WHEN WHAT GOES UP DOESN’T COME DOWN:
Have you ever watched a rocket launch? Months, sometimes even years, go into planning, all to come together for the countdown: 3 … 2 … 1 … BLAST OFF!
At first, rockets might not seem to have much in common with other projectiles, such as baseballs or bullets. Rockets are powerful and complicated machines that don’t need a slingshot or a catapult to get them moving. And they can fly higher, faster, and farther than any other kind of projectile there is!
Rockets have sent space probes across the solar system, and even carried people to the moon and back. The same forces that shape the paths of balls and arrows work on rockets, too. And although rockets might seem like a recent invention, they’ve been around for a while—they’re even older than the slingshot!
ESSENTIAL QUESTION
How are rockets similar to pebbles shot with a slingshot? How are they different?
probe: a spaceship or other device used to explore outer space.
solar system: the collection of eight planets and their moons in orbit around the sun, together with smaller bodies in the form of asteroids, meteoroids, comets, and dwarf planets.
liquid-fueled rocket: a rocket that uses liquid propellants to create thrust.
propellant: a combination of fuel and oxidizer that burns to produce thrust in a rocket.
ballistic missile: a missile that is at first powered and guided but is then pulled by gravity to its target.
HISTORY OF ROCKETS
Rockets have been around a lot longer than you might think. For centuries, they were used as fireworks and weapons of war. The first rockets appeared in the thirteenth century, when the Chinese filled bamboo tubes with an early kind of gunpowder to create a fire lance.
DID YOU KNOW?
During the War of 1812, British forces used rockets during their attack on Fort McHenry. The attack inspired Francis Scott Key (1779–1843) to write a poem with the line “the rockets’ red glare,” which later became a line in “The Star-Spangled Banner.”
Lighting these first rockets was very dangerous. Sometimes, the tubes would fly toward the enemy, but they were just as likely to explode in the hands of their creators.
As the use of rockets slowly spread to Europe and the Middle East, they were used in battles and wars, and were eventually replaced by more powerful firearms. The earliest rockets behaved more like fireworks, often scaring the enemy with flashes of light and a lot of noise. Despite the ability of rockets to occasionally fly to great heights, people didn’t think of using rockets to explore space until the end of the nineteenth century.
Robert Goddard stands next to his liquid-fueled rocket in 1926.
credit: U.S. Air Force
In 1898, Russian schoolteacher Konstantin Tsiolkovsky (1857–1935) described how rockets might be used to reach space. He wrote that a rocket using a liquid fuel, such as kerosene, instead of a solid fuel, such as gunpowder, could carry rockets much higher than the only aircraft of the time—balloons. This idea excited people around the world, including an American named Robert Goddard (1882–1945). In 1926, Goddard launched the first rocket to use liquid propellant. While it flew to a height of only about 40 feet, it’s considered the beginning of modern rocketry.
Goddard built and flew larger and more powerful rockets, even making a gyroscope system to help steer rockets in flight.
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He used his rockets to study the atmosphere, and built a recovery system to bring his experiments safely to the ground by parachute.
The biggest advance in rocketry came during World War II with the invention of the first ballistic missile, the German V-2 rocket. The V-2 was a terrifying weapon. At its fastest, it could travel 3,500 miles per hour—faster than the speed of sound and much faster than any airplane of the time. The V-2 became the first manmade object to reach space.
A V-2 rocket
credit: U.S. Air Force
warhead: a weapon that is the explosive part of a missile.
Soviet Union: a country that existed from 1922 until 1991. Russia was part of the Soviet Union.
concentration camps: large camps where Jews and members of other groups were imprisoned by the Nazis during World War II. Prisoners were forced to perform hard labor and millions were killed.
artificial satellite: a man-made object that orbits the earth, moon, or other object.
antenna: a metal rod that sends and receives radio waves. Plural is antennae.
During flight, once its fuel was used up, the V-2 followed a ballistic trajectory to its target. It was capable of destroying an entire city block with its 2,000-pound warhead. The V-2’s designer, Wernher von Braun (1912–1977), surrendered to the Americans at the end of the war and became an important part of the race between the United States and the Soviet Union to put a human on the moon.
These early rockets still couldn’t travel very far. Just like other projectiles, they always fell back to Earth—or exploded. Although the V-2 could reach the edge of space, it couldn’t stay there. To reach space, even more powerful rockets were needed.
DID YOU KNOW?
