Depth Charge - Underwater Explosions

Underwater Explosions

The high explosive in a depth charge undergoes a rapid chemical reaction at an approximate rate of 8,000 meters per second (25,000 ft/s). The gaseous products of that reaction momentarily occupy the volume previously occupied by the solid explosive, but at very high pressure. This pressure is the source of the damage and is proportional to the explosive density and the square of the detonation velocity. A depth charge gas bubble expands to reach the pressure of the surrounding water.

This gas expansion propagates a shock wave. The density difference of the expanding gas bubble from the surrounding water causes the bubble to rise toward the surface. Unless the explosion is shallow enough to vent the gas bubble to the atmosphere during its initial expansion, the momentum of water moving away from the gas bubble will create a gaseous void of lower pressure than the surrounding water. Surrounding water pressure then collapses the gas bubble with inward momentum causing excess pressure within the gas bubble. Re-expansion of the gas bubble then propagates another potentially damaging shock wave. Cyclical expansion and contraction continues until the gas bubble vents to the atmosphere.

Consequently, explosions where the depth charge is detonated at a shallow depth and the gas bubble vents into the atmosphere very soon after the detonation are quite ineffective, even though they are more dramatic and therefore preferred in movies. A sign of an effective detonation depth is that the surface just slightly rises and only after a while vents into a water burst.

Very large depth charges, including nuclear weapons, may be detonated at sufficient depth to create multiple damaging shock waves. Such depth charges can also cause damage at longer distances, if reflected shock waves from the ocean floor or surface converge to amplify radial shock waves. Submarines or surface ships may be damaged if operating in the convergence zones of their own depth-charge detonations.

The damage that an underwater explosion inflicts on a submarine comes from a primary and a secondary shock wave. The primary shock wave is the initial shock wave from the depth charge, and will cause damage to personnel and equipment inside the submarine if detonated close enough. The secondary shock wave is a result from the cyclical expansion and contraction of the gas bubble and will bend the submarine back and forth and cause catastrophic hull breach, in a way that can be best described as bending a plastic ruler back and forth until it snaps. Up to sixteen cycles of the secondary shock wave have been recorded in tests. The effect of the secondary shock wave can be reinforced if another depth charge detonates on the other side of the hull in a close proximity in time of the first detonation, which is why depth charges normally are launched in pairs with different pre-set detonation depths.

The killing radius of a depth charge depends on the payload of the depth charge and the size and strength of the submarine hull. A depth charge of approximately 100 kg of TNT (400 MJ) would normally have a killing radius (hull breach) of only 3–4 meters (10–13 ft) against a conventional 1000-ton submarine, while the disablement radius (where the submarine is not sunk but put out of commission) would be approximately 8–10 meters (26–33 ft). A higher payload only increases the radius by a few meters because the effect of an underwater explosion decreases with the distance cubed.