[dcposch]

One simple reason - separate from the less fun possibilities involving defense procurement politics - is that the F117 was a transition technology.

It proved that super low radar signatures were possible.

The design used those big low poly triangles not because it’s optimal (aerodynamically, very much not!) but because of the limits of computer simulation at the time.

There’s a whole fascinating story about how the theory behind low observable was developed by a Soviet scientist, published, ignored there, then implemented here.

But computer technology quickly advanced to where low-poly aircraft made airworthy by brute force were no longer necessary. See the B2 Spirit, also a very special simulation derived shape but streamlined.

[nomel]

> but because of the limits of computer simulation at the time

Is this true? I thought the low poly was that a curve always has an area whose norma vector points back to radar, where flat pieces (and their intersections) only do if they're perpendicular to the radar. So, unless you're unlucky enough to have one plate shining pointing back at the radar, the reflection is completely broken up, with the small corners being too small to effectively reflect radar wavelengths back.

Or, maybe I'm just having trouble visualizing a smooth surface geometry that also has few normals back? I naively assumed the "smooth" planes were enabled by better absorption materials, rather than geometry. Maybe a mix?

[wahern]

> Or, maybe I'm just having trouble visualizing a smooth surface geometry that also has few normals back? I naively assumed the "smooth" planes were enabled by better absorption materials, rather than geometry. Maybe a mix?

You're not wrong. From a head-on perspective, especially from below, you'll find few if any curves. Newer stealth planes look curvier because photographs are often taken from above where the canopy is prominent, and where the top of the body and wings have some curvature. But look more closely. Even from above, the curves you see are usually on the trailing portion of surfaces or facing laterally; the leading edge of the wings on a B-2, F-22, or F-35 are actually flat and triangular, not at all like a typical diagram of an airfoil. This is especially true of the F-22 and F-35--if you look very closely, they're far more angular from more perspectives than the older B-2. The B-2 looks curvier from below, but the flying wing design isn't a coincidence; and I believe the B-2 also relies more heavily on radar absorbing skin than later aircraft, which rely more on simple geometry much like the F-117.

Moreover, beneath the skin of these planes it's widely believed that the framing uses sawtooth and other similar polygonal patterns you'd expect, a mitigation for radar that passes through the skin. And I would think that part of the engineering of these aircraft leverages radar transparent skin in some areas, not just absorbant skin, similar to a nose cone holding a radar.

Here's a really amazing slide presentation on the basics of stealth design: https://understandingairplanes.com/Stealth-Airplane-Design.p... (I first came across that document via an HN post several years ago.)

[wrp]

Absolutely true that the faceted shape was due to computational limits at the time. There is a book detailing the development process for the shape of the F-117, but I can't remember the title. Anybody?

Anyway, the basic principle of stealth design is not what people intuitively think. As I recall, it has more to do with refraction along edges than reflection from surfaces. It was originally figured out by some Russian guy. I think that story was also in the book I can't remember.

[schiffern]

  > maybe I'm just having trouble visualizing a smooth surface geometry that also has few normals back?

Just look at the B-2 Spirit and B-21 Raider.

https://en.wikipedia.org/wiki/Northrop_Grumman_B-2_Spirit

https://en.wikipedia.org/wiki/Northrop_Grumman_B-21_Raider

The interface where the top and bottom meet (ie the only surfaces normal to the horizon, since they intentionally deleted vertical control surfaces) have relatively sharp edges and curvatures.

Curves are okay, as long as you plan them correctly. The tighter the curve the smaller the radar return. After all, the F-117's corners are (in the limit) just curves with a very tight radius.

The other trick is that (when viewed from the top) all the lines along the perimeter are parallel. That means there are only ~4 azimuth angles where the edges are presented 'face on' to an observer. At all other azimuth angles the radar return will glance off the (importantly, singly-curved) outer edge, bouncing the return signal away from the radar.

Presumably they'll plan flight paths such that these four relatively high-observability vectors will "sweep past" known radar installations rapidly, ideally while the plane is making a turn. All the radar sees is a small, extremely brief blip, which could well be discarded as a bogus radar return (either automatically or by human operators).

No insider information of course, just looking at the shape and applying geometric reasoning.

[yobbo]

I have vague memories of reading about the coating on F-117. It was supposed to be multiple layers of epoxy paint with suspended ferrous particles, each layer cured under a different magnetic field to align them. To re-coat, the plane would have to be disassembled. The effect they wanted is most likely cancellation, analogous to anti-reflective coatings on glass. Idk.

Furthermore, I have seen pictures of F22 with brown rust stains on panels that should not have any steel in them.

[micah94]

I memory about this is rusty, but there was A LOT going on with this stealth tech. I vaguely remember color being important, too. The earlier (and stealthier!) designs had a camouflage look rather than black. But this looked "ugly". So they changed it. I think advancements in materials made this moot.

[justsomehnguy]

You have the advantage of seeing the successor tech. Nothing later of 117 uses the 'big poly' design, with B-2 as an immidiate successor and F-22/35 as a years later one.

There would be always a question why did they chose a flying iron design, but efficency clearly states it's not needed.

[teruakohatu]

I had never considered that the Northrop YB-49 [1] might have ultimately worked if computer simulation had been available at the time.

[1] https://en.wikipedia.org/wiki/Northrop_YB-49

[amethyst]

Another key requirement would have been computer-aided flight control systems. The only reason the B-2 can remain stable during maneuvers is thanks to its fly-by-wire system that translates the pilot's directions into control surface adjustments that achieve the desired results, and continually adjust those controls faster than the human pilot can react.

[mc32]

My favorite is the Grumman X-29: https://en.wikipedia.org/wiki/Grumman_X-29 (It worked and they built two, I think) but ultimately a dead-end.

[scns]

> The design used those big low poly triangles not because it’s optimal (aerodynamically, very much not!)

It was so bad, they called it The Hopeless Diamond. To keep it in the air instead of dropping like a stone, flight computers had to be developed. Another stepping stone for the B2 which faced similar challenges.

[lumost]

It’s always confused me, under the radar absorbent material is stuff like engines, cockpits, and other reflective material. I’ve always wondered if under the smooth aerodynamic shape was a polygon frame covered in carbon fiber. That way the polygons could reflect radar away from the more reflective parts.

[analognoise]

I've heard this but can't remember the name of the book!

Does anyone know?

[johncalvinyoung]

Skunk Works, by Ben Rich. It's a classic, and the origin of my own fascination with the A-12/SR-71, and the F-117.