Aircraft Survivability Newsletter List
Aircraft Survivability is published quarterly by the Joint Aeronautical Commanders' Group, Joint Technical Coordinating Group on Aircraft Survivability.
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The aircraft survivability discipline can be considered in terms of two distinct categories, susceptibility reduction and vulnerability reduction. Susceptibility reduction deals with the likelihood of an aircraft being detected, acquired, tracked, engaged, and hit by a hostile threat, expressed as probability of a hit, or Ph. Vulnerability reduction seeks to minimize the likelihood of an aircraft being rendered incapable of completing its combat mission as a result of such a hit and is described as the probability of a kill, given a hit (Pk/h).
This issue of Aircraft Survivability focuses on a particularly complex and challenging aspect of vulnerability reduction: fire and explosion suppression. Consider a combat aircraft in flight: large quantities of highly volatile fuel and munitions, together with a massive induced airflow. To add a potential source of ignition and keep the resulting mix from burning or exploding is a daunting task indeed.
The problem is further complicated by a number of factors. Weight, space, and safety considerations severely limit potential solutions for aircraft. Cost, as always, is a major constraint.
More recently, environmental considerations have led to the strict regulation, and even banning, of ozone-depleting substances such as halon. The impacts of the Montreal Protocol and the amended Clean Air Act on aircraft fire suppression systems have been enormous. We have included several articles in this issue which discuss the search for acceptable, and effective, halon replacements.
As always, we welcome your feedback on Aircraft Survivability. Tell us what we're doing well, and more importantly, let us know how we can improve. Remember, this is your journal. Help us make it the best it can be.
John N. Lawless, Jr.
On September 22, 1994, Mr. John J. Over received the Award for Meritorious Civilian Service in recognition of his distinguished performance as Air Force Representative to the JTCG/AS from March 1990 to April 1994 and as JTCG/AS Director from December 1992 to April 1994. The accompanying citation notes that Mr. Over's leadership and technical expertise contributed immeasurably to increasing the survivability of aircraft acquired and operated by all branches of the United States Armed Forces. The award was presented at Wright-Patterson Air Force Base, Ohio by Mr. James F. Bair, Director of Engineering and Technical Management at Headquarters, Air Force Materiel Command. Our sincere congratulations to John for being chosen to receive this well-deserved honor.
Due to an internal reorganization at the Naval Air Systems Command (through whom we receive our mail), both our internal routing code and ZIP+4 have been changed. EFFECTIVE IMMEDIATELY, ALL MAIL FOR THE JTCG/AS CENTRAL OFFICE SHOULD BE ADDRESSED AS FOLLOWS:
NAVAL AIR SYSTEMS COMMANDOur telephone and fax numbers remain unchanged. Please update your records accordingly.
Brig. Gen. Raymond A. Shulstad (l) presents Air Force Meritorious Civilian Service Award to Ralph Lauzze (r).
Dale Atkinson (l) receives the 1994 AIAA Survivability Award from AIAA President, Dr. Malcolm R. Currie (r).
The large photo on the cover shows a geometric simulation of a V-22 wing section suspended from a holding fixture being used in V-22 Gas Generator Fire Suppression Tests for the Wing Dry Bays. Photo of V-22 Osprey shown in lower left corner. Test photo provided courtesy of Leo Budd & Hardy Tyson, Naval Air Warfare Center, China Lake, CA. V-22 Osprey photo courtesy of Bell Boeing.
Q: Describe your affiliation with the JTCG/AS over the years.
A: The first exposure I had to the JTCG/AS was when I moved to the Combat Survivability Branch in 1978. At that time, and until 1989, the Central Office of the JTCG/AS was collocated with NAVAIR's Combat Survivability Branch. LTCOL Reemers was the Chairman of the JTCG/AS when I first arrived at the office. During this period I had peripheral involvement with the JTCG/AS and, at times, had some specific tasking such as trying to establish an ad hoc committee on integration of JTCG/AS developments into service aircraft. I assumed the duties as Navy Principal Member in August 1990 and became Chairman in August 1994.
Q: What changes have you seen in your organization during this time?
A: Until 4 or 5 years ago, the Naval Air Systems Command had not changed drastically; at least the changes did not have a significant effect on the Combat Survivability Branch. However, approximately 5 years ago, the Naval Air Systems Command was reorganized when Program Executive Offices were establish. These offices, although collocated NAVAIR, were no longer organizationally part of NAVAIR and reported directly to the Assistant Secretary of the Navy for Acquisition. Most Program Managers report to these Program Executives and, as such, do not directly fall under the cognizance of NAVAIR. However, NAVAIR still provides all support to the programs (e.g., engineering, logistics).
A change that directly affected the Combat Survivability Branch was in 1992 when the Combat Survivability and the Electromagnetic Environmental Effects Branch were combined due to downsizing. This combination created three Sections in the Branch, (E3, Vulnerability, and Susceptibility).
However, the biggest change is yet to come. Starting this October 1994, the Naval Air Systems Command along with its field activities are reorganizing into a competency aligned organization. In other words, the Naval Air Systems Command and its field activities will be aligned by technical area without regard to geographic boundaries. This is being referred to as a seamless organization. This reorganization will not be completed until October 1997 and includes base closures and realignments as specified by BRAC 91 and 93. NAVAIR specifically is scheduled to move to Patuxent River, Maryland, during the summer of 1997.
Q: What are the top two or three challenges your organization faces over the next 5 years? How will you meet them?
A: From a survivability standpoint, I believe our organization has two main challenges over the next 5 years. The first is the ability to maintain a core group of highly trained research and development engineers. With the DoD and the Navy downsizing and with base closures and realignments, it is going to be very difficult to keep our highly trained survivability people. I am already seeing a drain of some of these people due to the unwillingness of some people to move or because of the incentives being offered for early retirements. To met this challenge, we must ensure an adequate training program for new employees to include both classroom and on-the-job training in the survivability discipline. We also must develop adequate databases so the corporate memory of those who leave is not lost forever. We have initiated programs to establish survivability databases and to develop an adequate survivability training program. These two efforts will continue and be improved upon during the next 3 years.
