What is in the 'stealth paint' that coats stealth bombers e.g B-2 spirit, and stealth aircraft, e.g SR-71, and how does it absorb/deflect radar?
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10$\begingroup$ Reflecting radar is exactly what you don't want stealth paint to do... $\endgroup$– AakashMOct 4, 2016 at 14:15
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1$\begingroup$ Oops, I thought it reflected it! $\endgroup$– anonymousOct 4, 2016 at 14:23
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3 Answers
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Stealth paint doesn't reflect radar, it absorbs it by converting it to heat which is absorbed and dissipated by the airplane's structure. Paint is only one of several Radar Absorbing Materials available.
The paints known have suspended balls of ferrous material which absorb radar energy at certain frequencies and convert the radio energy into mechanical vibration, which causes friction and in turn heat.
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$\begingroup$ Reflecting is fine, as long as it scatters the reflection. $\endgroup$– fectinOct 6, 2022 at 14:40
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Formulae for stealth paint seem to be closely guarded secrets. In electromagnetic test chambers we use radiation-absorbent materials (RAM) to absorb stray fields. I have seen stealth paints offered to motorists wishing to avoid police radar speed traps (some hope there!). They appeared to be ferromagnetic powder mixed with some kind of binder (back in WWII the Horten brothers experimented with carbon soot in glue). Certainly, ferromagnetic materials tend to be less bulky than other RAM solutions and their operational bandwidth can be optimised by tailoring the formula in various ways. Early stealth coatings were also reported in the press to wash off in the rain and the planes were repainted for every mission, which gives some clue as to the difficulty of finding a durable binder which is also electromagnetically transparent.
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The physics fundamental is what happens when an incident electromagnetic waves changes mediums of different speed-of-light transmission index. In the case of the transition is very low energy microwaves going from air medium to metal medium which is millions orders of magnitude slower speed of light results in total reflection. Radar absorbent paints work by using a high refractive index (to microwaves) coating to try to exploit destructive interference but that success depends on many factors like angle of incidence, wavelength and specific coating thickness. Radar absorbent materials depend on the specific spectral absorbance to convert wave energy into heat energy (like water getting hot when it absorbs 2.4 Ghz microwaves). But again the success depends on the coating material and the frequency of the radar. The biggest factor is geometry. You can consider an aircraft with a shiny mirror surface and a radar as a flashlight beam. If you can make the plane with faceted surfaces the mirrors don't reflect the flashlight beam in a consistent manner, only intermittently. There is no such thing as true stealth, only "Low Observability". This is all knowledge one can gain from college level chemistry and physics degrees.
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1$\begingroup$ "millions orders of magnitude". Physics rarely deals with more than a dozen orders of magnitude, astrophysics perhaps with 3 dozen. In reality, this effect is about 1 order of magnitude, so that part of the answer is off by millions of orders of magnitude. Destructive interference only works if the coherence length is sufficient, and you can't rely on the enemy to cooperate here. $\endgroup$– MSaltersJan 4, 2022 at 14:15