The Terrifying Physics Behind The "Mother Of All Bombs"

The MOAB in all its frightening glory. US Department of Defence

As you’ve no doubt heard, the “Mother of All Bombs”, or MOAB, was used in a conflict zone for the first time this week. Setting the pros and cons of using such a stupidly named, $16-million-a-piece weapon aside, the science behind the device is itself weirdly fascinating.

First off, this isn’t what the US-developed weapon is actually called. The acronym stands for “Massive Ordnance Air Blast”, and massive is certainly an understatement. One single MOAB weighs 10.3 tonnes (22,600 pounds), and it’s around 9.2 meters in length (around 30 feet). Crucially, it has an explosive yield of 11 tons of TNT, or 46 billion joules.

Most defense analysts agree that it’s the most powerful non-nuclear military explosive currently in use – yes, there have been more powerful versions – but, compared to nuclear weapons, it’s still a peashooter. The US nuclear arsenal, for example, contains the B83, a thermonuclear weapon that’s 110,000 times more explosive, and that’s not including the radioactive fallout.

Still, it’s plenty energetic. In fact, the explosive energy released by a MOAB is roughly the same as that unleashed during a M6.0 earthquake. No wonder that those near the blast site in Afghanistan described the detonation’s effects as feeling “like the heavens were falling.”

Explaining the use of the MOAB. CNN via YouTube

This partly explains why the MOAB has been used up to this point as a form of psychological warfare – it’s genuinely scary, earth-shattering stuff.

The detonation itself is caused by a mixture of TNT (80 percent) and aluminum powder (20 percent), which is a highly explosive mixture known as Tritonal. The addition of aluminum helps the TNT ignite and reach an extremely high pressure far quicker than TNT would be able to alone.

Guided to the ground by GPS after being literally pushed out of a plane, the MOAB doesn’t detonate upon impact. Instead, it explodes in the air above its target at a height of just 1.8 meters (6 feet).

This so-called “air burst” technique was one also investigated during the development of nuclear weapons. When such weapons explode, they create a huge “overpressure” bubble that expands rapidly outwards. The lower part of this bubble will impact the ground and rebound upwards and outwards.

Thanks to the extremely fast-moving air already inside the bubble, this reflected wave front will then reach the rest of the outwardly expanding overpressure bubble. This so-called “mach stem” boosts the pressure even more.

The formation of a mach stem. SiriusB/Wikimedia Commons; Public Domain

This mechanism intensifies the destructive damage in a way that a ground-based detonation can’t – and, because of this quirk of physics, the MOAB dropped in Afghanistan was so powerful that it had a blast radius of well over 1.6 kilometers (1 mile) from the epicenter.

Russia has its own version, by the way. It is, rather predictably, called the “Father of All Bombs”.

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