WASHINGTON — Aerojet parent company GenCorp. Inc. said July 23 that it has signed a definitive agreement to buy Pratt & Whitney Rocketdyne from United Technologies Corp. for $550 million.

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Brian Berger is editor in chief of SpaceNews.com and the SpaceNews magazine. He joined SpaceNews.com in 1998, spending his first decade with the publication covering NASA. He was named senior staff writer in 2004, a position he held until being named...