How Stealth Works
The embarrassingly simple geometry of radar evasion
Stealth technology is cool. It’s what gave the US domination over the skies during the latter half of the Cold War, and the biggest component of the US’s information dominance in both war and peace, at least prior to the rise of global internet connectivity and cybersecurity. Yet the core idea is almost embarrassingly simple.
So how does stealth work?
When we talk about stealth, we’re usually talking about evading radar. How does radar work?
Radar antennas emit radio waves in the sky. The waves bounce off objects like aircraft. When the echoes return to the antenna, the radar system can then identify the object’s approximate speed, position, and size.
So how would you evade radar? You can try to:
Blast a bunch of radio waves in all directions (“jamming”). This works if you’re close to the radar antenna, but kind of defeats the point of stealth.
Build your plane out of materials that are invisible to radio waves (like glass and some plastics) and just let the waves pass through. This is possible, but very difficult in practice. Besides, by the 1970s, if the entire physical plane was invisible to radar, modern radar can easily detect signals that bounce off the pilot from miles away.
US and Soviet missiles can track a “live hawk riding the thermals from 30 miles away” (Skunk Works by Ben R. Rich, pg 3)
Build your plane out of materials that absorb the radio waves. This dampening effect is possible but expensive, heavy, and imperfect (some waves still bounce back).
What Lockheed’s Skunk Works discovered in the 1970s, and the core principle of all modern stealth planes, is something devilishly simple: make the waves reflect in a direction that’s not the receiver.
How do you do this? Think of a mirror. You can see your reflection from far away if and only if the mirror is facing you exactly (ie, the mirror is perfectly perpendicular to you).
Tilted a few fractions of a degree off, and you don’t see your own reflection!
In contrast, if an object is curved, at least some of it at any given point is tilted 90 degrees away from you.
This is why stealth planes all have flat surfaces. To the best of your ability, you want to construct an airplane, and any radar-evading object, out of flat surfaces1.
Put another way, the best stealth airplane is a plane. Not an airplane, a plane. The Euclidean kind. A flat airplane design trades off a tiny chance of a huge radar echo blast (when the plane’s exactly, perfectly, perpendicular to the radar antenna), against a 99.99%+ chance that the echo is deflected elsewhere. In other words, a flat plane design allows you to correlate your failures.
Unfortunately, a single plane (the Euclidean kind) can’t fly [citation needed]. Instead, you need to conjoin different planes together to form an airplane’s surface. Which creates another problem where the planes meet: edges. Edges diffract radio waves back, which is similar to but not exactly like reflection. Still detectable however! Hmm.
The solution comes from being able to predict edge behavior precisely. The Physical Theory of Diffraction (PTD)2 allows you to calculate exactly how much radar energy any edge will scatter, and in what direction. While implementing the theory is mathematically and computationally complex, the upshot is the same: PTD lets you design edges that are pointed in the same direction. This correlates the failures again, trading off a huge radar echo blast when the edges are pointed directly at you against the very high probability the radar waves are simply deflected elsewhere. Pretty cool!
This simple idea revolutionized much of conventional warfare. Stealth spy planes can reliably gather information about enemy troop movements and armament placements without being detected (and thus shot down) themselves. Stealth fighters can track enemies from far away while being “invisible” themselves, winning aerial dogfights before enemy fighters even recognize an engagement is afoot. Stealth bombers and missiles have a huge first-strike advantage, allowing nations to bomb military targets (and cities) long before the panicked defenders have a chance to react.
But while the idea is simple, the implementation is not. Building an airplane almost completely out of flat surfaces trades off aerodynamicity for stealth. How do you make a stealth plane that actually flies? How do you do so quickly, and, well, stealthily, without leaking your technological secrets to your geopolitical enemies? How do you run an organization that reliably generates such bangers as stealth planes without resting on your laurels or succumbing to bureaucratic malaise? And finally, what were the intelligence, military, and ethical implications of these deadly innovations?
To learn more, subscribe below to read my upcoming full review of Skunk Works by Ben R. Rich, the Director of Lockheed’s Advanced Research and Development department during the development of the world’s first stealth airplane, and the man arguably singularly most responsible for heralding a new era of aerial warfare for over 50 years.
To returning subscribers: The Linchpin is back! Over the last month, I’ve been away at Inkhaven, where I wrote a blog post every day! I was worried about crowding y’all’s inboxes, so I created a separate blog to host all the new posts! I originally intended to crosspost my favorite Inkhaven posts over here, but they never quite felt like they reached my Linchpin quality standards.
Still, I had some posts I liked. On a technical level, I consider How to Write Fast, Weird, and Well, How to Win Board Games, and Rock Paper Scissors is Not Solved, in Practice to be well-written and executed, with a clear through-line. On a conceptual level, I’m proud of the new ideas in Skip Traditional Phase 3 Trials for High-Burden Vaccines, Aging Has No Root Cause, and The Rising Floor, and How to Write Fast, Weird, and Well again.
What’s next for The Linchpin? For paid subscribers, I recently did a marathon Substack Chat livestream with Ozy Brennan for over 3 hours! I plan to upload the recording tomorrow or the day after. For everybody else, I am writing my full review of Skunk Works and would like it to be up by later this week or early next week.
Modern stealth plane designs are made out of computationally-optimized curves: shallow, blended surfaces that reflect radar away while improving aerodynamics. The core principle is still the same however, and if you look at pictures of them, while not as perfectly flat, they still look much more flat/angular than traditional non-stealth fighters or modern passenger airplanes.
The story behind PTD is actually really cool, and recounted in some detail in Ben R. Rich’s Skunk Works. PTD was originally discovered by the Russian engineer and mathematician Pyotr Ufimtsev in the 1960s, who published it in a declassified paper, seeing no practical or military applications. It was picked up by Denys Overholser, who realized the implications and used the theory to create a unified computer program (ECHO 1) that can model and predict the radar signature of any shape. This was the breakthrough that led to the first prototypes and later the first stealth planes.
I think you're overstating the impact of stealth aircraft on warfare a bit:
1. There are a couple types of stealth UAVs used for reconnaissance (like the American RQ-180 and the Chinese WZ-10) but this isn't a major use of the technology, in large part because the countries capable of producing stealth aircraft mostly do their strategic reconnaissance with satellites now.
2. Stealth aircraft often can't "track enemies from far away while being 'invisible' themselves", because to do that they would have to turn on their own radars, which would give away their presence. The F-35 tries to get around this with really good infrared imaging sensors, but those still have a limited ability to precisely track enemy aircraft at long distances. Also air-to-air missiles generally aren't optimized for stealth, so once you fire those your enemies will recognize they're being engaged.
3. Reflective stealth designs are less effective against the low-frequency emissions of large ground-based radars, so it would be hard for stealth bombers to achieve complete surprise in a strategic first strike against a capable adversary. Even Yugoslavia was able to detect and shoot down an F-117 using air defense systems from the 1960s.
More generally, there just hasn't been a war yet where a country using stealth aircraft went up against a peer competitor with modern countermeasures, so it's hard to know how well they would perform in practice. It's certainly a useful technology, but it hasn't been demonstrated to revolutionize conventional warfare quite yet.
Looking at that picture of a stealth plane, it just hit me that stealth plane might be the aesthetic the Cybertruck is trying to copy. I wonder if that makes them harder to clock by radar speed limit enforcement, or whether the many-flat-surfaces aesthetic is more surface level.