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A Very Basic explanation of
Rocket Science


"How rockets work" can be explained easily and without much fuss. When you see a spacecraft blasting off it might be tempting to think it could be something to do with fire, or maybe that the machine is somehow pushing against the ground, but the truth is much more simple and can be shown by some simple experiments that you can do to demonstrate the principle. Rockets work by REACTION, as in "action and reaction", and is like the RECOIL when firing a gun.

You can demonstrate this with a boat. It's a slightly unusual boat, because it's a rowing boat with no oars, floating on a perfectly calm lake, and onboard the boat there's a small cargo of medium-sized stones. You're not stranded there floating on the lake, even with no obvious form of propulsion. Take a stone and throw it at some speed as far as it will go off the back of the boat. Then take another stone and throw it, and so on. After a while you notice the boat is moving slowly forwards. The force you have put into throwing the stones has also pushed you in the opposite direction. It might seem incredible, but that's rocket propulsion!

Another example of the same kind of idea is to sit on a skateboard facing backwards and fire a gun. (Don't try this, as it's clearly silly, and someone might get hurt!). Obviously most of the energy goes into the bullet, but regardless of where the bullet ends up, the recoil on the gun causes the skateboard to move forwards. Again, that's rocket propulsion.

The principle is known by scientists as "Newton's Third Law of Motion", or as a much-misquoted philosophical principle "to every action there is an equal and opposite reaction".

Yes, it really is true; rockets move forwards by chucking stuff backwards. It's the recoil or "reaction" that does the trick. A similar trick is used by Dyno-Rod as their drain-cleaning nozzle pushes itself upstream by forcing water out of the back end.

To force a huge spacecraft up into space requires a vast amount of stuff to be chucked out of the back very fast. But that is, in effect, what is being done. rockets2.jpg

Extra things about this:

* The point about "most of the energy goes into the bullet", yes it's true. Although the momentum is shared out equally, the energy of the projectile is much more than that of the object being recoiled. Not good news for spacecraft designers, who would like to see loads of energy going into propelling their flying machine and not being wasted in clouds of flaming vapour.

* On your boat, you're going to run out of stones. Again, true. In fact, to get a rocket powered spacecraft into space, it needs about seven times as much fuel as the weight of the craft. That's like going to a filling station and trying to put into your tank as much fuel as seven times the weight of your car.

What makes it worse is that you're having to carry all the fuel up there. It's like climbing a mountain with several days' food in your backpack, and then when you're almost up the mountain, the food you eat has cost you all that energy to carry it up that far!

* As the idea is to get a spacecraft to get up into space, a vast amount of energy is required. Almost all of this goes into fire and into ejecting the stuff. Because of the vast amounts of energy required, the heat of the continuous explosion, and the requirement to carry highly inflammable fuel, the whole business of rocketry is quite dangerous and expensive.

For this and other reasons, spacecraft designers are very keen to find alternative forms of spacecraft propulsion systems!


A few other references: How ORBITS work, space and astronomy, science, flying, a picture showing the edge of space, boats, a picture of some rocket engines, skateboarding, science museum, very basic astronomy, gadget stores, etc. Also if you want to see a nice picture of a NASA space shuttle launch, there's quite a good one on the page about ESS Data Recovery, or at least it used to be there before there was something found in the image tags.