Presentation on theme: "John W. Thomas 1 April 2013. Overview Introduction Fire prevention in Space Low-gravity/Microgravity Fire Characteristics Fire Detection Technology."— Presentation transcript:
Overview Introduction Fire prevention in Space Low-gravity/Microgravity Fire Characteristics Fire Detection Technology / Detection in Microgravity False Alarms Fire Response Fire Suppression Technology / Suppression in Microgravity Post Fire Actions Fire Safety for Payloads Fire Safety needs for Beyond Earth Orbit Fire Safety in partial gravity Summary/Conclusion
Introduction NASA’s fire-safety record is excellent, longer future missions (Moon, Mars, Asteroids) increase the possibility of fires occurring and minimize mission termination options NASA’s primary goals as of this point have been based upon minimizing flammable materials and eliminating sources of ignition Research needs to be done in the suppression of fires in microgravity and low-gravity environments It is especially important to detect and suppress these fire incidents when they occur in the contained crew environments Even though there are strict safety policies in place fire incidents are possible during any space mission
Fire Prevention in Space The first thing done in space fire prevention is to eliminate one of the three-fire causing factors (fuel, oxygen and ignition energy). This method of prevention isn’t perfect so detection and suppression systems are always needed. Space fire suppression and detections systems follow standards set for transportation systems and aircraft. Spacecraft and Aircraft have similar fire issues Confined spaces, hostile outside environment And restricted mass, volume and power availability for these fire intervention methods.
Low-Gravity/Microgravity Fire Characteristics Considered to be combustion in a non-buoyant environment Flammability and flame-spread rate in microgravity are particularly sensitive to atmospheric flow. Flames propagate poorly in truly quiescent conditions, but they are enhanced vigorously by low-rate atmospheric flows (velocities up to about 20cm/s). Some burning plastic materials may induce flow to continue combustion through the action of boiling and vapor-jet ejection.
Low-Gravity/Microgravity Fire Characteristics This environment diminishes the effectiveness of the fire suppressant as it effects the buoyancy and natural convective flows of the typical agents used It changes the flammability characteristics of the materials used and it also effects the dispersal of suppressants Microgravity tends to complicate post-fire clean up No single method currently used works against all the different possible fire scenarios onboard a space craft
Fire Detection Technology The most reliable and original method of detecting fires onboard a spacecraft is by the crew itself. In the first manned space missions this was the only way to detect fires. However, modern spacecraft tend to have sections of the craft that are inaccessible and complex and therefore often require additional automated systems. There are several types of automated sensors aboard spacecraft and they use differing fire “signatures” to determine a fire event.
Fire Detection Technology Some of the types of fire “signatures” are acoustic waves, combustion gasses, light, particulates (smoke), pressure rise, temperature rise and other radiation. Most current spacecraft have automated early fire warning systems that detect smoke using light- scattering or ionization-current interruption. The US Space Shuttle achieves this with nine aspirating ionization-current interruption detectors. The U.S., European, Japanese, and Italian segments of the International Space Station (ISS) have detectors in each module that sense smoke through photoelectric light-beam obscuration and scattering. The ISS uses a variety of these specific detectors with some set up as area or spot detectors and others within air-ducts and aspirating detectors.
Fire Detection in Microgravity Burning tests of PMMA in microgravity done extensively. PMMA - Poly (methyl methacrylate) (Developed in 1928, bought to market in 1933 by Rohm and Haas Company under the trademark Plexiglas).
Fire Detection in Microgravity There was a shuttle based project called Comparative Smoke Diagnostics (CSD) that examined particulate emissions from typical well established pyrolysis of fire events in microgravity. The sources in this experiment included a burning candle and four overheated materials. Paper (Flaming in some tests), silicone, rubber and polytetrafluoroethylene-insulated wires and polymide –insulated wires.
Fire Detection in Microgravity CSD Experiment Near Field ○ Within the same chamber as the smoke generation ○ Smoke particulates collected on thermophoretic grids for later analysis and total smoke density is measured by laser-light extinction Far Field ○ In a separate chamber connected by a pumped hose line with smoke detector response determined for a Shuttle (STS) detector and a prototype ISS detector used in parallel.
From the CSD project the response of the ISS and STS detectors can be seen. The ionization detector is more sensitive to relatively small particles and is good for detecting a flaming fire. The photoelectric detector is more sensitive to relatively large particles and good for detecting smoldering fires. Additionally in microgravity smoke particles tend to agglomerate into larger groups due to the lack of buoyant motion. This study suggests that the ISS detector should be a good replacement for fire detection in space as it responds faster in microgravity.
