Power Producers - Two of the most powerful Jet engines yet developed for the U.S. Air Force near the end of the production line at the Pratt & Whitney plant in East Hartford, Conn. Known as the J57, the engine was announced by the Air Force today as the first to have an official rating in the 10,000-pound thrust class. The open circular areas are the front ends of the engines through which air enters. At 20,000 feet and a speed of 375 miles an hour one pound of thrust Is the equivalent of one ho