Getting to space is not a difficult task, but staying in space is an entirely different challenge. At the altitude of the International Space Station — 250 miles — a spacecraft must have a horizontal velocity of about 5 miles per second. That’s roughly 17,000 mph. As the mass of the object or vehicle being delivered to space increases, the necessary power of the rocket also increases.
Image Credit: NASA/STS-132 Crew
In October 1957, the USSR launched and successfully orbited the artificial satellite Sputnik 1. A month later, a second Sputnik satellite was launched. These events triggered the Space Race between the Russians and the United States to become the first country to put a human into space.
The rockets required to achieve this feat would be huge marvels of engineering. Instead of going straight up, they would require building horizontal speed almost immediately after launch. This is done by a “gravity turn” maneuver while the ascending rocket is still in the atmosphere. Traveling more at a diagonal angle, the rocket would continue to climb towards space while also beginning to get to the necessary speeds for orbit. Once in space, it would continue to build speed until it was going so fast that every time it fell back towards the Earth, it would miss.
The USSR won this race in April 1961, when Yuri Gagarin became not only the first human to orbit the Earth, but also the first to reach outer space. Less than a month later, the US launched Alan Shepard on a sub-orbital flight, but the US didn’t achieve orbit until February 1962 when John Glenn piloted the Friendship 7 for three full orbits around the Earth.
But this was only the beginning. Even before the United States had made it to orbit, President John F. Kennedy addressed Congress on May 25, 1961, declaring NASA’s next mission: getting humankind to the moon.
Every step you take is a tiny study in rocket science! Lift one foot and push backwards on the floor with the other. The floor exerts an equal and opposite (forward) force on the planted foot, and your body moves ahead.
Equal and opposite force pairs are everywhere. Stand on an ice rink and push against the wall; the wall pushes back, and you slide away. Sit in a chair, and your body pushes down against the seat, which pushes up with precisely the same force– if it didn’t, you’d fall! Birds push down on the air with their wings, and that air pushes their bodies upward.
Image credit: NASA
The same process propels a rocket in motion. Thrusters shove fuel out behind a spacecraft. The fuel pushes back, equally and oppositely, and the rocket moves forward. Actual rocket fuel is accelerated backwards by explosive chemistry. We can model spaceflight in the lab with any expanding fuel whose flow can be directed.
Liquid nitrogen boils into gaseous form at -321 degrees Fahrenheit. This means if it is warmer than that, it will turn from a liquid to a gas that takes up 694 times as much space. We trapped this nitrogen in a ping pong ball with two very small holes on the sides. When the nitrogen expands, this is the only way out! The holes are positioned in opposite directions on opposite sides, and as the gas is directed out of these makeshift nozzles, the equal and opposite force due to Newton’s Third Law accelerates it to truly incredible speeds!
Actual rockets are meticulously designed for optimum aerodynamics and equipped with fins for stability. Ping pong balls are pretty much the opposite of that! Our tabletop rocket is all over the place. With some structural modifications, more stable motion might be possible. For now, though, we’re just having fun with this nitrogen-powered turbo top!
Rockets blast off from earth with a rumble and cloud of smoke on their way through the atmosphere to the vacuum of space beyond. This is something that is accepted today, but it hasn’t always been that way. Doctor Robert Goddard is known as the father of modern rocketry, but when he first postulated the current method of rocket propulsion in space, he was ridiculed. Even the New York Times published an article on the preposterousness of his ideas. Although rockets had been around for centuries, trying to use them in space was seen as ridiculous at the time.
The tools for understanding the basis of how a rocket works had been developed by Sir Isaac Newton hundreds of years earlier. Using Newton’s Laws of Motion, most of rocketry can be understood with relative ease.
His first law, often referred to as the law of inertia, states that an object in motion or at rest will remain in motion or at rest, respectively, unless acted upon by an outside force. This means that a rocket in space can turn its engines off and not slow down or even turn– unless an outside force, like gravity, acts on it. The bottle rockets in the video run out of fuel after a tiny fraction of a second, but they continue traveling upward until Earth’s gravity overcomes their momentum.
This variation on the classic “pull out the tablecloth” trick is a great example of Newton’s first law. Nothing pushes or pulls the tennis balls sideways, so inertia mandates that they don’t move horizontally. For more cool inertia demonstrations, check out our blog post here.