Fire Detection Using Atmospheric Sampling An additional source of early warning in the case of a fire event is the rate of buildup of atmospheric gasses. This particularly includes carbon monoxide concentrations. Due to changes in the flame-zone temperature in microgravity environments the combustion and quantity of gaseous combustion products will differ than that of combustion in normal gravity conditions. Some early studies on smoldering in low-gravity indicated an increased quantity of light-gas production but this could be due to the scale and conditions used in the experiment as it has not been observed in later tests.
Fire Detection Using Atmospheric Sampling The ISS uses continuous atmospheric sampling for air-quality monitoring. This has the potential to detect the buildup of carbon monoxide as the confirmation of a fire event. Multiple-gas sensing is being considered for more effective interpretation of fire signatures, rather than the currently used single-gas sampling. In ground tests combined CO/CO2 detectors have been shown to have the ability to distinguish between non-flaming fires, flaming fires and non-fire events.
Fire Detection from Flame Radiation Originally the ISS was set to have flame- radiation sensors in the end cones for overall monitoring of the open spaces of the different modules. The need for conserved mass and electrical power eliminated this type of detector from the ISS design. The European Space Agency (ESA) is continuing to develop these flame detectors as a supplemental fire detection technology.
False Alarms False alarms can cause needless interruptions and waste suppressant in automatic systems and they can erode the confidence in the detection system. A study done by Cleary and Grosshandler reported that false alarms in aircraft cargo compartments are 100 times more frequent than actual fire events. NASA has had a better record with less than 20 false alarms or detector failures recorded during time of the Shuttle’s operation During this time only five potential fire-causing events consisting of component overheating or electrical short circuits occurred. For these five incidents the fire “signature” was not strong enough to cause a smoke detector to actuate and the crew was able to identify and correct the issue.
Fire Response In the event of a verified fire alarm, the automated or manual crew response is to first isolate the affected zone where the fire is present and then to remove the power and local or general air circulation. The assumption in this case is that without forced ventilation flow the fire in microgravity will not propagate. Research has shown that in most fire situations quiescent flames do self-extinguish. This implies that a minimum atmospheric flow rate is necessary in low gravity to maintain fire propagation (and conversely to ensure extinction). Limiting flows for relatively flammable materials are very low. Self-induced flows may be sufficient to continue combustion. U.S. investigators in a series of experiments on the MIR space station found that a candle would continue to burn in a quiescent environment for several minutes. This shows that the removal of air flow upon fire detection is a necessary response but it is not always adequate for the control of the initial fire.
Fire Suppression Technology While fire events onboard spacecraft may be rare during regular space operations, fire suppression technology must be made available for the security and safety of the crew and the mission integrity. Human-crew spacecraft have always been equipped with some way of fire extinguishment The Mercury and Gemini spacecraft a water gun that was used for food reconstitution was designed for the secondary purpose of an emergency fire extinguisher
Fire Suppression Technology In the Apollo missions there was a dedicated fire extinguisher available that generated a stable water-gas mixture propelled by an inert Freon and nitrogen gas mixture. The Shuttle and it’s payload bay lab had extinguishers that were charged with Halon 1301 Portable fire extinguishers have nozzles suitable for streaming discharge into open spaces or insertion through cover ports for flooding discharge within racks. The shuttle also has a fixed, remotely operated Halon 1301 system, for use during critical periods, such as re- entry when the mobility of the crew is limited.
Astronaut Marsha Ivins performing a Zero G evaluation of orbiter fire extinguishers while onboard the KC-135
Fire Suppression Technology The non-Russian segments of the ISS have portable fire extinguishers charged with carbon dioxide. No centralized fixed system is planned. The Russian segment of the ISS has water-foam extinguishers based upon technology already in service in other Russian spacecraft.
Fire Suppression in Microgravity Currently the key concern is to discern either the minimum quantity of suppressant needed or the minimum oxygen concentration needed to ensure suppression. The current studies base the minimum requirements for carbon dioxide as a flame suppressant in spacecraft upon the resulting oxygen concentration in the flames zone. The ISS suppression system reduces the ambient oxygen concentration to half of its original in less than 60 seconds while increasing the carbon dioxide to a minimum of 50%. The National Fire Protection Association (NFPA) standard 12 permits a minimum concentration of 34% carbon dioxide for flooding suppression systems.
