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Aviation

Our past clients include the Department of Defense (Air Force, Army, Navy, DTRA, etc.), FAA, NTSB and their contractors; aircraft manufacturers (Boeing, Airbus, Lockheed Martin, Northrop-Grumman, Embraer), airframers (GE, Pratt & Whitney), and aircraft sub-system manufacturers (Parker and Goodrich) and airlines. We provide software, hardware, component and system level technology/safety assessment, specialized testing, specialized training (aircraft fire course) and expert witness services. We have worked extensively in the lethality, vulnerability and survivability of aircraft systems and components. Examples of our past work are provided below:

Flammability of Advanced Aircraft Seat Materials

A major safety hazard in an on-board aircraft fire is the seat materials: both the fabric upholstery and the foam. R&D on polymer flammability has been going on for many years to develop materials with a low potential for ignition, fire spread and the generation of smoke and toxic combustion products. Major advancements have included brominating conventional materials to make them fire resistant as well as the development of thermally stable polymers (such as phenolics, polyimides, polysulfone, PEEK, and polyphenylenesulfide).

In a seminal study, BlazeTech's founder concluded that direct replace- ment of commercial fire-resistant materials with these "advanced" thermally stable materials cannot be contemplated in the immediate future because of the cost and inadequate mechanical performance of the latter (cf. Materials for Fire Resistant Passenger Seats in Aircraft, by G.C. Tesoro and N.A. Moussa, J. of Consumer Product Flammability, Dec. 1977, pp. 201-616). Instead, he recommended judicious selection of state-of-the-art fire-resistant materials, and the innovative use of a heat barrier between fabric and foam. Such a barrier is now a common practice in the airline industry along with fire-resistant materials.

Our forte is in bridging the gap between theory and practice to solve real problems and develop software and advanced technologies in a timely and cost-effec- tive manner. Technology development includes conceptual designs, proof-of-concept tests, proto- type development, field testing, and analysis.

BlazeTech provides consulting and testing services in both the physical and chemical aspects of material flammability. We even have the ability to model gas phase kinetics of halogen- dominated mech- anisms typical of fire supres- sion environ- ments. Our past experience with detailed inorganic halogen chemistry is state-of-the-art.

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Hot-Surface Ignition and Fire-Suppression Tests in an Aircraft Engine Bay

Hot-Surface ignition tests were conducted on full-scale aircraft (the F-18 and the AV-8B) and on simulated engine nacelles (F-16). For the AV-8B, the conditions in an engine bay that promote the ignition of flammable fluids, and the suppression of subsequent fires, were investigated. Ignition by hot surfaces has been studied in the past under laboratory conditions or in simulated aircraft engine bays. This test was conducted under full-scale and realistic engine-bay conditions using an operable turbofan engine installed in an aircraft. The engine consisted of a forward zone (fan, compressor, and accessories) and an aft zone (combustor, turbine, and exhaust plenum). In the aft zone, key metal surface temperatures exceeded the fluid Autoignition Temperature (AIT), and ignition was observed. In the forward zone, surface temperatures exceeded the AIT but not the Hot-Surface Ignition Temperature (HSIT), and ignition was not observed. These ignition results, coupled with an examination of the heating conditions of each zone, suggest that suitable temperature criteria for ignition are the AIT in the aft zone and the HSIT in the forward zone. Fires in the engine bay were repeatedly extinguished by an automatic suppression system. When a manually actuated suppression system was used, with its inherent time delays, the fire was not extinguished.

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Vaporific Effects in Aircraft Vulnerability and Survivability

Background

The vaporific effect refers to the flash fire observed with the impact of high velocity projectiles against metallic targets. The impact produces small, finely divided particles originating from either the projectile, the target, or both. These particles are heated by the impact forces and can burn in the presence of air (oxidizer). The result is a metal-dust-type explosion with potentially significant overpressures inside the structure. This effect is particularly pronounced with pyrophoric metals (that react immediately upon exposure to air) such as aluminum.

Due to their high strength to weight ratio, composites are being considered for the replacement of aluminum in aircraft structures. Composites were also believed to provide improved survivability under ballistic impact because of the absence of the vaporific effect.

