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Fire in the Air
An onboard fire ranks high on the list of things pilots don’t want to experience. Prevention, detection, protection and elimination are vital pieces in the onboard fire puzzle.
By Ginger Bennett, Mathias L. Kolleck and J. Michael Bennett, Ph.D.
In most cases, fire is either the primary cause, or a contributing factor, to loss of aircraft assets. This includes combat and noncombat situations. In many instances, injuries to personnel and loss of mission capability accompany a fire event. Methods and technologies to mitigate fires or "design them out" are imperative, not only to save aircraft, but also to save lives and prevent property damage.
The military fire protection community has been developing fire protection technologies for aircraft for years. Fire prevention efforts on military aircraft are focused on the engine nacelles (the region surrounding the exterior of the jet engine case, shrouded by an outer cover and typically ventilated), the dry bays (which can include wing leading/trailing edges, landing gear, avionics and weapons bays) and the fuel tanks. Historically, fuel fire and explosion have been a major cause of aircraft losses in combat. Data from Southeast Asia showed that more than half of the aircraft combat losses involved fuel fire and explosions where the combustion overpressure generated exceeded the structural strength of the tank. To help address this problem, fuel tank protection systems are used on military aircraft to protect the ullage. Ullage is the void space above the fuel level in a fuel tank and it can have a potentially explosive fuel-air mixture. If initiated by a combat threat, an explosion can result.
The goal of the survivability discipline is the early identification and successful incorporation of those survivability enhancement features that are cost-effective and allow the weapon system to accomplish its mission. Alternatively, if the loss of the aircraft is inevitable, the survivability enhancement features should allow a graceful degradation of system capabilities, giving the crew additional time to depart the hostile area.
There are three main categories of fire protection systems: passive, active and reactive. Passive protection systems (which generally require no electronics, wiring, brackets/hardware, power or crew interface) are activated upon the initiation of a fire event. Passive protection technologies usually only mitigate the potential for fire ignition, not extinguish it. Active systems respond to the activation of a fire through the use of fire detectors. However, these systems require the crew to be notified and to take the time to discharge the fire extinguisher-time that could increase damage. The final option is the use of a reactive system, which reacts to the initiation of an explosion and automatically discharges a substance that is intended to suppress the explosion by either physical or chemical means. Reactive systems monitor the occurrence of fire, and upon detection, it releases an extinguishing agent. However, reactive suppression systems can be complex and must integrate numerous subsystems. Often, there are increases in cost, weight and volume penalty and the potential for failure and false alarms exists. As a result, some programs have been forced to forego needed fire protection and accept vulnerability.
The Air Force has been exploring the strategic placement of intumescent materials (a passive technology) within the aircraft engine nacelle for fire protection. Intumescent materials respond to the impingement of a fire by swelling and forming a protective coating to physically and thermally protect the coated structure. Intumescent materials come in several different forms that include coating/paint, tape, caulk/sealant and putty.
The intumescent coating can be applied as a very narrow and thin strip in a form of one or more closed rings on the exterior of the engine core. These rings are located to swell against the enclosure at locations where clearance is minimal. This swelling would block the downstream airflow path in the vicinity of the fire, depriving it of a steady flow of oxygen and facilitating self-extinguishment. If the blockage is only partial, and the flame follows the redirected airflow around the sealed-off area, the local intumescent-covered portion in that region would also swell, creating a series of "fire-walls." If an extinguishing system is also used, the intumescent material can improve its effectiveness. Previous analysis performed by the Air Force suggested feasible application for engine nacelle spaces and intumescent materials have been used (or investigated for use) in various military platforms for all three services and for various commercial applications.
This technique may be sufficient in many cases to permit the omission of an extinguishing system altogether. This could prove enticing to platforms with weight and volume restrictions such as the Joint Strike Fighter and unmanned aerial vehicles.
Hot Surface Ignition Mitigation
The ignition of leaking fluids onto hot components, such as a bleed air duct in an engine nacelle, can be a significant contributor to fires and results in asset losses. Testing of this phenomenon has been extremely difficult to replicate consistently. In addition, existing techniques, such as the use of insulation, to mitigate hot surface ignition are heavy and costly. The preferred fire suppression approach is to keep fire from starting.
