Non-Flammable Fluids Boost Armored Vehicle Survivability
As the U.S. Army moves ahead with plans to deploy medium-sized brigades, it must address one significant vulnerability in its vehicles-the threat of fires caused by flammable fluids.
Improving the brigade's survivability, therefore, means reducing fluid flammability.
On-board fire suppression systems are designed to extinguish any fire the moment it starts and have been an important element of survivability for armored vehicles. However, there has been a continuing effort to develop a non-flammable hydraulic fluid.
The Middle East conflicts in 1973 revealed the magnitude of the hydraulic fluid's contribution to the fire threat for armored vehicles. Rupturing of hydraulic fluid lines under pressure generated a fine spray that was easily ignited by hot pieces of projectiles or incendiary fragments. The U.S.-manufactured M60 main battle tanks used by the Israeli military were more prone to these fires than were Russian tanks used by Arab nations.
At the end of the conflict, strong complaints by the Israeli military regarding their significant M60 losses due to fires led to a Pentagon task force being established to correct this deficiency. The task force was asked to come up with a replacement fluid for the highly volatile and flammable MIL-H-6083 hydraulic fluid-also known by its military symbol OHT-that was used in all ground vehicles and equipment. The replacement fluid had to be one that had fire retardant qualities.
The task force initially considered a variety of fluid candidates such as glycol water emulsions, phosphate esters, poly-alpha olefins, silicone fluids, halogenated hydrocarbons-all of which were reported to possess fire retardant qualities. Since the replacement fluid had to be one that would be compatible with the existing hydraulic and gun recoil systems, the poly-alpha olefins (PAO) became the logical choice. Further, ballistic testing on pressurized hydraulic cylinders had demonstrated the PAO to be 70 percent more fire resistant than the flammable OHT. Using the PAO, a new hydraulic fluid was developed and became MIL-H-46170, also known by its military symbol, FRH.
FRH was first introduced into M60s in 1974 and then into M1 Abrams tanks in early 1980. The FRH offered some improvement in safety, because of its higher flash-point (it was 400 degrees F vs. 180 degrees F for OHT). FRH, however, did not completely resolve the hydraulic fluid fire threat and was therefore only viewed as an interim fix. The original directive calling for changeover from OHT to FRH had identified other armored vehicles, such as self-propelled artillery, carrier, command post and others. These other armored assets, however, were never changed to FRH and have continued to use the flammable OHT.
In the mid-1980s, an Army requirement for a non-flammable hydraulic fluid led to the development, in the early 1990s, of MIL-H-53119, also know by its military symbol CTFE. It is a non-flammable hydraulic fluid based upon chloro-trifluoro-ethylene chemistry. A cost analysis on the use of CTFE in the M1 fleet provided some interesting statistics on the numbers of non-combat fires occurring in M1s directly attributable to hydraulic fluid.
During the 1980 to 1990 period, there were 51 non-combat fires caused by the hydraulic fluid. These fires cost more than $4.5 million.
However, as CTFE was not a petroleum or PAO base fluid, there were some systems modifications required prior to its proposed use in hydraulic and gun recoil systems. These changes were necessary because of CTFE's incompatibility with certain seals and non-ferrous parts. Because of this retrofit requirement and costs associated with the changeover, CTFE was put on the shelf to be considered for future vehicle systems design.
In the past, on-board fire-suppression systems used Halon 1301, a proven and effective fire suppressant. In the late 1980s, the Montreal Protocol Treaty identified chlorofluorocarbons such as Halon 1301, to be depleting the ozone layer, and required these substances to be phased out of production during the early 1990s.
Other suppressants have since been developed, but these do not appear to be equivalent to Halon 1301-leaving a question as to their ability to suppress hydraulic fluid fires. There have been other concerns with fire suppressant systems, such as the responsiveness of sensors in detecting a flame, protection against multiple hit scenarios, system malfunctioning, possible loss of pressurized suppressant, and others. The bottom line is: If the sensors fail to function, or the system does not operate, there is no backup, and the presence of either FRH or OHT will, literally, add fuel to the fire.
Some limited testing and experimentation has been conducted using small amounts of non-ozone depleting halogenated compounds blended into FRH-producing surprising results in reducing its flammability. These halogenated compounds do not violate the Montreal Protocol Treaty and are acceptable for use. It was found that as little as 2 percent to 5 percent of a halogenated compound blended into FRH was sufficient to suppress any potential for the fluid to burn.
Although this preliminary testing has shown promising results, additional work is needed to ensure this small amount of halogenated compound will not change FRH's performance characteristics. If successful, introduction of this enhanced FRH, in combination with the on-board fire suppressant systems, will go a long way towards guaranteeing increased crew and vehicle survivability for all armored vehicle systems.
Ralph B. Mowery is a senior chemist at the U.S. Army Tank-Automotive Research, Development and Engineering Center (TARDEC), in Warren, Mich. Bernie R. Wright is a staff scientist at TARDEC's fuels and lubricants research facility at the Southwest Research Institute, in San Antonio.
Maurice E. LePera, a consultant in Woodbridge, Va., also contributed to this report.