Recently, the Air Force at Wright Laboratory (WL/FIVS) in Dayton, Ohio, conducted ballistic tests of graphite-epoxy composites and aluminum panels in an empty tank (dry bay) and in open air under identical conditions. Under some conditions, the pressure and temperature rise in the tests with a composite panel exceeded the values with aluminum. This raised concern about the contribution of composites to aircraft survivability.

WL/FIVS wished to conduct additional tests in this area. BlazeTech was tasked to review the results of the Air Force and related work by other agencies (if any), and to identify the type of measurements that should be carried out in future tests. This report presents the results of this small project by BlazeTech.

Summary

Historically, ballistic studies have attempted to determine whether vaporifics is a chemical or mechanical effect, and conflicting claims have been published. The relative importance of these two processes depends on the characteristics of the target, projectile and environment -- and thus, it may differ from one set of circumstances to another.

Over the years, NASA has sponsored a significant amount of research on hypervelocity impacts, particularly on the development of shields (or bumpers) to protect spacecraft by absorbing the impact of meteorites or space debris. These studies are typically conducted in the absence of air (to simulate space conditions) and at much higher velocity than those used at WL/FIVS. (NASA's lowest velocity is near the upper end of the Air Force, about 2.8 km/s or 9,200 fps.) These two differences accentuate the mechanical over chemical effects in the NASA work. Since our results in this study indicate that chemical effect pre-dominate for the test conditions of WL/FIVS, the NASA results are generally not relevant. Still much can be learned from NASA's novel experimental techniques and their qualitative description of multi-wall impacts.

Basic flammability properties of composites have been measured at NIST including piloted ignition by thermal radiation, heat and combustion products generation rates, lateral flame spread rates and flame heights. In these tests, composites are typically subjected to longer-duration and lower-intensity heat fluxes than those of interest to our study. More relevant studies addressed the thermo-mechanical response of composites to intense heating from nuclear explosions, laser heating and the lining of rocket nozzles. Both experimental and analytical work have been conducted in this area. The analyses covered one-dimensional heat transfer, initially, and more recently three-dimensional models, including mechanical considerations. Such models show promise despite the complex response of the composite materials.

Ritter's work on vaporifics for WL/FIVS involved ballistic tests of composites and aluminum panels in a 1 ft3 empty box simulating a dry bay, at a striking velocity range of roughly 4,000 to 10,000 fps. He found impact with an aluminum panel produces a white flash fire characteristics of combustion of aluminum particles, while impact with a graphite-epoxy panel produces an orange hydrocarbon type flash fire. He attributed the latter to the combustion of volatiles originating from the resin in the composite. (Many of his composite test results were overshadowed by the use of an aluminum back panel, however.)These qualitative observations were corroborated by Blythe in ballistic tests involving single panels in free air. However, the two studies differed significantly in the reported flash durations: 3 to 10 milliseconds for Blythe vs. up to several 100 milliseconds for Ritter.

Furthermore, the static pressure rise in the tank was roughly twice as high for a composite than an aluminum panel, and the gas temperature remained hot for a longer period, which suggests a higher potential for tank damage.

On the other hand, in a few tests with pre-mixed fuel/air in the tank, Ritter found ignition to require a higher impact velocity for composite than aluminum (6800 vs. 3600 fps, respectively). Similarly, Blythe found that the occurrence of projectile shatter and a flash fire (on the exit side of the panel) require a higher projectile velocity for composite than aluminum targets.

Our project consisted of a characteristic energy analysis during ballistic impact, including: the kinetic energy of the projectile, the chemical energy of the projectile and panel debris, the heat of combustion of volatiles and carbon fibers the heat transfer from the flame to the panels and the walls, and the enthalpy loss during tank blow-down. We estimated the theoretical maximum energy, its time scale, and the amount of air consumed, drawing upon the test data of WL/FIVS.

