Rockets move by expelling mass (action) in one direction to move the rocket in the opposite direction (reaction). This is called a thrust force and is understood from Newton's third law of motion.

With chemical rockets such as the ones seen on the Space Shuttle, the action is the expelling of hot gas propellant out of the engine. This causes movement of the rocket, a reaction, as the rocket moves in the opposite direction. To enable a rocket to lift off from the launch pad, the action — or thrust— from the engine must be greater than the force from the downward acceleration of gravity on the mass of the rocket.

Rockets actually work better in space than they do in air. While in the Earth's atmosphere, a rocket actually moves slower due to the surrounding air. This phenomenon, called drag, is due to air resistance. Air resistance can be demonstrated using a piece of paper. If you hold a piece of paper flat above the floor and drop it, the paper will slowly float down to the floor. However, if you crumple up the same piece of paper and drop it, it will fall to the floor much more quickly. The reason this happens is the crumpled piece of paper is experiencing less drag because it has a smaller surface area. In other words, the air did not have as large of an area to resist the motion of the falling paper. When rockets make it all the way to space, there is no longer any air to slow them down as there is no air resistance in space.

To get more thrust out of slow moving rockets (e.g., paper rockets), the exit "nozzle" must be as small as possible to make the air accelerate faster.

Figure 2. A diagram of air moving through a nozzle. click for copyright

Because it is a gas, air can be compressed. Devices called air compressors use a pump to push air into a rigid container, storing a tremendous amount of air in a small space at very high pressure. The pump uses a large force to compress the air. When the air is released from the container, it generally blows out very hard and very fast (such as when you user a compressor to fill up bicycle or car tires). When air moves relatively slowly (approximately 200 mph or less) through a tube that is decreasing in diameter, it does not actually compress. Under these conditions, there is not enough force on the air to make it squeeze together. Instead, the air speeds up. This is what happens when you open up a door to your home or a building. If you are standing next to the door inside the house/building, you will feel a breeze since the air speeds up as it moves through the doorway. Another example is water flowing through a garden hose. If you place your thumb over the end of the hose to partially block the water from exiting, the water will spray out much faster than if you let it flow without blockage. There is no where for the water to go, so its only option is to speed up.

But why then do pictures of rockets usually show a bell-shaped nozzle (see Figure 3)? Why would engineers make the nozzle area larger? Based on what we have learned, wouldn't that mean that the hot gases coming out would move slower? No, rockets usually travel faster than the speed of sound and that changes how air behaves in a nozzle. Once air is moving faster than the speed of sound, it behaves differently than what we are used to living on Earth! When air moves this fast, it becomes compressible. It will slow down if the nozzle diameter gets smaller (see Figure 4). This explains the bell shape for rocket nozzles. In order for the hot gases to then speed up, the shape of the nozzle must expand (see Figure 5).

Space shuttle nozzles. click for copyright

Figure 4. Fast air moving through a nozzle click for copyright

Figure 5. Air moving through a bell-shaped nozzle. click for copyright

The weight of a rocket is the force that opposes motion and thrust. A heavier rocket requires more thrust to reach space, which ultimately increases the overall cost of the rocket. Engineers try to find ways to reduce the weight of a rocket by using lightweight materials. However, lighter is not always better. For example, a rocket made out of paper would not survive a launch into space since it lacks the necessary structural integrity to endure the harsh elements of space and the rigor of space flight. Furthermore, strong, yet lightweight materials are often expensive to design and make. A good engineer must come up with a solution that balances each of these competing needs.

Fun Fact: The weight of one solid rocket booster of the Space Shuttle (there are two total) is 1,252,000 pounds including the propellant (http://www.atk.com/rocketmotors/rocketmotors_rsrm.asp).

The weight distribution of a rocket is important also. The point about which a rocket's weight is equally balanced is called its center of gravity (CG). The concept of center of gravity can be easily demonstrated by balancing a meter stick horizontally on your finger placed at the 50 cm mark. The meter stick's CG is located at the 50 cm mark because all of the weight on one side of the mark is equal to the weight on the other side of the mark.

The motion of any naturally unstable object is almost impossible to predict. For example, if you pick up a football and throw it without the proper grip and technique, it will surely wobble through the air and will be difficult for someone of the receiving end to catch. The reason why a quarterback spirals a football is so that it completes a controlled trajectory, making it easy for his receiver to decide where to place his hands. Why does an untied balloon fly all over the place when let go? It has no control. The opening from which the air escapes is flexible and the round shape of the balloon means it will rotate in any direction as it pushes through the air.

