Over the years, the potential for in-flight fires has become an increasing area of concern for platform survivability in aviation. In addition to fuel, fluids such as hydraulic, lubricant, and coolant have contributed significantly to these fires. A common fluid used in aircraft due to its excellent properties is MIL-PRF-87252D—or polyalphaolefin (PAO) oil—which is currently found on the F-22, F-35, B-1B, F/A-18E, EA-18G, and RQ-4. Despite its low volatility, this oil can contribute significantly to fire risk in battlefield scenarios. If a fluid line is compromised, PAO can be ejected as a mist; and additional impact from fragments can atomize the oil, enabling rapid combustion from the impact flash. As a result, PAO fires have become a significant contributor to aircraft vulnerability. In fact, fires involving hydraulic and coolant systems, where PAO fluid is often used, are responsible for a significant portion of the vulnerable area in ballistic vulnerability analyses. With a limited quantity of tests and lack of PAO ignition curves, PAO attrition vulnerability is also a significant source of uncertainty in aircraft vulnerability analyses. To help address this uncertainty, the Joint Aircraft Survivability Program (JASP) enacted a multi-phase effort to provide a high-confidence characterization of the probability of ignition, as well as to develop and validate an advanced mist-control polymer with the ability to reduce probability of ignition given a compromised fluid line.
Test Setup
The U.S. Army Combat Capabilities Development Command (DEVCOM) Analysis Center (DAC) conducted three phases of testing at the Airbase Experimental Facility 6 located at Aberdeen Proving Ground, MD. The test setup (pictured in Figures 1–3) used an existing test fixture (Figure 2) with an aluminum line containing PAO fluid heated and pressurized to normal operating conditions located inside. Surrounding the PAO line were four Type K thermocouples to capture temperatures throughout the fixture. Flowmeters and pressure transducers were positioned before and after the test pipe to ensure conditions were adequate before the article was tested.
For the ballistic portion of testing, a steel cube, representative of a single fragment of a missile or high-explosive ballistic projectile threat, impacted a composite skin panel before impacting the pressurized PAO tube (Figure 3) being tested. After impacting the PAO tube, the threat exited the bay through an aluminum spar panel (Figure 2). Standard and high-speed video, as well as temperature data, were recorded for each event.
Figure 1. Full Test Setup.
Figure 2. Front and Rear View of the Test Fixture.
Figure 3. Internal View of Test Fixture.
Phase I Summary
Phase I was conducted in November 2020 and finished in late January 2021. This phase was composed of two subphases involving static and dynamic testing. Nine static tests were performed, followed by 72 ballistic tests. Static testing was used to confirm fire propagation in a worst-case scenario as well as limit nozzle sizing for Phase II laboratory testing. The static test matrix consisted of three events for three different hole diameters. The dynamic test matrix was made up of three fragment velocities and three fragment sizes. The data showed a significant correlation of probability of ignition with fragment velocity compared to fragment size. The probability of ignition was found to be higher at one specific velocity. This suggests that standoff distance from the PAO line and the back structural plate is a key factor in probability of ignition.
Phase II Summary
Phase II consisted of a partnership between the DEVCOM Army Research Laboratory and the California Institute of Technology to develop an additive using a similar approach to the one used to create megasupramolecules for JP-8. Megasupramolecules are polymer chains that are held together by intermolecular (noncovalent) bonding. The combination of pCOD molecular backbone and DA:DB associative end-groups increases the elongational viscosity of the fluid, thus changing the droplet formation. Additive concentrations of 0.05 and 0.1 weight-percent resulted in almost complete disappearance of observable droplets, and only long ligament structures were observed. The ligaments are also thickened 1.5 to 2 times that of untreated PAO. Additionally, the spreading angle of the spray is reduced by approximately half (as illustrated in Figure 4).
Figure 4. Comparison of the Spray Development Between Base PAO, Additive of Only Backbone Molecules, and the Final Additive With Associative End-Groups.
The additive was validated through laboratory flame propagation. To reduce cost, improve cold temperature solubility, and ease certification, the minimal amount of additive needed to inhibit ignition was pursued. Laboratory testing concluded that a treat rate of 0.05 weight-percent DA:DB was sufficient to reduce ignition. This amount is equivalent to 81 g per 55-gal drum of PAO fluid. After validation, the additive was added to the PAO fluid and retested using the dynamic ballistic setup in Phase I.
Phase II testing was completed in October 2021. This phase consisted of 32 ballistic tests (1 velocity, 2 fragment sizes). The additivitized PAO demonstrated a significant reduction in probability of ignition when compared to the baseline MIL-PRF-87252D PAO. A ballistic vulnerability analysis of the additivitized PAO was shown to reduce aircraft vulnerable area by a considerable amount.
Phase III Summary
The final phase of testing began in September 2024 and was concluded in October 2024. The objective of Phase III was to repeat the ballistic test matrix used in Phase I with the additivitized PAO fluid created in Phase II to thoroughly compare the additive effectiveness against the remaining fragment sizes and velocities that weren’t tested in the second phase. Additional fragment tests at a higher velocity were also conducted for both treated and untreated PAO after U.S. Air Force testing revealed that a higher velocity was important. With 10 test events of untreated PAO remaining, the test range suffered catastrophic equipment failure while trying to consistently reach this higher velocity. Nonetheless, these remaining test events were recently completed and are included in the final report, which will be released in the fall of this year.
While the additive showed a significant decrease in probability of ignition for the critical velocity, the remaining Phase III results showed the probability of ignition was similar to untreated PAO for the remaining fragments and velocities.
Conclusions
Building Joint Service knowledge surrounding PAO ignition and developing strategies to mitigate ignition ultimately help to reduce aircraft vulnerability and protect Warfighters. This program effectively collected probability of ignition data for MIL-PRF-87252D PAO, developed a polymeric fire mitigation additive, and demonstrated a reduced probability of ignition for additivitized PAO, completing its overall program objective. Because the reduction in probability primarily occurred at one velocity, further investigation into the fragment interaction with different mix weight is warranted to understand full benefits. Finally, this test program demonstrated the upper limits of existing test methodologies with respect to velocity; additional investment may be required to achieve higher fragment speeds.
About the Author
Mr. Nick Wojtysiak is currently an operations research system analyst at the U.S. Army Combat Capabilities Development Command (DEVCOM) Analysis Center. He began his career working on target acquisition analysis, developing computer-aided design models used to determine probability of detection in the thermal and visual spectrums. He then transitioned to the analysis of aviation systems, where he became the lead flight performance analyst, providing flight performance data for the Future Long-Range Assault Aircraft and Future Attack Reconnaissance Aircraft programs. In 2024, Mr. Wojtysiak moved from the flight performance aspect of aviation to the vulnerability side, where he currently leads test programs and provides inputs into vulnerability analyses. He holds a degree in mechanical engineering from the University of Maryland, College Park.
