The legendary F117. It started an era of stealth based air superiority. But here’s the most remarkable fact. The design was enable by a Soviet scientist. Yes, at the height of the Cold war, the F117 could be created in the US because a Soviet scientist showed the way. Thread.

Radar is the primary tool to detect enemy aircraft. Electromagnetic waves of various frequencies are sent toward aircraft and reflections are used to detect intrusion. For a sufficiently small frequency, reflection from metals can be approximated using Geometric optics.

What is geometric optics. Well, its more or less equivalent to light reflected from a mirror. In other words modeling using geometric optics is the same as ray tracing which is used everywhere from games to animation. So easy, right?

Not quite. While it is a good first approximation, a curved surface immediately causes errors. And this is where Russian physicist Pyotr Ufimtsev, Chief Scientist, Moscow Institute of Radio Engineering comes in.

In 1962 he published a paper titled “Method Of Edge Waves In The Physical Theory Of Diffraction”. In this paper he introduced the theory that allowed corrections to the calculations to take into account curvature. Remarkably, it was not considered important enough to censor!

This paper was later translated by the US Air Force and an engineer at the amazing Skunk Works figured out he could use it to calculate radar signature. Originally estimated at 6 months, the software, called Echo 1 was built in 5 weeks.

But why does it look the way it does, while later stealth aircraft like B2 look much smoother. I think the secret is to just look at it. It looks like a coarse triangle grid. And I am guessing the resources available could only do coarse grids. So resources defined the shape!

It was famously called the “Hopeless Diamond” since the stealth first (strange) design meant it performed really badly in terms of aerodynamics. But with “fly-by-wire” they made it work.

The F117 flew between 1983 and 2008 and the only known time it was shot down was in 1999. It was detected by very lone range radar. Remember, it was designed for stealth against short range radar making it more vulnerable to long range radar.

Pyotr Ufimtsev, sometimes called the Father of stealth, later moved to the US, where he learned what he had enabled. There is a fairly accessible paper on his theory with him as co-author. https://onlinelibrary.wiley.com/doi/pdf/10.1002/mmce.20725

This is quite a remarkable story. Many theories are developed with no application in mind, but this is a great example of why that can be worthwhile.

End of thread.