The term “rocket” can mean either the machine that carries people and things into space or the powerful engines used to get there!
Wernher von Braun
Despite his success building rockets, von Braun’s membership in Germany’s Nazi Party made him a controversial figure. To produce the V-2, the German Army relied on the slave labor of thousands of men and women held in concentration camps. Forced to work underground and in dangerous conditions, many thousands of people lost their lives constructing the new weapon. In fact, more people died building the V-2 than were killed by its use as a weapon. Although von Braun said he had no knowledge of the terrible working conditions, many historians doubt his claims.
Today, rocket flights might seem common, but getting to space is still very hard and very expensive. Private companies such as Space X and Blue Origin are making reusable rockets that can be flown many times, unlike most rockets in the past. Will these new rockets allow more people to catch a ride into space? What do you think?
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Both the United States and the Soviet Union wanted to have the most powerful rockets in the world.
On October 4, 1957, the world was stunned when the Soviet Union launched the first artificial satellite into orbit. Sputnik, a small, silvery ball with a few radio antennae, circled the globe every 90 minutes! Just 12 years later, Neil Armstrong (1930–2012) and Buzz Aldrin (1930– ) rode the massive Saturn V rocket into space and became the first people to walk on the moon!
Sputnik, the world’s first artificial satellite
credit: NASA
hydrogen: a colorless, odorless, and flammable gas that is lighter than air.
oxidizer: a substance that contains oxygen, which mixes with fuel in a rocket engine to produce thrust.
liquid oxygen: a liquid produced by compressing oxygen gas.
combustion: a chemical reaction that produces heat and light.
combustion chamber: the part of a rocket where liquid fuel and oxidizer are combined to create a chemical reaction.
solid-fueled rocket: a rocket that uses solid propellants.
exhaust: the hot gases produced from a rocket’s engine.
propel: to drive or move forward.
WHAT IS A ROCKET?
Rockets are different from catapults and rifles. Instead of using a separate machine to accelerate a projectile, rockets bring the machines along for the ride. Rockets regularly lift items such as satellites, space probes, and astronauts into space using powerful engines. These rocket engines provide a great amount of force to get their heavy payloads moving up and away from Earth. But how do rocket engines get things moving?
Just as with everything else, rockets use Newton’s third law to get going. When you blow up a balloon, the air pushes against the inside rubber walls, keeping it inflated. But when the air is released, it rushes out in one direction, and the balloon shoots off in the opposite direction!
This force is called thrust, and it is the same force that carries rockets into space. It takes a lot of thrust to lift heavy things. To do this heavy lifting, rockets turn the chemical potential energy of rocket fuels into kinetic energy!
The Saturn V
The Saturn V was the largest rocket ever flown. It weighed 6.2 million pounds at liftoff and stood 363 feet tall. That’s taller than a soccer field is long! At launch it generated 7.5 million pounds of thrust, which is more powerful than 85 Hoover Dams! The Saturn V took the Apollo astronauts to the moon!
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A rocket blasting off. Now that’s a lot of propulsion!
credit: NASA
PROPULSION
There are two main kinds of rocket engines. In a liquid-fueled rocket, a fuel, such as liquid hydrogen, is mixed with an oxidizer, such as liquid oxygen, in a combustion chamber, where they burn very quickly as soon as they meet. In a solid-fueled rocket, the fuel and oxidizer are mixed to make a solid propellant and carefully packed inside the rocket. When the propellant is ignited, it burns quickly at a very high heat.
Both kinds of rockets make extremely hot gases called exhaust. As the gases expand, they’re expelled out of the rocket. Just like a loose balloon letting out air, the rocket is propelled in the opposite direction of the fiery and quick-moving exhaust, and we have liftoff!
DID YOU KNOW?
Because rocket engines apply force as the rocket climbs, the rocket is not really on a true ballistic trajectory when the engines are running! But once the engines are off, only gravity and air resistance affect the rocket’s flight.
stage: a smaller rocket that is stacked with other rockets and detaches when its fuel is used up.
jettison: to throw something away.
throttle: to control the amount of thrust.
booster: a rocket used to give another craft the power needed for takeoff.
The way rockets are accelerated is different from other projectiles. When an arrow or bullet is fired, a force is applied to accelerate them. But that force is only applied for a very brief instant. After that, they are on a ballistic trajectory. The only forces acting on them are gravity and a little air resistance.
For rockets to reach the speeds and heights needed to get into space, rocket engines apply a constant force to accelerate them.
