The weather is getting nice and we have studied Force and Motion. It’s water rocket season! This pro          The GOAL:  build a rocket out of a soda bottle that uses only water and air pressure and Make it Fly as far at possible.  OR  create a parachute system and see how long it lasts in the air.

The rocket MUST:

1. use a bottle for a carbonated beverage. No flimsy water bottles. They will explode and give you no launch points. Also, the bottle must have smooth sides- not the COKE/Sprite bottles with the odd shape.

2. use a bottle with a standard 2 liter cap opening to fit our launcher.  Some green bottles and some Coke and Sprite bottles- will not fit the launcher. Bring in any imported bottles to test on the launcher BEFORE building. 

3. NOT be punctured, cut or weakened as it will not pressurize. No pressure…no launch.

4. be SAFE to launch.  Do not use any sharp materials like metal or pointed plastic.  If it isn’t safe, it won’t launch and you’ll lose out

5. only be powered by the water and air pressure provided at the launch site.  

6. have at least three evenly spaced fins for stability.  Fins = stability, but too many fins will just add mass.

7. stay together for the entire flight.

Rocket-building advice and FAQ:

1. What type of bottle should I use for my rocket?

The shape of the bottle near the opening (how tapered it is) will make a difference in the flight of the rocket.   A bottle that slopes gently to the opening will allow for less friction as the pressurized water rushes out.  The easier it is for the water to leave the bottle the greater the energy available to lift the rocket in the air. 

DO NOT use flimsy, thin water bottles. They will explode from the pressure on the launcher.

2. Your bottle needs to fit the launcher

Picture...

We don’t want our rockets to end up hurting someone so we won’t launch any rocket that isn’t safe. Metal fins and sharpened nose cones are some of the most common additions that will keep a rocket grounded.

You’re probably too young to remember or even know about “Jarts” but when I was your age we had a set of lawn darts “for the whole family” that were actually pretty dangerous to be tossing around.

4. What materials should I use to make my rocket?

Many students like to decorate their rockets and that is AWESOME. I love creativity. The problem occurs when the decorations add too much mass or air friction and keep the rocket from doing its thing. These will likely get ripped off the moment your rocket launches.

Fins, nose-cones, parachutes, and line should all be made of a waterproof material. These are WATER rockets

5. Water and air pressure power only

When you are doing research for your design online you may find really neat ways to deploy your parachutes. Some of those are using rubber bands or remote control devices to time the release of the parachute. You are welcome to build these as a side project but to enter our competition the rocket can ONLY use water and air pressure provided on site. We will use an air pump to bring our rockets to somewhere between 80 – 120 psi.

6. What about the fins?

A fin should be sturdy enough to resist the air.  If it is flimsy, the air will push the fin and it will not add any stability to the rocket.  Choosing a waterproof material is important as well.  These are WATER rockets and will get wet.  If the material soaks up water, it will lose integrity and gain in mass.  Both are bad for the rocket.  Corrugated yard sign material (vinyl) or an old folder can often be good choices.  Paper and cardboard should be avoided whenever possible.

The fin shape is mostly a personal choice. 

7. How long should my rocket be?

Depends on the center of your mass...  in #8

8. Where should the “center of mass” of my rocket be?

For your rocket you will want the center of mass to be higher up on the rocket (toward the nose cone).  With more mass at the top it will have more inertia at the top.  It will be more difficult to push off course by accelerating to the left or right during flight.  This means the upward direction of the rocket will be difficult to change.

Adding some mass (in the form of a golf ball or some clay) to the nose cone will also bring the center of balance forward on your rocket.  We don’t want it to be TOO heavy up front but we want the center of mass to be in front of the center of pressure (the middle of your pressurized bottle).

Use some string to balance your empty rocket.  Your rocket should balance more like the pictures on the right.  The center of MASS must be in front of the center of the bottle PRESSURE.

9. No pressure…no launch

DO NOT poke a hole in the soda bottle. DO NOT cut the soda bottle. BE CAREFUL not to weaken the soda bottle with hot glue guns. We will use a bicycle pump to increase the air pressure. If the air escapes through a hole or cut in the bottle it will not pressurize. Without air pressure to push on the water, your rocket won’t launch. FYI…duct tape will not keep air from coming out of the hole you have made.  If you are experimenting with merging two soda bottles together you will need a backup rocket that will certainly launch. The most common error in combining bottles together is that they will not hold the pressure of the pump. FYI…Duct tape will not hold bottles together at 100 psi by itself.

        10. Can I join two or more bottles together?

Yes. You can experiment with merging two or more bottles together but you will want to make certain you have a workable rocket for the competition. It would be a shame to put in all that work and not have a rocket that actually launched. Here is just one tutorial if you would like to try this. It takes several DAYS for the glue to cure however so don’t try this one “last minute”.

11. How do I keep the rocket up in the air longer than 5 seconds?

Parachutes aren’t required but they sure are FUN when they work.

Building a rocket using the above advice will certainly get yo past FIVE seconds in the air. If you’d like to shoot for the record time, you’ll likely need a good parachute.  Plastic bags from the grocery store or trash bags for small cans are both common materials for parachutes.

How to deploy the parachute

There are many ways to convince your parachute to open up mid-flight. These are just a few: The simplest parachute (figure 1) is placed under an attached cup or bottle top that loosely fits the pressurized bottle.  The bottom of the cup or bottle top can be cut into evenly spaced flaps (like a fringe).  The goal is for the flaps to stay flat on the way up and then grab the air on the way down, releasing the parachute.