How do you get a 45,000 pound plane up to launch speed on a 270-foot aircraft carrier runway? With a giant, steam-powered catapults of course. Jörg Sprave would be so proud.
The modern aircraft catapult, part of the CATOBAR (Catapult Assisted Take Off But Arrested Recovery) system, consists of a towbar, which attaches to the landing gear under the plane's nose, connected to a shuttle. The shuttle then connects to a metal lug that pokes up through a slit in the deck and is itself attached to a pair of below-deck pistons. These pistons sit in parallel cylinders, 18-inch diameter and about 100 yards long, which are "charged" with high-pressure steam from the carrier's reactors.
Once the cylinders are sufficiently charged — too little pressure and the plane takes a dip in the ocean, too much pressure and the shuttle will rip the front end of the plane off — the aircraft opens up its throttle (for added thrust) and the catapult officer releases a lock on the pistons. This causes the pistons to slam forward, accelerating the plane down the runway and off the end of the carrier — going from 0 to 165 MPH in two seconds.
To keep the shuttle from simply careening into the end of its track (or clear into the ocean) at 165 MPH, carriers employ a water brake system to slow it down with out damage. This consists of a horizontal dashpot filled with aerated seawater. The resistance this water provides slows and eventually stops the charging pistons. [Wikipedia - US Navy Aircraft History - How Stuff Works - Electronic Aviation]
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