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Hydraulic catapults where swapped out in the 1950’s

http://content.time.com/time/subscriber/article/0,33009,860952,00.html

“ The hydraulic catapults on U.S. aircraft carriers have figured in a long series of postwar accidents, e.g., the explosion that took 103 lives on the U.S.S. Bennington (TIME, June 7). Last week the Navy announced that it is abandoning the hydraulic catapult. A steam-powered model of British design, already tested successfully aboard the U.S.S. Hancock, will be installed on all American carriers. The steam catapult, utilizing a hooked piston riding in a slotted cylinder, is safer than the old hydraulic model because it uses no highly volatile, explosive liquids.

But the chief reason for the change is not safety. The Navy switched to the "steam slingshot" mainly because it is faster and vastly more powerful. On larger carriers like the Forrestal, four steam catapults will launch as many as 32 interceptors in four minutes.”