The results from the experiments that went into the previous table emphasize the strong fire-enhancing actions of low flows in microgravity. As an extinguisher carbon dioxide can not only work as an oxygen diluter but also though thermal effects by reducing the temperature of the fuel surface and in the flame zone. Due to the minimal nature of this temperature reduction, the approach to determining the minimum amount of suppressant is to define the minimum amount needed to dilute the oxygen for guaranteed fire suppression.
Fire Suppression in Microgravity PMMA surface temperature at the rear stagnation point as a function of mass flow rate of oxygen. Results from this experiment showed that increasing the sample temperature made the flame more difficult to extinguish by suppressant than those that were not
Fire Suppression in Microgravity PMMA samples in flowing oxidizer after flames became stable in the low-gravity environment At the higher velocities the flame does not wrap completely around the sample Results of this show the interdependence of the flow velocity and the flame intensity and structure on the suppressant performance
Halon Phase-out and Replacement CF 3 Br - Bromotrifluoromethane By international protocol the manufacture and production of Halon 1301 is now prohibited. Halon 1301 acts as a stratoshperic ozone layer depleter. A Chemical agent that inhibits combustion by chemical reactions to remove free radicals from the reaction zone. Many Halon replacements have great promise
HFC-227ea – Heptafluoropropane Is a highly regarded Halon replacement but it requires about twice the discharge quantity as Halon 1301
Suppression of Oxygen-Generator Fires February 1997 – Russian Space Station Mir A chemical fire resulted from a failed Vika oxygen generator Fortunately no one was hurt and it did little overall damage Module and atmospheric cleanup took several days The Vika Oxygen Generator It uses solid canisters of lithium perchlorate (LiClO 4 ) and it burns these to create gaseous oxygen Lithium perchlorate decomposes at around 400 Degrees Celsius to form lithium chloride and oxygen It has the highest oxygen to weight and oxygen to volume ratio of all perchlorates except beryllium diperchlorate which happens to be highly toxic and expensive
Suppression of Oxygen-Generator Fires Astronaut Jay Apt looking at a solid-fuel oxygen generator like the one that caught fire on Mir.
Suppression of Oxygen-Generator Fires Chemical Oxygen Generators are not used in most of the ISS. The Russian segment has one as a backup oxygen source In ground tests it was shown that water based foam (the extinguishers used in the Mir incident) is the most effective agent to extinguish these types of fires. The foam must be applied directly to the surface of the generating cassette. The carbon dioxide extinguishers used by the rest of the ISS modules has been shown to be ineffective against these fires. In some cases they have actually enhanced the fire.
Other Agents for Spacecraft Fire Suppression Nitrogen is used as an alternative to Halon and carbon dioxide in small, inhabited modules of space craft (ISS airlock) It is less effective than the other methods but is inert, easily available and non-toxic Water-based mists and foams are also used. The Russian segment will continue to use the Mir-type aqueous foam suppression system The foam penetration is different in low gravity compared to normal gravity. The foam sticks to surfaces and successfully smothers the fires Obvious disadvantages of non-gaseous agents are their removal from the atmosphere and other surfaces once the fire has been contained. Also accidental release of these agents can cause serious damage to other systems in the station
Other Agents for Spacecraft Fire Suppression Experimental results in the effect of several diluent atmospheres on microgravity flame-spread and flame-extinction limits. As can be seen from this about half the required carbon dioxide is needed to accomplish flame- extinction than nitrogen
Pressure Reduction and Venting for Extinguishment Venting the Atmosphere in the event of an uncontrollable fire event is possible on the ISS The U.S. Laboratory Module has a vent/relief valve that is designed to reduce pressure Two scenarios merit this action For fire suppression the module is depressurized to a limit of 6.9 kPa in 10 minutes For hazardous atmosphere removal the module is depressurized to a limit of 2.8 kPa in 24 hr.
Pressure Reduction and Venting for Extinguishment Experiments done in low-gravity have looked into the effects of depressurization and the final pressure on the suppression of a fire These experiments suggest that if a fire is to be controlled by depressurization, the pressure in the module in question must be decreased as rapidly as possible This induces high velocities and limits the flame zone heating Slow depressurization might lead the final pressure to lower limits and complicate the suppression efforts.
