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The F-117 Nighthawk has flat angled lines where most later designs (B2, F22, F35) have very smooth lines. Why the difference? Are the newer designs 'more stealth'? Do they use a different design to handle better? Is there another reason the distinct angled look is avoided in later designs?

The question, 'Why do many stealth aircraft have a similar shape and design?', is close, but doesn't really address why the F-117 stands out from the rest.

enter image description here
Left: F-117, right: F-22.

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Initial work on Radar Cross Section (RCS) reduction concentrated on absorbers and had started in Germany late in WW II. The Americans were not interested at the time, but British scientists documented the results and gained early insights in US RCS research when they pooled their results in the 1970s.

Lockheed started work on the systematical reduction of RCS when they discovered the academic papers of Petr Ufimtsev, a Russian mathematician who explained how to calculate the reflection of radio waves from the 1960s on. Their initial success with RCS reduction on the SR-71 was more luck than systematic work. At the same time, similar insights were gained in Germany where a manned prototype of an aircraft (MBB Lampyridae) similar to the Have Blue prototype proceeded up to wind tunnel tests. Then the program came to the attention of US authorities which quickly put an end to it.

In all those cases, the major idea was to restrict the surface to large, flat panels because the edges, not the total area, determine how much energy is radiated back. In that respect, the German design was much cleaner than both the Have Blue and the F-117, but I digress. Those designs can be called first-generation stealth aircraft.

When Northrop joined the RCS work, they found that even curved panels work well - after all, it is edge reduction and alignment which reduces RCS most. Their initial prototype was for a battlefield observation aircraft (in contrast to the Lockheed attack aircraft) which was called Tacit Blue.

When work on stealth prototypes was opened at the end of the Cold War, all engineers could pool their knowledge and combine the work on absorbers, edge reduction and edge alignment. This gave rise to the second generation of stealth designs which look indeed quite different to those of the first generation. What also influenced the designs was more understanding of the tactical value of stealth, so the second generation designs do not optimize RCS above all else but arrive at a compromise which combines low frontal RCS with good agility and multi-role capabilities.

Koyovis is right, but it was not only computer power but also the limitations of early RCS codes which were based - at least in case of Lockheed - on the ideas of Petr Ufimtsev, and he in turn used analytical methods which did not allow to investigate the RCS of curved surfaces. However, the computers at the time were already powerful enough to allow the F-117 to fly at all - in contrast to the MBB design, it is unstable and uncontrollable without artificial stabilization.

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    $\begingroup$ While studying aerospace engineering in university, I hypothesized that one could trace everything in our curriculum back to Germany, and apparently that holds true for stealth technology too. Very cool :) $\endgroup$ – techSultan Jan 20 '18 at 7:23
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    $\begingroup$ @techSultan: Well, if you ask the Russians, they will tell you that all comes from Russian pioneers. And Ufimtsev really did pioneering work. $\endgroup$ – Peter Kämpf Jan 20 '18 at 9:13
  • $\begingroup$ My understanding was that the paint on more recent aircraft significantly contributes to RCS reduction beyond what can be achieved with geometric considerations and modifications. Would that contribution help alleviate the need optimizing edge geometry etc.? $\endgroup$ – nodapic Jan 24 '18 at 1:20
  • $\begingroup$ @nodapic: No. Absorbing paint was among the first techniques used, so it is not restricted to more recent designs. The absorbers are frequency-specific, so they work only for/against some radar systems. For edge alignment they are no replacement, but they help for example to reduce backscatter in intakes. $\endgroup$ – Peter Kämpf Jan 25 '18 at 9:06
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The reason I had heard, and seems to be confirmed by this site, is that the computers at the time of design were not powerful enough to compute radar reflection of off curved surfaces.

enter image description here

The photo is from the same site - it sure looks like something only a 70s computer could love. It most definitely makes aerodynamicists weep.

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    $\begingroup$ That is also the answer I always heard , however that is not really a proof of it’s validity either. $\endgroup$ – rul30 Jan 18 '18 at 6:22
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    $\begingroup$ @rul30 it is indeed the reason. It was mentioned in several books about the ATF competition that led to the F-22 which I read. The available computer power dictated that only flat surface reflections could be calculated in reasonable time, so that's what they ended up with. $\endgroup$ – jwenting Jan 18 '18 at 6:53
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    $\begingroup$ It was more the limitations of the algorithms than of computer power. Early algorithms were based on analytical work which used no computers at all. $\endgroup$ – Peter Kämpf Jan 18 '18 at 8:47
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    $\begingroup$ @mins: remember we are talking about the 1970s and its not ray tracing but diffraction of electromagnetic waves. $\endgroup$ – rul30 Jan 18 '18 at 17:59
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    $\begingroup$ @mins, raytracing is the wrong tool for dealing with radar waves. As far as I know, nobody's ever managed to get electromagnetic radiation below about the terahertz frequencies to show particle-like behavior, so you need to use far-more-complicated wavefront simulation techniques. $\endgroup$ – Mark Jan 18 '18 at 22:57

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