Fire Safety in Spacecraft

FIRE AND MATERIALS, VOL. 20,235-243 (1996)

Fire Safety in Spacecraft* Robert Friedman NASA Lewis Research Center, Mail Stop 500-115, 21000 Brookpark Road, Cleveland, OH 44135, USA

Fire prevention, detection, and suppression requirements for spacecraft are based on those established for terrestrial and aircraft systems. In the weightless (or microgravity) environment of an orbiting spacecraft, however, the buoyant upward flow typical of fires in terrestrial environments is nearly absent; and this feature profoundly influences fire characteristics and responsive safety strategies. This paper reviews the findings of microgravity-combustion research that are relevant to techniques of spacecraft fire safety. These practical applications are further illustrated by descriptions of some fire-safety requirements and design features of the Shuttle and those in progress for the International Space Station.

INTRODUCTION

The spacecraft cabin is a confined volume with little room for fire-protection equipment and no means of escape while in orbit. Fire events, even though they have a very low probability of occurrence, are considered serious threats and greatly feared. No fires have occurred during US human-crewed space flights, but at least five minor incidents of electrical short circuits or component overheating have been reported on Shuttle missions.’ Crew members quickly responded to these potential fire- precursor events by disconnecting the affected circuits or components and alleviating the threats.

A major impediment to the assurance of fire safety in spacecraft is the limited understanding of the unusual characteristics of incipient fires in the low-gravity, weightless environment of orbiting spacecraft.’ This paper reviews the knowledge of fire behavior in the space environment, using analyses and data from low-gravity combustion research as a basis.3 The paper also describes the application of this information to fire-protection de- signs and operations for spacecraft, particularly in examples of the fire-safety technology on the Shuttle and that proposed for the International Space Station.

THE SPACE ENVIRONMENT AND FIRE-SAFETY RESEARCH

Fires typically have large density gradients created by the high temperatures in flames. On the ground, because of gravity, these density gradients generate a substantial upward buoyant flow. Orbiting space vehicles, in contrast, are in a state of near-equilibrium with a balance of centrifugal and gravitational forces. Residual gravitational accelerations are extremely low (of the order of

or less of the Earth sea-level value of 9.8 m s - ~ ) . Buoyant forces and flows are nearly eliminated in this ‘microgravity’ environment even when very large density gradients are present. Thus, heat and mass-transport

rates and, consequently, ignition, flammability, fire characteristics, and flame-spread rates differ considerably from those ordinarily encountered in terrestrial fire situations.

The study of combustion processes in microgravity has expanded considerably in recent years, motivated by the opportunity to improve scientific knowledge through analyses of simplified non-convective systems and by the necessity to resolve problems in on-orbit fire The application of this growing body of microgravity combustion science to useful spacecraft fire-safety tech- nologies and operations has proven to be difficult, however.5 Furthermore, fire prevention, detection, and suppression functions in spacecraft are as much constrained by the limitations in volume, mass, power, and funding as by the need to respond optimally to the peculiar at- tributes of fires in microgravity.

In themselves, spacecraft are the ideal laboratories for studies and scale-model demonstrations of combustion and fire behavior in microgravity. Indeed, the Shuttle and the forerunner US space station, Skylab, have provided access for some significant combustion experiments. The test opportunities are limited, however, by the infrequency of missions, the competition for payload space, the high cost of operations, and the severe safety restrictions. Instead, many investigations are conducted in ‘free-fall’ ground-based facilities that can furnish low-gravity environments approach- ing those in spacecraft, albeit for very short durations. These facilities include (1) drop towers, where experiment pack- ages fall for distances that provide up to 10 seconds of microgravity, (2) airplanes flown over parabolic trajectories that provide up to 25 seconds of low gravity (about lo-’ of sea-level gravity), and (3) sounding rockets, launched into sub-orbital paths (parabolas) and recovered, that provide 10 minutes or more of mi~rogravity.~

FIRE BEHAVIOR IN SPACE

Fire precursors and smoldering

Evidence has shown that microgravity environments can promote ignition in many cases or inhibit it in other

*This article is a US Government work and, as such, is in the public domain in the USA.

CCC 0308-0501/96/050235-09 0 1996 by John Wiley & Sons, Ltd.

