (PI) should be aware that spaceflight 

experiments may be limited by a combination of power, crew time, or volume 

constraints. Launch and/or landing scrubs are not uncommon, and alternative 

implementation scenarios should be considered in order to reduce the risk from 

these scrubs. Preliminary investigations using ground-based simulators may be 

necessary to optimize procedures before spaceflight. Also, many experiments require 

unique hardware to meet the needs of the spaceflight experiment. To understand 

previous spaceflight studies, prospective PIs should familiarize themselves with the 

NASA ISS Program Science Office database, which discusses research previously 

conducted on the ISS, including that of the International Partners. A detailed 

catalog of previous, current, and proposed experiments, facilities, and results, 

including investigator information, research summaries, operations, hardware 

information, and related publications is available at www.nasa.gov/iss-science 

through the NASA ISS Program Office. Additionally, details pertaining to research 

previously supported by the Space Life and Physical Sciences Research and 

Applications Division of NASA’s Human Exploration and Operations Mission 

Directorate can be located in the Space Life & Physical Sciences Research and 

Applications Division Task Book in a searchable online database format at: https://

taskbook.nasaprs.com/Publication/welcome.cfm. 

What Should Principal 

Investigators Know About 

Conducting Research on 

the ISS?
49

There are various avenues that can result in funding for research to be conducted 

on the ISS, and the source of funding often dictates the availability of launch 

opportunities. Generally, funding for microbiology-related research is awarded 

through NASA-sponsored research announcements (NRA’s), ISS National 

Laboratory awards through other government agencies, private commercial 

enterprise, nonprofit organizations, and research awards sponsored by the ISS 

International Partners. It is not the responsibility of a researcher awarded an 

ISS flight experiment to fund costs associated with launch or the ISS laboratory 

facilities. Greater detail concerning current funding opportunities for ISS research 

can be found through the NASA ISS research website http://www.nasa.gov/

mission_pages/station/research/ops/research_information.html.

The NASA Solicitation and Proposed Integrated Review and Evaluation System 

(NSPIRES) can be accessed via http://nspires.nasaprs.com/external/.

Funding Opportunities/ 

Points of Contact

Dietrich, D. L., Ross, H. D., Shu, Y., Chang, P., and T’ien, J. S. (2000), 

“Candle flames in non-buoyant atmospheres,” Combustion Science and 

Technology, 156:1-24.

Dietrich, D. L., Haggard, J. B., Dryer, F. L., Nayagam, V., Shaw, B. D., and 

Williams, F. A. (1996), “Droplet combustion experiments in spacelab,” Proceedings 

of the Combustion Institute, 26:1201-1207.

Dotson, K. T., P. B. Sunderland, Z. G. Yuan, D. L. Urban, “Laminar Smoke Points 

of Coflowing Flames in Microgravity,” Fire Safety Journal 46, 2011, pp. 550-555.

Friedman, R. and Ross, H. D. (2001), “Microgravity Combustion: Fire in Free 

Fall,” Chapter 8: Combustion Technology and Fire Safety for Human-crew Space 

Missions, pages 525-562, Academic Press.

Greenberg, P.S., Klimek, R.B., and Buchele, D.R. (1995), “Quantitative Rainbow 

Schlieren Deflectometry,” Applied Optics 34(19), 3810-3822.

Greenberg, P.S., Ku, J.C., (1997) “Soot Volume Fraction Maps for Normal and 

Reduced Gravity Laminar Acetylene Jet Diffusion Flames,” Combustion and Flame 

108, 227-230.

Citations
50

Greenberg, P.S., Ku, J.C. (1997a) “Soot Volume Fraction Imaging,” Applied Optics 

36(22), 5514-5522.

Hegde, U., Zhou, L., and Bahadori, M.Y. (1994) “The Transition to Turbulence of 

Microgravity Gas Jet Diffusion Flames,” Combustion Science and Technology 102, 

95-113.

