One of the most amazing endeavors man has ever undertaken is the exploration of space. A big part of the amazement is the complexity. Space exploration is complicated because there are so many problems to solve and obstacles to overcome. You have things like:

The vacuum of space

Heat management problems

The difficulty of re-entry

Orbital mechanics

Micrometeorites and space debris

Cosmic and solar radiation

The logistics of having restroom facilities in a weightless environment

But the biggest problem of all is harnessing enough energy simply to get a spaceship off the ground. That is where rocket engines come in.

Rocket Image Gallery

Rocket engines are, on the one hand, so simple that you can build and fly your own model rockets very easily. On the other hand, rocket engines (and their fuel systems) are so complicated that only three countries have actually ever put people in orbit.
When most people think about motors or engines, they think about rotation. For example, a reciprocating gasoline engine in a car produces rotational energy to drive the wheels. An electric motor produces rotational energy to drive a fan or spin a disk. A steam engine is used to do the same thing, as is a steam turbine and most gas turbines.

Rocket engines are fundamentally different. Rocket engines are reaction engines. The basic principle driving a rocket engine is the famous Newtonian principle that "to every action there is an equal and opposite reaction". A rocket engine is throwing mass in one direction and benefiting from the reaction that occurs in the other direction as a result.

This concept of "throwing mass and benefiting from the reaction" can be hard to grasp at first, because that does not seem to be what is happening. Rocket engines seem to be about flames and noise and pressure, not "throwing things". Let's look at a few examples to get a better picture of reality:

If you have ever shot a shotgun, especially a big 12-gauge shotgun, then you know that it has a lot of "kick." That is, when you shoot the gun it "kicks" your shoulder back with a great deal of force. That kick is a reaction. A shotgun is shooting about 30 grams of metal in one direction at about 1,000 kilometers per hour, and your shoulder gets hit with the reaction. If you were wearing roller skates or standing on a skateboard when you shot the gun, then the gun would be acting like a rocket engine and you would react by rolling in the opposite direction.

If you have ever seen a big fire hose spraying water, you may have noticed that it takes a lot of strength to hold the hose (sometimes you will see two or three firefighters holding the hose). The hose is acting like a rocket engine. The hose is throwing water in one direction, and the firefighters are using their strength and weight to counteract the reaction. If they were to let go of the hose, it would thrash around with tremendous force. If the firefighters were all standing on skateboards, the hose would propel them backward at great speed!

When you blow up a balloon and let it go so that it flies all over the room before running out of air, you have created a rocket engine. In this case, what is being thrown is the air molecules inside the balloon. Many people believe that air molecules don't weigh anything, but they do. When you throw them out the nozzle of a balloon, the rest of the balloon reacts in the opposite direction.

Action and Reaction: The Space Baseball Scenario

Imagine the following situation: you are wearing a space suit and you are floating in space beside the space shuttle; you happen to have a baseball in your hand.

If you throw the baseball, your body will react by moving in the opposite direction of the ball. The thing that controls the speed at which your body moves away is the weight of the baseball that you throw and the amount of acceleration that you apply to it. Mass multiplied by acceleration is force (F = m * a). Whatever force you apply to the baseball will be equalized by an identical reaction force applied to your body (m * a = m * a). So let's say that the baseball weighs 0.5 kg, and your body plus the space suit weighs 100 kg. You throw the baseball away at a speed of 9.81 meters per second (35 km/h). That is to say, you accelerate the 0.5 kg baseball with your arm so that it obtains a velocity of 35 km/h. Your body reacts, but it weighs 200 times more than the baseball. Therefore, it moves away at two-hundredth the velocity of the baseball, or 0.05 meters per second (0.18 km/h).

If you want to generate more thrust from your baseball, you have two options: increase the mass or increase the acceleration. You can throw a heavier baseball or throw a number of baseballs one after another (increasing the mass), or you can throw the baseball faster (increasing the acceleration on it). But that is all that you can do.

A rocket engine is generally throwing mass in the form of a high-pressure gas. The engine throws the mass of gas out in one direction in order to get a reaction in the opposite direction. The mass comes from the weight of the fuel that the rocket engine burns. The burning process accelerates the mass of fuel so that it comes out of the rocket nozzle at high speed. The fact that the fuel turns from a solid or liquid into a gas when it burns does not change its mass. If you burn a kilogram of rocket fuel, a kilogram of exhaust comes out the nozzle in the form of a high-temperature, high-velocity gas. The form changes, but the mass does not. The burning process accelerates the mass.