Received I0 July 1995 Accepted 10 March 1996

236 R. FRIEDMAN

situations. Since natural convection is nearly absent, heat transport from objects to the surrounding atmosphere is greatly reduced. Hence, the temperature of thermally stressed components may continue to rise rapidly, increasing the chances of overheating and ignition. A 1992 Shuttle experiment, Wire Insulation Flammability (WIF), demonstrated that the heating rates and maximum temperatures attained by overloaded electrical wires under microgravity conditions were greater than those reached in reference normal-gravity air condi- tiom6 (In fact, temperatures were comparable to those reached under non-convective vacuum conditions.)

The microgravity environment also greatly reduces typical density-driven motions in space. That is, the expected downward settling of particles or upward buoyancy of gases does not occur, creating a hazard following a liquid or powder spill. The resulting mixture remains as a 'cloud' in the atmosphere, which disperses very slowly. For many substances, this aerosol can be highly flammable. Another unique space hazard is that of the ejec- tion of hot liquid or vapor globules from small fires. The Shuttle WIF observations showed such bubbles and molten fragments issuing from the burning wire insulations, as clearly seen in Fig. 1.6 In normal gravity, these fragments would drip downward and cool, but in microgravity they are propelled outward and may serve as hot ignition sources.

On the other hand, research has shown that two burning surfaces may interfere with one another in micrograv- it^.^ Ignited parallel sheets of paper separated by distances of 1 cm or closer show no flame in the gap although a stable flame spreads on both external surfaces. In normal gravity, the flame envelops the sheets completely. The microgravity behavior is likely due to the accumula- tion of combustion products, a phenomenon also noted by the extinguishment of candle flames when a second candle is brought into proximity to a n ~ t h e r . ~

The non-convective microgravity environment may also promote smoldering (a slow, low-temperature pyro- lysis and oxidation reaction, often manifested in overheated porous materials). In a 1992 Shuttle experiment, cylinders of polyurethane foam were ignited to produce self-sustained ~moldering.~ The measured temperatures were low, with peaks of 350-400°C, much cooler than typical combustion temperatures. These temperatures were slightly higher (of the order of 15OC) than those in corresponding normal gravity, no doubt due to reduced convective cooling. The most significant difference between microgravity and normal-gravity behavior was the large increase in light-gas (e.g. carbon monoxide, carbon dioxide, and methane) evolution in microgravity. Ana- lyses of carbon monoxide, for example, showed concentrations of 100-4000 parts per million (ppm) in microgravity compared to traces of a few ppm in normal gravity, due possibly to oxygen-transport deficiency in the space environment.

Flammability and flame spread

As noted, normal-gravity fires always generate substantial buoyancy-driven flows around the flame zone. This natural-convective flow, typically with speeds of 1 m/s or greater, promotes flame spread and continued oxygen

Figure 1. Polyethylene-insulated wire burning in microgravity. Flame spread and air flow are from left to right. Note the dark molten insulation around the wire and the bright globules in the flame zone.

transport into the flame zone. In microgravity, a truly quiescent flame environment can exist. Despite the static conditions, tests show that ignition and flame spread can occur in initially quiescent microgravity, at least for thin sheet materials. In these quiescent environments, the flame spread is uniform but slow, at rates as low as 15% of those expected in corresponding normal-gravity conditions.'~~ Also, the minimum, or limiting, oxygen concentration for flame spread is greater in quiescent microgravity than in normal gravity (i.e. the flammability range is reduced). These differences are illustrated in the plots of Fig. 2, which summarize experimental data on burning rates of thin paper, taken from several sources.2310-12 For reference, curve A shows typical downward flame-spread rates in normal gravity as functions of atmospheric oxygen concentration. The corresponding spread rates under quiescent microgravity, curve B, are considerably lower. The flammability range under quiescent microgravity, as indicated by the higher limiting oxygen concentration, is also reduced. The differences in flame-spread rates between quiescent-microgravity and normal-gravity environments diminish for higher atmospheric-oxygen concentrations, where the flame zone is well supplied with oxygen and replenishment by convective flow is less important.