It is necessary to control a rocket in the atmosphere before it reaches space. We know that there are wings on airplanes, but usually there are smaller fins on the tail as well. Similar small fins are most often seen on rockets. Why, do you suppose?

Wings only provide lift perpendicular to the motion of an object, and they create drag — which is okay for airplanes flying horizontally (so lift is generated vertically). Rockets, however, are trying to get to space, and the most efficient way to do that is by heading straight up with as little drag as possible. Rockets do not need lift from wings. Instead they get all of their lift from engine thrust. The smaller fins help provide the necessary control a rocket needs immediately after lift off (or launch).

The trade-off between wing lift and drag is seen in Figure 6. If an object with wings has a constant thrust (like an airplane), it can stay aloft longer. The extra thrust counteracts the drag and keeps the wings generating lift. Now, look at the wing with no extra thrust. At first it seems better than a rocket with fins, but the wing creates a lot of drag; however, without the extra thrust to compensate, all the forward motion is lost, which decreases the lift and gravity quickly pulls it down! Small fins do not generate lift, but they more than make up for it by not adding much weight or drag.

Figure 6. The comparison between a fin and a wing. click for copyright

Since rockets fly through the air, another important point must be considered in addition to the rocket's center of gravity (CG): the center of pressure (CP). The CP is different from the CG in that it is the point about which the surface area of the rocket is equally balanced on both sides, not the weight. For a stable rocket, the CP should be behind the CG - meaning if the rocket is pointed upward, the CP should be below the CG. This can be accomplished by either adding fins near the back of the rocket (which increases surface area in the back and thus moving the CP back) or adding mass near the front of the rocket (which moves the CG up further towards the top). However, if the center of gravity and the center of pressure are at the exact same spot, the object will rotate randomly in either direction at any time. Engineers must always decide where these two points should be located on the rocket, and in relation to each other, to design the best rocket possible.

Figure 7. The comparison between a stable rocket and an unstable one in relation to its center of gravity. click for copyright

Cooperative Group Brainstorming: Arrange students in groups of four. Ask them to write down everything they know about rockets and report to the class how they think rockets actually get into space and if they are heavy or light. Have them draw a sketch of a rocket (any one that comes to mind) and have them explain how the different parts of the rocket might be used.

Venn Diagram: Have students create a class Venn Diagram or T-chart to compare what they know about airplanes and rockets.

Question/Answer: Ask the students and discuss as a class:

• Why do explorers and exploration teams utilize engineers? (Answer: Engineers design the equipment that helps explorers and teams conduct successful missions. Engineers have helped, designed, built and used most exploration vehicles.)
• How are airplanes different than rockets? (Answer: Airplanes have wings and sometimes propellers to help them fly through the air. There is no air in space, so rockets cannot use the same parts as airplanes.)
• What are three things that engineers need to consider when designing a rocket that can get into space? (Answer: weight, thrust and control)
• How do rockets use thrust to get into space? (Answer: Thrust directly relates to Newton's third law of motion. It tells us how rockets move into space by pushing out fuel in one direction to move in the opposite direction.)
• Can you change the weight of a rocket without changing the thrust and control? (Answer: No, all three factors are interrelated.)
• On an actual rocket, if you add propellant (fuel) to get more thrust, what other key factor have you affected? (Answer: The weight; fuel is heavy.)

Cooperative Group Brainstorming Revisited: Pass back the papers that were used in the Pre-Lesson Assessment. In the same groups, have the students read over their answers. Ask them to update their answers, and if they want to, sketch another rocket on a new sheet of paper. Then, call on volunteers to make a brief presentation to the class about what they thought before the lesson and what they have learned now.

Interesting Facts/Discussion Questions: Solicit, integrate and summarize student responses.

• Is there a vehicle that flies both through the air and in space? (Answer: Yes, the space shuttle does both but it really only glides in the atmosphere. Right now, there is no vehicle that takes off like an airplane and can make it to space, but engineers are currently working on this.)
• Spacecraft can escape the Earth's gravity and travel to other planets. Can a spacecraft escape the Sun's gravity and leave the solar system? (Answer: Yes, the Voyager spacecraft has already done it.)