Supersonic speeds could cause big problems for the F-35′s stealth coating | World Defense

Supersonic speeds could cause big problems for the F-35′s stealth coating


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Nov 17, 2017
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Supersonic speeds could cause big problems for the F-35′s stealth coating
By: Valerie Insinna and David B. Larter
12 June 2019

An F-35C flies over the U.S.s Navy's stealth guided-missile destroyer Zumwalt as the ship transits the Chesapeake Bay on Oct. 17, 2016. (Andy Wolfe/U.S. Navy)

WASHINGTON — At extremely high altitudes, the U.S. Navy and Marine Corps’ versions of the F-35 jet can only fly at supersonic speeds for short bursts of time before there is a risk of structural damage and loss of stealth capability, a problem that may make it impossible for the Navy’s F-35C to conduct supersonic intercepts.

The Defense Department does not intend to field a fix for the problem, which influences not only the F-35’s airframe and the low-observable coating that keeps it stealthy, but also the myriad antennas located on the back of the plane that are currently vulnerable to damage, according to documents exclusively obtained by Defense News.

The F-35 Joint Program Office has classified the issues for the "B" and "C" models as separate category 1 deficiencies, indicating in one document that the problem presents a challenge to accomplishing one of the key missions of the fighter jet. In this scale, category 1 represents the most serious type of deficiency.

Both deficiencies were first observed in late 2011 following flutter tests where the F-35B and F-35C both flew at speeds of Mach 1.3 and Mach 1.4. During a post-flight inspection in November 2011, it was discovered the F-35B sustained “bubbling [and] blistering” of the stealth coating on both the right and left sides of the horizontal tail and the tail boom.

During similar tests of the F-35C in December 2011, “thermal damage” that compromised the structural integrity of the inboard horizontal tail and tail boom were apparent.

Vice Adm. Mat Winter, who leads the F-35 program on behalf of the Pentagon, told Defense News that the department has taken steps to mitigate the problem with an improved spray-on coating, but added that the government will not completely fix it — instead accepting additional risk.

As justification for the decision, Winter noted that the issue was documented while the jet was flying at the very edge of its flight envelope. He also said the phenomenon only occurred once for both the B and C models, despite numerous attempts to replicate the conditions that caused the problem.

“How often do we expect something like that to occur?” he said. “It's very, very small.”

Greg Ulmer, Lockheed Martin’s F-35 program head, said there have been no cases of this problem occurring in the operational fleet and that incidents have been limited to the “highest extremes of flight testing conditions that are unlikely replicated in operational scenarios.”

Winter acknowledged that the deficiency could keep the Navy from accomplishing its supersonic intercept mission — as the documents charge — if similar issues were being experienced more widely across the F-35C inventory.

“If you had that performance on all of your fleet, then you would have a problem. That’s not the case,” he said.

“We have put into place what we believe are the appropriate technical fix to ensure that our F-35Cs have the full envelope and capability to do the high-speed mission, if needed. That’s where we are. Right now, our United States Navy and Marine Corps flying the sea agree with that,” he said.

The new coating, which was introduced in Lot 8, allows the jet to withstand hotter temperatures caused by the afterburner, the documents stated. Winter characterized the material as able to withstand “what we call the thermal shock wave,” but declined to specify how the coating works or how much protection it provides.

“It may be some future advanced materials that can withstand the pressure and the temperature,” Winter said. “Then we see that, and we go, ‘Hey, look, we've got this on the book,’ [and] we do a test check to see if that new material solves that problem.”

The Defense Department has also instituted time limits on the number of seconds the F-35B and F-35C can fly at speeds in excess of Mach 1.2 while at full afterburner.

However, those restrictions are somewhat complicated, and it is unclear how pilots are expected to monitor their compliance to the limits while in flight.

For example, an F-35C can only fly at Mach 1.3 in afterburner for 50 cumulative seconds, meaning that a pilot cannot clock 50 seconds at that speed, slow down for a couple seconds and then speed back up. However, the time requirements reset after the pilot operates at military power — an engine power setting that allows for less speed and thrust than afterburner — for a duration of three minutes.

The F-35B can fly for 80 cumulative seconds at Mach 1.2 or 40 seconds at Mach 1.3 without risking damage.

But for both the C and B models, flying at Mach 1.3 over the specified time limits poses the risk of inducing structural damage to the aircraft’s horizontal stabilizer.

