| One of the most amazing endeavors man has ever | | | | occurs in the other direction as a result. |
| undertaken is the exploration of space. A big part of | | | | This concept of "throwing mass and benefiting from |
| the amazement is the complexity. Space exploration | | | | the reaction" can be hard to grasp at first, because |
| is complicated because there are so many problems | | | | that does not seem to be what is happening. Rocket |
| to solve and obstacles to overcome. You have things | | | | engines seem to be about flames and noise and |
| like: | | | | pressure, not "throwing things." Let's look at a few |
| The vacuum of space | | | | examples to get a better picture of reality: |
| Heat management problems | | | | If you have ever shot a shotgun, especially a big |
| The difficulty of re-entry | | | | 12-gauge shot gun, then you know that it has a lot |
| Orbital mechanics | | | | of "kick." That is, when you shoot the gun it "kicks" |
| Micrometeorites and space debris | | | | your shoulder back with a great deal of force. That |
| Cosmic and solar radiation | | | | kick is a reaction. A shotgun is shooting about an |
| The logistics of having restroom facilities in a | | | | ounce of metal in one direction at about 700 miles |
| weightless environment | | | | per hour, and your shoulder gets hit with the |
| But the biggest problem of all is harnessing enough | | | | reaction. If you were wearing roller skates or |
| energy simply to get a spaceship off the ground. | | | | standing on a skateboard when you shot the gun, |
| That is where rocket engines come in. | | | | then the gun would be acting like a rocket engine and |
| Rocket engines are, on the one hand, so simple that | | | | you would react by rolling in the opposite direction. |
| you can build and fly your own model rockets very | | | | If you have ever seen a big fire hose spraying |
| inexpensively (see the links on the last page of the | | | | water, you may have noticed that it takes a lot of |
| article for details). On the other hand, rocket engines | | | | strength to hold the hose (sometimes you will see |
| (and their fuel systems) are so complicated that only | | | | two or three firefighters holding the hose). The hose |
| three countries have actually ever put people in orbit. | | | | is acting like a rocket engine. The hose is throwing |
| In this article, we will look at rocket engines to | | | | water in one direction, and the firefighters are using |
| understand how they work, as well as to understand | | | | their strength and weight to counteract the reaction. |
| some of the complexity surrounding them. | | | | If they were to let go of the hose, it would thrash |
| When most people think about motors or engines, | | | | around with tremendous force. If the firefighters |
| they think about rotation. For example, a | | | | were all standing on skateboards, the hose would |
| reciprocating gasoline engine in a car produces | | | | propel them backwards at great speed! |
| rotational energy to drive the wheels. An electric | | | | When you blow up a balloon and let it go so that it |
| motor produces rotational energy to drive a fan or | | | | flies all over the room before running out of air, you |
| spin a disk. A steam engine is used to do the same | | | | have created a rocket engine. In this case, what is |
| thing, as is a steam turbine and most gas turbines. | | | | being thrown is the air molecules inside the balloon. |
| Rocket engines are fundamentally different. Rocket | | | | Many people believe that air molecules don't weigh |
| engines are reaction engines. The basic principle | | | | anything, but they do (see the page on helium to get |
| driving a rocket engine is the famous Newtonian | | | | a better picture of the weight of air). When you |
| principle that "to every action there is an equal and | | | | throw them out the nozzle of a balloon, the rest of |
| opposite reaction." A rocket engine is throwing mass | | | | the balloon reacts in the opposite direction. |
| in one direction and benefiting from the reaction that | | | | |