The Federal Trade Commission on Monday said that it has closed its investigation into the planned acquisition of Pratt & Whitney Rocketdyne by GenCorp, paving the way for the $550 million purchase of the longtime San Fernando Valley rocket-engine manufacturer.

The commission’s action paves the way for Sacramento-based GenCorp to complete its $550 million purchase of Rocketdyne, a longtime San Fernando Valley rocket-engine manufacturer. The deal was announced nearly a year ago.

In making the decision, the commission cited the Pentagon’s desire to complete the deal for national security and other reasons. The FTC, however, found that the deal does give GenCorp a monopoly on rocket-propulsion systems used on missile-defense interceptors. And it would create barriers to entry for competing manufacturers of similar engines.

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.

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.