It is infeasible for the Navy or Marine Corps to operate the F-35 against a near-peer threat under such restrictions, the documents acknowledge.

“Pilot observed timers are not practical/observable in operationally relevant scenarios,” one document read. Another document said that “pilots will be unable to comply with time limit in many cases due to high mission workload, resulting in lost missions due to aircraft damage.”

And when those timer violations occur, they will result in “degradation of [stealth], damage to [communications, navigation and identification] antennas, and/or significant [horizontal tail damage],” one document explained.

How significant is this problem?

The limitations on the afterburner, when combined with another deficiency pertaining to the plane’s maneuverability, could prove deadly in close-combat scenarios.

The concept of operations for the F-35 is to kill an enemy aircraft before it can detect the fighter jet, but relying on long-range kills is a perspective that, for historical and cultural reasons, naval aviation distrusts. In the Vietnam War, when air warfare began heavily relying on missiles and moved away from the forward gun, it caused a spike in air-to-air combat deaths.

The lesson naval aviation took away was to prevent the latest and greatest technology from offsetting the learning of fundamentals, and it was the impetus behind the formation of Top Gun 50 years ago, a naval strike fighter course for training and tactics development.

“The solution is: ‘Hey, we’ll just limit the afterburner to less than a minute at a time,’ ” one retired naval aviator said, when told of the issue. “Which, with what the aircraft is supposed to do and be capable of, that’s a pretty significant limitation.”

“If you want to use it on the first or second day [of a conflict], it has to be stealthy, so you can’t hang a lot of external stores, which means you have to use internal fuel and internal weapons. And that means you have to launch fairly close in and you’ve got to be close enough to do something to somebody. And that usually means you are in a contested environment,” the aviator said.

“So you’re saying that I can’t operate in a contested environment unless you can guarantee that I’m going to be however far away from the thing I’m trying to kill,” the aviator added. “If I had to maneuver to defeat a missile, maneuver to fight another aircraft, the plane could have issues moving. And if I turn around aggressively and get away from these guys and use the afterburner, it starts to melt or have issues.”

The issue is compounded for the Navy, which must operate forward for months at a time, because any significant issues with coatings or the structure of aircraft would require a depot-level repair. And so a damaged aircraft would remain damaged until its host ship return to home port, reducing the combat effectiveness of the air wing.

“We might have to be operating at sea for eight months, so if you damage something on week one, guess what? It’s damaged for the rest of the deployment. And it affects your ability to evade detection by the enemy — you just degraded that asset permanently until you can get it somewhere where it can be fixed, at great expense and time,” the aviator said.

However, a naval aviator currently in service said the afterburner problem may not be that troubling to pilots, who must frequently work around a jet’s limitations. The key, he said, is understanding how often the issue occurs.

"I think you'd do well to go back and look at all the times they used the afterburner and that didn't happen," he said. "We're talking about tens of thousands of sorties at this point that this aircraft has flown."

Other aircraft that the Navy operates also have afterburner limits, he explained.

“I think that number needs context,” he said. "It looks scary on its own, but [the Super Hornet] has afterburner limits. They’re not that restrictive, but they have them. The aircraft has an afterburner, you want it to work.

“But I would want to get context for that number: Does this represent 0.002 percent of all sorties? If that’s the case, I don’t give a sh--, and I’ll probably have 15 other things fail before that."

Bryan Clark, previously a top aide to former Chief of Naval Operations Adm. Jonathan Greenert and now an analyst with the Center for Strategic and Budgetary Assessments, likened the limitation on the afterburner to similar restrictions on submarine and ship operations.

“I think the operational impact is not huge, since it only applies during a small fraction of the jet’s operational profile. In subs and ships, we have a ‘safe operating envelope’ that defines where the platform is engineered to operate reliably for a long time. We can operate outside the safe operating envelope for a short time, but there are risks to doing so. The operator or commander needs to balance those risks against the benefits," he said.

“That is similar to this situation," he added. "The pilot can be on afterburner as long as needed to evade a threat but has to know the risk of structural damage increases. The pilot can balance that against the risk of getting shot down because he or she didn’t evade fast enough.”

The most important piece will be how well trained the pilot is on the aircraft, he continued.

“As a submariner, I knew the risks of being outside the safe operating envelope and how those risks increased over time and would impact ship performance.”