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