Post edited 11:18 pm – July 6, 2010 by Luke Maurits
I have posted previously that I observed most sounding rockets use a two-parachute system, and included a diagram and admitted to not quite understanding how it works. I've done some more reading and learned some more about this. The trick was finding out the name that this approach is most often refered to by – a "dual deployment recovery system"/p>
The logic is as follows: apogee is a great time to deploy a parachute because the rocket is moving very slowly compared to any other point in its flight, so the deployment is fairly non-violent and stable. However, if you deploy a large enough chute at apogee to land the rocket at a safe vertical velocity by itself, the descent takes a very long time, and the rocket can drift horizontally a very long way from the launch site, making recovery a pain. So instead one deploys a smaller drogue chute at apogee. This isn't enough to softly land the rocket by itself, but it slows it down enough that one can land safely by deploying a larger main chute at a much lower altitude, closer to the ground. Since the rocket still falls relatively quickly under the drogue chute alone, it doesn't drift too far during the longest part of the descent. Perfectly sensible, and something we should do.
The reason I couldn't understand how the deployment works seems to be that the diagram I copied when making that earlier post had the parachutes arranged in the incorrect order. The standard arrangement seems to have the drogue chute packed above the main chute, i.e. nearer to the nose cone – I can absolutely see how this works.
This leaves the overall composition of the recovery section of the rocket body specified, which is a good move forward.
EDIT: Here's a good diagram: