Science & Tech
Rockets: Atmospheric Drag
Lesson time 6:41 min
Chris breaks down the equation for drag and shows how rockets are designed to overcome the biggest hurdle of launching into space—the atmosphere.
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Topics include: Atmospheric Drag
The essence of you being an astronaut is the ability to leave Earth, the ability to fly a rocket through our atmosphere and get into orbit or beyond. But flying a rocket through the atmosphere is the hardest part. It's the most dangerous part of a space flight. And the reason is because of the equation for drag, unfortunately, which is 1/2 rho v squared S-- 1/2 rho v squared S. It sounds maybe complicated or it maybe sounds deceptively simple, but to push yourself through the atmosphere-- because to stay in orbit, you have to be going 5 miles a second, 8 kilometers a second, 17 and 1/2 thousand miles an hour, 25 times the speed of sound. That's how fast you have to go to orbit the world. Those speeds are incredible. How do you get through the atmosphere and accelerate out fast enough that you can successfully stay coasting in orbit from then on? And the main impediment to doing that is to shoulder your way through the drag of the atmosphere. When you're driving a car down the highway, I'm sure at some point in your life you've stuck your hand out the window, maybe when you were a kid in the backseat, and you stuck your hand out the window. And if you're going slowly, it's kind of fun. You can feel the air. You can fly your hand. But if the car gets going faster, it becomes pretty hard just to stick your hand out there. And if you stick your hand out at highway speed, you know, your hand will get whipped right back. If you're on board a spaceship coming through the atmosphere, you stick your hand out the window, it's going to break your arm and burn your arm off. That's how much drag there is at that high speed. And it's because of the equation 1/2 rho v squared S. So let's just think about that. The 1/2 we can sort of ignore. That's just to make the math right. But rho is an ancient alphabet symbol for density. How thick is the air? Down here, close to the ocean with all the air up above me, the air is pretty thick. The higher you get, the thinner the air gets. So as you go up, rho, density, gets less. S-- rho v squared S-- S is just a matter of area, like inches by inches or meters by meters. It's really how big is your ship. What type of blunt object-- you know, if you measured my hand, you know, it's got this width by this height. That would give you the area, which would be S. The S of my hand is whatever that is, 4 inches by 3 inches, 12, we'll say, square inches. That's the S of my hand pushing through the atmosphere. If I could make by hand like this, it'll go through the atmosphere a lot easier because the S is a lot smaller. If my hand is the size of a house, it's going to be really hard to push through the atmosphere. So it helps you think about what you're trying to do with your rocket ship, that rho, the density, is getting less as you go up. And S, the size of your ship, is a really big, important factor, the cross-sectional size. But v squared, that's the big surprising part. v is just your velocity or your spe...
About the Instructor
Impossible things happen. At age nine, Chris Hadfield knew he wanted to go to space. He eventually went there three times, becoming a commander of the International Space Station. In his MasterClass, Chris teaches you what it takes to explore space and what the future holds for humans in the final frontier. Learn about the science of space travel, life as an astronaut, and how flying in space will forever change the way you think about living on Earth.
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