MythBusters Episode 238: The Explosion Special

If a mail truck containing a bomb is filled with wet cement, the explosion will not injure bystanders or completely destroy the truck. (Based on a scene from the television series MacGyver.)

busted

After analyzing the scene, Adam and Jamie determined that 84 pounds (38 kg) of dynamite had been placed in the mail truck in the MacGyver episode. To begin their tests, they set up a similar mail truck with explosives. To gauge shrapnel injuries, they surrounded the truck with several human-shaped plywood cutouts at a distance of 30 feet (9.1 m). They also placed pressure sensors at distances up to 250 feet (76 m). A control test with an empty truck shredded the cutouts and broke the sensor cables, throwing shrapnel as far as 675 feet (206 m).

For the second test, they secured the dynamite to a wood frame in the cargo compartment and loaded the truck with 32,000 pounds (15,000 kg) of cement. When some of the dynamite came loose and began to float up, Adam cut a hole in the back wall of the driver’s cab and climbed in to re-secure it. As cement began to leak out around the back doors, they set off the explosion and were amazed to find that all of the cutouts were intact and the truck only suffered moderate damage. However, with closer inspection, they discovered that only a small quantity of the dynamite had exploded, due to the weight of the cement dislodging the primer cord.

For a final test, they secured the dynamite in a plywood box before filling the truck with cement in order to ensure that it would all explode. The blast caused the cement to shoot upward in a jet, destroyed the truck, and left a shallow crater in the ground underneath. Debris was thrown to a maximum of 250 feet (76 m), less than the control test, but the cutouts were knocked down and peppered with small shrapnel holes. Adam and Jamie classified the myth as busted, noting that although the cement redirected most of the shock wave of the blast, it did not protect the truck or bystanders from destruction or injury.

It is possible to cross a minefield safely using a hovercraft.

confirmed

After taking hovercraft lessons over land and water, Adam and Jamie built 20 artificial, programmable, pressure-sensitive landmines to deliver a blast of compressed air and foam debris when triggered. They then set up a minefield measuring 50 feet (15 m) by 80 feet (24 m), with the mines laid out in an S-curve to ensure that a straight run down its length would cross over most of the mines. Adam tested an anti-personnel trigger setting of 35 pounds (16 kg) by walking the field blindfolded, and Jamie tested an anti-vehicle setting of 300 pounds (140 kg) by driving a jeep through it. Adam and Jamie each hit and triggered several mines during their test runs.

Jamie then drove the hovercraft across the field and back, with the triggers set to the anti-vehicle pressure, but he did not set off any of the mines. When the mines were set to the anti-personnel pressure setting, Adam was unable to set them off in the hovercraft as well. Jamie then made one last run in the hovercraft with the triggers set to 5 pounds (2.3 kg) and completed it safely. Explaining that the hovercraft distributed its weight over a large enough area to avoid setting off the mines, they declared the myth confirmed.