Posted on 11/25/2012 6:38:56 PM PST by andyk
"Is it possible to build a jetpack using downward firing machine guns?” —Rob B
I was sort of surprised to find that the answer was yes! But to really do it right, you’ll want to talk to the Russians.
The principle here is pretty simple. If you fire a bullet forward, the recoil pushes you back. So if you fire downward, the recoil should push you up.
The first question we have to answer is “can a gun even lift its own weight?” If a machine gun weighs ten pounds but only produces eight pounds of recoil when firing, it won’t be able to lift itself off the ground, let alone lift itself plus a person.
In the engineering world, the ratio between a craft’s thrust and the weight is called, appropriately, thrust-to-weight ratio. If it’s less than 1, the vehicle can’t lift off. The Saturn V had a takeoff thrust-to-weight ratio of about 1.5.
Despite growing up in the South, I'm not really a firearms expert, so to help answer this question, I got in touch with an acquaintence in Texas. (Judging by the amount of ammunition they had lying around their house ready to measure and weigh for me, Texas has apparently become some kind of Mad Max-esque post-apocalyptic war zone.)
(Excerpt) Read more at what-if.xkcd.com ...
LOL, I love that guy.
oh, this is one myth that definitely needs to be busted!
Good to see there’s another XKCD fan around here. The guy’s a hoot! I don’t always understand the math behind some of his cartoons, but he’s always a lot of fun.
Yosemite Sam does this all the time :)
The principle is sound for space flight (Mass driver) but going against gravity is a lot tougher.
I know it slows down an A10 warthog a fair amount.
It would work on the Moon.
XKCD rocks.
Sounds like a proposal that Konstantin Tsiolkovsky put together a few years back.
Probably slows down whatever is on the receiving end of those Gatling guns as well; just a hunch! *grins!
Ya stole my thunder, I’ve seen him achieve lift off many times.
Early A10 Warthogs would fall out of the sky when firing long bursts.
I saw a show about A10’s on the history or military channel awhile back. One of the pilots they interviewed said that the plane would fly backwards if it had enough ammo and you could hold down the trigger long enough with out melting the barrels.
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