Our results showed that under the effect of the high-intensity, short duration flash fire, a very steep temperature gradient develops in the composite panel. Volatiles out-gas from a very thin outer layer in the resin, exposing carbon fibers to the air inside the box. The volatiles burn in the flash fire raising the tank pressure (over the case of an aluminum panel). They can also produce a fuel rich condition inside the box, extinguishing the fire therein, while combustion continues outside where air is plentiful. This latter finding is corroborated by Ritter's time-sequence photographs. (In a fuel tank, air starvation would be even more severe.)

After flash extinction and box blow down, carbon fibers react with air arriving through the entrance and exit holes in the box. Based on our calculation, this combustion mode is the only exothermic process that provides sufficient energy release over the long time interval (500-1000 milliseconds) observed experimentally. This is a more likely explanation of the observed temperature- time histories for composite than the volatile combustion proposed by Ritter

The maximum theoretical energies were tractable by our analysis. Even more important are the actual energies released and their rates, but they are more difficult to calculate. Accordingly, we propose to measure them in future ballistic tests.

First, we recommended that future tests include: (1) use of a box (not free panels) with volumes representative of real aircraft structures to faithfully duplicate the availability of oxygen; (2) use of test panels along all walls in the path of the projectile; (3) better simulation of heat transfer to the tank wall; (4) testing with and without fuel vapors inside the box (which affects air starvation inside the box); and (5) use of two boxes (simulating a dry bay in front of a fuel tank) to incorporate multiple impacts effects which are important.

Second, in addition to standard ballistic measurements, we proposed calorimetric measurements to determine the heat released in the gas phase during the vaporific event. They include dynamic measurements (with sub-millisecond resolution) of heat flux, static pressure, and temperature inside the tank during the test, as well as key measurements before and after the test. A simplified method is presented to perform the required mass and heat balance calculations based on the measurements.

In summary, the data suggested that a composite panel requires a higher speed to break up the projectile and produce an impact flash fire. Until then, it offers an advantage over aluminum. Once a flash fire is produced, radiant heat from the burning debris cooks off a thin outer layer in the rest of the panel (which does not occur with aluminum). The resin devolatilizes leaving behind carbon fibers. The volatiles burn in the flash fire raising the overpressure in the dry bay, while the remaining carbon fibers burn more slowly and sustain the temperature in the tank for several 100 milliseconds. Under these conditions, a composite panel would provide higher overpressure than an aluminum one. The implication of this result to dry bay survivability has not been addressed in this study.

Finally, we recommended that a modeling effort be developed to describe the above processes. The model can be integrated with impact mechanics model such as FATEPEN 2, adding modules that handle combustion, heat transfer and overpressure. The modeling effort should also be integrated with the experimental program to serve as a tool for test design and result interpretation, and as well as a basis for rational decision on survivability improvements.

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Ullage Flammability in Aircraft Fuel Tanks

For the Navy, BlazeTech reviewed a model and test data on the flammability of ullage in an aircraft fuel tank.  The model assumed well-stirred ullage and equilibrium thermodynamics, which limited its applicability.  We showed that a zonal model is more realistic and provides the generality sought after by the Navy while being simple enough to run on a personal computer.  In this model, stratification was related to tank inclination and a Rayleigh number. Out-gassing of dissolved oxygen and nitrogen was related to heat and mass transport processes in the fuel.  These effects enable predictions of dynamics effects (climb/dive rates, altitude and fuel temperature) during a mission profile as well as fuel tank design features such as nitrogen inerting and oxygen scrubbing. (See also BlazeTank.)

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Simulation of External In-Flight Aircraft Fires

For the Air Force, we conceived of a novel approach to simulate fires external to aircraft in flight using scale model planes in a wind tunnel. We formulated a plan to validate this approach in a unique test facility (500 lb/sec) operated by the Air Force. Also, we carried out the fire safety aspects of a national study on the impact of switching aircraft fuels from JP4 to heavier hydrocarbons. The study was led by General Electric. We also participated in a study led by Boeing on the ignition potential of aircraft fuels by friction and impact sparks.

