Aerospace engineer Benjamin Hagen looks inside of the Air Force Research Laboratory's trisonic wind tunnel where he is studying the fluid dynamics of transonic air flow in Dayton, Ohio. Due to a large number of retirements over the last few years the research lab is looking to hire about 300 new scientists.

DAYTON, Ohio — The number of scientists and engineers retiring at the Air Force’s top science research agency has doubled in the last five years, and defense experts say the trend could lead to a shortage because a growing number of highly trained workers are eligible to leave.

The Air Force Research Laboratory, headquartered at Wright-Patterson in Ohio, has a workforce with about half the employees age 50 or older. This year, 20 percent of the agency’s scientists and engineers were eligible for retirement; by 2018, that figure will reach 33 percent.

The Air Force reportedly has lost nearly 30 percent of its top senior scientists the last two years, as well.

Former Lockheed Martin Corp. Chairman Norman R. Augustine said he expects a future shortage of engineers and scientists, which could impact national security. For decades, the United States has relied on superior technology to maintain an edge against adversaries.

“I do think it puts us at risk, and one of the greatest dangers is, it takes a long time (to find replacements),” said Augustine, a co-chairman of a National Academy of Sciences committee in 2012 that reviewed the status of the science, technology, engineering and math, or STEM, workforce in the Department of Defense and U.S. defense industry.

“You don’t just turn the spigot on and say we’ll have more engineers.”

A 2010 National Academy of Sciences study projected a shortage of scientists and engineers between 2015 and 2020, said George K. Muellner, a former Boeing Co. executive who was a cochairman of the review.

Budget instability caused last year by sequestration — from civilian furloughs to grounded jets — could hurt Air Force recruitment of civilian scientists and engineers, the retired Air Force lieutenant general said.

“To be frank, if they’re not able to start providing some stability to the folks they hire, they’re not going to compete well at all,” said Muellner, a past president of the American Association of Astronautics and Aeronautics.

The status of the Department of Defense science and engineering workforce has attracted the attention of Congress. As part of the fiscal year 2014 National Defense Authorization Act, lawmakers required the Pentagon to report on STEM workforce needs by last March.

The Defense Department missed the deadline but says a report will be released. In a Senate hearing last week, U.S. Sen. Rob Portman, R-Ohio, pressed the Pentagon to release the data so Congress can assess the issue.

“We want them to define the problem and then tell us how they recruit, how they retain and then what tools they need,” Portman said.

The military and defense and national security contractors face the challenge of competing for a limited number of graduate school students. Many students in U.S. graduate schools are foreign citizens not eligible for security clearances.

“Now you’ve cut the pool of graduate students in half that we’re eligible to go after, and of the half that’s left, we’re competing with industries that are more lucrative,” said Scott Coale, a retired colonel and former vice commander of the Air Force Research Lab, or AFRL.

To work on a classified project at a Department of Defense lab, a scientist or engineer must be a U.S. citizen with a security clearance, said Pamela Swann, AFRL deputy director of personnel.

In limited circumstances, AFRL may employ foreign-born scientists or engineers who have a green card, or permanent U.S. residency but who do not work on classified projects, she said.

The 2010 study that reviewed the Air Force’s STEM needs noted “reason for concern as to whether the supply of scientists and engineers who can obtain a security clearance will be adequate to meet the future needs of the Air Force.”

The report said that while science and engineering degrees awarded increased 8 percent between 2000 and 2005, the number of those degrees awarded to U.S. citizens and permanent residents fell 5.5 percent. It also said women and minorities were a growing segment of potential recruits. It urged the Air Force to take a “proactive role” to address shortfalls in math and science skills among middle and high school students.

Augustine said U.S. high school students fare poorly in international science and math tests and often have not shown the kind of interest in STEM careers their counterparts in other countries have demonstrated.

“That’s the real problem,” he said.

Throughout the Air Force, 21 percent of scientists and 17 percent of engineers who are eligible retire every year. Forty-four percent of scientists and 40 percent of engineers are older than age 50, and the Air Force expects the retirement of 250 scientists and engineers every year until 2019.

Within AFRL, the agency reported that 311 scientists and engineers retired between fiscal years 2009 and 2013. In fiscal year 2009, 35 scientists and engineers retired at AFRL, and that number more than doubled to 76 in 2013, agency figures show. Retirements reached a peak of 96 in fiscal year 2012. The agency anticipates 400 more will opt for that path from this year through 2018.