Pressure Reduction and Venting for Extinguishment PMMA cylinder ignited along its axis with atmospheric crossflow This figure is a combination of the experimental data and analytical results Notable is that low-gravity suppression is much more difficult with increasing fuel temperature Extinction appears to be most difficult around a flow of 10cm/s
Post-fire Actions Burned material remains hot in the non-convective microgravity environment this means that embers may reignite if prematurely exposed to fresh air Atmospheric revitalization is necessary to remove even trace quantities of fire and suppressant contamination This may tax the environmental controls and require the use of portable air-breathing equipment for extended periods of time Despite all of the clean up when normal conditions are restored there will be trace amounts of toxic and corrosive aftereffects of the fire on the equipment, systems and payload. After the discharge of the Halon systems in the Shuttle the mission was terminated and all of the post-cleanup was done groundside.
Fire Safety For Payloads One of the most serious concerns for fire safety in the ISS and onboard the STS missions are the payloads Payloads can be anything from furnaces, energetic experiments and sensitive biological systems. There are currently many proposed methods for fire detection in payload areas Parameter monitoring (continuous monitoring for fire signatures) Automated air-cooling shutoff for preventing fires
Fire Safety For Payloads A proposed ISS payload safety system is the Combustion Integrated Rack (CIR). Common facility to provide the majority of chamber, power, diagnostics, flow and control for combustion –experiment packages Uses the standard ISS smoke detector and internal cooling-air flow
Fire Safety Needs for Beyond Earth Orbit Can be broken into two phases Travel / Transit Phase ○ Assumed unpowered and without gravity ○ Fire safety on these long term missions is crucial ○ Amounts of suppressant and spare atmosphere is limited Surface Base / Habitat Phase ○ Systems and the crew are exposed to the local gravity which will be greater than microgravity but less than Earth
Fire Safety in Partial Gravity Even less is known about the exact effects of partial gravity upon fire characteristics (0.01 to 0.6 g) Flights on the KC-135 give a short period of time in which to analyze this phenomenon From the studies that have been done, it seems that from the fuels tested that flammability range increases to a maximum between normal-gravity and microgravity levels.
Fire Safety in Partial Gravity These studies also show that flame spread also increases to a maximum at this partial gravity The increase in flammability and flame spread are thought to be caused by the generation of optimum buoyant flow velocities at this low yet finite convective environment. This effect is similar to the effect that was observed during the low-velocity forced air- flow experiments in microgravity.
Summary Fire prevention in space is usually done by limiting the possible sources of ignition. Fire Characteristics are quite different in microgravity as it is a non buoyant environment. Fire detection in microgravity is vastly different than fire detection in normal gravity as the smoke particulates can act as liquid droplets under quiescent conditions. Detectors currently used in microgravity use light obscuration and ionization-current interruption
Summary False alarms are a serious issue in space as they can cancel missions prematurely. Responding to fires in microgravity as soon as they start is very important as low air flow rates can accelerate the flame spread. Not all quiescent flames in microgravity self-extinguish
Summary Fire suppression technology in microgravity meet different requirements than suppressants that are used in normal gravity The most efficient way to extinguish flames in microgravity is by smothering rather than cooling. All post fire cleanup for the shuttle missions was done Earth-side. This however, isn’t possible for space stations.
Conclusions Fire response and detection systems need to be greatly improved for microgravity environments, especially as the shuttle detectors seemed not to function during their fire incidents. Many of the studies examined here show data that conflicts with itself. More funding needs to be given to NASA to study the effects of microgravity and partial gravity on fire characteristics in order to build a better knowledge base about this topic.
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References 6 Olson, S.L., Beeson, H., and Haas, J., "An Earth Based Equivalent Low Stretch Apparatus to Assess Material Flammability for Microgravity & Extraterrestrial Fire-Safety Applications," NASA CP-2001-210826, 2001, pp. 409-412. 7 Ruff, G. A., Hicks, M., Mell, W., Pettegrew, R., and Malcom, A., “CO2 Suppression of PMMA Flames in Low-Gravity,” NASA CP- 2003-212376, 2003. 8 Takahashi, F., Linteris, G.T., and Katta, V.R., "Physical and Chemical Aspects of Fire Suppression in Extraterrestrial Environments," NASA CP-2001-210826, 2001, pp. 417-420. 9 Urban, D.L., Mulholland, G., Yuan, Z.G., Yang, J., Cleary, T., ""Smoke:" Characterization of Smoke Particulate for Spacecraft Fire Detection," NASA CP-2001-210826, 2001, pp. 401-403.