The second challenge that I see is ensuring that adequate survivability requirements are specified for new and modified weapon systems in light of the DoD policy not to use military specifications and standards. Although I believe that waivers should be obtained for some of our survivability specifications since there are no commercial equivalents, it is obvious we will be unable to do that for all specifications. Accordingly, we must be able to specify clear survivability standards as performance requirements that are easily measured.
I think the two challenges, although discussed from a survivability standpoint, are challenges to all disciplines, not just survivability.
Q: With the almost certain continuing dramatic reductions in defense spending, how are survivability requirements faring now in Navy acquisitions?
A: Although Naval Air has fewer acquisitions, survivability requirements are faring very well for the programs we have. Both the V-22 and F/A-18E/F have extensive survivability requirements as well as survivability analysis and testing programs. Program Managers now realize the importance of the survivability of their weapon systems and the survivability discipline in general. With smaller inventories of aircraft, survivability becomes even more important as a force multiplier. The one concern I do have, as I indicated previously, is out ability in the future to ensure adequate performance-type survivability requirements without using military specifications or standards.
Q: Where do you see the emphasis being placed on modeling and simulation over the next 5 years?
A: Well, I would hope it would be in the following areas:
Q: What impact has/will the SMART program made/make in system acquisition?
A: I have not seen an impact on system acquisition yet due to the SMART program. However, as additional objectives of the SMART program are achieved, I anticipate two impacts on system acquisition. The first is more trusted and credible analysis of survivability to support design decisions by program managers. The second is the eventual reduction in actual testing being performed due to the confidence of our models.
Q: What will be the primary emphasis in model VV&A over the next few years?
A: I think the primary emphasis will be in moving from the pilot SMART program to a definitized procedure for V&V of all survivability models. By that time the SMART program will have defined the process for V&V models and the structure and content of accreditation support documentation. I would hope that the JTCG/AS would play a major role in this new definitized procedure.
Q: What impact, if any, will the base realignment and closure (BRAC) commission actions have on survivability analysis and testing capabilities.
A: That's hard to say since we don't know what's going to be the result from BRAC 95. However, I don't believe the BRAC 91 and 93 decisions have had any significant impact on survivability analysis or testing capabilities. Even as bases close, those required testing facilities are moved. However, it will be interesting to see the decisions made in BRAC 95. I look for some survivability testing and analysis facilities to be combined, at a minimum.
Q: What influences will affect aircraft survivability testing in the next 5 years?
A: Aircraft survivability testing may be affected in many ways over the next 5 years. The future successes of the SMART program and any other VV&A programs as well as the general improvement of our methodologies may reduce the amount of testing required because of more confidence in our modeling techniques, thus reducing the testing costs to Program Managers. Obviously the political influences will always remain which can have a major effect on how we perform survivability testing.
Q: What are your perspectives for the future of the survivability discipline as we face military drawdown? Has the survivability "war" been won?
A: I believe that the future for the survivability discipline is bright if we remain vigilant. As the total aircraft inventory shrinks, survivability becomes even more important as a force multiplier. As mentioned earlier, we must ensure that we have an adequate number of trained survivability engineers to support program offices and research and development. Also during this downsizing, we want to ensure that corporate knowledge is not lost as experienced engineers retire and leave the Government. The survivability battle has been won, but the war is not over. We must ensure that the accomplishments achieved by the survivability community over the years are not significantly affected by downsizing.
Fire and explosion pose major threats to weapon systems and their personnel, both in combat and in training. Halons are the predominant chemical agents used for suppression of fires and explosions in fixed-wing aircraft and helicopters, ships and maritime craft, ground armored combat vehicles, and mission-critical ground installations (such as communications centers and command and control facilities). Currently, installed (or fixed) fire and explosion suppression systems employ Halon 1202 and 1301 as total flooding agents, while portable systems use Halon 1211 as a streaming agent. Modern weapons systems also utilize complex and sophisticated technologies such as lasers, optical guidance systems, inertial guidance systems, digital communications systems, and computers, which generate heat during operation and require cooling. The most widely used refrigerants today are chloroflurocarbons (CFCs), primarily CFC-11, CFC-12, CFC-114, and blends such as R-22. In addition, the production and maintenance of these weapon systems require the use of solvents and other cleaning agents to maintain the highest level of cleanliness from particulate matter and lubricants, especially in aircraft oxygen breathing systems, hydraulic systems, and electronic systems. The most widely used cleaning agents are CFC-113 and 1,1,1 trichloroethane.
All of these fluids have one thing in common: they deplete the stratospheric ozone layer that protects the earth from ultraviolet B solar radiation.
In response to the Congressional mandate for research and development of ozone-depletion substance (ODS) alternatives and the cessation of ODS production, the Deputy Director, Defense Research and Engineering (DDR&E) (Research and Advanced Technology) created the Halon Alternatives R&D Steering Group (HASG) in September 1991. Its objective is to formulate and execute a technology strategy for identifying alternatives to ODSs for weapon systems use. The HASG is structured to facilitate coordination among the various stakeholders in DoD's search for technology solutions. The HASG includes representatives from the Army, Navy, and Air Force science and technology (S&T) communities; the DoD weapon systems acquisition community; the Military Department environmental communities; the Deputy Under Secretary of Defense for Environmental Security (DUSD(ES)); and the Defense Logistics Agency (DLA).
As its first task, the HASG formulated a technology strategy for identifying near-term alternatives to ODSs. This work culminated with the issuance of the "Technology Strategy for Alternatives to Ozone-Depleting Substances for Weapon System Use" by the Director, Defense Research and Engineering (DDR&E) in August 1992. The strategy goal is to identify and/or develop feasible alternatives that would allow ODSs to be eliminated from all weapon systems. Feasible alternatives are considered to be those that are friendly to the environment and minimize the acquisition resources required to implement their use. The process of identification and/or development is considered complete when the generic technical know-how exists to design alternative ODS-free systems.
