2. Students are then given an additional day or 2 to redesign their rocket for a longer flight time. Take them out for day 2-3 for another test flight.
I usually repeat steps 1 and 2 for about 1 to 2 weeks before having the whole class competition described in the attached rubric.
As students are working through their various build sessions, we pause ocasionally to talk about the variables they are changing (independent), the expected outcome (dependent variable), and the records and/or graphs they are keeping on the flight times of their rocket (data collection and repeatable expeirmentation).
NOTE: In order to do this project with your students you will need a water rocket launcher, which uses pressurized air (provided by a bike pump or an air compresser) as the energy source. You can search "water rocket launcher" on Google for plans to build your own (costs around $20) or you can purchase a professional high grade launcher for around $140 from Pitsco Education (http://www.pitsco.com/store/detail.aspx?CategoryID=86&ID=1179&c=2-1|17-2|86-3&t=&l=). I built my own launcher the first year, and it fell apart after continued student use. I wrote a grant and purchased the professional launcher and it has lasted through 7 years of students and classes. Even though it's 7 times the cost, it's well worth the added expense.