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