The strategy focuses on developing alternative technologies for the following seven ODS functional applications within specific, near-term time frames:
With the technology strategy in place, the HASG created a technology development plan (TDP) which codifies and integrates the military's S&T activities in focused technology efforts corresponding to the strategy's seven functional ODS applications. Technical Plans I-IV contain activities for alternatives to halons for fire extinguishment and explosion suppression in ships, fixed-wing aircraft, helicopters, and ground armored combat vehicles; Technical Plan V addresses replacing Halon 2402 in Minuteman III booster rockets (in which halon is used for thrust control, not fire extinguishment); Technical Plan VI consists of research activities for investigating alternatives to CFCs in refrigeration and environmental control; and Technical Plan VII contains activities aimed at identifying alternatives to solvents and cleaning agents used in general and precision cleaning applications during the production and maintenance of weapon systems. The first edition of the TDP was issued by DDR&E in June 1993.
Since the TDP focuses on near-term technology solutions, efforts being pursued do not address development of new fluids but are somewhat constrained to investigate existing (suboptimal) fluids--an approach that recognizes that reasonable, near-term, ODS alternatives may result in weight, volume, cost or performance penalties. Alternative processes and/or techniques for performing the requisite functions are also addressed in the TDP (and, in fact, form the predominant approach used in identifying replacement solvents and cleaning agents for the production and maintenance of weapon systems). Because of the strategy's time frame, the TDP intentionally does not address any comprehensive long-term basic research and technology development efforts that might result in optimal replacements, such as one-for-one replacements for current ODSs. Therefore, it entails a moderate degree of risk, particularly with respect to identifying alternatives for halons.
The estimated program cost is approximately $160 million for FY93 to FY99. Since research activities are inevitably changed as they progress toward their goals, the HASG publishes annual updates to the TDP.
Progress to date toward finding environmentally friendly CFC replacements in weapons systems is evolving in a straightforward manner, with replacements already available for many systems. New solvents and cleaning agents are also being identified by industry, and processes are being revised to eliminate ODS use.
Efforts toward a solution for fire and explosion suppression have been less straightforward and are progressing more slowly. For example, although perfluorohexane has been identified as a potential streaming agent replacement for Halon 1211 in Air Force tests, Environmental Protection Agency (EPA) restrictions on its use have prompted new initiatives to develop alternative solutions. Tri-Service research efforts to identify alternatives for engine nacelles and dry bays will lead to a down-selection of one or two fluids in FY95. These fluids will be considered primarily for weapon systems currently in development, such as the F-22 and the V-22, but will likely have weight, volume, or performance penalties associated with their use. It is apparent that the current candidates for replacement of halons are suboptimal, and different fluids or approaches may be required for different applications. This situation adds another dimension of complexity to the halon alternatives problem since, unlike Halon 1301, "one size will not fit all" in meeting our total flooding fire suppression needs. This factor could complicate backfit decisions.
Because current TDP research efforts will not identify drop-in replacements for halons in fire and explosion suppression systems, and available (suboptimal) replacement may require costly backfits or system penalties, DDR&E initiated a national planning effort in July 1994 to develop a comprehensive next-generation fire suppression technology research plan that seeks to develop environmentally safe, penalty-free, Halon 1301 replacements. This long-term basic research effort, which is a natural and logical follow-on to the current TDP activities, will span approximately 8 years and address the new fluids, processes, and/or techniques needed for our next generation weapon systems. This activity is planned to start in the FY95-96 time frame.
Faced with the cessation of Halon 1301 production, NAVAIR instituted a comprehensive program to focus on:
These four elements embody the Navy's program, which plays a significant role in the overall Government/industry effort to identify aircraft Halon 1301 alternatives. Figure 1 illustrates the Navy's program and its relationship with the efforts of other services and agencies.
Figure 1. Relationship of the Navy Halon Program with Other Services and Agencies
While significant efforts are ongoing in the areas of responsible use, reserve management, and planning studies, this article will focus on ongoing Halon 1301 alternatives research and development efforts under NAVAIR's program.
Several Government and industry groups have efforts underway to find substitutes to Halon 1301. These efforts primarily concentrate on identifying halon-like chemical replacements. While co-sponsoring halon-like chemical research with the Army, Air Force, and Federal Aviation Administration (FAA), NAVAIR's program also focuses on non-halon-like chemical and non-agent-based alternatives to solve this problem. The Navy's research and development efforts in the area of Halon 1301 alternatives is directed not only at future aircraft designs, but also at existing aircraft.
NAVAIR's thrust in using non-halon-like chemicals has centered around the innovative use of traditional fire-fighting agents such as H2O, CO2, and N2. One area under investigation is the use of micron-sized water droplets or fine water mists (FWMs). When applied to a fire, FWMs can extract large amounts of heat energy while utilizing small quantities of water. Fire extinguishment is achieved by:
These mechanisms are influenced by water droplet surface area, velocity, direction, and droplet flow stream momentum. Naval Air Warfare Center Aircraft Division, Warminster, (NAWCADWAR) has evaluated several commercially available, air-assisted and hydraulic FWM nozzles. However, these nozzles could not provide an FWM with enough momentum without using relatively large amounts of water. Aircraft applications will require high momentum streams, but must minimize overall system weight. Therefore, the Navy developed nozzles that were optimized for aircraft applications. Preliminary test results indicate that these prototype nozzles are significantly more efficient than commercially available nozzles. Full-scale FWM testing on engine nacelle and dry bay simulators is scheduled for FY95 and FY96 to compare performance with other halon alternatives.
The Navy is also pursuing the use of a CO2 unit to replace the existing 2.75 pound aircraft Halon 1301 portable. Navy studies have determined that, for small crew compartment fires, CO2 portable units have demonstrated satisfactory performance when compared to Halon 1301. Prototype units have been successfully tested and are currently undergoing qualification.
NAVAIR is pursuing the use of inert gas generators to produce non-halon-like extinguishing agents for aircraft engine nacelle and dry bay fires. Gas generators produce high yields of hot inert gases that deprive the fire of oxygen. Having successfully demonstrated a gas generator for dry bay applications, Naval Air Warfare Center Weapons Division, China Lake, is currently evaluating the performance of this technology in engine nacelle fires. Using Halon 1301 as a comparative baseline, the gas generators will be evaluated by varying fire location, distribution, and quantity of propellant in a realistic mock-up of an F/A-18E/F engine nacelle. These test results will determine the potential for gas generator use in both existing and future aircraft platforms.