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By using rocket engines, spacecraft can accelerate gradually and safely on their way to space!
The rocket engines and the fuel to power them make rockets very heavy. In fact, most of a rocket’s weight is fuel. To help them reach space, many rockets have separate parts called stages. Each rocket stage has its own engine and fuel. When a stage’s fuel is all used up, the stage is jettisoned, and falls back to Earth. This makes the rest of the rocket lighter—and the less mass, the better!
DID YOU KNOW?
Not all rockets discard their stages. The SpaceX Falcon 9’s first stage can return to Earth and land under its own power! Check out this video to see two boosters land at once!
Falcon Heavy and Starman
Most rockets have two or three stages, or booster rockets, that fall away when their fuel is used up. This makes it easier for a rocket to reach the incredible speeds and heights needed to get to space.
Why Two Kinds of Rocket Engines?
Both liquid-fueled and solid-fueled rockets have advantages and disadvantages. Liquid-fueled rockets are very complicated, but they can be throttled up or down to change their amount of thrust. Solid-fueled rockets are very simple, but once they’re ignited, they can’t be turned off! Most rockets today use liquid-fueled rocket engines. Solid-fueled rockets are used mostly as boosters, which help lift the main rocket at the beginning of its flight. When they’re used up, they separate from the main rocket and fall back to Earth.
orbit: the path of an object circling another object in space.
altitude: the height of an object above sea level.
How do rockets reach the heights and speeds they do? And how do they stay in space? After all, what goes up must come down, right? Even with the help of slingshots or catapults, every projectile you’ve ever launched eventually came back down to Earth.
Is it possible to get something moving so fast that it doesn’t come back to Earth?
A CANNON ON A MOUNTAIN
Imagine climbing to the top of a very tall mountain, where the air is thin and cold. At the peak, you find a huge and powerful cannon, just waiting to be used. How far can this giant cannon fire?
First, you fire the cannon at its lowest setting, and watch as the cannonball disappears. The cannonball travels thousands of miles before it comes crashing back to Earth. That’s good, but you want the cannonball to go farther. You increase the power of the cannon and try again.
This time, you hear reports that the cannon ball has traveled more than 12,000 miles—halfway around the world! Still not satisfied, you turn the cannon’s power to maximum, and fire one more cannonball.
This time, there are no reports of the cannonball crashing to Earth. Nervously you wonder, where did it go?
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The cannonball is traveling so quickly that even as gravity pulls it back to Earth, the earth’s surface curves away from it. The cannonball is falling around the earth! This is called an orbit!
Where does the atmosphere end and space begin? As you climb higher and higher, the air gradually becomes thinner and thinner. There’s no exact line, and no sign that says, “Welcome to Space!” Scientists consider something to have entered space when it reaches an altitude of 62 miles above the surface of the earth. Does that seem high to you? It’s 10 times higher than most passenger jets fly!
An orbit is the path an object takes around another object in space. In our solar system, planets, asteroids, and comets all orbit the sun, while natural satellites, or moons, orbit planets. Artificial satellites, such as the Hubble Space Telescope and the International Space Station, orbit the earth And some space probes even orbit other planets!
Getting into orbit isn’t easy. First, a projectile must be traveling fast—at least 17,000 miles per hour. That’s more than 10 times faster than a speeding bullet, and much faster than any cannon or rifle can fire a projectile.
Second, incredible speed isn’t enough. When a projectile is moving that fast, the force of drag becomes very large! For an object to orbit the earth, it must be above the earth’s atmosphere—in space. Otherwise, even the thinnest air will slow a satellite or spacecraft down, causing it to eventually fall on a ballistic trajectory back to the earth.
momentum: the force that a moving object has in the direction that it is moving.
ellipse: an oval shape.
retro-rocket: a rocket that is fired opposite the direction of motion to slow a spacecraft down.
THE PATH OF A ROCKET
When a rocket is launched, it starts its flight pointing straight up. But as it accelerates, it begins to tip. This is normal! Just as with any projectile, a rocket needs a combination of horizontal and vertical motion to go as far and as fast as needed to get into orbit.
As its stages are left behind, the rocket becomes lighter and faster. After only a few minutes, the rocket’s remaining engines shut down and the rocket has enough momentum to carry it into space. At this point, the rocket is on a ballistic trajectory. But a ballistic trajectory is a parabola, and that will bring it back to Earth! Instead, a rocket will fire its engines one more time, just long enough to change the shape of its path from a parabola to an ellipse. An ellipse is an oval shape. It’s not perfectly round like a circle, but it’s close.
