About cellular polyurethane in fuel tanks to prevent fires
There are many reasons why equipment, often together with the crew, can be irretrievably lost as a result of combat damage. However, one of the main ones is a fire that occurs after a shell hits and penetrates the armor. Its fragments are quite capable of igniting both propellant charges and fuel, the supply of which often amounts to many hundreds of liters.
Unfortunately, today there are no means capable of effectively extinguishing flaming gunpowder, much less preventing the detonation of shells, so the only way out of this situation can only be complete isolation of ammunition stowage from the crew compartments. But as for fuel, there are quite a few solutions in general - they concern not only the improvement of automatic fire-fighting systems and the production of water-fuel emulsions (we wrote about this here), but also the use of open-cell polyurethane.
The latter method is by no means new - one way or another, it has already been repeatedly used in a variety of equipment, and not only land-based. Therefore, it would be interesting to talk about what it is, even without reference to specific machines, including its effectiveness and shortcomings. This is what we will do.
Cellular polyurethane foam
Conventional types of fuel have a very unpleasant feature from the point of view of fire hazard, which consists in the formation of vapors. They appear as a result of gradual fuel consumption (emptying of fuel tanks), and their concentration directly depends on the temperature - the higher it is, the stronger the vaporization. It is especially strong if the tanks are located in the engine-transmission compartment, where the temperature can often reach hundreds of degrees Celsius or more.
Mixing with air, fuel vapors (gasoline and kerosene are the most dangerous in vaporization, but diesel fuel is not non-flammable either) form a highly flammable and sometimes even explosive mixture, which can ignite when the fuel tank is penetrated by bullets and shrapnel, including large fragments of shells. This problem can be solved in several ways, including filling empty volumes of tanks with inert gas, as well as blowing them with air and even cooled exhaust gases to reduce the concentration of the mixture.
Open cell structure of polyurethane
However, the simplest is to use open-cell polyurethane foam, which resembles a sponge or foam rubber in appearance. This is a light, porous material with a mesh structure, where 95-98% of the volume is made up of small open cavities. Such foam, made in the form of blocks or plates, fills almost the entire internal volume of the tank, soaking with fuel, without interfering with its circulation and reducing the fuel supply by no more than 5-7%. It is resistant to aggressive fuel environments, vibrations and high temperatures, thanks to stabilizing additives.
The compositions of such polyurethane foam vary, as do its porosity or "cellularity". But even "foam rubber" in the form of polyurethane PPU-EO-100, which is actively produced in our country today for the manufacture of air filters, as well as fuel and wastewater filters, already shows quite good results. It is enough to "fill" 75% of the fuel tank with it, and in most cases you can forget about the ignition of the vapor-air mixture. And why?
The first most important effect of polyurethane is the reduction of the volume of free fuel vapors. After all, the greater the volume of emptiness in the tank, the higher the chance of accumulation of vapors in a critical concentration. Polyurethane foam, filling the internal space with its cells, leaves significantly less space for their formation. Thus, the probability of the occurrence of a flammable environment capable of igniting from the impact of fragments and other attacking bodies is reduced to a minimum.
The second protective function is related to flame propagation. In a conventional tank, fire can spread instantly. But polyurethane divides the internal volume into many microcavities, and any flame is forced to collide with polyurethane barriers. In essence, the heat transfer of combustion products is disrupted, and the ignition process is interrupted. This effect turns a potential fire into a localized, dying combustion.
One of the variants of "foam" fuel tank filler in the form of mesh polymer foam
It is equally important that polyurethane dampens internal fuel vibrations. When a tank is punctured, a hydraulic shock can cause intense aerosol formation – tiny droplets that easily ignite. Polyurethane foam acts like a sponge: it holds the fuel in its cells, preventing massive fuel ejection outside the tank and thereby reducing the likelihood of a “fuel torch”. Moreover, polyurethane foam itself reduces the shock wave energy, which reduces the destruction of the fuel tank.
Added to this is another feature: partial localization of holes. Of course, cellular polyurethane is not able to "seal" a hole left by a bullet or fragment, but it significantly reduces the rate of fuel leakage.
Efficiency and disadvantages
Tests conducted both in Soviet times and in modern Russia have clearly shown that fuel tanks filled with cellular polyurethane are not prone to either ignition or explosion, even with direct hits. Where under normal conditions one spark is enough to ignite and cause a major fire, conditions for stable combustion do not arise inside such a tank.
This applies to many calibers. The tanks were fired at by large-caliber machine guns, including those with armor-piercing incendiary bullets, and such "small arms" as 5,45 and 7,62 mm machine guns. Moreover, the stability of the polyurethane filling was also demonstrated when hit by armor-piercing shells from 30 mm guns. In such conditions, the damage to the tank body was quite serious, but the "explosive effect" itself or combustion never occurred. The same result was achieved when grenades exploded in the immediate vicinity of the fuel tank - the fragments did not ignite.
Such fuel tanks with polyurethane filling can be used on any combat equipment, regardless of whether it runs on gasoline, diesel fuel or kerosene. Moreover, given that the engines of modern combat vehicles are most often multi-fuel, the versatility of this protection is of particular importance. Ultimately, polyurethane significantly reduces the likelihood of fire in the event of combat damage.
Of course, cellular polyurethane foam has its weaknesses. The main one is its limited effectiveness in case of multiple damage to the tank. For example, if it “doesn’t notice” one hit, extinguishing the danger of ignition, then in case of a series of holes due to shrapnel, bullets or shells, the fuel leakage in any case becomes the cause of the fire. Experiments have shown that in case of multiple hits, the protection completely loses its effectiveness.
Therefore, it cannot be considered a universal remedy in itself. In practice, the use of polyurethane foam is always supplemented by other solutions. Most often, we are talking about self-healing coatings based on polyurethane or multi-component compositions that tighten holes and reduce the leak rate or completely eliminate it. In this case, the foam plays the role of an internal barrier, and the outer layer - a sealing shell.
This combination certainly increases efficiency, but it also has a downside - more complicated design and higher production costs. While a simple metal tank can be made relatively easily, a fuel tank with polyurethane foam inside and a coating on the outside becomes a more complex and expensive product. However, it is this complication that allows for a radical increase in the vehicle's chances of surviving a hit.
There is another factor that cannot be discounted: polyurethane is not eternal. In an aggressive fuel environment, it gradually degrades over several years, and therefore requires periodic replacement. Designers must take this into account already at the design stage, laying in the possibility of servicing and updating the filler. Yes, all this requires costs and increases the price, but the result is obvious - the survivability of equipment and, most importantly, its crew increases many times over.
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