The Navy is investigating non-agent-based alternatives to Halon 1301, such as airflow modification and enhanced containment designs and materials. Airflow modification focuses on controlling the fuel/air ratio in a fire zone. Physical devices (e.g., airbags), flaps, and doors can be used to create a fuel-rich environment, starving the fire of oxygen. Bleed air or increased ram air can also be used to create a fuel-lean state, essentially blowing out the fire. These airflow modification methods will be tested at NAWCADWAR in FY95.
Another non-agent-based approach to fire protection is the use of state-of-the-art fire-resistant materials in the design of fire zones. These advanced materials and designs attempt to produce compartments that will contain the fire for extended periods, allowing the aircraft to land safely. Tests are currently underway at NAWCADWAR to evaluate the integrity of several compartment designs.
The aircraft safety and survivability communities must find ways to protect aircraft from fires without Halon 1301. New aircraft need effective fire protection systems that meet the requirements of the international Montreal Protocol and the U.S. 1990 Clean Air Act Amendments as implemented by new Environmental Protection Agency (EPA) regulations. While existing aircraft Halon 1301 systems are "grandfathered," the track record of the international community, Congress, and EPA indicates that phase-out dates will continue to move up, and requirements will become more stringent. A complete ban on Halon 1301 in aircraft is a possibility.
Although much research into halon alternatives has been directed at finding another "wonder gas" like Halon 1301 that can be used in all applications, it now appears that there will be no second wonder gas. The search for alternative agents has been more difficult and less fruitful than originally hoped. This failure to find another silver bullet should not be surprising, since retrospective analysis of the process that selected Halon 1301 in the 1950s indicates that even if ozone depletion was ignored, Halon 1301 would not meet all the other new environmental and safety requirements. Barring an unexpected breakthrough, no single agent will replace Halon 1301 in the foreseeable future.
Certain characteristics of all currently known replacement/alternative agents give them one or more of the following undesirable qualities:
While many potential alternatives to Halon 1301 exist, no single new agent appears able to replace it effectively in more than one or two areas. However, military specifications such as MIL-F-87168 (Fire and Explosion Hazard Protection Systems) and MIL-E-22285 (Aircraft Fire Extinguishing Systems) still specify Halon 1301 extinguishers rather than describing a fire protection requirement that must be met, e.g., "the aircraft must have an engine nacelle fire extinguishing system that will extinguish a fire within X seconds." To develop appropriate plans for halon replacement, specific criteria must be established for each application, and each potential replacement must be evaluated to determine its suitability for that role. Regulations and specifications must reflect the lack of a single agent for all uses and recognize that new agents that better meet our needs will come along. Requirements should spell out performance parameters rather than requiring a specific agent be used in a specific way, so new and better agents/approaches can be used as they are developed.
Establishing agent performance parameters will require an optimization process that considers the needs for effectiveness, safety, size/weight, cost, and cleanup. The community cannot afford to exclude an effective, environmentally safe agent because it has a perceived undesirable characteristic that does not relate to real requirements. For example, in recent tests of alternative agents for ground vehicles, available clean agents were ineffective in putting out the fires in simulated and surrogate engine compartment tests without large quantities that created unacceptable weight/space claim penalties in the vehicles. A hybrid gas/powder agent showed acceptable effectiveness but it created a light film of powder residue. When the level of residue was compared to the amount of dust, dirt, mud, leaves, and other debris found in vehicle engine compartments in normal use, however, it was determined to be insignificant in its effect on engine compartment cleanliness.
The survivability community faces two challenges. First, we must identify all the alternatives to the Halon 1301 fire/explosion protection systems currently installed in combat aircraft, including nontraditional agents such as hybrids and powders. Second, we must review and overhaul the specifications for aircraft under development to allow the flexibility to tailor fire/explosion protection systems and agents to provide the safest, most effective, environmentally acceptable protection for each individual application. By developing requirements-based standards, we may avoid rejecting without proper cause less costly and more effective halon alternatives in new combat aircraft and in modifications to current aircraft. The goal posts have been moved several times already and may be moved again. We must continue to expand our efforts if combat aircraft are to have affordable, effective fire protection both in the short term and into the 21st century.
Halon fire extinguishing agents, specifically Halon 1301 and Halon 1211, are used extensively in commercial aircraft and by airport firefighters. Selection of halons is based on a number of factors, most notably extinguishment effectiveness per unit weight, effectiveness over a wide range of operational and fire conditions, low toxicity, low corrosivity, and virtually no cleanup. In commercial aircraft, Halon 1301 total flooding systems protect cargo compartments, power plants, and lavatory trash receptacles. Halon 1211 is required in portable extinguishers for use against cabin fires. At airports, Halon 1211 is an approved extinguishing agent for flight line extinguishers and crash fire rescue vehicles.
Over the past 10 years, concern has increased about the role of halon (and chlorofluorocarbons) in stratospheric ozone depletion. This concern culminated in the production ban on halon in developed countries on January 1, 1994. The Federal Aviation Administration's (FAA's) policy has been to substantially reduce the discharge of halon in non-fire situations. FAA also believes it is necessary to continue using halon to maintain the current level of aircraft fire protection and airport firefighting capability.
Figure 1. Halon Replacement Program for Aircraft
With the future availability of halon uncertain, FAA has aircraft and airport programs underway to evaluate replacement and alternate agents and to develop the basis of approval for suitable agents. Figure 1 shows the Aircraft Halon Replacement Program. Full-scale fire tests are the key element. To date, FAA testing has focused on the lower cargo compartment because this application utilizes, by far, the largest Halon 1301 quantity and presents the most challenging fire threat. Pyrotechnically generated aerosols, water spray, and a hydrofluorocarbon are being evaluated. The Airport Halon Replacement Program also stresses full-scale fire tests of likely airport flight line scenarios. Candidate "clean" streaming agent replacements for Halon 1211 are being evaluated.