A recent study developed a predictive tool to determine the combination of heated surface temperatures, fuel types and flow rates that would result in ignition and fire and could assist in modifying equipment to avoid operating under such conditions.
By far, the most interesting ignition mode of this study was the occurrence of suspended, airborne ignition kernels located a substantial distance above the surface, which quickly spread downward to engulf the entire fume hood region (defined as "air" ignitions).
The Los Alamos National Laboratory discovered a new rocket propellant that is impact insensitive, nonexplosive, nonpyrotechic and an inflammable solid that decomposes rapidly without flame and produces nitrogen-Bis(aminotetrazolyl)- tetrazine, or BTATZ. Because of these qualities, BTATZ has been identified as a composition highly suitable for fire suppression applications.
The potential for its use provides possibilities of entirely new "out of the box" fire suppression systems. The properties of BTATZ suggest system simplification and lightweight packaging are possible. This could be accomplished using vacuum packed molded bricks, powder packs, or conceivably even no packaging (with the use of propellant paint, etc.).
Several issues, such as toxicity, impact of the hydrogen production, stability and sensitivity, need to be resolved before applying BTATZ in "real world" situations.
The Navy is investigating propellant "scale-up" production methods, effluent analysis and species measurement, and chemical suppression enhancement/additives. There is additional work underway to investigate the application of BTATZ as a powder pack enhancement and further development of BTATZ "paint." Additional work was cosponsored by V-22 research and development to investigate the powder pack enhancement with the use of BTATZ, to include a conductive binder that would reduce static sensitivity, to produce test quantities of the propellant, and to demonstrate the concept in a full-scale aircraft fire scenario. BTATZ will also be investigated for its ability to withstand the aircraft engine nacelle environment.
Reactive Powder Panels
Standard powder pack technology includes a lightweight, brittle, honeycombed panel filled with a fire suppressant powder-usually aluminum oxide. The panel is normally affixed to a dry bay wall adjacent to a fuel tank. Projectile damage to a powder pack results in release of some powder into the dry bay to prevent ignition of leaking fuel. However, some limitations exist with this current design. Fire suppressant powder is dispersed solely through kinetic energy transfer from the projectile to the powder panel and the amount of dispersed powder is limited to the region of projectile penetration. Most of the suppressant can remain encased within the powder panel, unused and "wasted." Fuel and incendiary dispersion can be much more extensive than the powder dispersion. The application of powder packs must then usually be restricted to smaller dry bays, with little or no airflow. Usually, additional passive technologies (such as self-sealing fuel cells) are combined with the powder packs to achieve a more effective protection level. This results in increases in cost and weight penalties.
Recently, the Navy demonstrated two alternative technologies (reactive and enhanced) that provide dramatic improvement over current fire protection powder panels. The reactive powder panels are commercial powder panels with reactive energetic backing. The enhanced powder panels are totally redesigned powder panels.
The reactive powder design incorporates a small amount of impact sensitive pyrotechnic (BTATZ) thinly painted on the surface of the commercial powder panel. The powder panel is then affixed on top of this painted surface. When a round impacts the panel, the pyrotechnic is initiated and results in removal, breakup and discharge of the "entire" powder panel from the wall. Pyrotechnic gases effectively disperse the fire suppressant powder.
To be effective, BTATZ must be initiated by bullet impact almost simultaneously along its entire surface. However, BTATZ is impact insensitive and does not react immediately. To solve this problem, a dual layer of BTATZ and an additional impact sensitive initiator material can be sandwiched between the powder panel and the dry bay wall. This energetic initiator activates on impact and initiates the entire surface of the BTATZ.
Baseline powder panel tests were performed for comparison to the reactive and enhanced powder panel designs. This testing included demonstration testing of the concept versus actual dry bay simulator fires. Both the reactive and enhanced powder panels showed dramatic improvement in fire protection performance over standard commercial panels.
Simple Passive Extinguisher (SPEX)
The Simple Passive Extinguisher (SPEX) concept focuses on fire protection system simplification with minimal, or no, supporting subsystems. An ideal application of the SPEX concept would place an agent, such as BTATZ, within the volume to be protected. The reactive agent would be sensitive to the characteristics of a fire (heat, smoke and potentially light).
