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xkcd #3013: Kedging Cannon
(xkcd.com)
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In a typical 4-stroke engine, there is a process for resetting the engine between power strokes. The energy for the other three strokes (exhaust, intake, compression) comes from inertia in some sort of flywheel.
The "power stroke" in this system is not the gunpowder. It is the winching in of the cable.
In this system, the cannon is analogous to the flywheel: It merely resets the system between power strokes.
More specifically: