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Context:

In this semi-futuristic war history, some militaries looked at the ultra-high performance concrete used in bunkers and thought: "hum, the only problem with bunkers is that they can't move, let's fix that".

And decided to make air planes made out of thick walls of ultra-high performance concrete that are heavy and fly at "incredible" speeds of 50-100 km/h (31-62 miles per hour).

Since these flying bricks of aircrafts aren't flying as fast as air liners (mach 0.8), they could use more delicate radiation-absorbent materials around its surface.

Although concrete (and the other additives) are somewhat already considered radiation-absorbent materials, since this brick fly so slow, they could become essentially invisible to radar.

In summary, normally these flying bunkers fly really low to hide in the curvature of the earth, and on top of that, are covered in at least a meter or more of this material. But, these are very slow and normally the length of a bus, around 20-30 meters (65-98 feet) long.

(Don't worry about the logistics, their autonomy and so on, that's not the question.

There are a lot of types of radiation-absorbent materials with varying degrees of effectiveness, but I'm specifically referring to the pyramid sponges.

From the wikipedia article itself:

"Pyramid RAM. The grey paint helps to protect the delicate radiation-absorbent material."

Photo of radiation-absorbent material pyramid sponges

They are really fragile and wouldn't withstand the airflow, but, if someone were to cover an entire aircraft with such material and not get it ruined, would it turn the aircraft invisible to radar?

I say that because these sponges are not meant for this function, they are used to absorb the radiation noise from the outside; And the real world stealth aircraft designs put a lot of care on its shape in order to have a really low radar cross-section.

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    $\begingroup$ (1) There is no such thing as "invisible to radar". The best that can be done is make the thing harder (not impossible, just harder) to detect by certain specific kinds of radar. (2) There is no way to make an object the size of a bus hard to detect by any kind of half-decent military radar at short range, say maybe 5 km. (3) Which is fine for modern stealth war aeroplanes, because they can cover 5 km in 10 seconds; but your slow moving target will spend 5 to 10 minutes in the detection envelope. $\endgroup$
    – AlexP
    Commented Jan 25, 2023 at 11:56
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    $\begingroup$ No need, @L.Dutch provided the answer. $\endgroup$
    – AlexP
    Commented Jan 25, 2023 at 12:16
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    $\begingroup$ Let's assume your sponges could absorb 100% of radar emissions. (a) Against an open sky, your plane would have no radar signature. (b) Against a mountain side, your plane would be a suspicious hole against the reflection of the mountain. Indeed, objects that would reflect radar, passing behind the plane, would produce a suspicious plane-shaped "hole" in the reflected image. So while the solution would be harder to spot, it's still not impossible to spot by radar. $\endgroup$
    – JBH
    Commented Jan 25, 2023 at 21:24
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    $\begingroup$ BTW, did you really mean to ask for hard-science answers? Because neither of the two answers provided so far meet the expectations of that tag. $\endgroup$
    – JBH
    Commented Jan 25, 2023 at 21:26
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    $\begingroup$ It's hard to request a hard science answer when the scenario is absurdly non-hard science. $\endgroup$
    – Michael Hall
    Commented Jan 25, 2023 at 21:54

2 Answers 2

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Current stealth airplanes are covered in radiation absorbent materials and paints, adding also geometry designed to deflect radar away from the receiver.

This doesn't make them invisible, they only make them lowly, very lowly visible. I remember reading, years ago, that a stealth airplane would have had the radar signature equivalent to a dove.

Since air forces usually do not go after doves crossing the borders of their airspaces, it becomes evident the advantage that such airplanes have. By the time they are close enough to be actually detected, there is not enough time to react and prevent them to do any damage.

However for this to happen, a decent velocity is needed. A stealth snail will still be a snail: if it takes it several minutes to reach the target, there will be time to take appropriate countermeasures, zeroing the tactical advantage of being "invisible".

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    $\begingroup$ Not to mention that a dove moving at 800 km/h would be rather unusual... $\endgroup$
    – AlexP
    Commented Jan 25, 2023 at 12:16
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    $\begingroup$ @AlexP Although to be fair to the stealth industry, a radar's tracking software relies on "signal to noise ratio" and "radar cross section" to help it pick things out of the random garbage that is constantly going on (clouds, trees, bouncing signals, general interference). The less your target bounces back to the radar, the more likely the radar's tracking software will simply not be able to pick it out of the noise. $\endgroup$
    – JamieB
    Commented Jan 25, 2023 at 18:34
  • $\begingroup$ From memory, I think the B2 bomber has the radar return equivalent of a tennis ball. And that the F117 shot down over Bosnia was done over "open sights" rather than getting a radar lock. $\endgroup$
    – Criggie
    Commented Jan 26, 2023 at 3:13
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You could. But I think that would make your aircraft slower and more visible. The better approach is to make your aircraft with no upright surfaces to reflect sideways-looking radar back where they came. The Vulcan bomber had a very low radar signature by accident. The B-2 Spirit 'Stealth bomber' did this by design.

If this is not a combat aircraft, you might go for an ultralight aircraft with a bamboo skeleton sheathed in transparent plastic. It is non-conductive so it would not reflect a lot of radar, and it might be hard to see too. Or one of those really thin-walled weather balloons.

The U-2 spy plane had a different strategy - it flew at 70,000 feet. There was no Concorde, so it pretty much had that atmospheric band to itself. Radar could spot it, but it wasn't looking there.

All of these approaches use a skinny, flat aircraft. There are radar absorbing materials, but geometry is the main weapon against radar. A flying brick does not sound like a good start IMHO.

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    $\begingroup$ Oh, U-2 spy planes were very visible to Soviet radar. (Radar does not really have the notion of "not looking" at aircraft flying high.) It's just that for a few years the Soviet Union did not have the ability to shoot them down. The American feeling of invulnerability was terminated rather abruptly on May Day 1960 by an S-75 Dvina surface-to-air guided missile (which the Americans, who for some specifically American reason never call Russian weapons by their Russian names, called the SA-2 Guideline). $\endgroup$
    – AlexP
    Commented Jan 26, 2023 at 1:12
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    $\begingroup$ Yes, the Russians could see the U-2s by radar. But there was a weird period when they did not seem to know they were there. Perhaps they did not want to disturb them until they had something that could bring them down. My info probably dates from the 80's so isn't very fresh. $\endgroup$
    – Richard Kirk
    Commented Jan 26, 2023 at 12:01

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