Up to this point, we have discussed explosives that contain their own oxidizer, or have no oxidizer (they simply break up into simpler molecules). But in many military applications, there is plenty of oxidizer around, in the form of the oxygen in the air. Not having to carry around all of the oxygen means the weapon can be smaller, and thus easier to carry around, perhaps even man-portable, or huge, consisting of a tanker-load of fuel.
The term thermobaric weapon has been applied to this type of explosive. It refers to the heat and pressure they produce (as opposed to the shock wave of a conventional explosive).
As we saw with gunpowder, mixing the fuel and the oxidizer very closely is key to making an explosion. In the case of gunpowder, even that was not enough, the mixture had to be confined for a while, until it could build up enough pressure to suddenly burst the container, in the case of a bomb, or move the projectile, in the case of a gun. In the open air, containment is obviously a problem. But in a cave or a bunker, a mix of fuel and air can have the same explosive effect as we see in mine explosions, where methane or coal dust mixes with air in a confined space.
If the amount of fuel is large enough, and it is well mixed with just the right amount of air, the containment issue is less relevant. The target will be hit from all sides, and the inertia of the blast will produce its own confinement.
In a typical fuel-air bomb, the fuel (such as a hydrocarbon like gasoline) is spread into the air by one conventional explosive, and then ignited by a second explosive that is timed to allow the fuel and the air to properly mix. The fog that results from the first explosion can drift into foxholes and buildings where it will be confined until the second explosion ignites it. Thus, these types of weapons are very effective in destroying bunkers, caves, and buildings, and any people in them.
If the fuel is preheated enough by the first explosion, usually by enclosing it in a strong casing to delay the actual spread until the fuel has been heated, it may be above its ignition temperature when it encounters the air, and no second ignition charge is needed.
One thermobaric weapon, the Russian Aviation Thermobaric Bomb of Increased Power (ATBIP), is thought to use powdered aluminum and ethylene oxide as the fuel. 7.1 tons of the fuel is contained in the bomb, which delivers the explosive equivalent of 44 tons of TNT when detonated. The blast radius is 1,000 feet. Due to secrecy and the propaganda value of a weapon such as this, the details are not easily verifiable.
The US BLU-118/B thermobaric weapon was designed to destroy tunnel complexes in Afghanistan. It could be detonated at the entrance to the tunnel, or set to detonate after skipping on the ground into the tunnel, or set to detonate after penetrating into the tunnel from above. Insensitive explosives are needed for the last two methods, as the bomb takes a beating getting through the rock above the tunnel, or skipping on the ground after being dropped from a jet.
The insensitive explosive used in the BLU-118/B was PBXIH-135, a mix of HMX and fine aluminum powder in hydroxyl-terminated polybutadiene (a polyurethane rubber used to make skateboard wheels and Spandex). The fuel was a solid fuel, mostly aluminum powder. The BLU-118/B is a single-stage detonating device, and relies on the confinement inside the tunnel to increase the shock wave of the explosion. The explosion begins small, but builds inside the tunnel, and lasts longer than a conventional explosive detonation would.
In tests in tunnels in Nevada, the bomb worked as designed. In actual use in Afghanistan, the first use missed the target, hitting a ridge in front of the tunnel instead. Since the explosive was smaller than a normal bomb (not counting the solid fuel), it did less damage in the open-air miss, which is considered an advantage of the weapon.
The BLU-73/B was a fuel-air explosive used in the Vietnam War by the US, developed at the China Lake facility. It weighed 100 pounds, and contained 75 pounds of ethylene oxide fuel. Its fuse was set for 30 feet above the ground, creating a cloud of fuel 60 feet in diameter. This was then ignited by an embedded detonator. The BLU-73/B was a sub-munition, carried in the 550-pound cluster bomb called the CBU-72/B. The US Marine Corps dropped 254 of these from A-6-E attack jets. Designed to attack trenches and clear minefields, it is unclear if it was actually effective at clearing mines.