In a practical sense, the spacecraft atmosphere is often in motion due to ventilating flows. A superimposed (forced) flow, even at a very low speed, has a strong influence on microgravity fires. In Fig. 2, curve C shows experimental data on thin-paper flame-spread rates and flammability under microgravity with air flow (at speeds no greater than 6 cm s- ') in a direction opposed to the flame spread. With the forced air flow, the microgravity flame-spread rates exceed those found in quiescent microgravity, and they approach the rates noted for normal gravity. Low-speed air flows in a direction concurrent to the flame spread (curve D) increase the microgravity flame spread further and can cause rates, at least over part of the oxygen-concentration range, that exceed

FIRE SAFETY IN SPACECRAFT 237

A B C D

2.4

2.0

I .a

n

Normal gravity, downward Microgravity, quiescent Microgravity, 6 cm/s opposed flow Microgravity, 5 cm/s concurrent flow

Limiting oxygen

concentration

D

- 10 20 30 40

Oxygen concentration in atmosphere, vol. YO Figure2. Summary of test results on the burning of thin paper at a pressure of 101 kPa. Data points are shown only for cases C and D.

those measured in normal-gravity downward flame spread. (Upward normal-gravity flame-spread data are not shown for comparison, since these flames spread non-uniformly .)

The cited comparisons are taken largely from ground experiments on the burning of thin materials, which are necessary as test fuels to accelerate the burning processes in the short time durations of available microgravity. A few tests in spacecraft, however, have studied flames over thicker, more representative materials, including smoldering cotton and flaming wire insulations and plas- tic sheets.6.’ Preliminary data from a multiple-mission Shuttle program, the Solid Surface Combustion Experi- ment, showed that the flame spread over relatively thick (3.2 cm) sheets of polymethylmethacrylate (PMMA), in contrast to the uniform spread observed for thin materials, is non-uniform, decreasing as the flame progressed along the fuel and leading to potential self-extinguishment.I4

Flame characteristics and detection

Flame appearance in microgravity is variable and dis- tinctly different from that in normal gravity. Thin materials burning under quiescent conditions generally exhibit pale blue, almost invisible, flames. In contrast, those burning under fire-promoting conditions of forced flow or higher-oxygen concentrations often have bright, sooty flames. A typical example of a flame propagating over thin paper is shown in Fig. 3. The flame forms separate lobes on each side of the fuel surface with distinct standoff distances, in contrast to the usual enveloping flame of normal g r a ~ i t y . ~ The leading edge of the flame (on the right in the figure) is pale, but the body of the flame is bright because of the oxygen-rich, high-pressure

Figure 3. Edge view of flame over thin paper (invisible) in microgravity with the flame spreading from left to right. The leading edge of the flame is pale blue, but the remainder is bright yellow.

conditions of this Shuttle test example. A different microgravity flame configuration is shown for the burning wire-insulation example (with forced air flow) in Fig. 1. The flame surrounds the fuel and is roughly spherical in shape with an open trailing edge. Recent European Space Agency flammability tests conducted in an airplane facility also observed that, while paper and cloth fires in quiescent air are barely visible, temperature measurements indicate nearly complete combustion.I5

The radiant, gaseous, and particle emissions from incipient fires serve as ‘signatures’, or critical indicators for detection, and their constituent analysis and morpho- logy is important. As noted, smoke and radiation signatures from fires in spacecraft can show great variability from near invisibility in quiescent conditions to strong sooting and bright color under fire-enhancing conditions. Research also indicates that the component particles in microgravity smoke plumes differ in size and shape from those in normal-gravity plumes. For example, analyses of smoke particles from overheated fluorinated-polymer wire insulations in microgravity showed a mean particle size about a factor of two greater than that in normal-gravity emissions.16 The overall range of particle-size distribution is also greater in microgravity, with a larger proportion of aggregates formed from the primary particles. This agglomeration is prob- ably the result of the longer residence time and the low mass-transport rate in microgravity. The nature and quantity of gaseous products from incipient and established combustion in microgravity can also differ from those usually observed in normal gravity. As cited above, Shuttle smoldering tests showed the interesting result that a considerably increased quantity of light gases is released in microgravity compared to that in normal g r a ~ i t y . ~

238 R. FRIEDMAN

Fire extinguishment

Early tests conducted on the US Skylab spacecraft investigated fire extinguishment in space through small-scale demonstrations of water sprays and atmospheric venting.* The water spray was effective but difficult to control. If insufficient water struck a burning material, the momentary disturbance scattered hot particles. Extin- guishment by vacuum was also effective, but flame intensification caused by the forced-convection flow was noted prior to extinguishment. Other research has been conducted to determine the effect of inert gases added to the atmosphere on the reduction of flame-spread rate and self-extinguishment of thin-paper fuels.' ' These tests are useful in evaluating the relative influence of dilution gases on flammability limits in microgravity, but they do not reproduce the physical processes of agent addition and suppression of established fires.

