Rocket Engines 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 interesting 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 Restroom facilities in a weightless environment And so on.. 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 engines are on the one hand so simple that you can build and fly your own model rockets very inexpensively (see the links at the bottom of the page for details). On the other hand, rocket engines (and their fuel systems) are so complicated that only two countries have actually ever put people in orbit. In this edition of How Stuff Works we will look at rocket engines to understand how they work, as well as to understand some of the complexity. The Basics 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”. So 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 guage shot gun, 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 an ounce of metal in one direction at about 700 miles per hour. Therefore your shoulder gets hit with the reaction. If you were wearing roller skates or standing on a skate board 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 firemen holding the hose). The hose is acting like a rocket engine.
The hose is throwing water in one direction, and the firemen 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 firemen were all standing on skateboards, the hose would propel them backwards at great speed! When you blow up a balloon and let it go so 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 (see the page on helium to get a better picture of the weight of air).
When you throw them out the nozzle of a balloon the rest of the balloon reacts in the opposite direction. Imagine the following situation. Let’s say that you are wearing a space suit and you are floating in space beside the space shuttle. You happen to have in your hand a baseball. If you throw the baseball, your body will react by moving away in the opposite direction.
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 1 pound and your body plus the space suit weighs 100 pounds. You throw the baseball away at a speed of 32 feet per second (21 MPH). That is to say, you accelerate the baseball with your arm so that it obtains a velocity of 21 MPH.
What you had to do is accelerate the one pound baseball to 21 MPH. Your body reacts, but it weights 100 times more than the baseball. Therefore it moves away at 1/100th the velocity, or 0.32 feet per second (0.21 MPH). If you want to generate more thrust from your baseball, you have two options. You can either throw a heavier baseball (increase the mass), or you can throw the baseball faster (increasing the acceleration on it), or you can throw a number of baseballs one after another (which is just another way of increasing the mass). 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 pound of rocket fuel, a pound 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. 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 one 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). So if you were floating in space with a bag of baseballs and you threw 1 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 1 baseball every second at a speed of 2,100 MPH. That means that you have to start with 3,600 one 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 this 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 shuttle itself the big external tank the two solid rocket boosters (SRBs). The shuttle 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 millio …