Figure 2. NASA's Space Shuttle Discovery click for copyright

Liquid propellant is composed of liquid fuel and oxidizer. Rockets use many different combinations of these two substances. One example of liquid propellant is liquid hydrogen with liquid oxygen. The large, orange external fuel tank on NASA's Space Shuttle holds both of these two chemicals (see Figure 2).

You cannot see it from the outside, but the chemicals are separated from one another until launch time. Then during the launch procedure, the chemicals flow through a bunch of pipes and valves until they reach a chamber where they are burned. The burning of this propellant creates a high pressure and high velocity gas. This gas travels through an exit nozzle which makes the gas move even faster. The faster the gas can leave the nozzle, the more thrust the rocket will have.

The liquid propellant on the Space Shuttle is used to fire the three main engines on the orbiter. These main engines can move in a specified order to control the direction of the Shuttle. Liquid propellant is good to use because it can provide a lot of thrust. Also, rockets using liquid propellant can be quickly shut down should there be a problem — an extremely important safety feature. However, because of how complex they are, these types of chemical rockets can be very expensive to design and build. This is why other rockets use a different type of propellant, called solid propellant.

Many people do not think of fuel as being solid. However, there are many ordinary examples of solid fuel all around you, such as wood and coal. Even candle wax is a solid fuel. Energy is slowly released as the wax melts and is consumed by the flame. Solid propellant that is used in rockets utilizes substances that are a bit more complex and burn at a much faster rate, but the idea is still the same. The most famous solid rockets are the two white solid rocket boosters (SRBs) that are attached to the orange external tank on the Space Shuttle (remember, the orange external tank uses liquid propellant. The solid fuel used in the SRBs is atomized aluminum, and the oxidizer is ammonium perchlorate.

Solid propellant rockets may have a hole drilled through the center that is shaped like a circle or star (or other possibly complicated shape). The interior shape of the hollowed out core is an important factor in determining a rocket's performance. The amount of propellant exposed to burning flames is called surface area. An increase in surface area will increase thrust but will reduce burn-time since the propellant is being consumed faster. Typically, solid rockets will start out having a large propellant surface area because rockets need the greatest amount of thrust to get off the ground. After the propellant has burned for some time, the surface area will decrease since the required thrust is less. For example, a star configuration would accomplish this necessary surface area change over time in that as the propellant burns, the star will eventually become circular and the surface area will be less. The SRBs on the Space Shuttle actually use an 11-point star configuration.

Figure 3. Solid rocket with star configuration. click for copyright

Solid propellant rockets are relatively simple. Unfortunately, once a solid rocket is ignited, it will consume all of its propellant without any option to turn it off. Therefore, if anything goes wrong, this can potentially pose a dangerous situation. Because of this, the SRBs are the last component to be turned on before launch so that if anything goes wrong before then, the mission can be aborted.

Thrust for a rocket continues as long as its engines have propellant that can be burned. Furthermore, the mass of the rocket changes during flight. Its mass is the sum of all its parts, including engines, fuel tanks, payload, control system and propellant. By far, the largest part of the rocket's mass is its propellant (note: earlier in this lesson, we learned that the propellant accounts for 90% of the weight of the rocket). This mass constantly changes as the engines burn and eject the propellant — ultimately decreasing the rocket's mass during flight. The acceleration of the rocket will increase as its mass decreases due to Newton's second law of motion, which states that the force of an object is equal to its mass times its acceleration.. A rocket is accelerating fastest when its propellant is almost gone. Newton's second law of motion can be restated as such when explaining rockets: the greater the mass of propellant ejected, combined with the speed with which is it is ejected from the engine, then the greater the thrust force which pushes the rocket in the opposite direction.

Did you know that fireworks that shoot up into the air are actually rockets? Gunpowder is often used in fireworks as a solid propellant. You could launch a model rocket into the air with gunpowder to simulate a real rocket launch. A safer alternative, however, is to use the energy stored in an antacid tablet (such as Alka-Seltzer®), which contains aspirin, sodium bicarbonate (NaHCO₃) and citric acid (C₆H₈O₇). When the tablet is dissolved in water (or some other liquid), a chemical reaction occurs between the sodium bicarbonate and the citric acid: the hydrogen ions (H⁺) and carbonate ions (CO₆^-2) are freed to collide and react in the solution. The products of the reaction are carbon dioxide (CO₂) gas and water. The carbon dioxide can be seen as bubbles in the gas. If you find a way to capture the carbon dioxide created, you can release this pressure and use it to create thrust.