The International Halon Replacement Working Group, an integral part of the program, provides for industry (and Government) input and participation in the FAA's program. Meetings are held three times a year: at the FAA Technical Center, elsewhere in North America, and in Europe.
The July 26-27, 1994 meeting was hosted by Boeing in Seattle, Washington. At the working group meeting, which was chaired by Dick Hill, FAA's Aircraft Systems Fire System Program Manager, FAA presented the status of its program. The highlight of the meeting was the spirited discussion evoked by FAA's draft proposal of a minimum performance standard for cargo compartment halon replacement agents/systems. These meetings are very informal to promote dialogue. If issues cannot be resolved on the spot, a task group is created. Ten task groups have been formed to date; three groups have completed their work (halon recycling standards, cargo compartment agent toxicity, and cargo compartment fire load). Open dialog is undoubtedly a major benefit of the working group. FAA prepares and mails meeting minutes, copies of viewgraphs and other related materials to participants (as well as those on the mailing list who are unable to attend the meeting).
The most recent working group meeting was held at the FAA Technical Center on November 15-16. Participation in these working groups is open to anyone interested in aircraft halon/fire suppression systems.
Halon 1301 and 1211 have been the agent of choice for aircraft fire extinguishing systems because of their high fire suppression efficiency as well as their low weight, low volume, low corrosiveness, toxicity, and cost. Unfortunately, halons are fluorocarbons that contain bromine--a known ozone-depleting chemical. As a result of the Montreal Protocol on Substances that Deplete the Ozone Layer, halon production stopped in January 1994. This production ban has forced aerospace users of halon to rely on the finite amount of existing halon to meet their needs while suitable substitutes are evaluated. The substitute chemicals must not only be effective fire suppression agents with attractive weight, volume, and other technical characteristics--they also must be environmentally acceptable. Environmental acceptability is currently measured in terms of Ozone Depleting Potential (ODP).
Halon alternatives for aircraft are being studied at Wright Laboratory in a joint Army, Navy and Air Force research program in cooperation with the Federal Aviation Administration's (FAA's) Technical Center. Part of the study includes a consideration of each candidate chemical's ODP. One of the most promising agents being considered is a chemical called trifluoromethyl iodide (CF3I). This new chemical's ODP is currently being evaluated by atmospheric scientists. CF3I is a close chemical cousin of Halon 1301, trifluoromethyl bromide (CF3Br) with the bromine replaced with an iodine. The iodine chemical bond is very weak and can be broken by naturally occurring solar radiation, making its atmospheric life very short. Theoretically, this should produce a very low ODP since, unlike Halon 1301, CF3I should decay before it can reach the ozone layer.
But what if CF3I was directly introduced into the ozone layer by high flying aircraft? Atmospheric scientists at Lawrence Livermore National Laboratories predicted an extremely large ODP for such an occurrence.
Best approximations of total halon discharge and the altitudes corresponding to actual fire incidents for Air Force aircraft were made using information gathered from the Air Force Safety Office (AFSO) Incident Reports covering 1983-1993. The total number of false alarm discharges and subsequent release of Halon 1301 also was estimated based on Air Force maintenance records for the period 1991-1993. The Naval Air Systems Command (NAVAIR) provided altitude/discharge data, including false alarm discharge incidents, from the Naval Safety Center for the period February 1990 to December 1992. Data on Army fire incidents was provided by the U.S. Army Aviation and Troop Command (ATCOM) Safety Office for the time period August 1992 to June 1994. False alarm discharge incidents were not included in these data.
For the 10-year period investigated for AFSO incidents, 105 actual fire discharge incidents involving 1301, 1202, 1211, and 1011 were found. Fifty-eight of these were Halon 1301, 13 were Halon 1202, 12 were Halon 1011, and 21 were Halon 1211. One incident specified nitrogen as the extinguishing agent. The 58 Halon 1301 discharges released a total of 175.8 kg over the 10-year period; the 13 Halon 1202 incidents, a total of 88 kg; the 12 Halon 1011 incidents, a total of 145.0 kg; and the 21 Halon 1211 incidents, a total of 23.9 kg.
Over the period of January 1991 to May 1993, 101 false alarm events were identified from maintenance data. Of this number, 27 resulted in discharges totaling 94.5 kg of Halon 1301 and 1202, most of which was Halon 1301.
Data on the specific date of each fire incident were not available. As a consequence, information on the amount of halon discharged per year cannot be calculated; only an average per year over the 10-year period can be determined. The average yearly amount of Halon 1301, 1202, and 1011 discharged for actual fire incidents over the 10-year period was 40.9 kg per year.
A similar situation applies to the Air Force false alarm data. Discharge amounts due to false alarms have been annualized to provide a time unit comparable to the fire incident discharges. Furthermore, no data are available on the altitudes at which the Air Force false alarm discharges occurred. Therefore, the total annualized discharge amount of 40.2 kg due to false alarms is distributed among the various altitudes in proportion to that altitude's percentage of the total halon discharged for the known fire incidents.
The Navy reported a total of 88 incidents for the 3-year period investigated. Eleven of these incidents involved actual aircraft fires; the remaining 77 incidents involved discharges due to false alarms. These 88 incidents all involved Halon 1301. A total of 393 kg was released over the time period considered. Of this amount, 45 kg were discharged for actual fire events and the remaining 348 kg due to false alarms. On an annualized basis, the total Navy halon discharged was 131 kg per year--15 kg per year for fire events and 116 kg per year because of false alarms.
The Army reported a total of 15 fire incidents for the 22-month period investigated. All incidents involved Halon 1301. A total of 30.5 kg was discharged over the 22 months, giving an annualized rate of 16.6 kg per year. No false alarm data were provided. A significant number of reported incidents did not provide an altitude associated with the incident, nor was there any description of the incident to determine where the aircraft was in its mission. Therefore, the halon associated with discharges at these unknown altitudes was again distributed among the various altitudes in proportion to that altitude's percentage of the total halon discharged for the known fire incidents.
