Is it possible to build a jetpack using downward firing machine guns?
It’s a simple enough question, one that just about every gun owner had pondered at one point in his or her life. Luckily, one curious Internet user posed that question to the clever mind behind XKCD’s What-If physics discussion.
The basic idea is fairly simple: shooting a gun causes recoil, so pointing a gun downward should cause the gun to create upward thrust. Strap enough of those to a backpack and you should be able to build a gun-powered jetpack!
All you have to do is get a gun that creates enough thrust to overcome its weight, plus the weight of a passenger. What-If tackles that problem with an AK-47, which has an approximate thrust of 13 pounds of force. That’s more than enough to overcome the rifle’s 10.5 pound weight. All you had to do is add up a bunch of those rifles, and suddenly all that extra thrust really starts to add up!
So, yes: you could build a jet pack out of AK-47s. What-If estimates that an aircraft fueled by 25 to 300 AK-47s could hit speeds of 100 meters per second (approximately 223 miles per hour) and soar half a kilometer into the air.
AK-47s are not ideally suited for flight, however. They don’t create a tremendous amount of recoil and the weight of ammunition limits their effectiveness – anything more than 250 bullets would make the craft too heavy to fly.
What-If attempts to solve that problem by bringing out the big guns – literally. The GAU-8 Avenger Gatling-type cannon, for example, would generate more than enough thrust to send you flying. What-If explains, “If I mounted a GAU-8 on my car, put the car in neutral, and started firing backward from a standstill, I would be breaking the interstate speed limit in less than three seconds.”
As if that wasn’t good enough, the Russian-built Gryazev-Shipunov GSh-6-30 weighs less than the GAU-8 and has an even greater rate of fire. That means a better thrust-to-weight ratio, which translates into more air time.
What-If states, “But if you somehow braced the human rider, made the craft strong enough to survive the acceleration, wrapped it in an aerodynamic shell, and made sure it was adequately cooled … with a GSH-6-30, you could jump mountains.”
Aren’t physics neat?
We need to combine the world of guns and racing. Who needs an internal combustion engine when you can just strap 300 guns to the back of your car? Can you imagine how much fun it would be to see Glock and Smith & Wesson drag cars compete against each other to see which one could cross the finish line first?