Discussion Question: Solicit, integrate and summarize student responses.

• Ask the students if candy lasts longer when you chew it or just suck on it. (Answer: Sucking lasts longer. If you chew it into many pieces you have actually increased the candy's surface area, and your saliva will dissolve it faster!) Do you get more candy if you chew it up? (Answer: No, it is the same amount of candy. It just does not last as long.) These are important concepts to understand for learning about solid fuel.
• Shake up a can of soda pop. Open the can over the sink or outdoors. Ask the students what happened to cause the pop to explode. (Answer: The drink is carbonated with carbon dioxide gas. The gas is usually dissolved into the liquid because the can is pressurized. When you shake the can, some of the gas is no longer dissolved and bubbles form on the sides and bottom of the can. When you open the can, the undissolved gas bubbles try to escape and push liquid out in the process since it is in the way.)

Question/Answer: Ask the students questions and have them raise their hands to respond. Write their answers on the board.

1. Does launching a rocket into space require energy? (Answer: yes)
2. What material is used to generate thrust in a rocket? (Answer: propellant)
3. What are the two types of propellant? (Answer: liquid and solid)
4. What is the difference between a propellant and fuel? (Answer: Propellants are made up of a fuel and an oxidizer.)
5. What is used as propellant in cars? (Answer: Gasoline and air: gasoline is the fuel and air is the oxidizer.)
6. Why do engineers need to know the properties of different propellants? (Answer: Engineers need to know about the properties of propellants in order to launch a rocket successfully into space.)

Class Discussion: Show pictures or video clips of rocket launches (from NASA's website at http://search.nasa.gov/nasasearch/search/search.jsp?nasaInclude=launch+video+clips or from v2Rocket.com at http://www.v2rocket.com/start/others/aud_vid.html). In groups of 2-3, have students discuss what powers a rocket. (Note: fire and fuel are popular answers.) Fire is the result of fuel burning, but how exactly does the fuel store and release energy? (Answer: Fuel stores chemical energy which is released when the fuel burns, causing an expansion of gases and a pressure increase. These gases are expelled out of the back of the rocket very quickly through a nozzle which makes the rocket move in the opposite direction because of Newton's third law of motion).

Mental Math/Pairs Check: Have students work in pairs to answer following questions.

Ideally, any rocket, is approximately 90% propellants; 4% tanks, engines, fins, etc.; and 6% payload (spacecraft, satellites and astronauts). Think of a pie graph when picturing the distribution; the biggest chunk is propellants.

• If a satellite weighs 5 tons, and the tanks, engines, and body of the rocket weigh 5 tons, how many tons would the propellants weigh? (Answer: 5 + 5 = 10 tons. 100 - 10 = 90 tons. The answer is 90 tons.)
• Challenge: If the total rocket weighs 200 tons, how much of that weight should be propellant? (Answer: 90% of 100 tons = 90 tons, so 90% of 200 tons = 180 tons. The answer is 180 tons.)

Toss-a-Question: Ask students to independently think of an answer to each of the questions below and write it on a half sheet of paper. Have students wad up and toss the paper to another team member who then adds his/her answer. After all students have written down ideas, have them toss the paper wad to another student, who then reads the answers aloud to the class. Discuss answers with the class.

• Can any material be a fuel? (Answer: No, only materials that react with something and cause a change in energy of its particles can be a fuel.)
• How is pressure created in a rocket engine? (Answer: A reaction takes place that gives particles more energy causing them to move more and take up more space which increases the pressure.)
• How does the amount of surface area affect the rate at which fuel burns? (Hint: Remind students to think about the piece of candy example from the Pre-Assessment section.) (Answer: More surface area means a faster reaction; less surface area produces a slower burn.)