The U.S. Air Force and wider U.S. military are committed to building and deploying roughly 2,500 F-35 stealth fighters in the years to come. However, never to be outdone, the U.S. military would love to have hypersonic stealth fighters to outclass anything China or Russia build. Here is how one expert sees it:
Stealth fighters are widely seen as the pinnacle of modern tactical aviation technology. But in terms of both sheer performance and operating cost, they often fall short of older, more dated platforms like the F-15 Eagle, F-16 Fighting Falcon, and Su-35 Flanker-E. While fighter design is an incredibly complex process, a fair portion of the blame for both underwhelming performance figures and seemingly untenable operating costs can be attributed to a single technology: Radar Absorbing Materials, or RAM.
While often thought of as little more than “stealth paint,” radar absorbing materials are essential to the operation of stealth aircraft, but also account for a large portion of the operating costs associated with 5th generation fighters. Worse still, the physical limitations of these materials often become the physical limitations of the aircraft they treat, stifling the performance of the world’s most advanced fighters in the air, while simultaneously limiting both fleet size and readiness rates on the ground.
But that may not be the case for long. A new type of ceramic radar absorbing material could not only drastically reduce the limitations current materials place on low observable aircraft, but also could launch stealth into a new era of aviation — paving the way for stealth to enter the hypersonic realm.
Yes, that might mean hypersonic stealth fighters.
In the short term, such material could dramatically reduce the costs of operating existing stealth fighters and even allow for the development of lower-cost platforms with similar performance to today’s high-dollar jets. In the longer term, it could allow America’s next-generation stealth fighters to be faster, more aerobatic, and stealthier than anything else that’s ever taken to the sky.
What are radar-absorbing materials?
Modern stealth aircraft leverage radar-reflecting designs meant to deflect electromagnetic waves away from them, rather than directly back at the receiver. But these designs alone aren’t enough to make a modern jet truly “stealth.” They’re also covered in layers of Radar Absorbing Materials (RAM) that dramatically reduce their radar returns.
In technical terms, radar absorbing coatings are a special class of polymer-based materials that literally absorb electromagnetic energy. In other words, this advanced (and often troublesome) material literally eats radar for breakfast.
Even with an advanced stealth design meant to deflect radar waves, the leading edge of an aircraft’s wings, its jet inlets, parts of vertical tail surfaces, and other parts of a fighter all tend to produce radar returns. These facets of a fighter’s shape are essential for aerobatic performance. As a result, you’ll often see a radar absorbent edge treatment over these portions of the aircraft. More radar absorbent material is often incorporated into a honeycomb or similar structure inside the turbofan intakes for the same reason.
“RAM works on the principle of the aircraft absorbing the electromagnetic wave energy to minimize the intensity of the reflected signal,” wrote Adrian Mouritz in the textbook “Introduction to Aerospace Materials.”
“It is possible to reduce the radar cross-section of a fighter aircraft to the size of a mid-sized bird through the optimum design and application of stealth technologies.”
The RAM used by modern American fighters is incredibly important, as it’s been rated to absorb upwards of 70-80 percent of inbound electromagnetic energy. But it’s also expensive and time-consuming to maintain (part of the immense expense associated with maintaining the F-22 and F-35).
RAM and the financial limits of hypersonic stealth fighters
“Maintaining [the] radar-absorbent coating on the surface of the F-35 is a job that takes very detail-oriented, sometimes tedious work — masking every small area, properly mixing chemicals, applying them precisely, smoothing, and assessing the smallest imperfections. It’s time-consuming, but it’s vital to get it right,” Master Sgt. Francis Annett, 388th Maintenance Squadron Fabrication Flight Noncommissioned Officer-in-Charge explained in an Air Force release.
In 2005, the world’s first stealth fighter — the F-22 Raptor — entered service, ushering in a new era in tactical aviation and serving as the basis for an entirely new generation of fighter aircraft — now known as the 5th generation of fighters. In the 17 years since, only three more operational aircraft have earned a place within this elite fraternity of fighters, with just a handful more in development.
The only nations in the world to develop their own operational stealth fighters to date are the United States, China, and Russia — but that observation can be rather misleading. When taking a tally of operational stealth fighter fleets the world over, Russia doesn’t actually even break into the top 10. The United States unsurprisingly ranks first with over 600 total stealth fighters (F-22s and F-35s combined), followed by a steep drop off to number 2 China, with perhaps 100 to 150 operational Chengdu J-20 Mighty Dragons. From there, however, follows another steep decline and seven nations operating American-sourced F-35 fleets before finally reaching Russia’s mixed fleet of just 16 prototype and serial production Su-57s.
It should come as no surprise that the only nations on the planet to operate stealth fighter fleets that extend into the triple digits are also the two nations that top the global list of military spenders (and as we’ve covered in the past, China actually spends quite a bit more on defense than these lists actually reflect). In fact, American and Chinese fleets combined represent about 75% of all stealth fighters in the skies today, and American-sourced jets represent more than 80% of the world’s 5th-generation numbers.
It’s clear that stealth fighters are a rich nation’s game, but the reasons for that aren’t as transparent. With F-35A purchase costs now dipping below the pricetag on modernized F-15s, it would seem that it’s never been cheaper to fly into the stealth era — but the truth is, acquisition isn’t the biggest financial hurdle for aspiring air forces; sustainment is.
The Pentagon intends to procure a total of 2,500 F-35s by the end of the program, which represents about $400 billion, but operating those fighters throughout their service lives is projected to cost three times as much, currently estimated by the Government Accounting Office at $1.27 trillion.
In 2020 figures, it cost approximately $7.8 million per year to operate each F-35A, with higher figures associated with the more specialized B and C variants. And a sizeable portion of that annual expense comes down to repairing, maintaining, or replacing the radar-absorbing materials coating these aircraft.
