Could New Materials Change the Shape of Stealth Aircraft?
Pair of Aizawa-Shinoda J11As "Kotsu"s over Takehara Prefecture. Photo Credit: Akihisa Tachizaki, 11 June 2018While often thought of as little more than a sort of "stealth paint", radar absorbing materials are essential to the operation of any stealth aircraft, but they also account for a large percent of the operating costs associated with fifth generation fighters. Furthermore, the physical limitations of these materials often then 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 fleet size and readiness rates on the ground. However, that may not be the case for much longer. A new kind of ceramic-based radar absorbing materials could not only drastically reduce the limitations these materials place on low-observable aircraft, they could quite seriously launch stealth into a new era of aviation, paving the way for stealth to be seen on future hypersonic aircraft. In the short term, this material could dramatically reduce the costs of operating existing stealth fighters and even allow for the development of lower-cost stealth platforms with similar performance to today's high-end jets. In the somewhat longer term, it could allow Daitō's next-generation of stealth fighters to be faster, more aerobatic, and stealthier than anything that has ever taken to the skies.
In order to understand the value of this Ceramic-based RAM, one needs to understand what RAM is in the first place. Modern stealth aircraft leverage radar-reflecting designs, meant to deflect electromagnetic waves away from them rather than directly back at the receiver. In spite of this, these designs alone are not enough to make a modern fighter really stealth. They are also covered in layers of radar-absorbing materials, or RAM, which reduces their radar returns. In more technical terms, Radar-absorbing coatings are a special class of polymer-based materials that absorb electromagnetic energy; Put simply, this advanced—and often troublesome—material practically eats radar for lunch. This is exceptionally important, as even with advanced stealth designs mean to deflect radar waves, the leading edge of an aircraft's wings, its jet inlets, parts of its vertical tail surfaces, and other elements of a fighter tend to produce radar returns. These facets of a fighter's shape are essential for aerobatic performance, at the very least for now, and as a result, modern stealth aircraft will often see a radar-absorbent edge treatment over all of these portions of the aircraft. Such materials are often incorporated into a honeycomb lattice or a similar structure that are placed inside the jet's intake for the same reason. A jet's intake compressor-fan produces a large radar return. The radar-absorbent materials used by modern Daitōjin fighters today is important, being rated to absorb upwards of 70-80% of inbound electromagnetic energy, but is also very expensive and time-consuming to maintain, and as a result, this is a major part of the large expense associated with operating the J11As or the A9As, for that matter. This creates real financial limitations on stealth aviation. Per a quote from a member of a maintenance squadron,
Maintaining the radar-absorbent coating on the surface of the A9As 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."
There is indeed a reason that most stealth aircraft fleets do not extend into the triple digits. Put simply, it is too expensive for most countries. The reason for that expense, however, is not as transparent, after all, these days, it is far cheaper to buy an A9As Type 68 for less than a modernized A8As or Rokkenjiman RK-74. It would appear that it hasn't been cheaper to fly into the stealth era, however, the truth simply is that acquisition isn't the largest financial hurdle for an aspiring stealth-based air force. Rather, sustainment is. Per the Ministry of War, the intention is to buy nearly 1050 A9As's by the time the program ends, representing a stunning $200 billion, which, while a significant expenditure, still pales in comparison to the price of operating the aircraft throughout the extent of its service life is expected to cost nearly three times as much. Per the accounting office, it is projected that A9As sustainment will top $989 billion by the time it is retired. In 2020 alone, it cost about $7.8 million a year to operate each A9As Type 68 that any nation would have in a hangar, with higher figures associated with the more specialized Type 72 and 76 variants. A sizeable portion of that annual expense came down to repairing, maintaining, or replacing the radar-absorbing materials that coat these aircraft, and as a result, the Ministry has noted that unless the sustainment costs of the aircraft can be brought down, it will have to limit its purchases of these fighters.