Several fire suppression vendors have products already designed to utilize heat as a primary activation mechanism. Commercial off-the-shelf technologies can be applied to SPEX at the simplest system level-heat reactive fire bottles and pyrotechnic suppressors with heat sensitive initiators. System retrofit would involve installing SPEX fire suppression canisters or bottles near fire-prone regions; the squadron maintenance level could perform the work.
Assuming the benefits (due to lack of detector, activation hardware) of the SPEX/BTATZ concept, the fire suppression system could be approximately one-sixth the weight of an equivalent active system. Since the SPEX/BTATZ concept has not been commercially produced to date, the cost benefits are unknown but should be similar. Using a SPEX/BTATZ concept with a chemical suppression additive could result in a synergistic enhancement. The fire suppression system could be approximately one-twelfth the weight of an equivalent active system.
Linear Fire Extinguisher (LFE)
Projectile-induced ullage explosions are usually generated by a specific sequence of events. The elapsed time from ballistic impact to a full-on explosion is milliseconds. The LFE system, initiated by detection of projectile function or fragment impact flash, operates within the same millisecond time frame and is expected to create a "protected" ullage space before damaging overpressures are developed from the ensuing explosion
The LFE system consists of an optical sensor, a hollow thin-wall stainless steel tube for extinguishant storage and a combination detonator and flexible linear shaped charge (FLSC) mounted over the exterior of the tube for extinguisher discharge.
Some advantages of the LFE system include: speed (response within five milliseconds); suppressant speed-1,000 feet per second; detectors; one channel IR fiber optic; efficient distribution; and low weight (mostly suppressant). Some disadvantages of the LFE system include: power consumption; detector technology lags; ullage overpressure with halon; and reaction forces from tube.
Discussions with government personnel indicate that a LFE test program is scheduled to be performed at Wright-Patterson AFB, OH.
The Parker Reactive Explosion Suppression System (PRESS), by Parker Hannifin, is designed to be installed in aircraft fuel tanks and react to and suppress fuel tank explosions. It consists of an optical detector, transmission lines and a suppression tube(s) containing a water/brine solution. This system is designed to respond within a few milliseconds to engage the flame front and reduce pressures below damage causing levels.
Some advantages of the PRESS system include: fastest responding system-allows less suppressant; lighter weight; system designed for liquids like water; tank overpressure problem not evident; and nozzles allow directed flow of suppressant. Some disadvantages of the PRESS system include: requires large scale proof-of-concept testing; more complex system-chance for malfunction despite high reliability components; and possible expense in manufacture.
Parker Hannifin representatives stated that the PRESS technology has been shelved due to technical and funding issues.
Ionomer Self-Healing Fuel Containment
The ionic forces in ionomer plastics provide a "self healing" capability. When the plastic is destroyed at impact, it instantly reforms in a similar manner as when a bullet passes through water. This characteristic lends itself to fluid containment applications.
Current materials used in "self-sealing" backing boards adjacent to fuel tanks incur some damage (a hole) after a ballistic impact. This may allow fuel leakage into the dry bay through this hole. An ionomer backing board would be expected to "self-heal" after impact and could provide additional containment of fuel. Other potential applications include: self-healing fuel lines; self-healing hydraulics containment covers/linings; and self-healing gearbox oil containment covers/linings.
The objective is to develop and demonstrate a simple and low-cost alternative/enhancement to current self-sealing fuel cell technologies. Commercially available ionomers (properties, types and suppliers) are being investigated, materials are being acquired, the ballistic response and containment are being tested, and the analysis and results are being documented.
Recent events have demonstrated the need for cockpit hardening of aircraft. The high impact strength of ionomers also suggests possible applications in cockpit hardening. Should time and funding permit, additional testing may be conducted to evaluate ionomers' resistance to bullet impact as a function of its thickness and layers.
In most cases, fire is either the primary cause, or a contributing factor, to loss of aircraft assets. Aircraft fires are a significant cost to the Department of Defense. Methods and technologies to mitigate them or "design them out" are imperative, not only to save aircraft, but also to save lives and prevent property damage. The military fire protection community has been developing fire protection technologies for aircraft for a number of years. Most of these fire protection technologies are light weight, effective, and can be applied to military aircraft.
The authors extend special thanks to Joseph A. Manchor and Richard B. Mueller, both from NAWD-WD, China Lake, CA, and to Peggy Wagner, 46OG/OGM/OL-AC, Wright-Patterson Air Force Base, OH.