DAMAGE to aircraft fuel tanks doomed roughly half of the 5,000 or so American warplanes and helicopters destroyed during the Vietnam war. Some crashed or blew up after only a few bullet hits drained or ignited their fuel, says Robert Ball, the author of a textbook on the combat survivability of aircraft. But such “cheap kills” are becoming increasingly rare, says Dr Ball, a former engineering professor at the Naval Postgraduate School in Monterey, California. Thanks to clever engineering, fuel tanks in aircraft, vehicles and even storage facilities can now withstand direct hits from enemy fire or tremendous impacts without exploding.

Armies like to keep their fuel trucks far from the enemy. But that can be difficult, as Western forces in Afghanistan and Iraq have learned. Even lightly armed insurgents can torch a tanker truck. Bullets shot into liquid fuel rarely ignite it. But a tank riddled with bullets spews fuel, and when fuel and its vapours mix with oxygen, a spark can create a firestorm. Between 2003 and 2007 attacks on fuel convoys in Iraq killed or seriously wounded more than 1,400 people, according to the US Army.

Strong steel armour would be prohibitively heavy on tanker trucks. So in 2005 High Impact Technology (HIT), a small firm based in Oregon, proposed using lightweight plastic instead. It developed a polyurethane material that is sprayed as a foam a few centimetres thick onto a fuel tank and dries into a rigid plastic shell. When pierced with a bullet, fuel spurting out of the hole reacts with a secret catalyst in the polyurethane, causing it to absorb fuel and expand, plugging the leak within seconds. The system, called BattleJacket, now protects more than 3,400 fuel-hauling trucks in conflict zones.

In 2008 more than 600 bullets were removed from the reservoir of one of them in Balad, Iraq. The truck was still moving fuel for the US Army but the bullets and shrapnel had begun to clog the reservoir’s drainage valve. HIT charges up to $22,000 to spray each tanker (or twice that if the job is done in a war zone). The material has also been applied to the far-smaller tanks of more than 8,000 of America’s fighting vehicles. There are now BattleJacket plants in America, Canada, Germany and Kuwait, with others being set up in Singapore, Taiwan and Turkey.

Bullet and shrapnel holes can also be sealed by rubber bladders placed inside fuel tanks. In a similar fashion to HIT’s polyurethane shell, leaking fuel reacts with chemical additives in an inner layer of the rubber, causing it to absorb the liquid and expand. Such bladders have been used in warplane fuel tanks for decades. The latest bladders, however, can even seal the bigger holes made by bullets that pass right through the fuel tank.

Meggitt, a British company, manufactures self-sealing rubber bladders that can seal holes up to 7cm across in about two minutes. They are not cheap: bladders for aircraft cost more than $20,000 each, and aircraft generally have multiple fuel tanks. America’s V-22 Osprey transporter, for example, has a Meggitt bladder in each of its dozen or more tanks. The company’s bladders are used on more than 10,000 of America’s military aircraft and more than 1,700 ground vehicles.

An extra benefit of the bladders is that they can withstand pretty much any impact that a flight crew might survive. When a fuel tank’s rigid shell splits open on impact, the bladder inside stretches to absorb the impact without bursting. This is good, because splattered fuel is likely to be ignited by a spark: more than 40% of American soldiers who survived a helicopter-crash impact used to be burned alive in an ensuing fuel fire. Today it is less than 1%, according to Dennis Shanahan, a doctor and retired colonel who studied the matter for the US Army Aeromedical Research Laboratory.

Even humble aluminium alloys, cleverly used, can prevent fuel-tank fires and explosions. Coils of nearly paper-thin aluminium mesh can absorb lots of heat very fast. Place enough of them inside a fuel tank, and the heat created by a projectile or crumpling tank will generate fewer sparks, or none at all. If sparks do ignite vapours, flames may not spread because the mesh restricts airflow. Jiangsu Ampute Explosion Prevention Technology, based near Shanghai, reckons that aluminium mesh will become widely used in car petrol tanks. It costs only $20 or so, says Yelian Ju, Ampute’s deputy manager.

Furthermore, by absorbing heat, aluminium mesh keeps fuel cool. This cuts in half the 5% or so of fuel that above-ground storage tanks in hot countries lose each year to evaporation through cooling vents. Ampute expects its sales of aluminium mesh to exceed $9m this year.

Yet another approach is that taken by Firetrace, based in Arizona. It has designed a plastic fuel-tank shell that is packed with fire-suppressant powder, which is released if the shell shatters. More than 55,000 shells have been installed in police and military vehicles, at a cost of around $4,500 each. Advances in fire safety, then, are spreading quickly. Fuel-tank explosions will doubtless continue in Hollywood movies, but they are becoming less frequent in real life.