The weather is getting nice and we have studied Force and Motion. It’s water rocket season! This project ties together your understanding of unbalanced forces, acceleration, and Newton’s Laws of Motion.

The GOAL:

build a rocket out of a soda bottle that uses only water and air pressure and have it last in the air with a parachute for as long as possible.

The rocket MUST:

1. use a bottle for a carbonated beverage. No flimsy water bottles.  They will explode and give you no launch points. 
2. use a bottle with a standard 2 liter cap to fit our launcher.  Some green plastic bottles will not fit the launcher. Bring in any imported bottles to test on the launcher BEFORE building.  They can always be made to fit but it will take some time.
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.  No moving parts, lighter- than- air devices, or stored potential energy of any kind may be used. (this includes but is not limited to propellers, rubber bands, battery powered motors and Helium balloons)
6. have at least three evenly spaced fins for stability.  Fins = stability, but too many fins will just add mass.
7. have your name(s) and hour clearly displayed. You may work by yourself or in pairs.
8. stay in the air for at least 5 seconds. 
9. 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.  Rounded bottles like a Pepsi 2 Liter will still fly well but all other things being equal, a Sprite or Dr. Pepper bottle usually launches a bit smoother. 

DO NOT use flimsy, thin water bottles. They will explode from the pressure on the launcher. SOME Aquafina and Desani waters are packaged in actual Pepsi and Coke bottles. They will be fine. Check with the instructor before you build if you aren’t certain.

2. Your bottle needs to fit the launcher

The launcher we use has three Aluminum prongs that hold on the pressurized bottle in place. The plastic cork MUST be able to fit into your rocket to make a waterproof seal or we will not be able to put air into your rocket for launch.

There are some imported bottles that look like they will fit the launcher and will even fit a typical soda cap but need more work than most to launch. Perrier, for example, has bottles that are a great shape for rockets but the inside diameter of the mouth will not fit the plastic cork of our launcher. Using Perrier will require a significant amount of sanding around the inside of the nozzle to make work.

3. Your rocket must be safe to launch

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. Keep your decorations to a minimum and make sure they don’t stick out away from the rocket. 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 and will get wet. Soaking up water will add more mass to the rocket. Wet paper and cotton materials will often not survive to launch a second time. Scotch tape usually doesn’t stick to wet plastic. Stay waterproof.

Raid the nearest recycle bin for flat pieces of plastic that you can cut through. Some plastics are easier cut with a Dremel tool or even a saw if you have that available. Never cut toward yourself and get permission for those power tools.

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 a bike 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.  Just make certain that ALL fins are cut from the same material and are exactly the same size.

7. How long should my rocket be?

A good estimate is about 1.0 to 2 times as long as the pressurized bottle by itself. That said, the information in #8 is more important in deciding this question for your design.

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

Try balancing a wooden match on your finger…now a wooden meter stick.  Why is the meter stick easier to balance?  A match is so short that even the slightest motion makes a big difference in its center of mass (balance point).  Adding length moves the center of mass.   A meter stick is much easier to balance because the balance point is higher.

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.

Often, students will place the parachute under the nose cone for the trip up into the air.  You’ll need to make sure that all pieces are attached to the body of the rocket.  The parachute must bring the entire rocket back to the ground.

The parachute should be folded in a way that makes it easy to open up. The diagram to the left demonstrates one way a parachute can be folded.

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.

One way is to attach the nose cone so that it falls off to the side (figure 2).  The diagram to the left show a blue folded parachute under a nose cone.  The cone is attached to the body loosely and can easily fall off when given a push.  A flap of plastic is attached (drawn on the right of the rocket) that will stay down on the way up due to air friction and then be lifted up when the rocket starts to fall back down.  The plastic flap help to push the nose cone off, releasing the parachute.

Another common idea is to place small plastic tabs all the way around the rocket so that it holds the nose cone in place on the way up (figure 3).  The nose cone is to be so loosely placed on these tabs that it will easily fall off to the side when the rocket descends.  This will release the attached parachute.  The tabs could be made from other soda bottles or other strong plastic.

12. How much water should I put in my rocket?

It’s a great question. Too much or too little water will produce a rocket with a very small lifting force. You want the sweet-spot. Watch the video below to get an idea for why we put in the correct amount of water.

13. How is the rocket launched?

1. Have your rocket filled and ready to go
2. Take the cap off your bottle (but hold on to it)
3. Lift the rubber “cork” and hose from the middle of the launcher
4. Hold your rocket upside down (so as to not spill), plug the cork into the rocket
5. Feed the hose back through the launcher and lower the rocket into its seat
6. Hold the three Aluminum prongs on the plastic lip of the bottle
7. Gently lock the launcher lever
8. Safety first!  Make certain no one is in the launch circle before pressurizing
9. Pressurize the bottle between 80-120 psi using the bicycle pump
10. GENTLY raise the launch cord over your head to release the rocket
11.  Newton’s Laws take over and the rocket takes off!

All time student water rocket records from class

Rank	Name	year	time
1	Rachel Strutmann	2003	40 seconds !!
2	Gavin Barret and Luke Munson	2025	26 seconds
3	Grant Johnson	2017	25.53 seconds
4	Robbie White	2025	24 seconds
5	Lucas Lovellette	2014	21.55 seconds
6	Kevin McCole	2017	19.32 seconds
7	Evan Kreith	2013	15.52 seconds
8	Matt Purcell / Jack College	2011	14.28 seconds
9	Allyson Hoff	2016	13.39 seconds
10	Ahnika Wegeng	2017	13.34 seconds