Five! Kirby inhales. Her breath is big and deep. Four! She exhales sharply, emptying her lungs of air. Three! She sips a slow, steady breath, stopping short when she hears the audience chant Two!

It’s at this point that Kirby assumes a rigid plank position. From her head to her toes, every fiber of her physique is tightly flexed in anticipation of what comes next. The crowd chants “one!” The announcer says “go!” And suddenly Kirby is gaining momentum, accelerating toward the mouth of the cannon. By the time she exits, she’s traveling between 60 and 66 miles per hour.

The inner workings of these cannons may be closely guarded, but we do know that they pack a wallop. Kirby says the strain on her body is enormous, but that the brunt of it is absorbed by her ankles, knees and glutes. “There’s enough power in there to make peanut butter out of you,” David Smith has said of his home-brew artillery. How much power, exactly? I asked Rhett Allain, a physics professor at Southeastern Louisiana University who has also written extensively on the physics of unconventional artillery, like the cannons used to lob drugs across the U.S./Mexico border.

#### Above: A photo of a border-defying drug cannon, via the Mexicali Public Safety Department

Allain says we can start with the following kinematic equation. It may look scary to those with a phobia of physics, but it’s actually pretty straightforward:

## vf2 = v02 + 2a(x – x0)

To translate: The square of Kirby’s velocity exiting the barrel is equal to the square of her velocity at the base of the barrel, plus twice the distance she travels along the barrel multiplied by her acceleration. If we assume a final velocity of 66 mph (29.5 m/s) and an initial velocity of 0 m/s (remember, she’s not moving in any direction while she waits at the base of the barrel), and plug in the length of the barrel (24 feet, or about 7.32 meters), we can solve for Kirby’s acceleration, which comes to to 59.6 m/s 2, or about 6 gs.

The video above claims that Kirby experiences a g-force of 7. That’s certainly possible—likely, even. Our back-of-the envelope calculation makes a few assumptions, but the biggest of these assumptions is the distance that Kirby travels while inside the barrel. For simplicity’s sake, we’ve used the barrel’s overall length, but it’s entirely possible that Kirby, when she lowers herself into the cannon, does not slide down the cylinder’s full, 24-foot extent. Perhaps the launching platform at the base of of the tube is positioned such that she slides only 20 feet. The difference may seem small, but the bodily strain one experiences accelerating to 66 miles per hour over 20 feet is significantly more than one experiences over 24. In fact, if we plug the former distance into our equation, we get a g-force of around 7.2.

“You want to keep the acceleration down,” says Allain, “because high accelerations kill humans.” A longer barrel keeps your acceleration small, by allowing you build up speed over a greater distance.

The average person can withstand maybe 5 gs before passing out. Granted, Kirby only endures those 6 or 7 gs for a split second, as opposed to, say, the sustained strain that a pilot feels while pulling out of a nose dive (a maneuver that can lead to “G-induced Loss of Consciousness,” or G-LOC).

“I’ve heard urban legends of cannonballs losing consciousness in the air,” says Kirby. “I don’t know if those legends are true or if they’re just invented,” she adds. But personally? No. She’ll often feel dizzy, but she’s never blacked out mid-flight.

### Stick The Landing

As soon as Kirby exits the barrel, she stretches into a bird-like position, thrusting her arms wide, arching her back, and swinging her legs up and over her head as she reaches the peak of her trajectory. Her fingers, she says, are “super straight, super stretched, and super pretty.”

It’s here, about forty feet in the air, that Kirby’s superhuman aerial abilities come into play. The most dangerous part of the cannon act, she says, is the landing; alighting flat, and face up, is key. It’s also easier said than done. Her elapsed flight time is only about 2.6-seconds. “When you’re that high up, and you have that little time, deciding how to have a controlled landing is not something you do intellectually,” she says. “It’s something your body does on feel.” Kirby honed these skills while training for the trapeze. “After years of executing tricks and missing them, intentionally or by accident, trapeze artists develop instincts where we’ll figure out a way to land on our backs,” she says.

On her way down, Kirby keeps her body elongated until right before she hits the airbag. In the last few hundredths of a second, she tucks her chin to her chest so that she lands flat on her back. If she needs to create more rotation mid-flight, she does a pike, quickly touching her toes before opening her body, like so:

“It’s crazy the way we relate to time,” says Kirby, reflecting on her first flights as a human projectile. “The first, maybe, hundred launches that I did, I remember there being all this prep, and it feeling like an eternity,” she says. “Then I’d climb in, and— fivefourthreetwoonefire!—the next thing I knew I was in the airbag. I wouldn’t even know what had happened.”