Can I cover my car in low density foam - with high density nuggets to make it invisible to radar ?

Question

As far as I know - this isn't illegal is it? It's intended to be a safer coating if I should hit anyone - a nice soft landing.

The fact it absorbs radar is completely coincidental.

As it has a proven safety feature (used on playground surfaces) I think it should used on more vehicles as standard.

Answer 1

It's probably illegal  to evade radar detection.

That said, the radar cross section of your car would still be large, as you can't cover the windows and other critical sections.  The radar detector (automatic sign) in our area faithfully records the speed of me on my bicycle - surely, a radar cross section of only a few square feet at most.

Answer 2

Better idea. Slow down!

Answer 3

Well , I don't know what this stuff looks like, but this is America (this is America , isn't it ?) and you're free to do anything that doesn't infringe on others rights, right? So, if it doesn't bother your neighbor, go for it !!

Answer 4

I think you may misunderstand how radar works.  Though the foam may be invisible to radar something of your car won't be, even if the foam covers that.  As long as that something is moving with your car, it will give a doppler speed return to a radar gun.

Think of radar not as being a kind of sonar, but as a shining light.  Light and the radio waves used by radar are both forms of electromagnetic radiation, so they operate by similar rules.

When all forms of EM radiation shine on any given material they do one of a number of things.  They either reflect, go through it, or are absorbed by it.

If the foam reflects the radar pulse (which not knowing the exact material involved who's to say it wouldn't) it would give a return as normal.

Radiation going through something is as good as that object being transparent for that kind of radiation.  Stained glass windows are transparent for some colors but not for others.  Your foam may likewise be transparent to radio, in which case the radar may get returns of the nuggets, your engine, or chassis.

On the other hand, materials that are transparent to a given radiation do refract that radiation (unless they have an equal index of refraction with the surrounding medium).  For a sophisticated structure such as a foam this could translate to diffraction, which could hypothetically scramble the radar pulse to the point of it's no longer being coherent on its return.  Personally I think that's very unlikely however.  Given the wavelength and the speed of light you'd need a heck of alot of intervening material for that effect.

If radiation is absorbed by a material there's myriad effects that could happen, the most likely of which being the electromagnetic energy is transformed into heat.  Whatever the effect the material will be black to the radar, which is essentially how the RAM coating on stealth warplanes works.  You're not out of a ticket if that happens though, as most likely the radar gun will still get a return from your windshield, the interior of your cabin, or even yourself.

I don't in the meantime recommend this idea of yours for purely safety reasons.  Given the highly kinetic nature of automobile impacts it would be very easy for a whiplash effect to send one of these high density nuggets flying as a deadly projectile.

And for that matter foam is not up to snuff considering the energy we're talking about.  Sacrificable crumple zones do the same thing, and far better since they're larger and that's what they're designed to do.

Edit2:  I thought you were thinking of it like sonar.  Yet, your sonar-man's brain thought of something I didn't.  What if BOTH the nuggets and the foam are transparent to the radio of the RADAR pulse?  Well for one thing, your gambling alot on the small chance multiplied by another small chance that BOTH these materials happen to be transparent the band of radio used by police RADAR guns.  The radically different indices of refraction of the denser and lighter materials would in that case play havoc with the signal though?  Well yes, but I still think you'd need alot of intervening material.

As for the nuggets reflecting the signal but not the foam.  Some of the signal would penetrate the material and be thrown out of phase with the rest of it, while most of it would be reflected by the nuggets closest to the surface.  However, we're talking about the same effect you get from RADAR pulses returning on surfaces nearer and farther from the RADAR at the same time, which you get alot of on a car anyways.

As for the windshield thing, the role of the geometry of warplanes in their stealth, I think, is rather exaggerated (probably on purpose).  What their geometry does is present lots of flat surfaces, usually parallel with all others to a side.  The idea here is that what of RADAR that isn't absorbed by the RAM will all be returned in one direction.  What this means is if a receiving RADAR picks up a return pulse from its, or another RADAR's, active signal, it will only be for the briefest of moments, due to the movement of the aircraft.

The difference between your car windshield and a stealth's canopy is that the stealth's canopy is free of imperfections with surfaces of each layer being perfectly smooth and also all homogenous and free of impurities.  Some stealth canopies are even flat panels, while your car windshield is I'm sure not.  (Although you'll note that the Raptor's canopy which is supposed to be now the penultimate in deployed stealth, only loosely conforms to the parallel flat panel maxim.  On the other hand, given the distance between a RADAR gun and your windshield, the fuzz would get a bit more than the "briefest of moments" to get a read on your speed.)  In short, your car is not a stealth aircraft.

In the meantime, Kit cars only meet the minimum standards of automotive safety required by law, which in my opinion is not enough.  The reason why such things are allowed in this country is because we're not communists who legislate the misadventures by which people kill themselves out of existence.  And, we have a romantic notion of the self-reliant builder, who through his enterprise defeats the stranglehold of big business.  I'm not inclined to get into a debate on which is better for the interests of the public, because quite frankly I can see the merits of both sides.

Returning to comparison of your foam to crumple zones in each case we're talking about the kinetic equivalent of a spring.  The desirability of a spring in this scenario is defined by the spring force it exerts, as well as the distance it has to work with.  Both factors enter into how much deceleration occurs before the spring is compressed entirely, and the remaining deceleration that hasn't yet occured happens all at once!

Your foam, as you wrote, is "rigid".  (It has just occured to me, this may actually be a liability more than anything.  The foam may be too rigid for it's own good, as the forces we're talking about may, instead of compressing it, crush it, tearing the fibers, and destroying its internal structure entirely.  This might not seem like a worse thing.  But believe me, the foam in that case would offer negligible resistance to the impact.)  Even if the foam remains uncompromised by the forces we're talking about, it can not offer more resistance than automotive steel.

Crumple zones for their part, in high speed impacts, transmit 10s of Gs between the impacted surface and the cabin frame, up to the limits of what a human body can endure without serious or lasting injury.  And, they do this for as long as they can.  Which is a good thing since as I wrote before all the deceleration that you don't get during that time, you get all at once, when there's nothing left to crumple.

Keep in mind that sacrificable crumple zones are not some feature that car-makers have created to tout in their ad's and sell cars.  These people are taking this very seriously, if only to attain a good crash test safety rating, with which they can sell more cars.  Really you can put a price on any other car feature.  But you can't put a price on safety.

EDIT:  O.K. even after my incredibly involved answer, kuhnem you win "Best Answer" as far as I concerned.

See 2nd edit above.

Answer 5

Why would you cover your car in chicken nuggets.

Answer 6

You're onto something.

I first thought, yes.. this guy is nuts. But, you know.. the more I thought about it, the more I went.. Hmm.. you know this guy might be onto something.

Not so much the nuggets, or the coating on playgrounds -- but recycled tired shavings? You know they also use them on playgrounds, it's super absorbent [when the kid falls they're protected from hurting themselves] -- & it's bouncy.

I don't know about the longevity of the material, but I wonder why car's are not made out of something more shock absorbant & safer... cause really what we drive around in is a soda can. :|