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.
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.