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Effect of Clutter on Aircraft Fire Suppression Systems

For the Air Force, we modeled the thermodynamics and dispersion of fire suppression agents in aircraft bays.  Our focus was on candidate Halon replacement agents with minimum ozone depletion potential such as FC-31-10, PFC-614, FC-218, and HFC-227; and on dry bays and engine nacelles containing "clutter".  We modeled liquid flashing, droplet formation, spray behavior, droplet impingement on clutter surfaces, drag effects of clutter, and flame stabilization in the wake of clutter.  The model was used to compare agent performance with and without clutter. Also, we designed a clutter simulant consisting of a uniform network of thin, orthogonal, baffles placed normal to the direction of flow in a staggered fashion.  Tests were conducted at the Air Force using this network with various baffle widths and spacings.  The results showed significant improvements in the simulation of clutter effects over previous configurations.

Conclusions:

  • Effects of clutter depend on physical and thermodynamic properties of agent
  • Tests showed that clutter had a significant effect on agent selection
    • ranking of agent performance with/without clutter was not the same
    • clutter requires more agent mass to extinguish fire
  • Real life performance is complex and requires detailed evaluation by engineering and computational models such as MIFAC

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Hazards to Fuel Tank from Uncontained Engine Burst

For the FAA, BlazeTech examined two potential hazards of engine debris impacting fuel tanks aboard commercial aircraft: fuel vapor ignition and hydrodynamic ram.   The first can occur when sufficiently hot and large debris (such as a turbine blade) enters the ullage of a tank containing flammable vapors.  The second hazard can occur when debris enters the liquid fuel with sufficient kinetic energy. Either of these events can produce overpressure inside the tank with a potential for tank rupture and an immediate loss of the aircraft. The conditions resulting in these hazards were characterized.

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Jet Fuel Fire Hazards

For a commercial client, we conducted a comparison of the properties for a number of aircraft fuels: JP-4, JP-5, JP-8, up to JP-20.  We examined two potential fire scenarios:  flammability of the aircraft fuel ullage and pool fires due to spill during refueling.  For the first scenario, we used a thermodynamics model of the ullage to related flammability to fuel properties such as volatility, LFL and UFL, etc. For the second scenario, we modeled heat/mass transfer for an evapo­rating fuel films. Thus, we related the evaporation rate to fuel properties, ambient conditions, and a newly defined "fuel wet bulb" temperature.

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Aircraft Fire and Explosions

For several clients, we conducted major studies on fires and explosions in aircraft engines, fuel tanks and dry bays.  This included the design of critical tests that were conducted in national test facilities using simulated engine components and full-scale aircraft (AV-8B and F-18).  We analyzed the data and developed theories to interpret and generalize the results.  We also evaluated the effectiveness of quick-response explosion detection and suppression systems. A novel approach was conceived to simulate fires external to aircraft in flight using a scaled model of planes in a wind tunnel. The scaling requirements between model and prototype were geometric similarity; matching the Damkholer number; and placing limits on the Reynolds and Mach numbers and the ratio of chemical reaction to droplet evaporation time.

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Hydrodynamic Ram Effects on Aircraft Structures

For the Navy and the FAA, we analyzed the response and failure of metal aircraft structural panels subjected to hydrodynamic ram pressure loading.  Transient in-plane and out-of-plane displacements of the panels were calculated using COSMOS/M and simplified engineering models.  Furthermore, we developed a new method to protect tanks against hydrodynamic ram.  Based on our design, a brass-board system was built and proof-of-concept tests were carried out successfully at a Navy facility. 

An Assessment of the Crash Fire Hazard of Liquid Hydrogen Fueled Aircraft

In this study, we compared crash fire hazards of mission equivalent, 400 passenger, Mach 0.85, 5500 n. mile range aircraft for three types of fuel. These fuels were liquid hydrogen, liquid methane, and conventional jet fuel. The two cryogenic-fueled designs had tanks located in the fuselage; the conventional fuel aircraft had wing fuel tanks. All the designs were based on published Lockheed studies.

For purposes of comparison, we considered four crash scenarios ranging from minor releases to a catastrophic crash. In each scenario, the potential fuel-release and crash fire consequences were compared for the three types of fuels.

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