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Here's why the 'mother of all bombs' exploded 6 feet above its target in Afghanistan

Rafi Letzter
MOAB bomb
MOAB bomb

(DoD Photo)

The US just dropped a bomb on an ISIS target in Afghanistan that had never before been used in war.

The GBU-43/B Massive Ordnance Air Blast, sometimes grimly called the "Mother of All Bombs" or "MOAB" because of its acronym, was loaded onto a pallet, flown in a C-130 cargo plane, then dragged out the back of the plane — pallet included — by a parachute.

The 21,600-pound bomb free-fell toward earth.

The MOAB isn't the heaviest nonnuclear bomb in the US arsenal — that's the 30,000-pound Massive Ordnance Penetrator — but with a length of 30 feet and an estimated cost of $15.7 million, it's the largest, and a significant piece of military equipment.

The Air Force is said to have as many as 20 total MOABs, so the deadly machine could represent more than 6% of the total US stockpile of such weapons.

In free-fall, the MOAB quickly detached from its pallet and deployed course-adjusting grid fins. Hooked up to a computer and a GPS system, those fins bent the MOAB's earthward course toward a preselected target. In this case, that target was an underground ISIS tunnel complex, according to the Department of Defense.

The MOAB itself never touched the ground at the complex, however. Instead, like most large weapons, including nuclear bombs, it detonated in the air moments before impact. There was probably about 6 feet between the MOAB and the ground when it unleashed its destructive force, which was equivalent to about 11 tons of TNT.

moab test
moab test

(A prototype MOAB an instant before detonation on Eglin Air Force Base's Range 70.US Air Force)

Why didn't the MOAB strike the ground before detonating?

"The main attribute of the MOAB is that it causes overpressure," Adam Lowther, the director of the US Air Force's School of Advanced Nuclear Deterrence Studies, told Business Insider reporter Alex Lockie.

"Overpressure" is a term for the sharp spike in air pressure that a bomb causes. That change in pressure moves away from the bomb in a wave in all directions. Any mines, tunnels, or bodies caught in the blast would be unrecognizable after it passed.

If the MOAB were to detonate on contact with the ground, a lot of that pressure wave would be sent into the dirt, digging out a small crater around the bomb. Dirt is a lot harder to move than air, so the energy wouldn't travel far before petering out.

Detonating a bomb of that scale on the ground would still do plenty of damage within a certain range, of course, but weapons engineers use physics to expand the reach of such bombs.


(An illustration of the effect creating a mach stem in a nuclear explosion.Wikimedia commons)
When a bomb explodes in the air, the overpressure waves still spread out in all directions, including downward. But the downward-moving waves bounce when they strike the earth, flying back up through the hot region of thin air that the detonation created just moments earlier.

In hot, thin air, pressure waves move faster. So the bounced waves can catch up with the ones created in the initial explosion as they spread outward and sideways above the ground. Those waves combine, forming a "mach stem" that can increase the initial force of the explosion by as much as a factor of two.

That mach stem wildly expands the bomb's kill zone and creates a larger downward force on the earth that can collapse tunnels and explode mines below the surface without wasting energy carving out craters.


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If this bomb had exploded somewhere flat, unlike the mountainous Nangarhar province, where the bomb was dropped, it would have created a blast radius a mile wide.

MOABs are not the only explosives built to detonate in the air. Nuclear bombs are often built to explode miles, not feet, above their targets.

It takes a lot of complex science to build machines this deadly.

NOW WATCH: The first nuclear bomb test in history is helping scientists unlock secrets of how the Moon formed

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