GenCorp Inc. is making a 10 percent reduction in the staff of its Aerojet Rocketdyne division, which includes its Chatsworth rocket-engine development and manufacturing facility. The cuts will amount to about 500 jobs and will be completed over the next four years. It is not known how many will take place in the San Fernando Valley, but 250 to 300 are expected to be carried out at the company’s Sacramento facility. The company expects an annual cost savings of $145 million by 2019. GenCorp, in Rancho Cordova, acquired Rocketdyne in 2012 and combined it with its own rocket engine division, Aerojet. A round of layoffs in early 2014 affected 225 employees at the Chatsworth and Sacramento facilities. Aerojet Rocketdyne has about 1,300 employees in Chatsworth working on multiple projects, including developing a hypersonic space engine and building a rocket engine to replace the Russian RD-180, which is now used to power the Atlas V rocket that launches U.S. military and spy satellites. In addition to the staffing cuts, GenCorp is downsizing its Sacramento plant by 1 million square feet by moving work to sites in other states. Chief Executive Scott Seymour called the moves essential to the near and long-term health and performance of the business. “This competitive improvement program represents a fully-integrated, long-term business strategy that preserves the company’s accelerated momentum and assures our ability to continue driving the inherent growth and profitability of our business,” he said in a prepared statement. Share of GenCorp rose 30 cents, or more than 1 percent, to $20.51 on the New York Stock Exchange.