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The physics of Jetpack Joyride's machine gun jetpack explained by xkcd

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If one were to strap a pair of machine guns to one's back, could they fly through the air, perhaps to collect scores of floating coins? Is such a thing possible? That idea, the flight concept featured in Halfbrick Studios' Jetpack Joyride (formerly Machine Gun Jetpack), is theoretically possible, according to former NASA robotocist and xkcd webcomic creator Randall Munroe.

In the latest "What If?" entry from xkcd, in which Munroe tackles hypothetical physics questions, the webcomic author answers the question "Is it possible to build a jetpack using downward firing machine guns?"

It is, Munroe says, explaining that the AK-47 assault rifle would emit enough force to launch itself into the air, if the trigger were held down with a piece of tape.

"So what does all this mean for our jetpack?" Munroe writes. "Well, the AK-47 can take off, but it clearly doesn't have the thrust to spare to lift anything weighing much more than a squirrel." Add additional rifles, emitting more force, and machine gun jetpacks become more plausible.

Munroe goes on to explain the limitations of firearm-based flight, complete with informative stick figure illustrations, which should be read in full to properly appreciate.