|
A recovery system is used to return a rocket to the ground without damage to the rocket or objects on the ground. The typical recovery mechanism is a parachute. The components of one rocket's recovery system are shown below.
This rocket uses an altimeter to split the rocket at about the midpoint at apogee. A small (drogue) parachute is used to stabilize the rocket's descent, but not to significantly slow the rocket. Causing the rocket airframe to fall horizontally to the ground maximizes drag, slowing the rocket. At a low altitude like 300' or 600' the altimeter causes the main parachute to be deployed from the top section of the rocket. The ejection charge for the main parachute is located above the altimeter bay and cannot be seen.
The main parachute is stuffed into a Nomex cloth deployment bag. The bag protects the parachute from the ejection charge.
Some rockets use only a single (main) parachute, which is deployed at apogee. An ejection charge activated by the motor, or an electronic device like an altimeter or timer may be used to deploy the parachute.
Two stage recovery uses a drogue parachute deployed at apogee and the main parachute deployed at a low altitude like 300' or 600'. This is done to minimize the distance a rocket drifts. This technique uses an electronic device like an altimeter or timer to deploy the main parachute, and it also usually deploys the drogue parachute. Sometimes a streamer is used instead of a drogue parachute.
If a drogue parachute is deployed at apogee and the rocket is suspended from the parachute rather than falling horizontally, a larger drogue parachute will be needed than if the airframe falls horizontally, which adds weight. A horizontally falling airframe has maximum drag, which helps slow the rocket. Therefore a smaller, and lighter, drogue chute can be used.
A deployment bag is literally a bag into which a parachute is packed. A bag can have one or more purposes, depending upon how the recovery system works. The two common purposes in a high power rocket is to protect the parachute from hot ejection charge gases and particles, and for orderly deployment. A common material for deployment bags is Nomex cloth, which is fire resistant.
Orderly deployment means that the parachute's suspension lines are fully extended, and the harness is tight before the parachute inflates. This reduces the opening shock forces. A large parachute opening force can tear a rocket or recovery system apart.
A pilot parachute (a small parachute) may be used to pull the deployment bag off the parachute. Some deployment bags are designed to serve as its own pilot chute.
Submitted by Dean Roth
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 parasheet 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 the 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
|
