The "Coney"
Backslider Water Rocket
Please see Ulrich Hornstein's site for more information on Backsliding:

The cone is 2.5 feet long and made from paper twisted into the cone shape. The paper I used was labeled for use as school book covers. It is 15 inches wide and 20 foot long. The cone is simply taped to the bottle with scotch tape.

Reason may indicate that the Center of Gravity is way to far back on this rocket for it to have stable flight. It was discovered years ago that in long skinny rockets the relationship of the Center of Gravity and Center of Lateral Area can be very different than the required one caliber (dimeter) distance of Center of Gravity ahead of Center of Lateral Area.

A fellow name Barroman mathmatically calculated a new point which has become know as the Barrowman Center of Pressure. In long skinny rockets with the Center of Gravity  located behind the Center of Lateral Area, stable flight is still possible as long as the Center of Gravity is ahead of the Barrowman Center of Pressure.

This rocket will fligh well and will maintain vertical flight to apogee. At apogee, rather that nose over and head for the ground, the rocket slides downward tail first. At a point the fins catch the air and put the rocket into a horizontal recovery. This ballance is maintained by the angle of the fin. If the tail is lifted up too much the fins stall and the back of the rocket goes back into the horizontal position until the fins catch enough air to get back into the horizontal glide angle.

It is a beautiful thing to watch and a dependable form of recovery.
This rocket has 4 fins which are almost too much surface area to keep the Center of Lateral Area ahead of the Center of Gravity. When you think about it, another fin or larger fins add little weight, but the additional cross sectional area does dramatically move the Center of Lateral Area backward while the Center of Gravity stays in place. When the Center of Lateral Area gets behind the Center of Gravity you are almost insured a nose down attitude as the rocket progresses through apogee. You may find if you used three fins or smaller fins you would get a smooth recovery with a slightly steeper backward glide angle.
The paper fins used on this rocket are of my own design and you can find instructions on how to make them by clicking on the picture of the fin next to this text.