Water Rockets

“A good Cub program should not be rocket science … but it can involve Rocket Science!”

A bottle rocket is a 2-liter (soft drink) bottle with compressed air and water released in an upward direction. We can use this model to learn many concepts about motion, forces, energy and flight.

Why do bottle rockets fly? The air pressure propels the bottle rocket skyward.

Launch Pad
The launch pad needs to be solid, stable and a distance of at least 5m from the ‘pilot’.

Rubber stopper method: A rubber stopper, with a ball inflation needle passing through, is placed in a bottle and air pumped into the bottle. At a certain pressure the stopper is forced out and the water inside the bottle is able to escape. The main weaknesses of this launching method is that the exact time of release cannot be determined and it is less efficient and does not provide the directional guidance of a launching tube. This is the most simple method when dealing with a large number of rockets, as it keeps the rockets themselves simple.

Pump
This application is punishing: you need a good, heavy duty pump if you plan to repeatedly pump your rockets to 100 psi or higher. The usual wimpy bike pump just can’t take it! You will need a track pump, or a car pump.

Rockets made from typical 2 litre softdrink bottles.

The main part of your rocket is the body or PRESSURE CHAMBER. We will be using a 2 liter bottle for our project. Almost any 2 liter bottle will work, however, some bottles have a mouth or opening (nozzle) that is too small to accommodate the launch tube.

DON’T Cut or puncture the pressure chamber! Don’t use… hot/boiling water, Knives/Razors or Chemical solvents

Peel the label off your bottle and try to clean the glue residue the best that you can. Be careful not to use hot water on the bottle because it might shrink the sides and deform the bottle.

When launching, the tremendous pressure inside the bottle will cause the walls to expand. This expansion leads to a loss of energy and will make for a lack luster performance. To solve this problem pre-wrap the center of the pressure chamber with the silver tape (or duct tape). One and a half times around should do the trick. You don’t want the tape to be too bulky and watch for wrinkles. This will strengthen the walls of the bottle without adding too much mass and launch altitude will increase overall.

Rockets with nose cone from carboard, fins from corrugated plastic card and the silver tape used for joining air-conditioning pipes.

Fins should be firm; If they flop around they are useless. Fins should be securely attached.

Nose Cones
Nose cones and fins are not only for performance but add character and style to your rocket.

The cone is made from poster board (card board). You can make the cone by simply cutting a large circle out of the board (about a 15cm radius). Cut a line from the outer edge of the circle to the center on the radius. Overlap the cut edges and turn the circle while holding one edge stationary until you get the desired cone shape. Secure the cone with staples or tape. Attach to the bottle with tape or similar adhesive.

As it stands thios cone lacks flight stability, this can be fixed to a degree by pressing a small lump of clay/bluetack to the inside of the nose cone. This will add mass to the cone and keep your rocket from flipping end over end while in flight.

Fins
Fins are the guidance system for your rocket. Without them a rocket would tumble end over end. With the incredible speeds and frightful acceleration generated at launch, many fins get ripped off the rocket body within a fraction of a second. We have found that tape (silver, duct, gaffa or cloth) works the best at holding the fins on the rocket as this allows for the flexing of the rocket body.

To ensure stability and safety, the minimum number of fins on a rocket is three (3). Many people choose a 3 or 4 fin design. There is no maximum number of fins you may have but keep in mind that the more fins you have the more drag you will create and drag slows a rocket down. Test the movement of the fins. Your fins shouldn’t move more than a few centimeters from side to side.

Parachute Systems
We don’t use these in our basic rocket, but you may wish to add another compartment and a parachute to yours.

Ensure that people in the launch area are aware of the pending water rocket launch and can see the water rocket’s lift-off before beginning an audible five-second countdown. Do not launch a water rocket so its flight path will carry it against a target.

If a water rocket suffers a misfire, do not allow anyone to approach it or the launcher until you have made certain that the pressure has been relieved (preferably from a remote release point).

Add Fuel:
Fill the rocket approximately 40% with water. This will vary due to different techniques, too heavy or too light will produce less flight.

This kind of rocket launcher is made by simply stuffing a rubber stopper in the neck of the bottle, inserting an inflation needle through a hole in the stopper, and pumping it up until it blows!

Launch Pad
The launch pad needs to be solid, stable and a distance of at least 5m from the ‘pilot’.

Rubber stopper method: A rubber stopper, with a ball inflation needle passing through, is placed in a bottle and air pumped into the bottle. At a certain pressure the stopper is forced out and the water inside the bottle is able to escape. The main weaknesses of this launching method is that the exact time of release cannot be determined and it is less efficient and does not provide the directional guidance of a launching tube. This is the most simple method when dealing with a large number of rockets, as it keeps the rockets themselves simple.

