WINDSOR LOCKS, Conn. — Pratt & Whitney expects military and commercial aerospace business will improve, but not for a while, the president of the jet engine manufacturer said today.

David Hess, president of the subsidiary of United Technologies Corp., told reporters at the company’s annual media day that sales are expected to double, to $24 billion, by the end of the decade.

“This is not some aspirational goal,” he said. “This is math.”

But the East Hartford, Conn., company must first get through a few years in the short-term as the military shifts to new fighter jets that require different engines, he said.

“We just have to navigate through a couple of transitional years here in the business,” Hess said.

High fuel costs and the weak economic recovery also are pressuring airlines and military spending cuts threaten to slice into sales by defense contractors.

Pratt & Whitney’s purchase of Rolls-Royce from a joint venture that makes engines for the Airbus A320 will boost engine deliveries from 5,000 in 2011 to 8,000 in 2018. The higher engine sales also are expected to establish a “very lucrative aftermarket” that will continue to grow through the end of the decade, he said.

On the military side, he said Pratt & Whitney’s engine for the F-35 joint strike fighter will generate revenue long-term, even amid Pentagon budget cuts.

Just last week, a Pentagon official signaled that the Army could lay off as many as 24,000 enlisted personnel and up to 5,000 officers within five years to meet a projected reduction in the force driven by budget cuts and the winding down of two wars.

The Defense Department last year halted development of an alternate engine being developed by General Electric Co. and Rolls Royce, leaving Pratt & Whitney the sole supplier.

“Regardless of where the debate ends up on … some of the defense budget cuts, that is going to be a growth program for Pratt & Whitney through the end of the decade and for decades to come,” Hess said.