(Photograph courtesy of Raytheon)

A new project could supercharge cruise missiles with several times more bang, and all without changing the warhead.

The Tomahawk cruise missile, launched from subs, ships, or aircraft, is the tip of the spear against opponents with air defenses. More than a hundred were fired in the opening round against Libya in 2011. While the basic design has been around for decades—they were used as far back as the 1991 Gulf War—the Tomahawk has seen numerous upgrades over the years. This new tweak could improve the Tomahawk's striking power through the power of what you might call extreme mixology. It's all about fuel-air explosions.

Ordinary high explosives such as TNT do not require any oxygen. The big molecule simply breaks apart, releasing energy. By contrast, a fuel-air explosion is a form of combustion in which the fuel combines with oxygen in the air and burns more rapidly. As any gearhead will tell you, the fuel-air mixture is all-important for efficient combustion.

Vaporized

Eleven years ago, I experienced a first-hand demonstration of the fuel-air effect. My wife and I were woken at 6:02 am one Sunday morning by the rattling of the roof tiles, as though something immense had just landed on the house. Along with thousands of other Londoners, we did not discover the cause until it came on the news later that morning. There had been a fire and explosion at an oil storage terminal at Buncefield, more than 20 miles away. The shock that woke us had registered 2.4 on the Richter Scale and was heard as far away as Belgium. Amazingly nobody was killed, though several were injured.

Fuel air explosions are not uncommon, but the one at Buncefield was exceptionally powerful, far more powerful than experts would have been predicted. That's because the fuel vapors had somehow been mixed with the air.

In this handout image released by the Hertfordshire Police Force, the devastation caused by the Buncefield oil depot explosion and resulting inferno near Hemel Hempstead is clearly visible on December 14, 2005 in Hemel Hempstead, England. The fire was fin Getty Images

The Buncefield incident puzzled the investigators because the cloud from the 60,000 gallons of oil should not have caused such a strong blast. Normally, vapor will burn rather than explode, giving that characteristic "whoomph" sound, but Buncefield showed all the signs of a huge pressure wave. Cars, including a new Porsche, had been flattened. It turns out, the investigators found, that the key factor was a row of trees by the storage tanks. Burning vapor produces an exhaust like a jet engine, and when the flame front reached the trees, it accelerated to high speed. The irregular branches and twigs made the smooth flow turbulent, mixing the vapor cloud with air so it burned far more rapidly and with much greater force.

The same science that woke me up in 2005 is now being harnessed to make the Tomahawk more deadly. In this case, weapons designers are turning unused fuel into a second warhead via controlled mixing with air.

Missiles usually have fuel left over when they reach the target. Some missiles have a fuze to ignite this fuel after impact; in other cases, it may burn anyway. For example, when an Exocet missile hit the British destroyer HMS Sheffield during the 1982 Falklands War, the explosive warhead did not go off , but burning rocket propellant started fires that destroyed the ship anyway. And, of course, we cannot forget the importance of burning fuel in the 9/11 attacks.

As any gearhead will tell you, the fuel-air mixture is all-important for efficient combustion.

The Tomahawk cruise missile is unusual in that it uses turbine powered by a liquid fuel known as JP-10. Normal aviation fuel, JP-5 or Jet-A kerosene, produces about 125,000 BTUs per gallon, 10 percent more than gasoline. JP-10, otherwise called exo-tetrahydrodicyclopentadiene, pushes this number up by another 10 percent. It's the best around, but costs around $25 a gallon.

The Tomahawk Block III is loaded with more than a thousand pounds of JP-10 on launch, giving it a range of more than 800 miles. So, if the target is only 400 miles away, the missile may have some five hundred pounds of fuel left on impact. That leftover could make quite a bang. A rough calculation suggests the total energy content of that much jet fuel is several times greater than the Tomahawk's explosive warhead (approximately a thousand pounds of PBXN-107 plastic-bonded explosive). However, creating such an explosion would mean turning all the fuel into a vapor cloud and detonating it efficiently. And therein lies the trick.

Fuel-Air Fireball

Fuel-air explosives are already used as weapons. The Russians, in particular, have a range of "thermobaric" fuel-air weapons that make ferocious blasts, including the tank-mounted TOS-1 rocket launcher that could destroy eight city blocks with one salvo. U.S. thermobaric weapons are generally based on powdered solid fuel in powdered form, as liquid explosions are a different challenge.

Enter Blaine Asay, formerly of Los Alamos National Laboratory, and his colleagues at Energetic Materials Research and Engineering based in Atchison, Kansas. Under a contract with the U.S. Air Force, Asay, expert in the field of non-shock initiation of explosions, is developing a system that will implode a missile's fuel tank to generate a cloud of vapor and ignite it in a rapidly burning fireball.

This is quite a challenge, as the fuel-air mixture has to be just right. The precise engineering cannot be done by trial and error, but requires computer modeling with a package called ALE3D (Arbitrary Lagrangian-Eulerian 3D and 2D Multi-Physics Code)—software that allows the simulation of complex high-speed reactions in three-dimensional space.

Asay says he's not at liberty to discuss all the details, but published results show the team succeeded in creating a cloud of JP-10 that burned in 30 milliseconds. In the next phase, the researchers will use their previous results to improve the burning speed by a factor of 100, aiming to hit the jackpot: a detonation in which virtually all the fuel is burned.

If their research succeeds, then a simple, cheap add-on could make existing cruise missiles far more powerful. The same technology would also enable a new generation of small, liquid-fueled missiles or jet-powered attack drones with a powerful punch. Some of these might not even have warheads in the usual sense, but rather, would simply carry a dual-use fuel tank so that striking power can be traded off against greatly extended range. Otherwise-unarmed scout drones fitted with a fuel-air device could be used as missiles if they encountered a high-value target. As the understanding of fuel-air mixing grows and our ability to model it gets better, such devices will get increasingly powerful.

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