Law, C. K., Faeth, G. M., “Opportunities and Challenges of Combustion in 

Microgravity,” Progress in Energy and Combustion Science, Volume 20, Issue 1, 

1994, pp. 65-113.

Miller, F.J., Ross, H.D., Kim, I., and Sirignano, W.A. (2000) “Parametric 

Investigations of Pulsating Flame Spread Across 1-Butanol Pools,” Proceedings of 

the Combustion Institute 28, 2827-2834.

Musgrave, Gary E., Axel Larsen, Tommaso Sgobba, Safety Design for Space 

Systems, Butterworth-Heinemann, 2009.

Nayagam, V., Haggard, J. B., Colantonio, R. O., Marchese, A. J., Dryer, F. 

L., Zhang, B. L., and Williams, F. A. (1998). Microgravity n-heptane droplet 

combustion in oxygen-helium mixtures at atmospheric pressure. AIAA Journal, 

36(8):1369-1378.

Nayagam, V., Dietrich, D.L., Ferkul, P.V., Hicks, M.C., and Williams, F.A. (2012) 

“Can cool flames support quasi-steady alkane droplet burning?” Combustion and 

Flame, Vol. 159, 3583-3588.

Olson, S.L., Baum, H.R., and Kashiwagi, T. (1998) “Finger-like Smoldering over 

Thin Cellulosic Sheets in Microgravity,” Proceedings of the Combustion Institute 

27, 2525-2533.

Olson, S. L., Ferkul, P. V., and T’ien, J. S. (1988). Near-limit flame spread 

over a thin solid fuel in microgravity. Proceedings of the Combustion Institute, 

22:1213-1222.

Olson, S.L., Hegde, U., Bhattacharjee, S., Deering, J.L., Tang, L., and Altenkirch, 

R.A. (2004) “Sounding rocket microgravity experiments elucidating diffusive and 

radiative transport effects on flame spread over thermally thick solids,” Combustion 

Science and Technology176(4), 557-584.
51

Pearlman, H.G., and Ronney, P.D. (2994) “Near-limit Behavior of High Lewisnumber Premixed Flames in Tubes at normal and Low Gravity,” Physics of Fluids 6, 

4009-4018.

Qi, C., Chen, D., and Greenberg, P.S. (2008) “Fundamental Study of a 

Miniaturized Mini-disk Electrostatic Aerosol Precipitator for a Personal 

Nanoparticle Sizer,” Aerosol Science and Technology 42(7), 505-512.

Ronney, P.D., Wu, M.S., Pearlman, H.G. and Weiland, K.J. (1998) “Experimental 

Study of Flame Balls in Space: Preliminary Results from STS-83,” AIAA Journal 

36(8), 1361-1368.

Ross, H.D. and Miller, F.J. (1996) “Detailed Experiments of Flame Spread Across 

Deep Butanol Pools,” Proceedings of the Combustion Institute 26, 1327-1334.

Ross, H. D. (2001), Microgravity Combustion: Fire in Free Fall, Academic Press, 

London, England.

Ruff, G. A., Urban, D. L., Pedley, M. D., and Johnson, P. T. (2009), Safety Design 

for Space Systems, Chapter 27, Fire Safety, pages 829-883. Elsevier.

Urban, D.L., Yuan, Z.-G., Sunderland, P.B., Linteris, G.T., Voss, J.E., Lin, K.-

C., Dai, Z., Sun, K., and Faeth, G.M. (1998) “Structure and Soot Properties of 

Nonbuoyant Ethylene/Air Laminar Jet Diffusion Flames,” AIAA Journal 36(8), 

1346-1360.
52

A summary of available hardware and mini-facilities relevant for combustion 

research is given in the table below with some details as to the descriptive specs and 

operational parameters.

