A "Steam Catapult" sounds like something that would be used by hordes of mutant marauders in some post-apocalyptic feudal warfare, but it is actually quite contemporary. It is the system that provides the massive acceleration necessary to launch jet aircraft from the decks of aircraft carriers.
Before the advent of the steam catapult, aircraft carriers used a cable-driven system, with pulleys and a hydro-pneumatic rotary drive mechanism. As the planes grew heavier, the size and weight of these systems began to overwhelm even the magnificent bulk of the aircraft carrier.
In 1952, the first tests of the steam catapult, invented by Cdr. C. C. Mitchell, a volunteer reserve officer for the Royal Navy, took place aboard the HMS Perseus. It was a resounding success, and it ushered in the age of the modern carrier-borne jet fighter.
The concept is pretty simple. It uses a pair of cylinders, which run the length of the flight deck. Each one contains a piston, and the pistons are connected together by a bar with a hook in the middle, which sticks up and runs along a slot in the deck. The steam from the carrier's boilers pushes the pistons, and the hook pushes the aircraft. The odd thing about the cylinders is that they are slotted, so that the pistons can connect to the bar. What makes it all work is a linear seal that can follow the piston, preventing the pressure from being lost through the slot.
These things can get a 60,000 pound aircraft moving about 170 mph in under two seconds, and repeat that feat every 30 to 45 seconds. Still, they are not perfect. For one thing, the steam impulse shoves about 6 G's at the beginning of the thrust, but it dwindles to about 2 G's toward the end. This requires pilots to endure quite a shock to become airborne. A system that could increase the force applied to the airplane as it moves along the launch track would be a big improvement.
The US Navy is funding research into linear electromagnetic motors that will someday replace the steam catapult, but they are not scheduled to select a candidate for production until 2004, and it is unlikely that one will be ready for duty until 2010 or so. Until then, we'll have a steam-powered naval air defense.