Newton’s second law is most easily understood as an equation.
The equation says that if you apply the same force to objects of two different masses, the lighter one will accelerate more than the heavier one. This shouldn’t be a surprise. If you kick a car with all your might, and then go kick a soccer ball with the same force, you will see (and feel) the difference! It’s easier to move or stop a light object than a heavy one.
The third and final law is what we like to call the law of interaction: for every action, there is an equal and opposite reaction. When a person stands, they push down on the ground, and the ground presses up equally on them. Imagine being on a frozen, icy lake, so slippery that you can’t walk. Normally, when you take a step, you push backwards against the ground, which in turn propels you forwards– an equal and opposite reaction. On an icy surface, you don’t have enough traction to push and move ahead this way. So, are you stuck? Not quite! Imagine standing on the ice and throwing a heavy rock. The rock exerts an equal and opposite force on you, pushing you in the other direction! Thinking this way makes a rocket easy to understand. Just like you can move across a slippery surface by throwing a heavy object away from you, a rocket can travel through the vacuum of space by shooting fire in the direction opposite its desired flight path.
A rocket doesn’t have to push against the air. It just throws fire out behind it, and the equal and opposite force thrusts it forward. As you can see, the basic idea behind rocket propulsion isn’t so complex after all.
So, what makes rocket science difficult in practice? Well, a rocket is basically a gigantic cylinder of explosive fuel with a nozzle to direct the explosion. The whole system moves at a very high speed. Without the right knowledge to control it, that’s a recipe for disaster! A rocket is also constantly launching its fiery fuel out behind it, so its mass is always changing. This makes calculations less than straightforward. As is the case in many fields, the core concepts in rocketry are simple. Complications arise when applying these ideas in real-world situations that require a lot of precision.
Blast-Off for the first space shuttle in 1981 from Kennedy Space Center. Credit NASA
Temperature is a measure of energy. Adding energy to a substance makes it hotter; removing energy makes it colder. Warm, energetic molecules move faster and farther, spreading out over a larger volume of space.
This balloon has been cooled to hundreds of degrees below zero (Fahrenheit), condensing the gas molecules inside. At room temperature, the condensed gas spreads out and expands, stretching the balloon back out to its original size!
We can make a gas less dense by heating it up. Less dense substances float in denser substances. This is how hot air balloons work! The warm gas inside is thinner and lighter than the air outside, so the balloon rises up through the thicker, heavier air around it. In this experiment, we’ll harness the temperature-dependence of density to turn ordinary tea bags into miniature rockets.
Step 1: Cut the staple, string, & folded paper away from the top of the tea bag. Step 2: Empty & unfold the bag to form a cylinder. Step 3: Ignite the rocket from the top.
Tea bags work well for this demonstration because they’re light, flammable, and conveniently shaped. Emptying and unfolding the bag yields an open-ended cylinder. As the delicate paper burns, the air inside the cylinder heats up and becomes less dense. At the same time, some of the tea bag is converted to smoke, leaving a super-light skeleton of ash behind. Takeoff occurs when the structure becomes so light– and the air inside so thin– that the rocket is, overall, less dense than the air around it.
WARNING: flaming tea bags follow unpredictable flight patterns. If you try this experiment at home, be sure to choose a non-flammable setting, and keep a fire extinguisher handy.
Up, up, and away!!! From the ground of Cape Canaveral to the edges of our atmosphere and into the reaches of outer space, humans have pushed the field of rocketry to the final frontier. Many people believe that rocket design reached its peak with the development of NASA’s space shuttle, but the designs of current rockets are continuously changing and improving.
NASA is currently developing the Space Launch System, or SLS, to be an improved model of the space shuttle system. The emergence of private companies who are designing and building their own unique rockets is truly revolutionizing the space industry. Space X is currently one of the leading companies in the private space race, already using their Dragon and Falcon 9 rockets to send supplies up to the International Space Station. Virgin Galactic is also changing the game by developing spacecraft that will take people who can afford the cost of a ticket into outer space. As the field of rocketry is forever changing, who knows where the next big breakthrough in design will come from? Maybe it will be you. Here at AstroCamp, we create our own unique small scale rockets. The creation and design of each rocket is limited only by the mind of the builder.
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