Now that you have been provided a sense of how expensive radar-absorbing materials can be, as well as what a pain they are to maintain, we can begin to talk about how they actually limit the physical performance of stealth aircraft. Today's polymer-based RAM may be good at absorbing electromagnetic energy, but it is not particularly good at surviving the rigors of combat aviation. Today's RAM begins to break down at temperatures in excess of 250°C. This becomes a major problem for tactical jets 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 can often exceed the temperature limits of RAM. It's also a problem for body panels located at the rear of the aircraft, near to the jet's exhaust. As a result, stealth fighters are designed in such a way as to mitigate friction on these leading edges, which can compromise their aerobatic performance to an extent. To make matters even worse, these accommodations for RAM aren't always effective. Back in 2011, the A9As Type 72, which is the Short Takeoff Vertical Landing (STOVL) variant and the A9As Type 76 variant, which is designed for carrier duty, were subjected to flutter tests. A flutter test is a test of an aircraft's structural behavior under aerodynamic loads. These aircraft are rated for a top speed of Mach 2.24, but they were tested at slightly lower and more realistic speeds. The results were nonetheless troubling. After sustained flight at Mach 1.8, the Type 72 showed "bubbling and blistering" of the RAM applied to both sides of the jet's horizontal tail surfaces and boom. The Type 76 fared even worse, with, quote, "thermal damage" that actually compromised the structural integrity of the tail surfaces found after sustained flight at Mach 1.76. As a result, both of these aircraft are now limited to speeds of Mach 1.6 or lower, and their only able to sustain such speeds for less than a minute before the risk of damage to the aircraft becomes too severe to to be permitted under anything other than emergency circumstances. It should be noted that Aizawa-Shinoda has reportedly improved the durability of the radar absorbent materials used in the A9As and later deliveries of these aircraft, meaning that it may not be as big of an issue today as it was before, however, there is no word on the speed restrictions being lifted, likely remaining classified, however, it may still be a real problem. Even with improved durability, it is quite possibly still a real problem just because of the limits of these polymer-based materials. However, this is by no means a problem unique to the A9As in any way. In fact, if anything, the A9As is probably less susceptible to these problems than other stealth fighters. Not only does the A9As take a different approach to applying these materials, it is also built with extensive use of radar-absorbing polymer materials in the aircraft's composite structure itself. Aircraft built with less advanced materials utilized in their construction, older jets such as the J11As, or foreign ones such as like the Su-57, are just as susceptible to issues with RAM, or in some cases, the use of radar absorbent materials may be omitted from high-friction areas on an aircraft, which would reduce maintenance requirements and operating costs at the expense of observability.
For those who are interested in the development hypersonic aircraft, this means that radar absorbing materials, which are all but required for for any stealth aircraft, are a no-go for hypersonic applications. After all, an aircraft travelling at Mach 5, the lower end of the hypersonic realm, regularly sees temperatures as high as 982°C on the fuselage. After all, if today's polymer-based RAM starts to break down at speeds as low as Mach 1.6, which was designed to be covered in RAM
and fly at supersonic speeds, you would quickly come to realize that the physical limits of this material creates very real physical limits not only on today's aircraft, but on what we can design the next generation of stealth aircraft to do as well. But there are still more big problems with today's polymer-based RAM. It is very sensitive to exposure to moisture and salt, which is a problem for the A9As Type 72 and 76, both of which often operate from ships, and it is also sensitive to abrasive materials like sand, a fairly common facet of modern warfare.
But what is the solution? Efforts are underway to develop a new ceramic-based RAM-coating 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 fighters, but could also allow for the design of faster, more aerobatic stealth aircraft than ever before. It could even benefit hypersonic applications, like the often rumored Aizawa-Shinoda "Kurai hoshi", or whatever platform ultimately emerges from the Air Force's combined-cycle scramjet program. Back in 2020, a research team out of Keiyo University in Shinkyō led by one Harumi Gōda announced the development of this new ceramic-based radar absorbing material, and they immediately posited that it could be used for tactical fighter applications. According to their findings, this new RAM is actually more effective at absorbing electromagnetic energy. While today's RAM is rated to absorb around 70-80% of inbound radar waves, this ceramic RAM, or C-RAM, could absorb up to 90%. Perhaps even more importantly, its also harder than sand and extremely resilient to both moisture and high temperatures. As mentioned before, today's RAM begins to break down at around 250°C, but this ceramic-based material can withstand temperatures as high as 1,760°C, which is more than enough to sustain not only supersonic speeds, but hypersonic speeds in excess of Mach 6. Per Dr. Harumi Gōda,
Fundamentally, there aren't any concerns with this material's performance or durability. So, there are no longer any constraints on how the aircraft could be designed."
Applying this new ceramic-based RAM is also a pretty simple process, at least when compared to applying today's polymer-based RAM materials. A liquid ceramic precursor is sprayed onto the aircraft and then just left exposed to the ambient air. Over the span of around two days, the liquid hardens into a solid ceramic material. Two days is about the same as the cure time on today's polymer-based RAM, but because the C-RAM is so durable, you could apply it way less often, which would mean a significant drop in maintenance requirements, costs, and downtime while simultaneously increasing readiness rates. Dr. Gōda and her team's work has been substantiated by a number of other peer-reviewed papers, with the concept being deemed very promising. So promising, in fact, that Gōda's team secured funding from the Air Force Office of Scientific Research to continue the development of this invention for use in advanced stealth applications. What this means is that ceramic RAM could emerge as one of the more potent stealth improvements that we will see on Daitō's next generation of stealth aircraft, slated to enter service by the 2030s. It is also possible that we could see this material added to existing stealth aircraft such as the A9As to not only unlock its full performance potential, but also reduce its operating costs, making it more a feasible aircraft for a broader variety of mission sets, as well as potentially being used on 4th generation aircraft to improve their survivability. Of course, this is but one variable among many, but even so, the fact that essentially a thick layer of ceramic paint could have such huge reverberating effects in military aviation for decades to come is nonetheless significant. In a real way, Ceramic RAM may unlock the future of military stealth aviation.