From 1990 to 1992, the average annual discharge of Halon 1301, 1202, and 1011 by U.S. Air Force, Navy and Army aircraft at altitudes approaching the ozone layer is very small. If the ozone layer is defined as beginning at 10 km (33,000 feet), our study shows that an average of 4.6 kg (10.1 lb) is discharged at altitudes exceeding this height. Of this amount, 1.4 kg are discharged for actual fire incidents, while the remaining 3.2 kg are discharged as a result of false alarms. These data clearly show an extremely small amount of fire suppressant agent is released from aircraft near the ozone layer. Based on this conclusion, possible substitute chemicals, such as CF3I, would not present an environmental risk to stratospheric ozone, given the historical use pattern.
Mathias L. Kolleck is an Associate at Booz-Allen & Hamilton Inc. and Director of Advanced Technologies in the Dayton Office. He holds a B.S. in Aerospace Engineering from the University of Cincinnati, as well as an M.B.A. in Finance and an M.S. in Economics, both from Wright State University. Mr. Kolleck can be reached at (513) 429-9509.
Besides being noted for its beautiful scenery, Monterey, California is the home of the Naval Postgraduate School and was this year's site for the annual JTCG/AS Methodology Subgroup meeting with industry. The meeting in Monterey on August 1 enabled Government and industry fuel ullage explosion experts to meet to establish a joint road map. Ullage, by the way, is that part of the fuel tank that is not filled with liquid fuel. In this region, fuel vapors can combine with air to create a very explosive mixture. For that reason, it is often said that a nearly empty gas can is far more dangerous than a full gas can.
Fuel ullage explosion prevention plays a large role in the design of survivable air vehicles. For tactical jet aircraft, every nook and cranny not otherwise occupied is a place to put a fuel tank. If a fuel-thirsty jet is placed in a combat environment, the fuel tanks will be increasingly filled with ullage or, if you prefer, the fuel tanks will approach empty. As a result, the potential for a fuel ullage explosion in tactical jets is great. In the past, a number of methods have been used to reduce the chance that a fuel ullage explosion will destroy an air vehicle. These methods range from placing foam in the fuel tanks to inerting systems that replace the oxygen in the ullage with halon or nitrogen. Large air vehicles such as air transports typically do not have ullage inerting systems. Recent vulnerability assessments have shown that if steps are not taken to protect the ullage, fuel ullage explosion will be the major contributor to the aircraft's vulnerability and a leading source of potential catastrophic loss for military air transport aircraft.
A recent study, sponsored by Mr. Richard Ledesma, Deputy Director, Test and Evaluation for Air and Space Programs (DDT&E(A&SP)), and conducted by SURVICE Engineering, applied five existing fuel ullage explosion prediction methods to air transport aircraft to assess their vulnerability. The study found wide variances in predicted vulnerability from each of the different models. As a result of the study, a new impetus has emerged to establish a standard fuel ullage prediction methodology.
The SURVICE Engineering study compared five methodologies. Generally named by their source, they include the Naval Air Warfare Center Weapons Division (NAWCWPNS) methodology, the Falcon methodology, the South West Research Institute (SWRI) methodology, the old NAWCWPNS methodology, and the Ballistic Research Laboratory (BRL) methodology (BRL no longer exists, and its functions have been assumed by the Army Research Laboratory). Of these methodologies, most break the fuel ullage explosion problem into three parts: characterizing the explosiveness of the ullage, the effectiveness of the ignition source, and the resulting over-pressure.
Characterizing the explosion potential of an air vehicle's ullage is not a trivial task. It depends on the amount and distribution of oxygen and fuel vapor in the tank--if the mixture is too lean or too rich, no explosion will occur. In practice, the fuel mixture varies throughout the ullage and depends on a number of factors including characteristics of the fuel; current altitude and temperature; fuel tank size, shape and venting; flight profile history; and amount of fuel sloshing/splashing. Some of these variables--such as fuel sloshing/splashing resulting from flight conditions, pilot technique, or projectiles hitting the fuel--are very difficult to quantify.
Once the explosion potential of the ullage has been calculated, the ignition capability of the threat must be evaluated. A more intense spark of longer duration increases the chance of a damage-causing explosion. The spark may be created by fragment interaction with the skin of the aircraft or walls of the fuel tank, or by an incendiary flash or explosive charge in the projectile.
Finally, the over-pressure of the explosion must be evaluated to determine if sufficient structural damage is incurred to destroy the air vehicle. This step is largely system dependent.
Although the existing methodologies are similar in overall structure, differences in implementation make the results very sensitive to the scenario details and, therefore, the methodology used. For example, in methodologies that stratify the ullage into regions of differing fuel-air ratios, spark location within the ullage is critical. Given a particular fuel tank, a stratifying methodology might consider one region of the tank explosive, another too lean, and yet another region too rich. Thus, for this sort of methodology, minute changes in the spark location can have a large impact on the survivability of the air vehicle, and this changes the net vulnerable area of the platform. In another example, some existing methodologies take an otherwise over-rich ullage mixture and lean it with air outside the tank via the hole created by the projectile (e.g., fragment, bullet). Conversely, some methodologies allow ullage that is otherwise too lean to be enriched by the splash of projectile fragments impacting the fuel.
Therefore, given a specific profile, the choice of methodology can have a big impact on the predicted survivability of the air vehicle. This concerns OSD, since fuel ullage explosion vulnerability assessments are not even vaguely consistent or comparable between methodologies. Flight test data to validate these ullage models are scarce. If good data existed, the community could pick the methodology that best matched test results as the accepted standard. The challenges of collecting flight data to validate the methodology, however, are numerous. Aside from the expense of instrumenting an air vehicle to characterize the ullage, differences in flight profiles and smoothness of the ride vary from flight to flight and are believed to significantly affect the explosiveness of the ullage. Due to the elusive nature of ullage explosiveness, existing methods will be difficult to validate.
