GenCorp completed its acquisition on Friday of storied San Fernando Valley rocket-engine manufacturer Pratt & Whitney Rocketdyne. The $550 million purchase doubles the size of Sacramento-based GenCorp . The company will combine Rocketdyne with its AeroJet-General Corp. subsidiary in a business until named Aerojet Rocketdyne Inc., with headquarters in Sacramento. Rocketdyne has a history of more than 50 years in the Valley developing rocket engines used on the Saturn V for the moon landings, and on the Space Shuttle. GenCorp. is considered an industry leader in rocket-propulsion technology. The company’s Aerojet rocket- and missile-propulsion division dates back to the 1930s and has done work for the U.S. Air Force and Navy. “Combined, we bring decades of history that launched the first space age and put mission-critical technology into the hands of our warfighters,” said GenCorp Chief Executive Scott Seymour in a prepared statement. The completion of the deal comes just days after it passed a Federal Trade Commission anti-trust review. Rocketdyne was put up for sale last year by owner United Technologies, of Hartford, Conn., to generate cash for its $18.4 billion acquisition of airplane-parts manufacturer Goodrich Corp. Rocketdyne employs more than 1,000 people on separate campuses on Canoga Avenue and DeSoto Avenue. The acquisition also gives GenCorp a 50 percent stake in RD Amross LLC, a Rocketdyne joint venture with NPO Energomash, a Russian company that builds rocket engines.