Thrust

The "strength" of a rocket engine is called its thrust. Thrust is measured in pounds of thrust in the U.S. and in Newton under the metric system (4.45 Newtons of thrust equals 1 pound of thrust). A Newton of thrust is the amount of thrust it would take to keep a 1 kilogram object stationary against the force of gravity on Earth. So on Earth, the acceleration of gravity is 9.81 meters per second² (127,008 km per hour²). If you were floating in space with a bag of baseballs and you threw one baseball per second away from you at 9.81 m/s, your baseballs would be generating the equivalent of 1 Newton of thrust. If you were to throw the baseballs instead at 19.62 m/s, then you would be generating 2 Newtons of thrust. If you throw them at 981 m/s (perhaps by shooting them out of some sort of baseball gun), then you are generating 100 Newtons 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 Newtons of thrust for an hour by throwing one baseball every second at a speed of 981 m/s. That means that you have to start with 3,600 one kg baseballs (there are 3,600 seconds in an hour), or 3,600 kilograms of baseballs. Since you only weigh 100 kilograms 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 (74,850 kg). The external tank weighs 78,100 pounds empty (35,450 kg). The two solid rocket boosters weigh 185,000 pounds empty each (84,000 kg). But then you have to load in the fuel. Each SRB holds 1.1 million pounds of fuel (500,000 kg). The external tank holds 143,000 gallons or 1,359,000 pounds of liquid oxygen (550 m³ - 616,500 kg) and 383,000 gallons or 226,000 pounds of liquid hydrogen (1,450 m³ - 102,500 kg). 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 (2 million kilograms). 2 million kilograms to get 74,850 pounds in orbit is a pretty big difference! To be fair, the orbiter can also carry a 29,500 kgpayload (up to 4.5 x 18 meters 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.

Solid-fuel Rockets: Fuel Mixture

Solid-fuel rocket engines were the first engines created by man. They were invented hundreds of years ago in China and have been used widely since then. So you can see that rockets have been in use quite awhile.

The idea behind a simple solid-fuel rocket is straightforward. What you want to do is create something that burns very quickly but does not explode. As you are probably aware, gunpowder explodes. Gunpowder is made up 75% nitrate, 15% carbon and 10% sulfur. In a rocket engine, you don't want an explosion -- you would like the power released more evenly over a period of time. Therefore you might change the mix to 72% nitrate, 24% carbon and 4% sulfur. In this case, instead of gunpowder, you get a simple rocket fuel. This sort of mix will burn very rapidly, but it does not explode if loaded properly. Here's a typical cross section:

On the left you see the rocket before ignition. The solid fuel is shown in green. It is cylindrical, with a tube drilled down the middle. When you light the fuel, it burns along the wall of the tube. As it burns, it burns outward toward the casing until all the fuel has burned. In a small rocket engine the burn might last a couple of second. In a Space Shuttle SRB containing over a million pounds of fuel, the burn lasts about two minutes.

Solid-fuel Rockets: Channel Configuration

When you read about advanced solid-fuel rockets like the Shuttle's solid rocket boosters, you often read things like:

The propellant mixture in each SRB motor consists of an ammonium perchlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent). The propellant is an 11-point star-shaped perforation in the forward motor segment and a double- truncated- cone perforation in each of the aft segments and aft closure. This configuration provides high thrust at ignition and then reduces the thrust by approximately a third 50 seconds after lift-off to prevent overstressing the vehicle during maximum dynamic pressure. [source: NASA]

This paragraph discusses not only the fuel mixture but also the configuration of the channel drilled in the center of the fuel. An "11-point star-shaped perforation" might look like this:

The idea is to increase the surface area of the channel, thereby increasing the burn area and therefore the thrust. As the fuel burns, the shape evens out into a circle. In the case of the SRBs, it gives the engine high initial thrust and lower thrust in the middle of the flight.

Solid-fuel rocket engines have three important advantages:

Simplicity

Low cost

Safety

They also have two disadvantages:

Thrust cannot be controlled

Once ignited, the engine cannot be stopped or re­started

The disadvantages mean that solid-fuel rockets are useful for short-lifetime tasks (like missiles), or for booster systems. When you need to be able to control the engine, you must use a liquid propellant system.

DIY Rocket Propellant! How to Cook solid Rocket Fuel

Cooking isn't my main pastime, unless it results in a fast burning fuel and a successful rocket launch!

WARNING: Ignition of an incendiary or explosive material may not be legal in your area, so check local laws before attempting. Use of this information is at your own risk.

Using some kitchen chemistry and a few common household items, I tested a few different methods for DIY rocket fuel. The main components of this composition are a brand of fertilizer, which is 100% potassium nitrate (KNO₃), and plain white table sugar. When mixed together in ratios of 60/40 by weight, and placed on medium heat, they melt into a creamy brown liquid. This is because the sugar caramelizes and absorbs the KNO₃. The smell is similar to that of making candy, and that's why this is sometimes referred to as Rocket Candy or R-Candy. I tried some more batches with other ingredients added, like water, corn syrup and even a little home made rust powder. All the fuels burned a little differently, but overall I was most impressed with the batches using the homemade rust.

When it's runny enough, it can be poured into a casing to cool down and solidify. I've used empty toilet paper rolls, since they fitted nicely into my designed PVC fuselage. Watch out, it's hot!
The rocket motor casing had a nozzle made from ordinary clay. I was impressed to see it actually worked! I think this rocket shot up quite a distance.