When people discuss stealth, especially in relation to aircraft, it’s not uncommon to hear them treat it as though stealth is some singular thing a fighter or bomber just has or doesn’t. The truth, as truths tend to be, is quite a bit more complicated than that.
It might be better to think of stealth as a spectrum of capability that anything can be on (but let’s keep our discussion limited to aircraft for today). On the left, we have the Low Observable side. This is where you’ll find famed stealth platforms like the F-117 Nighthawk, B-2 Spirit, and F-35 Joint Strike Fighter.
On the right, we have the Observable side. This is where you’ll find everything from the B-52 Stratofortress to the Good Year Blimp.
While the complexities of stealth could fill volumes of books (in fact, they have), the simplest way to think of it with respect for that complexity may be this: Stealth is a series of technologies, design and production methodologies, and combat tactics that combine to postpone or limit detection in order to make an aircraft more survivable.
Stealth isn’t even close to invisible
Stealth aircraft like the B-2 Spirit or F-35 Joint Strike Fighter do go to some lengths to limit their visual signature, or how easy they are to spot against the sky, but they’re certainly not invisible.
Contrary to the popular misconception, stealth doesn’t really make an aircraft invisible to radar either — in fact, many stealth aircraft are plainly visible on radar designed to leverage certain lower frequency VHF and UHF bands. The goal of stealth, then, isn’t always to avoid being spotted at all… it’s more or less about avoiding being shot down. Spotting an aircraft isn’t the same as securing a weapons-grade lock, or a lock you could use to actually engage the aircraft.
The most commonly understood facet of stealth is how it can defeat, prevent, or postpone detection from enemy radar arrays both on the ground and aboard enemy fighter aircraft. Modern stealth programs take a multi-faceted approach to the problem, leveraging advanced designs that are digitally modeled to deflect radar waves away from the aircraft, as well as radar absorbant materials to capture the energy of radar waves that aren’t safely deflected.
Having a design that is specifically suited for deflecting radar waves away from the aircraft is an intrinsic part of 5th generation fighter design, but it can be most easily appreciated in the now-dated F-117 Nighthawk. The Nighthawk’s unusually angular exterior represents the best computers of the day could manage when it came to calculating radar deflection, whereas the sleek and smooth F-35 demonstrates how far our computational power has come in the interim.
Today’s F-35 Joint Strike Fighter has more curves than hard angles, but the premise remains the same.
“The plane’s shape is designed to deflect radar energy away from the source like a slanted mirror,” said Harold Carter of the F-35. Carter was a senior research science manager at Lockheed Martin’s legendary Skunk Works.
“Its surface is also blended and smoothed to enable radar energy to smoothly flow across it—similar to water flowing across a smooth surface.”
Radar Absorbent Materials
But that stealthy design alone simply won’t cut it. 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, but can’t be eliminated through advanced designs. As a result, you’ll often see a radar absorbent edge treatment over these portions of the aircraft. More radar absorbent material (RAM) is often incorporated into a honeycomb or similar structure inside the turbofan intakes.
“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 academic 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% 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). Its inability to manage high heat is also an issue, and has even been known to limit some stealth fighters’ ability to sustain supersonic speeds without getting damaged.
It’s not just the complexity of the math and the expense of the RAM that makes fielding a stealth fighter or bomber so difficult. One of the least-discussed but most important elements of being able to field a stealth aircraft in numbers is the ability to build them with extremely tight production tolerances. Even a tiny gap between body panels on a stealth fighter can make it more detectable on radar, so the process of assembling a stealth aircraft is a painstaking process that requires a great deal of expertise and some fairly expensive equipment.
“If the aircraft external structural parts are precisely machined to fit together with exceptionally close tolerances, then the stealth requirements can be more easily fulfilled,” wrote Robert Jones in a piece published by the Society of Manufacturing Engineers.
“That is, reduction and near-elimination of gaps between structural parts is highly desirable in achieving stealth characteristics of an aircraft.”
The ability to build aircraft with incredibly tight tolerances is one of the ways the United States has maintained its stealth advantage even as national competitors have begun fielding their own stealth aircraft. Stealth, after all, is a spectrum, and the tighter your production tolerances, the further toward the “low observable” side of the spectrum you end up on.
As an example, the very stealthy F-22 Raptor was built in the 1990s and 2000s with manufacturing tolerances of around 1/10,000 of an inch. That was incredible at the time, but gaps still had to be treated with tapes, caulks, and RAM to maintain the aircraft’s stealth profile. The F-35 is said to be assembled with production tolerances that are tighter by “orders of magnitude,” but its still not uncommon to see any potential gaps in the aircraft covered by RAM.
The infrared and electromagnetic parts of stealth
Modern stealth aircraft have more than just enemy radar to contend with, and as a result, modern stealth programs take pains to limit a jet’s infrared and electromagnetic signatures as well.
An easy way to think of an aircraft’s infrared signature is that it’s the amount of heat the jet produces. The hotter something is, the easier it is to see, track, and potentially, shoot down. The problem, of course, is that we power our stealth aircraft by mixing air with fuel and creating an explosion… and explosions tend to be pretty hot.
In order to mitigate all that heat, stealth aircraft usually house their engines deep inside the fuselage with shrouds around the exhaust outlet to diffuse the heat as its released. This approach also helps to manage the acoustic and visual signatures of the aircraft (in other words, how loud it is and how easy it is to spot in the sky with the naked eye).
“The exhaust system of an unmanned fighter not only affects its aerodynamic characteristics and infrared (IR) signature, but also affects the electromagnetic scattering characteristics of its tail and sides,” wrote Beijing’s Ze Yang Zhou and Jun Huang in a peer-reviewed paper published by Scientific Reports earlier this year.
“Studying the comprehensive design of the exhaust system is of great significance to the infrared/radar stealth performance of the aircraft.”
Not all “stealth” aircraft effectively shroud their engines, however. That doesn’t make these drones and fighters “not stealth” necessarily, it just means they lack these elements of stealth, moving the platform further toward the “observable” side of the spectrum.
But that’s still not the end of the stealth story, because modern stealth aircraft also leverage electronic warfare suites designed to interfere with nearby means of detection or even communications. Because even a “stealth” fighter is still detectable in different circumstances, sometimes being stealth means taking a pro-active approach to survivability.
Finally, even with all the stealth-stuff you can cram into an aircraft, the job still isn’t done. America’s pilots spend countless hours planning out their combat operations to ensure they’re operating at an advantage whenever possible.
That means using maps of the terrain, known enemy positions and equipment, and an understanding of your aircraft to plot a course that minimizes exposure to enemy defense systems that may have a good chance at spotting or engaging you. It also means developing plans for how to get out of dodge if something goes wrong without flying your $100 million stealth fighter right into the waiting wings of another threat.
“Fifth-generation aircraft derive LO properties from five major areas: radar cross section, the infrared spectrum, the visual spectrum, acoustic emissions, and radio frequency emissions,” explained Air Force Captain Stephanie Fraioli, an instructor and course chief at the F-35A Intelligence Formal Training Unit.
“Because of these technological advances, these airframes are even more reliant on mission planning for effective employment.”
After all, as famed F-14 Radar Intercept Officer Ward Carroll once told me about F-35s and within-visual-range dogfights, “Stealth doesn’t work against bullets.”