In all cases the aim of the recovery system is to slow the rocket down. All recovery systems decrease the terminal velocity in some way, either through the properties of aerodynamic drag or aerodynamic lift. It might be possible to use buoyancy as well, but I’ve never seen it done.
Two things to consider. You need to bring the rocket down slow enough that it presents no danger to itself or others. At the same time, the longer it takes to get down, the further it will drift in the wind on the way down.
In most cases you’re aiming at a 20 fps or slower vertical terminal velocity. If you fly to 1000’ in a 5 mph wind and have a 20 fps descent rate, you can expect 367’ of horizontal drift. You’ll get about 734’ with a 10 fps descent rate. The longer it takes, the further it drifts.
The rocket is light enough (less than an ounce typically) that it won’t do damage when it hits (Estes Quark is an example). Or the rocket could have enough drag that it’s terminal velocity is very low (Estes Snitch). Often times the motor is ejected to make the rocket unstable too. Ejecting motors is not allowed in NAR contests unless a streamer or parachute is attached to the ejected motor.
Simply breaking the rocket in the middle and attaching the two sections by a shock cord will work for many small rockets. They won’t come in streamlined. It would be possible to make large rockets, with very large surface area and relatively low weight that would be safe to recover this way.
The streamer adds drag and slows the rocket. The bigger the streamer, the better. Anything over 10 oz will not really benefit much from a streamer. NAR requires 10 square cm of streamer area per gram of mass in contest models. Conversion to American units is left as an exercise to the reader. Streamers run afoul of the principle of diminishing returns when they are enlarged. Eventually, adding a bigger streamer will only add a small bit more drag.
Using a parachute or parachute for drag. Because of the efficiency of parachutes, this is the most popular way. You get more drag with less cloth than in any other way. NAR requires 5 square cm per gram of mass. Because of this efficiency they are used for virtually all high power projects.
Using rigid blades and auto-rotation to slow terminal velocity. Usually the whole rocket must be designed around this recovery method. This is usually limited to small rockets as the stresses of a rapidly spinning rocket touching down are enormous. I’ve seen and heard of only 1 J800 powered helicopter recovery rocket. Very spectacular and it sustained damage when it touched down.
Using lifting aerodynamic surfaces to control the terminal velocity. Since the aerodynamic requirements of vertical flight and gliding flight are usually mutually exclusive, there needs to be some sort of mass shift to allow transition between vertical flight and gliding flight. In addition, since a glider and a rocket are optimized in mutually exclusive ways, all gliding rockets represent a compromise between these two competing requirements. Even very large rockets can be glided down. Many folks use radio control to fly their gliding recovery rockets.
And this is not necessarily all. You could deploy a lighter than air balloon that slows the rocket’s descent. You could have huge air-brakes deploy from the rocket body. It depends on what you want.
Which one is best?
It all depends on the rocket and what you’re trying to do with it. For anything over a few ounces, though, parachute recovery is pretty much the baseline. They are the most efficient for their weight and bulk.
Submitted by: Dave Urbanek