Pump
This application is punishing: you need a good, heavy duty pump if you plan to repeatedly pump your rockets to 100 psi or higher. The usual wimpy bike pump just can’t take it! You will need a track pump, or a car pump.

All energy imparted to the rocket must originate from the water/air pressure combination.

A Track pump is a good way to get this up to pressure, using nothing more potent than simple tap water. All you have to do is partially fill the rocket body with water, then pump the track pump quite a few timess. The compressed air and water provides the power to send the Rocket flying by jet propulsion!

A 12v car pump is also pretty handy, but not as much fun for the Cubs as actually pressurising and launching manually.

The force acting on a rocket, called its thrust, is equal to the mass ejected (in this case, water and air) per second times its velocity as it is expelled through the nozzle. This force can be understood in terms of Newton’s third law of motion, which states that, “for every action there is an equal and opposite reaction.” In the case of a water rocket, the action is the backward-streaming flow of water and air and the reaction is the forward motion of the rocket. Another way of understanding rocket propulsion is to realize that tremendous pressure is exerted on the walls of the bottle (or other pressure vessel) except where the water and air exit at the rear; the resulting unbalanced force on the front interior wall of the chamber pushes the rocket forward.

Water rockets should be flown outdoors in an open area, away from neighbouring property and hazards such as power lines. Typically, water-rockets reach 30 to 50 metres and without a recovery device, return to earth quite swiftly. Access to the launch and landing area should be controlled for obvious safety reasons. If used properly, water-rockets pose no hazard to anyone.

Safety

Safety rule number 1: Never get in the way of one of these rockets…
The rockets made from these bottles are surprisingly powerful. A standard 2-liter bottle 1/3 full of water, pumped to 80 psi and then released, will eject all its water in less than one-tenth of a second, and at that point (“burnout”) will be only about 2 meters off the ground. Amazingly, its velocity at burnout is around 76 meters per second. That’s over 170 miles per hour! This means the average acceleration during thrust is 111 g’s! WOW!!

Safety rule number 2: Keep your distance when the rocket is under pressure…
A pressurized bottle can be a dangerous explosive. Always keep at least 5 meters away from any bottle filled with pressurized air, and insist that everyone else also keep 20 meters away from the bottle. If something goes wrong with the launch and the bottle remains filled and won’t launch, empty the bottle by disconnecting the hose from the pump before going anywhere near the bottle. At 150 psi pressure, the bottle has the explosive power of approximately 0.6 grams of TNT.

Always wear safety goggles/glasses if you are working closer to the pressurised bottle.

Safety rule number 3: Use a suitable bottle …
Use only PET plastic bottles that originally contained carbonated drinks. Never use bottles that didn’t, as PET water packaging is not designed to hold pressure. Never use metal or glass bottles, as there is the danger of shattering and shrapnel from pressurizing and from touchdown impact. Never use or re-use a bottle that appears damaged in any way. Never use a bottle that has visible stretch marks from previous use. Never re-use a bottle that has been filled to the stretch-failure point.

Safety rule number 4: Rockets need space …
Launch only in unpopulated areas where there is at least 150 feet clearance on all sides of launch site.

* Point the rocket straight up only. Do not aim it at an angle or to the side.

Safety rule number 5: Rockets don’t like wind …
Launch your rocket on a wind free day. Wind can have an adverse effect on the flight characteristics of your rocket. Flight may be unstable or your rocket may be blown into undesirable locations.

Links

How Stuff Works :: How Rocket Engines Work
http://science.howstuffworks.com/rocket1.htm

Water rocket book::
This booklet is written as an introduction to the essential concepts of water rocketry.
http://bradcalv.customer.netspace.net.au/wrbook.htm

N.E.R.D.S, Inc – Launch Pads
(Nebraska Educators Really Doinf Science)
http://www.nerdsinc.com/products.aspx?cat=0

Clifford Heath’s Water Rocket Links:
http://homes.managesoft.com.au/~cjh/rockets/links.html

Water Rocket Safety
http://www.hannibal.cnyric.org/TeacherWebs/Rjones/rockets/Safety.htm

Water Rocket Design Project
This document is provided for use by members of INTAD.
http://www.intad.asn.au/documents/store/Water_Rocket_INTAD.pdf [PDF]

Joseph M. Prusa
Hydrodynamics of a Water Rocket
SIAM REVIEW Vol. 42, No. 4, pp. 719–726
http://epubs.siam.org/sam-bin/getfile/SIREV/articles/34822.pdf [PDF]
Now that’s technical !!