Randall “xkcd” Munroe is answering your hypothetical questions with physics every tuesday on his blog "WHAT IF?":
TODAY: "Is it possible to build a jetpack using downward firing machine guns?”
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An AK-47 clip holds 30 rounds. At ten per second, we’ll get a measly three seconds of acceleration. We can improve this with a larger magazine—but only up to a point.
It turns out there’s no advantage to carrying more than about 250 rounds of ammunition. The reason for this is a fundamental and central problem in rocket science: Fuel makes you heavier.
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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.
To put it another way: 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.
read the whole thing here.
