Science Experiments at Home: Model Rockets
Only three countries have managed to send humans into space via rockets, but you can easily build simple model rockets at home. Ask Science explains the science behind how model rockets work and offers tips on how to construct your own with easy science experiments.
Hi, I’m Sabrina Stierwalt, and I’m Ask Science bringing you Quick and Dirty Tips to help you make sense of science.
Rockets are truly fascinating because they are both extremely simple and very complicated. They are also so complex that only three countries have actually managed to send humans into space in a rocket. At the same time, science experiments with model rockets at home is as easy as going to the nearest hobby shop to purchase your own kit.
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How to Perform the Experiment
The best way to get started with building model rockets is to purchase a kit. This will include all the parts you need to construct your rocket as well as detailed instructions.
Take advantage of the fact that you can launch and re-launch your rocket by experimenting with different constructions. Do the number or angle of fin attachments you apply change your rocket’s trajectory? What happens if you put a small weight in the nose of the rocket?
You will also need to purchase a commercially-made rocket engine. For first time rocket makers, the best engine to use is type A8-3. They are the lowest power, but don’t worry—they still provide an impressive launch. You can count on them to go so high that you lose track of them on a clear day.
There are also B and C type engines, but for those rockets you will need some kind of binoculars and a couple of extra friends so that you can keep track of your rocket. Don’t forget to put a stop inside your rocket so that the engine doesn’t just shoot right through it or catch fire. It also helps if you buy the rocket engines before you construct your rocket, so that you can tell exactly how big your rocket body needs to be.
Notes on Safety
- Always launch rockets in wide open spaces, far from houses and other buildings, and at a time when the wind is <20 miles per hour.
- Always use the remote to launch your rocket so that you (and everyone else) are at least fifteen feet away from the rocket when it launches.
- Learn when to let go—don’t climb trees to collect a lost rocket if it is too dangerous.
- Make sure no young children or pets have access to your rocket parts at home, especially the rocket engines.
For more tips on being safe, check out the Model Rocket Safety Code put together by the National Association of Rocketry.
The Science Behind the Experiment: How Model Rockets Work
Both small, easy-to-make model rockets and enormous space shuttles work thanks to the same principle, which was realized by Isaac Newton in the 1600s and discussed in several earlier episodes.
Newton’s Third Law says that for every action, there is an equal and opposite reaction. Let’s think about what this means in non-rocket terms by looking at two examples. First, when you let the air out of a balloon, the balloon doesn’t just sit there—it flies around the room. The action is the air rushing out of the balloon, and the reaction is the balloon being forced in the opposite direction.
Second, imagine you are standing on a skateboard and you throw a football as hard as you can to your friend. You won’t just sit there—you will roll a bit in the opposite direction of your throw. The action is your throwing the football, and the reaction is your movement in the other direction.
Rockets work in a similar way: mass in the form of fuel is accelerated out the back (the action), and thus the rocket is forced to move forward (the reaction). The strength of the force pushing the rocket forward is called the “thrust.” The faster the fuel is thrown out the back of the rocket and the more fuel that is thrown, the faster the rocket will be forced to move forward and thus the greater the thrust.
Similarly, if you throw the football to your friend gently, you won’t roll as far on the skateboard as you would if you threw the football really hard.
So, what makes model rockets so much simpler than actual space-traveling rockets?
Well, one reason is that they don’t need to be controlled once they are launched, so they can use solid fuel. Rocket fuel needs to be a substance that burns fast but does not explode—that’s why you can’t use gunpowder. The most basic rocket fuel is usually about 71% nitrate, 25% carbon, and 4% sulfur, but, of course, do not attempt to mix up anything yourself.
When the fuel burns, it turns to gas, which is then forced out the back of the rocket. Rockets that use solid fuel are simpler (and thus cheaper), but once you light the fuel, the rocket cannot be controlled. You can’t stop the burning or start it over again. This lack of control is why solid fuel rockets are only used for things like models and missiles.
Space shuttles obviously need to be controlled to be useful, so they have to use liquid fuel. The fuel (liquid hydrogen for example) is pumped into a combustion chamber with an oxidizer (like liquid oxygen) and then burned into a very high-pressure gas. The gas is forced out the back of the rocket, forcing the rest of the shuttle forward.
No mass is lost during the conversion, so however much mass in liquid you started with is how much will be converted into gas. The gas leaves the rocket typically at speeds between 5,000 and 10,000 miles per hour!
Remember how I said the two ways to get more thrust (i.e., a stronger rocket) were to use more fuel or to accelerate the fuel to faster speeds? Although 10,000 miles per hour is incredibly fast, a major problem with shuttles is that you need a huge amount of fuel to propel the average-sized shuttle. A typical shuttle could weigh around 200,000 pounds (including the people and equipment inside), and thus require about 4 million pounds of fuel to launch. You can see the fuel weighs much more than the actual rocket.
For those looking to go a bit deeper into how rockets work, the key lies in understanding conservation of momentum. Say you start off sitting on the skateboard mentioned earlier. Momentum is given by the equation:
Momentum = Mass x Velocity
If you’re just sitting still, your velocity is zero, and so that means your momentum is also zero. Now, suppose you throw something away from you, like that football. The football has some mass and you gave it some velocity away from you. That means the football has a momentum:
Momentum of Football = Mass of Football x Velocity of Football
But we started with zero momentum, and, according to the laws of physics, momentum must always be conserved. So, how do we balance out the football’s momentum and keep the universe happy? The only solution is for you to gain the same amount of momentum that the football has, but in the opposite direction, so that when you add them together, you get zero:
Total Momentum = Mass of Football x Velocity of Football + Your Mass x Your Velocity = 0
To get these components to add up to zero, your velocity must be in the opposite direction to that of the football. Remember: speed is just how fast you’re going; velocity also has a direction to it.
Now, before we get back to rockets, let’s pretend that instead of a football, you have a machine gun. What will happen if you start at rest, and then fire the machine gun for a few seconds? Every bullet that you fire has a mass and a velocity away from you, so that means every bullet gives you a little more velocity. Your mass is much larger than the mass of a bullet, so the amount of velocity you gain will be very small for each bullet.
Say we fire 100 bullets:
Total Momentum = 100 x Bullet Mass x Bullet Velocity + Your Mass x Your Velocity = 0
Same idea, we just add up the momentum from each bullet and give you that total momentum in the opposite direction.
Finally, get rid of the machine gun and get a fire extinguisher. When you give a blast from the extinguisher, compressed CO2 gas comes rushing out. You can think of every individual molecule as acting like one of the bullets from our earlier example. The molecule is tiny, but it is going pretty fast, and to balance out the momentum of all those molecules, you move in the opposite direction.
Atlas-V Rocket Launch and model rocket components images courtesy of nasa.gov. Model rockets image courtesy of Shutterstock.
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