At the meeting in Monterey, Ron Levy gave a briefing on the SURVICE study, and other attendees discussed ongoing or planned fuel ullage explosion vulnerability studies and work in the area of methodology development. Of particular note was Andy Pascal's work on a dry bay fire prediction methodology that is being developed for the C-17 Live Fire Test (LFT) program, which is also supported by the Joint Live Fire (JLF) program and the Navy [Editor's Note: See Mr. Pascal's article, Joint Live Fire/Live Fire Test Dry Bay Fire Model.]. This methodology differs in structure from the traditional ullage methodologies in that it is based on "physics first principles" and dynamically tracks the availability of resources needed to sustain ignition (fuel, oxygen and heat) by location and time. This very detailed model has enjoyed success in predicting dry bay fires and could be used as the core of a fuel ullage explosion prediction model.
The consensus of the experts that gathered in Monterey is that the air vehicle survivability community requires a better fuel ullage explosion prediction methodology. While each of the existing fuel ullage explosion models provides an explosion prediction, the models have different strengths. A model based on the strengths of each of the existing models and revolving around Mr. Pascal's dry bay fire model would resolve the discrepancies in today's suite of ullage models.
This proposed model has the added advantage of being capable of supporting engineering design of fuel ullage inerting systems. The capability to model the effectiveness of one inerting system versus another is particularly desirable, since many existing air vehicles will require environmentally friendly ullage inerting systems either to replace their existing halon systems or to enhance the survivability of platforms exposed to threats they were not originally designed to withstand. Incorporation of the first principles fire model also should support model-based design optimization of the selected ullage inerting system. There is also a growing interest in the effect of different jet fuels on the ullage contribution to the aircraft's vulnerability. In particular, the Air Force is interested in the net effect on the aircraft's vulnerability that results from switching jet fuel from JP4 to JP8.
In today's fiscal environment, funding for a large single developmental task is difficult if not impossible to come by. JTCG/AS wants to coordinate the smaller existing tasks in this area to yield a solution that is both affordable and acceptable to the community. John Manion from the Naval Air Warfare Center Weapons Division at China Lake has accepted the challenge of heading up this effort.
In summary, for aircraft without ullage explosion protection features, the ullage explosion contribution to the total aircraft vulnerability is very large. The predicted ullage explosion contribution to the total aircraft vulnerability varies tremendously with the choice of methodology. As a result, DDT&E(A&SP) has emphasized the need for a standard, accurate ullage explosion methodology. The JTCG/AS is developing a plan to coordinate ongoing Service efforts to support merging the best features of the existing methodologies into a single accepted fuel ullage explosion prediction model.
The Joint Live Fire (LF) Test program recently completed a major expansion of the dry bay fire model (DBM). The model's scope has been broadened to include additional threat types and fluids, altitude effects, and fire sustainment. The DBM originated in support of the C-17 Live Fire Test (LFT) program because the available empirical data were not applicable for the detailed test planning and post-test analysis required by LFT. The DBM simulates the effects of a single threat penetrating an aircraft dry bay and impacting a fluid-filled container (tank or line).
The model computes the probability of fluid ignition and a time history of temperature and pressure given ignition within three regions of the dry bay. Examination of these latter variables reveals whether an ignition would be sustained or self-extinguish. The model, which is PC-based, queries the user for information about the conditions to be analyzed. No off-line computations by the user are required; the model contains predetermined threat and fluid databases encompassing the range of conditions expected to be analyzed. The model can be run in a single shot or parametric mode. While the model was originally developed to assist in test planning, it generates the PK/H data required by vulnerability analysts for inclusion in such models as COVART.
The DBM is based on physical principles and allows the user to examine a broad range of elements:
Ignition Source: The user can select either spark or threat ignition. For threat ignition, the armor piercing incendiary (API) projectile incendiary cloud, or fragment "flash," acts as the ignition source. For spark ignition, the spark provides the energy for ignition.
Threat Type: The user can assess either an API projectile or a warhead fragment. If an API projectile is selected, the user can select one of four projectiles (i.e., 7.62 mm, 12.7 mm, 14.5 mm, or 23 mm). In addition, the user selects the type of incendiary function that occurred (i.e., partial, slow burn, delayed, or complete). If a fragment threat is chosen, the user specifies any fragment weight of interest. High explosive incendiary (HEI) projectiles are currently being added to the model.
Fluids: The user can select any one of three fuels (i.e., JP-4, JP-5, JP-8) or either of two hydraulic fluids (i.e., MIL-H-5606 or MIL-H-83282).
Target Description: A wide range of information to describe the target is required, including whether the target is a tank or line, the target and dry bay dimensions, and the ventilation air inlet/exit dimensions.
Environmental Conditions: Many environmental conditions are required, including fluid and ambient air temperature, ullage pressure, aircraft velocity, ventilation air velocity, and altitude.
Impact Conditions: Shotline distances, threat striking velocities, and the impact location are needed. The impact location divides the dry bay into three regions. Temperature and pressure time histories are computed for each region, allowing the user to "view" the fire spreading in the dry bay.
Ignition is concerned with the life of a single fluid droplet, while sustainment examines the mass, species, and energy flows into, within, and out of the dry bay. Fluid ignition requires three criteria to be met:
Temperatures and pressures are computed within each of the three regions of the dry bay by computing the conservation equations of mass, species, and energy. Mass flow between all three regions and into and out of the dry bay are computed. Mass flow in and out of the dry bay occurs through the ventilation air inflow and exit and the threat damage hole. Fuel mass is added to the dry bay by vaporization of the fuel assumed to be pooled over the floor of the dry bay. The model computes the mass present of five species: O2, N2, fuel vapor, H2O, and CO2. Finally, the model computes the energy flow between, out of, and into each dry bay region. The energy conservation equations compute the change in internal energy consisting of heat flow due to convection and vaporization, heat added due to fuel vapor reactions, and net flow work done on the system.
