What is a stealth plane?

0:46 Welcome to Science is Not Boring by Kidopoly.com. I'm Mira. And I'm Finn. A big hello to Leo, who is five, and Max, who is three, listening all the way from Devon in England. They asked how radar works and how the secret Nighthawk stealth plane could hide from it.

1:04 And a big hello to four-year-old Amari and three-year-old Romy in Bristol, who love listening to Sciences Not Boring.

1:11 If you haven't already, tap follow wherever you're listening so you never miss an episode.

1:16 Okay, Mira, before we get started, is it really true that there was a massive military jet, over 65 feet long, that looked smaller than a tiny hummingbird on a computer screen?

1:28 It is absolutely true. Today we are diving into the world of physics and engineering to talk about radar. We'll explore how it works, who invented it, and how the top secret F117 Nighthawk was specially designed to turn completely invisible to it.

1:45 That sounds like something straight out of a science fiction movie. Let's get into it. So, to understand how a plane can turn invisible to radar, we first need to know what exactly is radar?

1:58 Radar stands for radio detection and ranging. Basically, a radar dish shoots invisible radio waves into the air at the speed of light. That means those waves travel an incredible 186,000 miles per second.

2:15 Whoa! 186,000 miles per second? That is unbelievably fast!

2:22 It is! And if those radio waves hit a solid metal object, like an airplane in the sky, they bounce back to the radar dish like an echo. By timing exactly how long the echo takes to come back, you know exactly where the plane is and how fast it is flying.

2:40 That's super smart! Who invented that?

2:43 A Scottish physicist named Robert Watson Watt in 1935.

2:48 1935? Wait, did they even have advanced computers back then?

2:54 Not at all. He was working at a research station in Slough, England, initially trying to find a way to track dangerous thunderstorms. He wanted to listen to the radio waves naturally given off by lightning strikes.

3:10 Thunderstorms? How did tracking lightning turn into tracking airplanes?

3:15 Well, the British government actually asked him if he could invent a radio death ray. He told them a death ray was completely impossible. But he could probably build a system to find enemy planes from far away.

3:31 So how did he prove his idea actually worked?

3:33 On February 26, 1935, Watson Watt and his team set up a famous test called the Daventry experiment. They beamed shortwave radio signals at a giant Handley-Page Hayford bomber plane flying eight miles away. And it worked perfectly. The radio waves bounced off the metal plane and came right back to their receiver.

3:59 Eight miles away? That gave them a huge advantage in knowing what was in the sky. Exactly!

4:06 It was such a massive success that by 1938 they built a huge network of 360-foot-tall radar masts all along the coast of Britain.

4:18 So if radar works by bouncing off the curved metal body of a plane, how on earth did the Nighthawk manage to hide from it?

4:26 Well, by the 1970s, radar systems had gotten so powerful they could easily spot aeroplanes from hundreds of miles away. The United States Air Force realised they desperately needed a plane that was completely invisible to enemy radar.

4:43 But aeroplanes are giant chunks of metal. How do you hide a giant metal bird from radio waves?

4:50 A top secret engineering team at Lockheed, called the Skunkworks, had a totally wild idea. Normally, airplanes are built with beautifully curved, smooth surfaces so they can cut through the air and fly fast. But those curved surfaces act like a giant reflector, sending radar waves straight back to the detector.

5:13 Sure, what did they do differently for this new stealth plane? They decided to build a plane out of completely flat panels called facets. Imagine a jet shaped like a diamond with lots of flat, sharp, angled sides instead of smooth, aerodynamic curves. Oh, so it was basically like a flying diamond. Exactly!

5:38 In fact, when the engineers built the very first test model, it looked so strange and bulky that they nicknamed it the Hopeless Diamond. The Hopeless Diamond! Why did they call it hopeless? Because aerodynamically speaking, it looked like a heavy brick that shouldn't even be able to lift off the ground.

5:59 But those flat angular panels did something amazing. They acted exactly like the mirrors on a spinning disco ball.

6:08 Wait, a disco ball scatters light beams everywhere around a room.

6:12 Yes. When a radar wave hit the F117 Nighthawk, those flat facets bounced the wave away at weird, random angles, instead of reflecting it straight back to the radar detector.

6:26 So the detector gets absolutely no echo back, making the screen look totally empty. That is pure genius.

