How to Build a Water Rocket
Host Kevin O'Connor and a middle school teacher show some eager students how to build a model rocket launcher using commonly available household and hardware store items.
Water rockets are easy to build and fun to play with. Rockets demonstrate science concepts such as air pressure and equal and opposite reactions. There are many good water rocket web sites with various levels of complexity. This is a simple water rocket plan.
Safety is very important with any rocket. Rockets are safe when everyone understands and abides by safe behavior. Children need to be closely supervised when they are using rockets. Even if they understand and agree to the safety rules there will be lapses in concentration or judgment. Children cannot be made responsible for the safety of others. A child may feel it is enough to tell a two-year old to stay out of the way.
Water rockets should never be pointed at another creature or object. Water rockets take off with force from the air pressure and weight from the water. This simple water rocket with a rubber stopper does not blastoff with as much force as more elaborate water rockets that have release mechanisms but I have not been willing to intercept one myself to see how bad the damage would be. After the bottle has lost water it is light and safer. Be careful at blastoff time and do not walk in front of the launcher. It would be fine to station people out in the field to catch the bottles as they fall.
At times the rocket will not take off and will spray water around the rubber stopper. The bottle is pressurized and will blast off when you touch it. Be careful and do not let it hit you. This is the situation that kids need to understand or they will stand over it and get hurt.
2-liter bottles can weaken and explode. This is less of a concern for rubber stopper bottle rockets because the pressure is not high before the rubber stopper gives way. We have never seen a bottle explode with the rubber stopper rockets. Safety glasses should be worn for higher pressure water rockets.
Bicycle pump: A pump with a pressure gauge is nice but any bike pump will do.
Valve stem: The valve stem prevents the water from going into the bicycle pump. (Sources: Automobile tire centers remove these when they dispose of tires and can give you some. Bike shops may have old inner-tubes with valves.)
Plastic tubing - inside diameter 1/4". 1 to 6 feet. (Sources: Sears Hardware, American Science and Surplus, Home brew shops.)
Flexible copper tubing - 1/4" diameter. 1 to 2 inches. (Sources: Plumbing or Hardware stores.)
No. 3 black rubber stopper, 1-hole or plain. (Sources: Sears Hardware, American Science and Surplus.) The black rubber stoppers are softer than other stoppers and stay in longer.
Waterproof glue such as epoxy or contact adhesive (we used a Sportsman's GOOP brand)
Hacksaw or pipe-cutter
Utility scissors or knife
Build the water rocket:
Children will need some assistance.
If the rubber stopper does not have a hole drill an approximately 1/4" hole.
Cut the copper tubing with a hack saw or pipe cutter to a length of 1 or 2 inches.
Put glue around the copper tube near the end but make sure it does not clog the opening. Fit the copper tube into the rubber stopper hole in the fat end of the rubber stopper. Work it in by twisting it and rocking it back and forth.
The plastic tubing gives the operator some distance from the rocket. This adds safety and keeps people dry. Put glue around the other end of the copper tube. Fit the plastic tubing over the copper tube and work it up to the bottom of the rubber stopper.
Adults should do this part. Trim the rubber on the valve stem to fit the plastic tube. Tough shears such as linoleum cutters work. It can also be trimmed with a knife. Put glue around the trimmed end of the valve stem keeping the opening clear.
Fit the valve stem into the plastic tube.
Let the glue dry overnight.
Build a launch pad:
Launchers can be as simple as a couple of bricks. The rocket goes farther if it has something solid to push against (this may help the rubber stopper stay in). Our launchers were two pieces of wood connected with a hinge. This allowed us to easily adjust the angle of the launch. We then cut another bottle or waxed box so that the mouth of the bottle would not get snagged and glued or screwed it onto the board.
Launching the rocket:
Fill the bottle about 1/3 full of water. Fit the rubber stopper firmly into the bottle. Place the rocket in the launcher. Attach the bike pump to the valve stem. Put on the safety glasses. Make sure everyone is clear and the rocket is aimed safely. Pump! If the rocket does not blast off be careful. The rocket is pressurized and will take off when you jiggle it.
Try the rocket with different amounts of water and air. A rocket with just air does not go far. Add a cone to the nose of the rocket and see if it goes further.
Rockets demonstrate Newton's Laws of Motion.
1. Objects at rest will stay at rest and objects in motion will stay in motion in a straight line unless acted upon by an unbalanced force.
The rocket rests on the ground. Gravity forces the rocket down toward the center of the earth. The earth holds the rocket up and balances the gravitational force.
The bicycle pump compresses air in the bottle. The compressed air pushes against the sides of the soda bottle and water. Finally the rubber stopper is pushed out and the air and water can escape from the opening.
Air pressure in the bottle is unbalanced when the rubber stopper gives way . The pressure against the side walls cancel each other out. Pressure is released through the opening of the bottle when the stopper pops out. The pressure at the top of the bottle is unopposed and pushes the bottle up. Another site (I will credit if I can find it again) had a very good analogy of unbalanced forces and escaping pressure. Imagine a herd of excited (under pressure or hot) children (molecules) in a box that is missing one wall. When a child bounces off a side wall the box moves to that side. Another child bounces off the opposite side wall and moves the box back where it started. Children leave the box through the opening rather than bounce against the wall. The force against the opposite wall is unbalanced and the box moves away from the open wall.
2. Force is equal to mass times acceleration.
3. For every action there is always an opposite and equal reaction.
Water adds mass to the rocket. Without water the rocket does not go far. The momentum of the water (mass and velocity) expelled from the rocket causes and equal and opposite thrust in the direction the rocket is pointed. Thrust is force at takeoff and depends upon the speed and the mass of the water escaping the bottle. Higher air pressure causes greater thrust.
At blastoff the rocket has maximum force but is not moving very fast. The rocket accelerates as the water escapes.
Transfer of momentum can be visualized as people on ice skates (this eliminates friction as a factor). One of them is a bully and likes to push other people. If the bully pushes someone lighter, the bully will slide backward some but the victim slides forward farther and faster. If the bully picks on someone her own size she moves back the same amount that the victim moves forward. If the bully picks on someone larger (showing a lack of brains as well as manners) she moves back farther and faster than her victim moves forward.
Rockets demonstrate Aerodynamics.
Drag is the air resistance a rocket experiences. Adding a nose cone can reduce drag. When geese fly in a 'V' formation the lead goose does more work then the geese behind it. A nose cone cuts the air and parts it more efficiently then the flat end of the bottle.