RAM and the physical limits of hypersonic stealth fighters
The application and maintenance of radar-absorbing materials isn’t just a sizeable portion of the expense associated with operating stealth fighters. It also often creates very real physical limitations for stealth fighters as well.
The radar-absorbing materials leveraged by today’s stealth fighters are very good at absorbing inbound electromagnetic energy, but they’re not particularly good at surviving the rigors of combat aviation.
Today’s RAM begins to break down at temperatures that exceed 250 degrees Celcius, or about 480 degrees Fahrenheit. This becomes a significant issue for tactical aircraft that travel at supersonic speeds, where the combination of friction and air pressure on the leading edge of the wings and in portions of the tail surfaces can often exceed the temperature limits of the material. It’s also a problem for body panels located at the rear of the aircraft near the jet’s exhaust.
As a result, stealth fighters are designed in a way that’s meant to mitigate friction on these leading edges, which can compromise the aircraft’s aerobatic performance. But to make matters worse, these accommodations for RAM aren’t always effective.
In 2011, the F-35B and F-35C, vertical landing and carrier-capable variants respectively, were subjected to flutter tests, or tests to assess the aircraft’s structural behavior under aerodynamic loads. These aircraft are rated for a top speed of Mach 1.6, but they were tested at slightly lower and more realistic velocities. The results, nonetheless, were troubling.
After sustained flight at Mach 1.3, the F-35B showed “bubbling [and] blistering” of the RAM applied to both sides of the jet’s horizontal tail surfaces and tail boom (or where the vertical tail surfaces connect to the fighter). The F-35C fared even worse, with “thermal damage” that actually compromised the structural integrity of both the horizontal and vertical tail surfaces found after sustained flight at Mach 1.4.
As a result, both aircraft are now limited to speeds of Mach 1.2 or lower, and are only able to sustain these speeds for less than a minute before the risk of damage to the aircraft becomes too great.
This isn’t just an issue for the F-35. In fact, the F-35 is less reliant on these radar-absorbing materials than many other stealth fighters, including the F-22, thanks to the extensive use of radar-absorbing polymer materials in the aircraft’s composite structure itself. In other words, aircraft with less advanced materials science in their construction, including the J-20 and Su-57, are likely more susceptible to these problems (or omit the use of RAM on high friction areas, resulting in a compromised stealth profile).
This also means that radar absorbing materials, considered essential for modern stealth operations, cannot be leveraged by hypersonic weapons or aircraft, which can regularly reach temperatures as high as 1,800 degrees Fahrenheit in flight.
But temperature isn’t the only issue with modern radar-absorbing materials. They’re also sensitive to exposure to salt, moisture, and other abrasive materials. Salt and moisture, of course, are incredibly common for aircraft like the F-35B and C, both of which frequently operate from ships. Other abrasive materials, like sand, are also incredibly common in warzones.
Ceramic radar absorbing materials could change everything
With RAM representing both a sizeable portion of the cost of operating stealth fighters and serving as a significant limiting factor in their performance, efforts are underway in a number of nations to develop new ceramic-based RAM coatings that can sustain much higher temperatures and withstand significant environmental abuse.
This new form of RAM is so promising that it could not only resolve the issues inherent to the material on existing stealth fighter applications, it could allow for the design of faster and more aerobatic stealth aircraft than ever before. It could even benefit hypersonic applications like the rumored Lockheed Martin SR-72 or whatever platform ultimately emerges from the Air Force Research Laboratory’s combined-cycle scramjet Mayhem program.
In 2020, a research team out of North Carolina State University led by Chengying “Cheryl” Xu announced the development of a new ceramic-based radar absorbing material that they posited could be used for tactical fighter applications.
According to the team’s findings, this new form of RAM is actually even more effective than existing polymer-based materials, reportedly absorbing upwards of 90% of electromagnetic energy. But perhaps even more importantly, it’s also harder than sand and extremely resilient to both moisture and high temperatures.
Existing RAM begins to break down at approximately 480 degrees Fahrenheit, but Xu’s ceramic-based material can withstand temperatures as high as 3,200 degrees — which is more than enough to sustain flight not only at supersonic speeds, but at hypersonic ones in excess of Mach 6.
“Fundamentally, there aren’t any concerns with this material’s performance or durability,” Xu told the American Society of Mechanical Engineers. “So, there are no longer any constraints on how the aircraft could be designed.”
Applying this ceramic-based RAM is a fairly simple process. A liquid ceramic “precursor” is sprayed onto the aircraft and left exposed to the ambient air. Over the course of about two days, the liquid hardens into a solid ceramic material. This is about the same as the required “cure” times for existing polymer-based RAM treatments, but ceramic RAM could be applied far less frequently because of its inherent strength, drastically reducing operating costs.
Xu’s work is substantiated by previous findings from studies conducted in other nations. One study conducted in Italy in 2018, for instance, came to similar conclusions regarding spacecraft surviving reentry, without actually developing this ceramic material.
“The research results described here suggest that hybrid ceramic/polymeric structures could be employed as spacecraft-specific subsystems in order to ensure extreme temperature withstanding and electromagnetic shielding behavior simultaneously.”
“Advanced Radar Absorbing Ceramic-Based Materials for Multifunctional Applications in Space Environment” published by “Materials,” September, 2018
In 2020, Xu’s team secured funding from the Air Force Office of Scientific Research to continue development on their invention for use in advanced stealth applications. This makes it reasonable to suspect that we may see Xu’s RAM emerge as one of the most potent stealth improvements on America’s new fighters being developed in both the Air Force’s Next Generation Air Dominance and the Navy’s F/A-XX fighter programs.