How a stealth jet stays invisible to radar and flies

HostIf you go outside at night with a flashlight and point the beam at a mirror, the light hits that glass and zips right back at your eyes. It's very easy to see where that mirror is. But if you tilt that mirror just a little bit, the light bounces off into the trees, and the mirror stays dark. This is how we try to hide a giant hunk of metal in a sky full of radar beams. How do you keep something as big as a jet from showing up on a screen?

GuestYou have the right idea with the mirror and the flashlight. Radar is basically a big ear that sends out a loud shout of radio waves and then waits to hear the echo. When those waves hit a normal plane, which is full of round curves and soft shapes, some of that energy always bounces right back to the radar dish. It's like throwing a ball at a round pole. It doesn't matter where you stand, because the curve ensures the ball will find its way back to you. A stealth jet is built to break that rule. Instead of being round and smooth, it's made of flat panels and very sharp edges. It looks more like a jagged diamond or a folded piece of paper than a bird. When a radar wave hits one of those flat, tilted slopes, it zips away at a weird angle into deep space. To the radar dish on the ground, the sky looks empty because no echo ever comes back.

HostThat sounds like it would make for a very clunky ride. If you make a plane out of flat panels and sharp corners, it seems like it would be about as graceful as a flying kitchen table. Does a shape like that even want to stay in the air?

GuestIt really does not. In fact, if you tried to fly one of those early stealth jets like a normal plane, it would tumble out of the sky the moment you let go of the stick. The shape is so bad for flight that the air wants to flip it over or spin it around. We call this being unstable. A normal plane is built to stay steady on its own, like a boat with a heavy bottom. But a stealth jet is built to be broken. The only reason it stays up is because of very fast computers. These computers are the real pilots. They're making hundreds of tiny shifts to the flaps and the tail every single second. The person in the seat thinks they're flying, but they're just telling the computer where they want to go. The computer then does a frantic, high-speed dance with the wings to keep that weird, flat shape from falling like a leaf. It's a constant battle between the shape that hides from radar and the air that wants to knock it down.

HostI find it hard to believe that just tilting some metal plates is enough. Even if the shape is right, what about the gaps where the doors open or where the engine draws in air? Those look like big, dark holes that would catch those radar waves easily.

GuestThose gaps are a huge headache for the people who build these jets. Every little seam where two metal plates meet or where a door closes can glow like a lightbulb on a radar screen if it's not handled right. To fix this, the jet gets a special skin. It's a thick, heavy coating that feels a bit like tough rubber. We call this stuff radar absorbent material, but you can think of it like a sponge for radio waves. It has tiny bits of iron or other materials mixed inside it. When a radar wave hits this skin, it doesn't just bounce off. The skin traps the wave and turns that energy into a tiny bit of heat. It basically drinks the radar. It's not perfect, and it's very hard to keep it smooth, but it cleans up all those little echoes that the flat shape might have missed. If a tiny scratch appears in that paint, the whole jet might start showing up on screens again.

HostOkay, so the shape scatters the waves and the skin drinks them up. But these jets have huge engines that get incredibly hot. Even if a radar dish can't find it, wouldn't a heat-seeking tool see a giant glowing torch moving through the clouds?

GuestHeat is the big giveaway. Most jets have their hot engine parts sticking right out the back for everyone to see. If you're looking through a lens that sees heat, a normal jet looks like a bright white dot against a cold blue sky. To hide that, a stealth jet does a few clever things with its exhaust. First, it hides the engine deep inside the body so you can't see the hot metal parts from the outside. Then, it takes that hot air and mixes it with a lot of cold air before it ever leaves the back of the jet. It also uses a very flat, wide tailpipe. Instead of a round hole that lets out a concentrated beam of heat, it spreads the air out thin and wide, like a fan. This helps the heat blend into the cold air around it much faster. It's all about being as lukewarm and as jagged as possible.

HostIt sounds like a lot of work just to stay hidden. You have to fight the air to stay up, paint the jet with heavy rubber every time it gets a scratch, and hide all your heat in the basement of the plane.

GuestThe cost is huge because every part of the jet is a compromise between being a good ghost and being a good flyer.

HostThe flashlight beam never finds the mirror in the dark as long as those flat faces keep throwing the light into the trees.

GuestA stealth jet is really just a very expensive way to make a giant piece of metal look like a tiny bird or a patch of empty air.

HostThat mirror in the woods stays dark because the light is busy going somewhere else.