Current research is also investigating the effectiveness of venting for flame suppression." Horizontal, 1.9 cm diameter PMMA cylinders in upward air flows of nom- inally 10 cm s-' were ignited by a hot wire along the bottom surface, and the resulting fires were then sup- pressed by venting. Results of normal-gravity tests indicated that the minimum pressure level for extinction decreases with increasing fuel temperature, implying that fires become more difficult to extinguish with time. Cor- responding low-gravity tests showed similar extinction- pressure limits, given identical fuel temperatures (but note that fuel-temperature histories differ in microgravity compared to normal gravity). The flame intensification noted in the Skylab tests was prevented by a slow rate of venting.

APPLICATIONS TO SPACECRAFT FIRE PROTECTION

General policies

The cited research provides ample evidence that fires can initiate and spread in the microgravity spacecraft environment, particularly if low levels of forced or ventilation-induced flows are present. Thus fire protection is essential on human-crewed spacecraft. For the US Shuttle, the first line of defense is a strong reliance on fire prevention through strict material and ignition-source controls. The second line of defense, that of responsive measures, is also provided by an on-board fire detection and suppression system.

For the International Space Station (ISS), a permanently orbiting multinational habitat, laboratory, and workshop now under construction, fire prevention is also emphasized. The probability of fire-threatening events is obviously higher in the ISS than on Shuttle missions, because of the much larger orbiting volumes, more com- plex operations, and longer mission times. Thus, the definition of fire detection and suppression requirements and operations will be more critical for the ISS than for the Shuttle.

This section of the paper presents some general prin- ciples of fire protection for human-crewed spacecraft as

they relate to the understanding of fires in microgravity. Some examples will be cited from applications on the Shuttle. The following section will then concentrate on details of fire safety proposed or in process for the ISS.

Fire prevention

Materials used in habitable volumes of spacecraft are selected from those meeting prescribed criteria of resistance to flame spread. For example, common materials in the form of sheets, foams, and coatings undergo the Upward Flame Propagation Test, which is a normal- gravity test in a configuration where the flame spread is aided by the direction of buoyancy. These tests are conducted in a sealed chamber filled with air or a representative spacecraft atmosphere (Fig. 4).19 After exposure to a promoted ignition source for 25 s, a material is accept- able if it either fails to propagate a flame away from the igniter or burns for a distance less than 15 cm. Ac- ceptable materials also cannot scatter hot particles capable of igniting a paper sheet mounted below the specimen.

A databank of thousands of items permissible for ser- vice on-board spacecraft on the basis of fire resistance is available." Out of necessity, rather than mere expedi- ency, the testing of spacecraft fire-safety technology, including fire detection and suppression equipment as well as material-flammability screening, is performed on the ground. The normal-gravity flammability assessments are severe tests, and they have been assumed to be con- servative with respect to flammability in all environments. Nevertheless, current research shows that in some circumstances (under ventilation flows, as one example) materials may be more flammable and have similar, or even greater, flame-spread rates in microgravity as compared to the normal-gravity reference Hence, the safety factor assumed in the general reliance on the normal-gravity material database may be non- existent for some material applications.

,- Test t-l 5 cm typical Substrate for testing of coatings -

Typical specimen - 1

Paper t Limiting flame spread height

sheet below ' specimen ;

-A-

/

:hamber

T T ?y I

Figure 4. NASA Upward Flame Propagation Test apparatus for qualification of spacecraft materials.


Obviously, there are a number of articles necessary for use on-board spacecraft that cannot qualify as non-flammable by any criterion. These items include paper, film, cotton clothing and toweling, some foam padding, and some commercial instruments and components. The end- use locations and configurations of these articles are determined prior to each mission, and potential flame- propagation paths are minimized by separation and con- tainment.20

Spacecraft atmospheres

The standard atmosphere of the Shuttle and the ISS is air, i.e. a mixture of 21 vol% oxygen in nitrogen at 101 kPa total pressure. Control tolerances permit the oxygen concentration to rise to a maximum of 24%.22 In addition, for prescribed periods of time to accommodate crew preconditioning prior to extravehicular activities, the spacecraft atmosphere is altered by the removal of nitrogen to yield a maximum concentration of 30% oxygen at a total pressure of 70 kPa.