At this point, the spacecraft is in orbit! And it will stay in orbit until another force acts to change its motion, just as Newton’s first law says.
An artist’s rendition of a space capsule re-entering Earth’s atmosphere. That’s a lot of friction!
credit: NASA
To come back to Earth, a spacecraft needs to slow down to change its path back from an ellipse to a parabola. Retro-rockets fire in the opposite direction of the spacecraft’s motion, reducing its speed.
Once it enters the atmosphere, the spacecraft experiences a lot of drag. The friction between the air and the spacecraft is enough to heat the outside of the craft and the air around it to very high temperatures. Finally, the spacecraft is in freefall, once again acting like any other projectile, until its parachutes open to bring it down to a soft and safe landing.
Kepler
For centuries, astronomers believed that the moon and the planets all moved in perfect circles through the heavens. But in the seventeenth century, Dutch astronomer Johannes Kepler (1571–1630) proposed that their paths were not perfect circles, but ellipses. By carefully studying their motions, Kepler was able to use mathematics to determine his three laws of planetary motion, which are still used today.
escape velocity: the lowest velocity an object must have to escape the earth’s gravitational pull.
BEYOND EARTH
Have you watched the Apollo astronauts hop around on the moon? Have you seen pictures of the Curiosity rover on Mars, or closeup shots of Jupiter’s Great Red Spot? How did spacecraft get to Mars and Jupiter?
To leave Earth behind, a spacecraft needs to reach even greater speeds than what it needed to get into orbit. To escape the Earth’s gravity completely, projectiles need to travel about 25,000 miles per hour. That’s nearly 7 miles every second! This is called Earth’s escape velocity. But even that isn’t fast enough to travel to Mars or beyond.
DID YOU KNOW?
The temperatures on the outside of the Apollo spacecraft reached more than 5,000 degrees Fahrenheit (2,760 degrees Celsius) when it returned from the moon. Don’t worry though—the astronauts were protected from that heat by a heatshield and were safe inside the cabin.
The farther you want to go, the faster you need to move.
The fastest spacecraft ever launched was the New Horizons space probe. It was sent on a trajectory toward Pluto at an incredible 36,000 miles per hour, and it still took more than nine years to reach its destination!
By observing everyday objects such as arrows and apples, scientists, including Galileo and Newton, showed that the laws that determined ballistic motion applied not only to things here at home, but to everything, everywhere. And you can, too! Every time you hit or kick a ball, shoot an arrow, or throw a pass, you’re seeing the science of projectiles at work. From the smallest pebble shot with a slingshot to the biggest rocket sent into space, the study of projectiles can tell us a lot about the world around us, and even places beyond!
ESSENTIAL QUESTION
How are rockets similar to pebbles shot with a slingshot? How are they different?
TAKE FLIGHT
Have you ever wanted to build your own rocket and send it into space? The game Kerbal Space Program lets you build, test, and fly your own rockets through a series of challenges. Can you send your Kerbalnauts into orbit and bring them home safely? What about your own mission to the Mun, the large moon of Kerbal?
Note that this activity requires a paid service.
›With an adult’s permission, try the game. Go to kerbaledu.com.
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Questions to think about
Can you build a rocket with more than one stage?
Are the game’s laws of motion and gravity the same as they are in real life?
How do control surfaces affect the flight of your rocket?
How does using stages improve the flight of your rocket?
Are you able to get to orbit or beyond?
Does the game make spaceflight seem easy or hard?
›You can also build airplanes in Kerbal Space Program. Can you get one flying? Or get one into orbit?
›There are many videos available to help you get started, as well as in-game tutorials and help!
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Consider This!
How far can your rockets go? How many Kerbals can you launch at once and return safely?
You don’t need a special launch pad or a million-dollar spacecraft to understand how rockets work. You can study their flight at home! All you’ll need are supplies such as string, drinking straws, and balloons.
›Attach one end of a piece of string to a sturdy object, such as a chair or doorknob, or have friend or family member hold it.
›Thread the string through one straw. Attach the other end of the string to another sturdy object. Make sure the string is level and taut.
›Blow up a balloon and pinch the opening closed—don’t tie it!
›Attach the balloon to the straw so that the balloon’s opening points along the string.
