Unlike the coefficient of drag for the rocket, the coefficient of
drag for the parachute should be as high as you can make it
with the pros and cons being: increasing the area; using a
design that has a high coefficient of drag; weight; cost;
reliability; and, so on.
Typical Coefficients of drag are:
- Circular = 0.90
- X-Form = 0.32
In addition to the parachutes above, you can also use:
parafoil chutes, ribbons ... whatever takes your fancy.
The idea behind using a parachute is normally to slow down
the rocket before it hits the ground so that landing does the
rocket as little damage as possible on impact. Other ways of
doing this include: having nose cone that is soft and springy
such as a bouncy ball; and, using another, smaller water
rocket mounted so that it is set off when the ground gets
close - a retro-rocket.