The higher oxygen-concentration atmospheres can increase the flammability of many materials, both in normal gravity and microgravity.20 Other hazardous atmospheric compositions are also possible. Leakage from water electrolyzers and life-support regenerators and products from smoldering events may introduce flammable constituents, such as hydrogen, methane, or carbon monoxide, into the atmosphere. Not only can these contaminants exceed their lower flammability limits in a local zone, but their presence in the atmosphere may also enhance the flammability and flame spread of acci- dentally ignited solid materials.23

On the other hand, certain spacecraft atmospheric compositions can inhibit flammability while supporting life. Such modified atmospheres are those with minimum oxygen partial pressures or with high-heat-capacity dilu- ents replacing part or all of the n i t r ~ g e n . ~ . ~ ~ The utiliza- tion of 'fire-safe' atmospheres on spacecraft remains a future desire, since considerable study is necessary on the technology of maintaining these atmospheres and on their long-term effects on the crew and exposed materials. One must also recognize that many spacecraft experiments assume the presence of an unmodified air atmosphere in space for comparability to reference experiments on the ground.

Fire detection and response

The development of fire-detection systems capable of effective and efficient operation in spacecraft is severely hampered by the still-incomplete understanding of the characteristics of flame appearance and smoke density and the transport of these signatures in the microgravity environment. Of necessity, spacecraft fire detectors are adaptations of established terrestrial and aircraft types. Currently, the Shuttle and its pressurized cargo-bay laboratories use ionization-current-interruption smoke detectors. The detectors have integral fans for aerodynamic separation of large dust particles and for effective sampling in the absence of b~oyancy .~

The immediate response to any incident, whether de- tected by the crew or by automated detectors, is removal

of local power and air flow (cooling or ventilation). The air-flow shutoff is, of course, a commonsense approach to limit the escape of flames and gases. Fortuitously, research shows that air-flow removal makes true quiescent conditions possible in microgravity combustion, which inhibits (but does not necessarily prevent) flame spread.

The development of fire-suppression systems capable of effective and efficient operation in spacecraft also is hampered by the meager information on physical- dispersion patterns, extinguishment characteristics, and atmospheric cleanup procedures in the microgravity environment. As with detection, spacecraft fire-suppression measures are largely adaptations of established terrestrial and aircraft systems. Currently, the Shuttle and its pressurized cargo-bay laboratories have portable fire extinguishers charged with Halon 1301 (bromotri- fluoromethane). A crew member can flood the hidden interior of an equipment compartment by inserting the extinguisher nozzle through an access hole on the cover panel. The Shuttle also has fixed, remotely actuated fire- extinguisher systems in each of three electronic bays for protection during periods when the portable extinguishers are inaccessible (e.g. re-entry). The manufacture of Halon 1301, which has a high ozone-depletion potential, is now prohibited by international protocol; but existing installations such as those on the Shuttle may be retained indefinitely.

FIRE-PROTECTION PROVISIONS ON THE SPACE STATION

General features of the Space Station

The International Space Station (ISS), jointly managed by the USA and international partners, will be a collec- tion of interconnected volumes permanently orbiting the Earth for scientific, exploratory, and commercial activities in space. Figure 5 is an illustration of the ISS installation as partially assembled (the Initial Science Capability) to a degree of completion that permits the retention of an atmosphere and the support of a human crew. Figure 6 shows the configuration of one of the pressurized volumes, the US Laboratory module. Typi- cally, the US Laboratory will contain banks of racks in four orthogonal positions surrounding the open central core. The fairings between the racks and module walls, called standoffs, hold interconnecting plumbing and cabling. Figure 7 illustrates the general structure of a rack. The racks comprise the basic construction units in each pressurized ISS compartment, with common de- signs capable of accommodating varied applications, such as operating and environmental systems, scientific and commercial payloads, crew-habitation quarters, and storage.

Fire prevention is emphasized in the ISS safety strat- egy by strict material selection, low electrical-wire current ratings, fusing, and electrical grounding. Additional fire prevention is provided by barriers, such as sealed cases for energized equipment and fire-resistant graphite- epoxy rack wall construction, to limit potential fire-

240 R. FRIEDMAN

system radiator

Temporary truss ssian universa \ docking adapter

Russian research \ , module#l

U.S. node 1

Figure5. Sketch of the International Space Station, as assembled in orbit at the 'Initial Science Capability'.

