The "strength" of a rocket engine is called its thrust. Thrust is measured in "pounds of thrust" in the U.S. and in Newtons under the metric system (4.45 Newtons of thrust equals 1 pound of thrust). A pound of thrust is the amount of thrust it would take to keep a 1-pound object stationary against the force of gravity on Earth. So on Earth, the acceleration of gravity is 32 feet per second per second (21 mph per second). If you were floating in space with a bag of baseballs and you threw one baseball per second away from you at 21 mph, your baseballs would be generating the equivalent of 1 pound of thrust. If you were to throw the baseballs instead at 42 mph, then you would be generating 2 pounds of thrust. If you throw them at 2,100 mph (perhaps by shooting them out of some sort of baseball gun), then you are generating 100 pounds of thrust, and so on.
One of the funny problems rockets have is that the objects that the engine wants to throw actually weigh something, and the rocket has to carry that weight around. So let's say that you want to generate 100 pounds of thrust for an hour by throwing one baseball every second at a speed of 2,100 mph. That means that you have to start with 3,600 1-pound baseballs (there are 3,600 seconds in an hour), or 3,600 pounds of baseballs. Since you only weigh 100 pounds in your spacesuit, you can see that the weight of your "fuel" dwarfs the weight of the payload (you). In fact, the fuel weights 36 times more than the payload. And that is very common. That is why you have to have a huge rocket to get a tiny person into space right now -- you have to carry a lot of fuel.
You can see the weight equation very clearly on the Space Shuttle. If you have ever seen the Space Shuttle launch, you know that there are three parts:
- The Orbiter
- The big external tank
- The two solid rocket boosters (SRBs)
The Orbiter weighs 165,000 pounds empty. The external tank weighs 78,100 pounds empty. The two solid rocket boosters weigh 185,000 pounds empty each. But then you have to load in the fuel. Each SRB holds 1.1 million pounds of fuel. The external tank holds 143,000 gallons of liquid oxygen (1,359,000 pounds) and 383,000 gallons of liquid hydrogen (226,000 pounds). The whole vehicle -- shuttle, external tank, solid rocket booster casings and all the fuel -- has a total weight of 4.4 million pounds at launch. 4.4 million pounds to get 165,000 pounds in orbit is a pretty big difference! To be fair, the orbiter can also carry a 65,000-pound payload (up to 15 x 60 feet in size), but it is still a big difference. The fuel weighs almost 20 times more than the Orbiter [source: The Space Shuttle Operator's Manual].
All of that fuel is being thrown out the back of the Space Shuttle at a speed of perhaps 6,000 mph (typical rocket exhaust velocities for chemical rockets range between 5,000 and 10,000 mph). The SRBs burn for about two minutes and generate about 3.3 million pounds of thrust each at launch (2.65 million pounds average over the burn). The three main engines (which use the fuel in the external tank) burn for about eight minutes, generating 375,000 pounds of thrust each during the burn.
In the next section, we'll look at the particular fuel mixture in solid-fuel rockets.