After 20 years, a St. Louis company may finally get its chance to prove that the Pentagon used its technology to make stealth combat aircraft stealthy without paying for it.

An appeals court on Friday revived a patent-infringement suit that Zoltek Corp. first filed in March 1996. The court said a judge erred in ruling the patent invalid, and ordered a hearing on whether the F-22 fighter plane and B-2 bomber used a Zoltek invention.

The U.S. government is invoking the military and state secrets privilege on the advanced aircraft and their classified technology, contending it must keep information about them from being disclosed to protect national security.

Zoltek owns a patent applied for in 1984 and issued in 1988 -- the same year the bomber was publicly displayed for the first time -- for a method to make carbon-fiber sheet products more resistant to electricity. It contends the process was used in the sensitive coating that helps make the F-22 and B-2 hard to detect on enemy radar.

Lockheed, Northrop

The case has bounced between the U.S. Court of Federal Claims in Washington and an appeals court that shares the same building. Among the patent rulings was one that determined Lockheed Martin Corp., which built the F-22, wouldn’t be pulled into the case because it’s the government’s responsibility to pay for any use of a patented invention by contractors.

In 2014, the Federal Claims judge invalidated the Zoltek patent, saying scientists knew that temperatures used in making carbon fibers would affect how resistant they would become. The appeals court reversed that decision, citing a 1987 letter from an engineer at Northrop Grumman Corp., which built the B-2, who said he’d never seen a material of the type developed by a company later bought by Zoltek.

The case will be returned to the trial judge, who has to determine if the patent was infringed. That’s when the government’s assertion of the state secrets privilege may prove the ultimate roadblock for Zoltek.

One of Zoltek’s claims is that a fiber called Tyranno made by Lockheed for the F-22 infringes the patent. In a 2013 letter to the court, then- Air Force Secretary Michael Donley said the information that Zoltek seeks “would assist foreign governments to develop measure to counter the advantages of the F-22 stealth technology” and help them develop their own stealth planes.

‘Valuable Secret’

“How stealth is achieved in the aircraft is considered one of the Air Force’s most valuable secrets,” Donley wrote.

Allowing information to be provided, even under strict rules, and witnesses to be questioned “increases the risk of accidental disclosure to unauthorized persons,” he wrote. “Given the value and nature of the Air Force secrets relevant here, this would pose an unacceptable risk that the information would be compromised.”

“The Justice Department is reviewing the opinion,” Nicole Navas, a department spokeswoman, said in an e-mail.

Dean Monco of Wood Phillips in Chicago, the lawyer who’s been representing Zoltek through the history of the case, said there’s no way to tell how the case will go, or for how long.

When the complaint was filed, Bill Clinton was president, Monco was in his 40s and his two children were in grade school. Those kids are now in their 30s and Clinton’s wife Hillary is running for president. In the meantime, Zoltek was bought by Toray Industries Inc. in Japan for $584 million in 2014.

The 2013 trial on the patent’s validity involved evidence from “witnesses who are either deceased, advanced in age, or who otherwise were unable to appear live at trial,” according to the judge’s opinion.

Monco, 65, declines to blame either government lawyers or the courts for the long delay. Instead, he said it’s had “an awful lot of interesting issues.”

“I don’t know if I want to spend the next 20 years litigating the rest of the interesting issues,” he said.

The case is Zoltek Corp. v. U.S., 14-5082, U.S. Court of Appeals for the Federal Circuit (Washington)