›Move the balloon and straw to one end of the string. What do you think will happen when you release the balloon?
›Release the balloon! Write your observations and include a diagram in your engineering notebook.
›Vary the experiment by inflating the balloon with more air and then less air. How does the amount of air affect the balloon’s motion?
›Now make the string vertical. What happens when you release the balloon?
›Keep your balloon setup for another activity in this chapter!
Questions to think about
What direction does the balloon travel when the string is horizontal? When it’s vertical?
Can you explain what’s going on using Newton’s laws?
What forces affect the vertical launch more than the horizontal launch? Can you explain why?
Try This!
Try different sizes and shapes of balloons. What effect do size and shape have on the motion of the balloon? Is there a best size or shape to get the farthest distance? Think about the shape of rockets. Would making the balloon look more like a rocket change how far or fast it goes?
WORDS TO KNOW
taut: stretched tightly.
Build your own liquid-fueled chemical rocket! While the fuels that real rockets use are extremely dangerous, you can use some common materials at home to get an idea of how these fuels work!
Caution: Have an adult help you with this project, and do not perform this indoors! Use safety glasses.
›Find a large, open space outside to launch your rocket.
›Place a cork inside the mouth of a plastic bottle until you have a tight fit.
›To make legs for the rocket, securely tape three pencils equally spaced around the outside of the bottle at the mouth end. The cork should be about an inch from the ground.
›Cut a sheet of paper towel in half. Place a tablespoon of baking soda in the center of the paper towel.
›Fold the paper towel so that the baking soda doesn’t fall out but can still fit through the bottle’s mouth when it’s time to add it.
›Put on your safety glasses.
›Remove the cork and pour about 1 to 2 inches of vinegar into your rocket.
›Place the baking soda packet into the bottle, and QUICKLY put the cork back in the bottle.
›SHAKE the mixture for 2 to 3 seconds, and then QUICKLY set the rocket down on its legs with the cork end facing down.
›Stand back about 20 feet. Watch it fly!
WORDS TO KNOW
ratio: the relationship in size or quantity between two things.
What happens when the baking soda meets the vinegar? Can you describe it in terms of potential and kinetic energy?
How does the mixture cause the rocket to move?
What other forces are working on the rocket as it moves?
Try This!
Try different amounts of vinegar and baking soda. Is there a ratio that works best? Try different sizes and shapes of bottles. How do they affect the rocket’s flight? Can you describe the effect using the laws of motion? Can you improve the aerodynamics of your rocket?
Voyaging Voyagers
If an object is traveling with enough speed, it won’t just leave Earth behind—it will leave the entire solar system! So far, only a few robotic spacecraft have managed this incredible feat. Pioneers 10 and 11, Voyagers 1 and 2, and New Horizons are all on trajectories out of our solar system and into interstellar space. Launched in 1977, Voyager 1 is the manmade object farthest from Earth. In 40,000 years, it will pass the nearby star Gliese 445.
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How do stages help rockets travel farther and faster? To find out, you can make a two-stage rocket balloon!
›Thread the string from your first rocket balloon experiment (on page 108) through two plastic drinking straws. If you no longer have your rocket balloon track, turn back to the first experiment to set it up.
›Cut a small ring from a cardboard tube, about 1 inch wide.
›Blow up one balloon until it’s about two-thirds full. It shouldn’t be completely inflated.
›Use a binder clip to pinch the end closed or have an assistant hold it for you.
›Without letting any air out, pinch the first balloon’s nozzle to the inside of the cardboard ring.
›With its nozzle facing in the same direction as the nozzle of the first balloon, pull a second balloon about one-third of the way through the cardboard tube.
›Don’t let the first balloon deflate!
›Inflate the second balloon so that it pinches the nozzle of the first balloon inside the cardboard ring, keeping it from deflating. You might need someone to help you!
›Pinch the nozzle of the second balloon closed with a binder clip.
›Tape each balloon to one straw on the track.
›Release the first balloon to launch your two-stage balloon rocket! What happens?
What’s happening when you release the first balloon?
Can you explain its motion using Newton’s laws?
What happens when the first balloon runs out of air?
Do the balloons separate or stick together?
Try This!
Try taping the two balloons together so they don’t separate when the first is deflated. How does that affect the speed and distance of the balloon rocket? Is it better to have them separate or stuck together? How do you think Isaac Newton would answer?
Try to add more stages! How far can your balloon rocket go?
YouTube Goddard rockets
Kerbal Edu