Water Bottle Rocket Launch

As you can see, the theme this week is explosions and rockets.  Earlier in the week my daughters and I dropped Mentos candy in 2 liter bottles of soda and watched the soda explode out of the bottle.  The next day my older daughter and I launched model rockets.  See my previous Corn Field Walk blog post for full details.  The latest explosion/launch activity required a recycling of the two liter soda bottle from the Soda Explosion experiment.  

Water bottle rockets are fairly easy to make.  You need a bottle (2 liter, 20 ounce, 16 ounce, etc.) filled approximately half full with water.  If you have a way to seal the bottle top and pump air into the bottle, then you have the opportunity for a water bottle rocket launch.  The trick is sealing the cap so air doesn't escape.  There are several methods to do this and if you Google search for "Water Bottle Rockets" or "Water Bottle Launcher" you will find several different designs.  Some assembly is required as you'll see.  I decided to take the easy way out and bought a water bottle rocket launcher off Ebay for about $20, shown below.

Water Bottle Launcher

Fill the bottle 1/2 full

The bicycle pump was not included, so you'll need one of those.  So what launches the rocket?  The answer is air!  Due to the water tight seal, neither air nor water can escape the bottle.  As air is pumped into the bottle via the bicycle pump, the pressure inside the bottle builds up.  If it builds up enough, the bottle will push itself off the seal and fly very high in the air.  The tighter you put the bottle on the seal, the greater the pressure needed to blow it off and the greater the height it will reach.  Here's the video of one of our launches.  The launch was too quick to catch the bottle in the air, but you should get the idea watching the video.

This is a great follow-up experiment to the soda explosion.  Both of my daughters were with me to help out and they got a big kick out of the bottle flying very high in the air.  Although you shouldn't stand too close, as long as you aren't very far away, you'll likely get sprayed from the bottle.  This is very nice on a hot day.

There are several ways to design your rocket.  Some involve one bottle, others two.  As you saw from the video, we simply attached the bottle to the launcher and let it fly.  In terms of aerodynamics it was very poor, but check the web for instructions on building more aerodynamic water bottles by adding fins and cones to make it look more similar to a real rocket.  Have fun!