How do rockets work? (Intermediate)

Rockets are truly fascinating because they are both extremely simple and very complicated at the same time. They are so simple that you or I could go to the nearest hobby shop and build our own quite easily, but they are also so complex that only 3 countries have actually managed to send humans into space in a rocket.

Both the tiny, easy-to-make model rockets and the enormous space shuttles work thanks to the same principle which was realized by Isaac Newton in the 1600s. 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 first by looking at two examples. First, as mentioned in this previous question by Karen Masters, 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? Well, one reason is that they don't need to be controlled once they are launched so they can use solid fuel The solid fuel has to be something that burns quickly without exploding (for more info see this previously answered question). 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, gasoline, or kerosene 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 we 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) which would require about 4 million pounds of fuel to launch! You can see the fuel weighs MUCH more than the actual rocket!

Why can rockets move in space? A previous answer describes Issac Newton and his laws, but I do not find that this increases my personal comprehension. If space is essentially a vacuum, and contains very few particles, what are space-craft pushing against to propel themselves? Why does this work? What is space? If an astronaut were in space, they could not propel themselves(?), so why can a vessel?

First, carefully re-read the answer above. Ok, done? Maybe that helped and maybe not, rockets are a hard concept to wrap your brain around. The key to how they work is in Newton's laws, which are simple, but can be tricky to apply correctly. Bear with me, I'm going to use some simple equations to help explain.

The real key to how rockets work is in the conservation of momentum (or check out this simpler explanation for younger students). Say you start off sitting on the skateboard mentioned above (this is the same as an astronaut floating in space). 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 too. Now suppose you throw something away from you, like the football in the example. That 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 universe, you always have to have the same amount of momentum. 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

This adds up to zero because the velocity away from you is opposite your velocity, so you can call one of them negative. 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. In equations:

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 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 above, it is tiny, but it is going pretty fast, and to balance out the momentum of all those molecules, you move in the opposite direction. (check out this video if you're skeptical)

That fire extinguisher is a rocket! If you were out in space and you opened the valve, the CO2 would still come rushing out and you would still go flying in the opposite direction! It has nothing to do with the extinguisher pushing on the things around it, it's all conservation of momentum. If you send lots of small things (molecules) really fast in one direction, it can add up to make one large thing (you) head off in the other direction.

Page last updated on June 25, 2015.

About the Author

Sabrina Stierwalt

Sabrina Stierwalt

Sabrina was a graduate student at Cornell until 2009 when she moved to Los Angeles to become a researcher at Caltech. She now studies galaxy mergers at the University of Virginia and the National Radio Astronomy Observatory in Charlottesville. You can also find her answering science questions in her weekly podcast as Everyday Einstein.

 Twitter: @galaxygirlguru

Similar Questions that might Interest You...

Search Our Q&A Archive

Most Popular

Our Reddit AMAs

AMA = Ask Me (Us) Anything