Episode 191: Trench Torpedo

Air Date: October 14, 2012

A battlefield trench built with sharp 90-degree corners can help stop a shockwave and save a soldier’s life.

plausible

This myth supposedly arose from the German army’s careful construction of its trenches during World War I. Adam and Jamie started by building small-scale trenches filled with water and colored oil to help visualize wave movement. Three trenches were built: one straight, one with two sharp corners, and one with two soft (curvy) corners. A motorized mechanism generated waves at one end of each trench and the amplitude of the waves was measured at the opposite end. The amplitude for the straight trench was 0.75 inches (19 mm), for the sharp cornered trench it was 0.125 inches (3 mm), and for the soft-cornered trench it was 0.25 inches (6 mm), lending credibility to the premise.

For full scale testing, three 50 foot (15 m) trenches were dug in the same shapes and lined with plywood to ensure straight edges (the soft corners were not lined). A 25 lbs (11 kg) charge of TNT was used for each explosion and force sensors were placed at 10 foot (3 m) increments from the charge. The following table summarizes the findings.

Trench Shape Pressure at 20 ft Pressure at 30 ft Pressure at 40 ft Pressure at 50 ft
Open-air (control) 39 psi (269 kPa) 12 psi (83 kPa) 7 psi (48 kPa) 5 psi (34 kPa)
Straight 397 psi (2737 kPa) 65 psi (448 kPa) 38 psi (262 kPa) 21 psi (145 kPa)
Sharp corners 60 psi (414 kPa) 19 psi (131 kPa) 12 psi (83 kPa) 7 psi (48 kPa)
Soft corners 76 psi (524 kPa) 21 psi (145 kPa) 13 psi (90 kPa) 8 psi (55 kPa)

Because the intensity of the blast was lower with sharp corners than with soft corners or no corners, the myth was deemed plausible.

A bunch of party balloons can act similarly to an airbag during a car crash and protect a passenger.

busted

In order to simulate a car crash at 35 mph (56 km/h), the Build Team dropped cars onto their front bumpers from a height of 41 feet (12.5 m). A dummy with force sensors was placed in the passenger seat for each drop. A control crash with no seat belt and no balloons gave a maximum force to the dummy’s chest of 640 g, which was well above the 100 g benchmark for lethality. A drop with normal latex balloons in front of the passenger gave a maximum force of 620 g, so they had little effect. Back in the workshop, the team experimented with different types and configurations of balloons including small balloons, large balloons, balloon animals, and giant, extra-thick balloons. The giant, extra thick balloons performed the best at cushioning an impact so the Built Team packed the car tightly with those balloons for another drop that resulted in a maximum force of 130 g (still lethal). For a final test, they tried taping many small balloons to plastic sheets in order to prevent movement and to prevent holes forming from only a few popped balloons. With this test the maximum force was 230 g, definitively busting this myth.

6 Comments

  1. James Jones says:

    The battlefield trench myth definitely looks plausible, but I do wonder about one thing. You put those trenches pretty darned close to one another. Wouldn’t the ones tested last suffer damage from the previous explosions, so that they might not give the results they would if they were tested in isolation from the others?

  2. Alex says:

    Did they find the deceleration with a proper airbag and no seatbelt? If the best balloons reduced the deceleration by 80%, isn’t that pretty good? Maybe it means that a 30 mph crash goes from five times lethal to survivable without a seatbelt.

  3. rik says:

    woudn’t the effect be less if in the balloon myth you had one big balloon to reduce the space?

  4. Robert says:

    After viewing the battlefield trench myth episode, I’m left wondering if the pressure wave could be reduced even more using slightly different configurations.

    The one I’m thinking off would have the trench continue pass the right angle turn (say the eqivalent of twice the width of the trench) making sure that any parallel sections between trench sections be separated by dirt walls that are at least twice the width of the trench.
    Here’s a crude representation:

    ———-|—-
    ……….|
    ……….|
    ……….|——–
    ……….|
    ……….|

    Hope you revisit this myth, and have a go using varations to see if any configuration can reduce the shock wave even more.

    Also, I can’t help but think of the movies where a fireball from an explosion is travelling down a tunnel and the hero jumps into a perpendicular hallway to escape death. So maybe the trench section needs to continue farther down past the perpendicular turn.

    Hope you have a blast testing this one !
    :-)

  5. Stephen says:

    German trenches were also dug deeper than the Allies. would that also make a difference?

  6. Ben says:

    The primary reason why trenches were designed in a zig-zag pattern was to prevent enfilading fire if the enemy got to the trench line. If the trench was straight, all one (conceivably) had to do was stand at one end of the trench and the soldier could shoot everyone inside. Zig-zagging the trench meant that if enemy soldiers did get to the line, only a small part of the trench was lost and the rest were safe.

Leave a Reply

(required)