* MDCA mini-facility supports the following experiments: FLEX, FLEX-2, 

FLEX-ICE-GA, FLEX-2J (future)

** ACME mini-facility is planned to support the following experiments: BRE, 

CLD Flame, E-FIELD Flames, Flame Design, s-Flame 

*** SoFIE mini-facility is planned to support the following experiments: Flame 

Spread, GEL, Narrow Channel, SMμRF, MIST

#

 MSG mini-facility supports the following experiments: SPICE, SLICE, BASS 

Appendix 

MDCA* ACME** SoFIE*** MSG#

Up Mass (kg) 254 250 250 40

Volume (m3) 0.48 0.5 0.5 0.1

Power (kW) 0.73 0.75 0.75 0.05

Data Volume (GB) 72 72 72 10

Diagnostics analog color camera, 

radiometer, low light level 

UV camera, two HiBMs 

cameras

radiometers, two color 

cameras, low light level UV 

camera, two HiBMs cameras, 

photomultiplier tubes, ion 

current measurement, gas 

chromatograph

same as ACME digital still camera, 

video camera, air flow, 

fuel flow, radiometer

TSC capable yes yes yes yes

On-orbit yes no no yes

POC Name Mark Hickman Mark Hickman Mark Hickman Robert Hawersaat

POC Number 216.977.7105 216.977.7105 216.977.7105 216.433.8157
53

ACME Advanced Combustion via Microgravity Experiments

BASS Burning and Suppression of Solids

BRE Burning Rate Emulator

CIR Combustion Integrated Rack

CFD Computational fluid dynamics

CLD Coflow Laminar Diffusion

CM Combustion Module

CNES French Space Agency

DECLIC DEvice for the study of Critical LIquids and Crystallization

E-FIELD Electric-Field Effects on Laminar Diffusion

ELF Enclosed Laminar Flames

EXPRESS EXpedite the Processing of Experiments to Space Station

FLEX Flame Extinguishment Experiment

GRC Glenn Research Center

HTI High Temperature Insert

HTI-R High Temperature Insert (refurbished)

ISS International Space Station

LOI Limiting Oxygen Index

LSP Laminar Soot Processes

MAMS Microgravity Acceleration Measurement System

MDCA Multi-user Droplet Combustion Apparatus

MSG Microgravity Science Glovebox

PIMS Principal Investigator Microgravity Services

PMMA Polymethylmethacrylate

POC Point of Contact

SAME Smoke Aerosol Measurement Experiment

SAMS Space Acceleration Measurement System

SCWM Super-Critical Water Mixture

SCWO Super-Critical Water Oxidation

SEA Space Exploration Atmospheres

SLICE Structure & Liftoff In Combustion Experiment

SOFBALL Structure of Flame Balls at Low Lewis-number

SoFIE Solid Fuel Ignition and Extinction

SPICE Smoke Point In Coflow Experiment

TGDF Turbulent Gas-jet Diffusion Flame

TSC Telescience Support Center

ZGF Zero Gravity Facility

Acronyms
54

The Complete ISS Researcher’s 

Guide Series

1. Acceleration Environment

2. Cellular Biology

3. Combustion Science

4. Earth Observations

5. Fluid Physics

6. Fruit Fly Research

7. Fundamental Physics

8. Human Research

9. Macromolecular Crystal Growth

10. Microbial Research

11. Microgravity Materials Research

12. Plant Science

13. Rodent Research

14. Space Environmental Effects

15. Technology Demonstration
55

For more information...

Space Station Science

http://www.nasa.gov/iss-science

Facilities

http://www.nasa.gov/mission_pages/station/research/

facilities_category.html

ISS Interactive Reference Guide

http://www.nasa.gov/externalflash/ISSRG/index.htm

Researchers/Opportunities

http://www.nasa.gov/mission_pages/station/research/

ops/research_information.html

ISS Research Customer Helpline

JSC-ISS-research-helpline@mail.nasa.gov

281-244-6187
56

National Aeronautics and Space Administration

Johnson Space Center

http://www.nasa.gov/centers/johnson

www.nasa.gov

NP-2015-10-034-JSC