\L Standoff Endcone

Figure 6. Sketch of a pressurized compartment of the Interna- tional Space Station, the US Laboratory Module.

propagation paths. System racks (those with environmental, communications, or power equipment, for example) will also have smoke detectors and ports on the rack covers for insertion of portable fire extinguishers. Fire-protection requirements for the payload racks with custom-designed contents are still being resolved. The installation of detection, suppression, or combined systems in payload racks will depend to a great extent on whether forced air cooling is present, whether the payload is contained and meets fire-resistance standards, or whether the payload is powered only under the control of a crew member.

Fire detection

The ISS is to have photoelectric smoke detectors, which operate on the principle of scattering and obscuration of a light beam by smoke particles. The photoelectric detector is selected over the ionization detector of the Shuttle because of the simpler construction, reduced mass, and lower power consumption for the photoelectric type. Figure 8 shows a model of the detector pr~totype. '~ A laser light beam originating in the electronic housing passes through the sampled duct and

Electronics housing, with laser source and detectors 7

Figure 8. Mockup of photoelectric smoke detector for the Inter- national Space Station (Allied-Signal Aerospace). Space Station.

FIRE SAFETY IN SPACECRAFT 24 1

reflects back to sensors in the electronic housing. One sensor in the direct light path responds to light attenu- ation due to obscuration by smoke particles. A second sensor offset from the beam path responds to light scattered by the particles. Upon the attainment of the detection setpoint, both a local visual indication and a general alarm actuate. The crew may also initiate an alarm if an abnormality is observed prior to detector actuation.

Atmospheric sampling for signatures is hampered by the absence of buoyant flows (essential aids to ceiling- mounted terrestrial detectors), although most ISS installations will be in ducts with fan-promoted flows. Note also that research has shown that incipient fires in microgravity under some conditions appear to be smoke-free. The sensitivities selected for the ISS smoke detectors presume that dilute smoke emissions from fire precursors exist and are detectible, even if invisible. Established alarm settings also must be trade-offs between levels for rapid response to real incidents and those to minimize false alarms. The optimization of alarm settings as well as the regulation of sensitivity and location of detectors requires information on microgravity fire precursors. Current data on microgravity smoke characteristics have been obtained largely in drop-tower tests, which can only model the early stages of fires from laminar diffusion gas jets and severly overheated wire insulations. A Shuttle experiment conducted in March 1996 investigated smoke emissions from model fires in microgravity, including those of a candle under concurrent ventilation flow (to model soot from a typical hydrocarbon fuel) and four overheated materials-flight-data file paper, silicone rubber, Teflon (polytetrafluoroethy1ene)-insulated wire, and Kapton (po1yimide)-insulated wire (Fig. 9).26 A Shuttle detector and prototype ISS detector were exposed to the smoke flow downstream of the combustion chamber to determine their sensitivities and response times to the incident and established fires. Data from the experiment are not yet available at the time of writing, however.

Sensors other than smoke detectors have also been proposed for the ISS, particularly flame-radiation sensors. Radiation detection relies upon line-of-sight observations for its effectiveness, and it is best suited for monitoring open cabin spaces. While flame detectors

Smoke generator for science experi- (overheated wire,

f=) 4 e-g.) 7

module (-J module

Figure 9. Concept of Comparative Soot Diagnostics project to determine smoke characteristics and evaluate detector response in microgravity.

were once installed on the early Skylab space station, they were eliminated from consideration as alternative detectors on the ISS because of the need to conserve mass and electrical power. The European Space Agency (ESA), however, is actively investigating the application of ultra- violet and broad-band optical flame detection for its spacecraft modules.27

Another supplemental means of fire detection proposed for the ISS is sensing for trace constituents, princi- pally carbon monoxide, that are indicative of incipient fires or smoldering. Again, the need to conserve mass and electrical power will likely limit gas sensing to discrete sampling for environmental control rather than to con- tinuous monitoring for early warning of fires.