6:33 And the crazy geometric shape wasn't their only secret. They covered the entire 65-foot-long plane in a highly classified radar-absorbing paint.

6:45 Radar absorbing paint? How does paint absorb powerful radio waves?

6:50 It was filled with microscopic iron balls. When the electromagnetic radar waves hit the paint, the magnetic energy actually turned into heat instead of bouncing off.

7:01 So between the disco ball angles and the magic iron paint, how well did it actually work in real life?

7:08 To find out, in 1975, they tested a 10-foot-long wooden mock-up of the hopeless diamond on a special pole in the desert. The radar operator turned on his equipment and couldn't see the model on his screen at all. No way. It just completely vanished. That's incredible. It gets better. Suddenly, a tiny blip appeared on the radar screen.

7:33 The engineers panicked, thinking their stealth design had failed. They looked at the wooden model through their binoculars and saw a little blackbird sitting right on top of it. A real live bird? Yes. The radar was totally blind to the 10-foot model, but it perfectly picked up the tiny bird sitting on it.

7:55 Once they scared the bird away, the radar screen went completely blank again.

8:00 That is one of the funniest things I have ever heard. So how big of a radar signature did the actual flying F-117 have?

8:09 The engineers calculated its radar cross-section was about 0.001 square meters. That means on a giant radar screen, this 65-foot twin-engine military jet looked the exact same size as a tiny little hummingbird.

8:25 A hummingbird! But wait, if it was shaped like a hopeless diamond and covered in heavy paint, how did it actually manage to fly?

8:35 That's the crazy part. It was so incredibly unstable in the air that the pilots who flew it gave it another hilarious nickname, the Wobbly Goblin.

8:45 Haha, the Wobbly Goblin! That doesn't sound very stealthy or intimidating.

8:51 It was actually so hard to fly that a human pilot couldn't do it alone. It needed four super fast computers, making constant split-second adjustments to the wing flaps just to keep it steady in the air.

9:05 What would happen if those computers ever failed during a flight?

9:08 The plane would just instantly tumble out of control, like a falling leaf. And here is another totally wild fact. The Nighthawk didn't even have a radar system of its own on board.

9:20 Why not? Don't all modern planes need radar to see where they are going and avoid crashing?

9:26 Usually, yes, but turning on a radar is exactly like shining a bright flashlight in a pitch black room. It would immediately give away the plane's secret position.

9:36 Oh, that makes total sense. So how did the pilots navigate in the dark?

9:41 They used highly advanced infrared cameras to see heat signatures on the ground, and they only ever flew their missions at night. Ah, so that is exactly why it was called the Nighthawk. Yep, they built 59 production planes in California and based them in total secrecy out at the Tonopah Test Range in the Nevada Desert.

10:01 They didn't even admit the plane existed to the public until November 1988, which was seven whole years after its very first flight.

10:10 I just can't believe they managed to keep a 65-foot jet-powered disco ball airplane a secret for that long? Science and engineering are amazing.

10:20 Alright, Finn, time for the quiz. Are you ready?

10:23 Okay, I'm ready. First question.

10:27 What year did Scottish physicist Robert Watson Watt invent his radar system? Next one, what everyday object did the F-117's flat panels act like to bounce radar waves away? Question 3. Because of its radar absorbing paint, what tiny bird did the 65-foot jet look like on a radar screen? Here's number four.

11:02 What animal landed on the test model that the radar actually picked up? Last question. What funny nickname did the pilots give the plane because of how unstable it was to fly? Great job! Let's see how you did. The first answer is, it was 1935. For question two, the flat panels acted just like a disco ball.

11:33 Number three, the answer is, it looked the size of a hummingbird. Question four, it was a bird, specifically a blackbird. And the final answer is, they called it the Wobbly Goblin. Hey, if you enjoyed learning about the Nighthawk with us today, please leave us a five-star review and make sure to follow this show wherever you're listening so new episodes turn up on their own.

11:58 Yeah, just scroll down and tap the stars. Do it for the wobbly goblin! Oh, and we have sister shows too. History and philosophy is not boring, and maths is not boring. Go give those a listen and a follow as well.

12:12 Want your name on the show? Send us an email at hello at kidopoly.com.

12:18 See you next time on Science Is Not Boring. Bye!