The projected ban of halons due to the Montreal Protocol and Clean Air Act has led the Navy to investigate chemical flame inhibiting gas generators. In the mid-1980's this organization first began investigating gas-generating compositions that flamelessly deflagrated with a broad range of rates to produce a comparatively cool nitrogen-rich exhaust gas. Nitrogen is an inert, low molecular weight gas with a zero heat of formation that produces a cooling effect by expansion. These gas generators are different from conventional compositions in that the driving force to combustion is provided by the energy released by the decomposition of N-N bonds in aliphatic azido binders and in solid tetrazoles. Products with high negative heats of formation (e.g., CO, CO2, and H2O) are produced from conventional gas generators and tend to increase flame temperature. The nitrogen-rich exhaust gas is capable of extinguishing fires in laboratory scale tests. Gas generators that largely produce nitrogen gas extinguish flames by reducing the oxygen concentration below that required to sustain combustion. Since nitrogen is not a chemical flame suppressant, a greater weight of gas generator may be required than for chemical suppressants, such as halons. However, nitrogen-producing gas generators yield comparatively non-toxic atmospheres that may allow personnel to survive. Halons are relatively non-toxic but, in the presence of a flame, they form toxic and corrosive gases. One such gas, hydrogen bromide (HBr), is a potent chemical flame suppressant.
Flame suppressing gas generator compositions typically contain an organic azide binder, a solid tetrazole high in nitrogen, in addition to a flame-suppressing organic bromine compound. A number of organic bromo-compounds have been evaluated, the most successful of which is decabromodiphenyl ether, a non-volatile, solid compound which is converted to HBr when the gas generator deflagrates. The addition of ammonium iodate also produces an exhaust containing compounds (i.e., hydrogen iodide/iodine) that dramatically enhance flame suppression.
A less toxic and relatively non-corrosive flame inhibitor, KOH, is produced by adding potassium nitrate to the high nitrogen compositions. Gaseous KOH has been found to be an effective flame suppressant. These compositions burn with a flame (calculated flame temperature ca. 2000 K). Potassium sulfate is widely used in gun propellants to suppress the flash emitted by combustion (in air) of hydrogen and other combustible gases. Potassium compounds are believed to be scavengers for flame supporting O and OH radicals. It is likely that the potassium compounds derived from KNO3 are less toxic and less corrosive than those derived from bromo-compounds; KOH would be readily converted to potassium bicarbonate, a benign slightly basic salt.
Calculated deflagration temperatures of the flame-inhibiting gas generators are relatively high (1500-2000 K). The use of coolants has been investigated to reduce the temperature of gases. The salts of perfluorooctanic acid decompose endothermically at relatively low temperatures (>=270 C for the sodium salt). Products are dense energy-absorbing gases (perfluoroheptene, CO2) and solid NaF. The salts of perfluorooctanic acid would be an effective inert coolant. Coolants such as ammonium bromide and ammonium iodide, which produce flame-suppressing species, decompose at a somewhat higher temperature (>=395 C). These sublime to produce an aerosol of micron-sized NH4Br and NH4I, salts that would release HBr and HI when exposed to a flame. Decabromodiphenyl ether (DBDPE) would also function as a coolant. Hot gases would produce HBr, while cooler gases would sublime the coolant to produce an aerosol of micron-sized DBDPE.
In view of the potent flame inhibitors that can be produced by these gas generators, it is likely that they may achieve flame suppression comparable to conventional halon systems and at even lower weights. This characteristic is particularly attractive with respect to aircraft fire suppression.
Currently, compositions are being tested under various conditions to evaluate the quantity of composition required to extinguish flames, as well as combustion characteristics. After refinement of extinction tests, the most efficient compositions are scheduled to be scaled up to produce 400 to 4000 g batches for use in larger scale tests.
Vicki L. Brady is a physical scientist with the Propellant and Explosives Technology Branch of the Ordnance Systems Department at the Naval Air Warfare Center, China Lake, CA. She holds a B.S. in Functional Biology with minors in Chemistry and Philosophy from Fresno State University. Ms. Brady can be reached at (619) 939-7342, DSN 437-7342.
Technical specialists concerned with the vulnerability and survivability of fuel-bearing aircraft structures attended the second Hydrodynamic Ram Workshop, held July 26-28 at the Naval Air Warfare Center, Aircraft Division in Warminster, Pennsylvania. The first workshop was held in 1989 prior to JTCG/AS Structures and Materials Committee-sponsored work on improved analysis and test methods. This second workshop, attended by 48 representatives of Government, industry, academia and the national labs, provided a forum for reassessing the state of technology in designing, analyzing, and testing structures subjected to hydrodynamic ram as a result of hostile threats. The Structures and Materials Committee also used the workshop as a planning tool for determining a course of action for future efforts in improving the design and validation of blast-resistant structures.
During the first 2 1/2 days of the workshop, 30 presentations were given on a variety of topics and issues related to the ram problem. At the end of the 3 days, each organization summarized its views and position. On the fourth day, a Government caucus was held to decide how to proceed with JTCG/AS-sponsored work in the Structures and Materials Committee in FY95. Their conclusions were:
1. The use of hydrocodes will provide a better basis for building the desired analysis tools than the approaches that have been pursued over the last few years. However, failure criteria, anisotropic modeling, and, possibly, strain rate effects need to be added to the existing codes. For composites, airflow effects are also important.
2. We need to be more realistic in what to expect from analysis. For vulnerability analysis and design trades, one level of sophistication is appropriate, whereas for detail design and live fire test sizing, more capability is needed. From a program acquisition office point of view, analysis needs to establish confidence in the design before expensive structural components are built and tested.
3. A more systematic, step-by-step approach needs to be taken whereby effects can be isolated and understood in both analysis and testing. In the past, we had tended to be ambitious in our goals at the expense of gaining a thorough understanding of the problem.
4. A team effort including Government, industry, the national labs and academia, with each doing what it does best in terms of modeling, design, and testing, would be most effective in developing the desired analytical capabilities.
The Structures and Materials Committee plans to pursue a 3-year effort leading to advances in analytical capability. They envision:
The 1994 Hydrodynamic Ram Workshop was organized and planned by Mr. Greg Czarnecki, Wright Laboratory, and Mr. Ed Kautz, Naval Air Warfare Center, Aircraft Division, Warminster. Ms. Dianne Charyton, Navmar Applied Sciences Corporation, provided assistance.
Event: Test and Evaluation Symposium and Exhibition IX
Event: 6th Annual TACOM Combat Vehicle Survivability Symposium
Event: SURVIAC Survivability Analysis Workshop
Event: JTCG/AS Air Combat Survivability Symposium: "Challenges in Air Combat Survivability - The Next 25 Years"
