Pneumatic Rocket Launcher (Richard Hanley)

AuthorRichard Hanley, 8th grade Physical Science Teacher at Del Sur Middle School in Lancaster, CA.

Principles

Pressure, speed, acceleration and velocity.

Standards

CSS 1b. Students know that average speed is the total distance traveled divided by the total time elapsed and that the speed of an object along the path traveled can vary.

1c. Students know how to solve problems involving distance, time, and average speed.

1d. Students know the velocity of an object must be described by specifying both the direction and the speed of the object.

1e. Students know changes in velocity may be due to changes in speed, direction, or both.

9f. Apply simple mathematic relationships to determine a missing quantity in a mathematic expression, given the two remaining terms (including speed = dis­tance/time, density = mass/volume, force = pressure × area, volume = area × height).

Materials needed

2-liter bottle Rocket Launcher (the wooden box does not come with the launcher at purchase and was made by our Wood Shop teacher - the amazing Mike Mann).

The link to the site for purchasing your own launcher is under References.

Procedure

1. Build a 2-liter water bottle rocket.

2. Attach the pneumatic launcher to an air compressor & plug the compressor in.

3. Attach the bottle rocket to 2-liter nipple and secure the collar.

4. Vary the pressure (75lb max) and amount of water to find the optimal conditions for speed or

height of launch.

5. Be the coolest science teacher any kid has ever had!

Explanation

The tendency of gases is to diffuse due to their constant random motion. As we increase pressure inside the 2-liter bottle, the gas wants to escape to normalize with the surrounding air. The nipple has two "O-rings" on it which do not allow gas to escape when being added to the bottle. The collar holds the bottle in place until we want it to launch. The shape of the 2-liter bottle narrows at the neck so that the part of the bottle which has the most pressure on it, also limits the amount of gas from escaping quickly. So that the gas escapes even more slowly, we add water to the system so it has to push against the more dense fluid and allows for a greater time the system is under pressure. Once fully pressurized, as seen when the bubbles inside the bottle are very small and the bottle makes a click noise as it slides up the nipple and presses against the collar, the launch button can be pushed. This causes the pneumatic cylinder to retract the collar. Due to the buoyant pressure of the surrounding air, as the pressure leaving the bottle pushes downward, the air pushes back. Since the object under pressure has such a small mass, it propels upward until the bottle is no longer under pressure, and the system has equalized, at which point the forward momentum comes to a halt and gravity overcomes the object due to its mass causing it to fall back down. Since some of the water under pressure turns to vapor upon launch and that vapor suddenly rises, it often condenses and forms a cloud inside the 2-liter bottle.

Questions

1. What do you think will happen to the launch if we do not add any water?

   • Answer: All of the air in the bottle will escape quickly and not allow the bottle to propel more than 10-15 feet in the air.

2. What do you think will happen to the launch if we fill the entire bottle with water?