Fire suppression

Each inhabited compartment on the ISS will have one or more firefighting stations containing a portable breath- ing apparatus charged with oxygen and a portable fire extinguisher charged with carbon dioxide (Fig. 10). The ISS extinguisher has two nozzles available: one is a coni- cal nozzle for streaming discharge in open volumes, and the other is a tube adapter for insertion into rack cover- panel ports for flooding discharge in concealed spaces. It is possible that some racks or free-standing experiment payloads will have fixed, internal suppression systems for localized fire control.

Carbon dioxide is specified as the suppression agent on the ISS as a replacement for the Halon 1301 used on the Shuttle. Carbon dioxide offers advantages of future avail- ability, a proven technology, and no halogenated products. At present, suppression quantity and discharge rate are specified conservatively, based on the dilution of atmospheric oxygen in an affected zone to half the initial concentration. As with detection, it is apparent that the direct application of normal-gravity technology is un- suitable. There are uncertainties regarding the control of the physical dispersion of the agent, the optimum

Fluid r Pressure \ outlet 1 gauge

Own area nozzle

View 14-14 attachments stowed

I 3 5 - O m a x . q - I 1 Enclosed I

Storage tank volume inter- 1 connect tube J

,

Figurelo. Design sketch of the carbon dioxide portable fire extinguisher for the International Space Station (dimensions in cm).

242 R. FRIEDMAN

discharge concentration for rapid extinguishment with minimum use of agent, and the removal of excess, possibly toxic, concentrations of agent from the spacecraft atmosphere and surfaces.

Nitrogen has been proposed as an alternative agent for suppression on the ISS to avoid the toxic hazard of excess carbon dioxide, but the ability to use existing technology and its superior efficiency favors the use of carbon dioxide.17 The performance of an alternative non-gaseous extinguishing agent, aqueous film-forming foam (AFFF), was demonstrated by ESA investigators in low-gravity airplane tests.I5 The AFFF adhered to surfaces and formed a film as it does in normal gravity, implying that it would effectively suppress fires in microgravity by excluding oxygen. However, the difficulty of removal of excess agent and its products dispersed in the atmosphere makes the use of such nonigaseous agents impractical for the ISS.

The ISS also has the option of venting the local atmosphere to the vacuum of space as a last resort to control a difficult or inaccessible fire. Compartment venting, actuated by the crew or from the ground, requires that a sufficiently low total pressure for suppression be attained within 10 min of initiation. This prescribes a rea- sonable but arbitrary limit on the venting rates. Too slow a rate obviously delays suppression, but too rapid a rate may cause initial flame intensification and physical de- compression damage.

Post-fire recovery

A space station is intended for permanent orbital status; the return to the ground for post-fire repairs is not an option. In particular, the restoration of the atmosphere by removal of fire and extinguishment products must be performed in orbit. While the environmental-control system is designed to remove trace condensed and gaseous contaminants from the atmosphere, the quantities released in even a modest fire event can overwhelm the existing system. Innovative large-capacity emergency cleanup systems are under consideration.28 An addi-

tional post-fire concern is that of the subtle, long-term toxic and corrosive effects from fire and extinguishment residues that may not be recognized until injury or damage occurs long after a fire event.

SUMMARY OF ISSUES AND NEEDS IN SPACECRAFT FIRE SAFETY

Fire prevention, detection, and suppression in spacecraft, of necessity, use techniques and standards adapted from those of terrestrial and aircraft systems. In the weightless, or microgravity, environment of orbiting spacecraft, flammability, flame spread, and combustion products can differ considerably from their counterparts in normal gravity. Microgravity-combustion research is actively investigating these aspects of fire behavior, but quantitative results applicable to practical spacecraft fire safety are, as yet, limited. As a result, current fire-safety provisions may well be, at best, overdesigned and wasteful, or at worst, inadequate for protection in certain fire situations.

The needs in spacecraft fire safety are summarized by the following critical topics that focus microgravity- combustion research and technology towards fire-protection applications:

(1) The correlation of material flammability in microgravity to results of normal-gravity flammability testing, to extend the existing database

(2) The improvement of fire detection in spacecraft by applying information on the detectible characteristics of fire precursors with effective sampling techniques and alarm criteria

(3) The improvement of fire suppression in spacecraft by applying information on physical dispersion, response time, and effectiveness of extinguishing agents

(4) The development of methods for the removal of combustion and extinguishment products from the spacecraft atmosphere and surfaces to ensure a safe environment for the crew and continued mission in- tegrity.

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Fire Safety in Spacecraft