Feasiblity Of A Jetpack Made From Multiple AK-47s

WHAT IF? XKCD Answering the question on all our minds:

“Is it possible to build a jetpack using downward firing machine guns?”

-Rob B

Hit up “What If?”over at XKCD for all the math and some other interesting info as well such as:

The GAU-8 Avenger fires up to sixty one-pound bullets a second. It produces almost five tons of recoil force, which is crazy considering that it’s mounted in a type of plane (the A-10 “Warthog”) whose two engines produce only four tons of thrust each. If you put two of them in one aircraft, and fired both guns forward while opening up the throttle, the guns would win and you’d accelerate backward.

hahaha wow.  Thoughts?

Also, if you have any interest in math, science, or engineering make sure you’re following the XKCD comic.

Hat tip: Stephan, Eric

9 COMMENTS

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Peter November 29, 2012 at 04:20 am

The Warthog actually slows in flight as the gun is fired. If it fired long enough the plane would eventually fall out of the sky but it doesn’t have enough rounds for that. Plus the ‘og is a ground attack plane so it doesn’t fire straight up, few warbirds do as that isn’t a very common attack pattern, especially for ground attack planes.

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bikemancs November 29, 2012 at 05:41 am

Round count isn’t the issue, they put an inturrupter in it to avoid firing long enough that would counter the thrust produced by the engine. Or at least that’s what I remember, can’t find a reference righ tnow. There’s also very few spots that you would need to fire the main gun long enough to get to that point though.

Good news, even though the A-10 was on the scratch list a few years ago, looks like the USAF is going to hang onto it for a while! http://www.airforce-technology.com/projects/a-10/

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nikonmikon November 29, 2012 at 07:20 am

It will never fall out of the sky, it produces roughly TWICE the thrust (18,130 lbf) that firing the GAU-8 does.

That means the GAU-8 only ‘neutralizes’ ONE of the TWO TF34 engines thrust output (9,065 lbf) on the plane and we all know that the A-10 can fly back on one engine without falling out of the sky.

From the Wiki:

“The recoil force of the GAU-8/A is 10,000 pounds-force (45 kN), which is slightly more than the output of one of the A-10’s two TF34 engines (9,065 lbf / 40.3 kN each). While this recoil force is significant, in practice cannon fire only slows the aircraft a few miles per hour.”

The most interesting thing about the gun to me is that it killed engines with its sheer volume of oxygen-free smoke.

“The A-10 engines were initially susceptible to flameout when subjected to gases generated in the firing of the gun. When the GAU-8 is being fired, the smoke from the gun can make the engines stop, and this did occur during initial flight testing. Gun exhaust is essentially oxygen-free, and is certainly capable of causing flame-outs of gas turbines. The A-10 engines now have a self-sustaining combustion section. When the gun is fired the igniters come on to reduce the possibility of a flame-out.”

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Rob C. November 29, 2012 at 03:32 pm

Thank you. This myth just refuses to die. The recoil of the GAU-8 is significant, but not significant enough to bother a 50,000lb aircraft traveling at 300kts being pushed by 18,000lbs of thrust.

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nikonmikon November 29, 2012 at 03:41 pm

for sure man.

i think the flameout potential is pretty wild myself, mind boggling exhaust.

did you know it runs all its cases back into the drum after they’re fired. obviously cant have those flying around lol

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Rob C. November 29, 2012 at 05:17 pm

Yeah, one of the test aircraft actually crashed from a gun-exhaust flameout. They changed the powder and made the engine modification that you mentioned. You can find some pictures of test aircraft with this funny-looking nose-piece designed to direct the gasses from the gun down away from the engines as a response to this crash. Eventually they realized that the other modifications made this unnecessary and dispensed with it.

Pic here: http://www.highgallery.com/USmilitaryAircraft/Fairchild/Fairchild-A-10-Thunderbolt-II.jpg

Most aircraft autocannons recycle the empties back into the drum for obvious reasons. I have a couple 30x173mm Avenger casings that have been cut down into shot glasses.

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nikonmikon November 29, 2012 at 06:36 pm

How do i get some of those cases cut or otherwise? I’d love to have a few for the same purpose! Also to show people exactly how big they are!

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DB327 November 29, 2012 at 07:06 am

XKCD FTW.

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Lew November 29, 2012 at 12:18 pm

I recall reading about WW2 Bristol Beaufighters of the RAF slowing in flight when all of their forward-facing weaponry (a high number of MGs and cannons) was fired at once. I’ve also heard that a full broadside from an Iowa (9×16” + a bunch of 5”s) would shift the battleship a metre sideways, though I might have mixed that up with the 105mm on the AC-130…
Recoil is a helluvva’ force.

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