| A recovery system is used to return a rocket | | | | Two things to consider. You need to bring the |
| to the ground without damage to the rocket or | | | | rocket down slow enough that it presents no |
| objects on the ground. The typical recovery | | | | danger to itself or others. At the same time, |
| mechanism is a parachute. The components of | | | | the longer it takes to get down, the further |
| one rocket's recovery system are shown below. | | | | it will drift in the wind on the way down. |
| | | | |
| Rocket with Recovery System | | | | In most cases you're aiming at a 20 fps or |
| | | | slower vertical terminal velocity. If you fly |
| This rocket uses an altimeter to split the | | | | to 1000' in a 5 mph wind and have a 20 fps |
| rocket at about the midpoint at apogee. A | | | | descent rate, you can expect 367' of |
| small (drogue) parachute is used to stabilize | | | | horizontal drift. You'll get about 734' with |
| the rocket's descent, but not to | | | | a 10 fps descent rate. The longer it takes, |
| significantly slow the rocket. Causing the | | | | the further it drifts. |
| rocket airframe to fall horizontally to the | | | | |
| ground maximizes drag, slowing the rocket. At | | | | Featherweight recovery: The rocket is light |
| a low altitude like 300' or 600' the | | | | enough (less than an ounce typically) that it |
| altimeter causes the main parachute to be | | | | won't do damage when it hits (Estes Quark is |
| deployed from the top section of the rocket. | | | | an example). Or the rocket could have enough |
| The ejection charge for the main parachute is | | | | drag that it's terminal velocity is very low |
| located above the altimeter bay and cannot be | | | | (Estes Snitch). Often times the motor is |
| seen. | | | | ejected to make the rocket unstable too. |
| | | | Ejecting motors is not allowed in NAR |
| The main parachute is stuffed into a Nomex | | | | contests unless a streamer or parachute is |
| cloth deployment bag. The bag protects the | | | | attached to the ejected motor. |
| parachute from the ejection charge. | | | | |
| | | | Break-Apart recovery: Simply breaking the |
| Single Stage Recovery | | | | rocket in the middle and attaching the two |
| | | | sections by a shock cord will work for many |
| Some rockets use only a single (main) | | | | small rockets. They won't come in |
| parachute, which is deployed at apogee. An | | | | streamlined. It would be possible to make |
| ejection charge activated by the motor, or an | | | | large rockets, with very large surface area |
| electronic device like an altimeter or timer | | | | and relatively low weight that would be safe |
| may be used to deploy the parachute. | | | | to recover this way. |
| | | | |
| Two Stage Recovery | | | | Streamer Recovery: The streamer adds drag and |
| | | | slows the rocket. The bigger the streamer, |
| Two stage recovery uses a drogue parachute | | | | the better. Anything over 10 oz will not |
| deployed at apogee and the main parachute | | | | really benefit much from a streamer. NAR |
| deployed at a low altitude like 300' or 600'. | | | | requires 10 square cm of streamer area per |
| This is done to minimize the distance a | | | | gram of mass in contest models. Conversion to |
| rocket drifts. This technique uses an | | | | American units is left as an exercise to the |
| electronic device like an altimeter or timer | | | | reader. Streamers run afoul of the principle |
| to deploy the main parachute, and it also | | | | of diminishing returns when they are |
| usually deploys the drogue parachute. | | | | enlarged. Eventually, adding a bigger |
| Sometimes a streamer is used instead of a | | | | streamer will only add a small bit more drag. |
| drogue parachute. | | | | |
| | | | Parachute Recovery: Using a parachute or |
| If a drogue parachute is deployed at apogee | | | | parasheet for drag. Because of the efficiency |
| and the rocket is suspended from the | | | | of parachutes, this is the most popular way. |
| parachute rather than falling horizontally, a | | | | You get more drag with less cloth than in any |
| larger drogue parachute will be needed than | | | | other way. NAR requires 5 square cm per gram |
| if the airframe falls horizontally, which | | | | of mass. Because of this efficiency they are |
| adds weight. A horizontally falling airframe | | | | used for virtually all high power projects. |
| has maximum drag, which helps slow the | | | | |
| rocket. Therefore a smaller, and lighter, | | | | Helicopter Recovery: Using rigid blades and |
| drogue chute can be used. | | | | auto-rotation to slow terminal velocity. |
| | | | Usually the whole rocket must be designed |
| Parachute Deployment Bag | | | | around this recovery method. This is usually |
| | | | limited to small rockets as the stresses of a |
| Typical deployment bag arrangement Deployment | | | | rapidly spinning rocket touching down are |
| Bag | | | | enormous. I've seen and heard of only 1 J800 |
| | | | powered helicopter recovery rocket. Very |
| A deployment bag is literally a bag into | | | | spectacular and it sustained damage when it |
| which a parachute is packed. A bag can have | | | | touched down. |
| one or more purposes, depending upon how the | | | | |
| recovery system works. The two common | | | | Gliding Recovery: Using lifting aerodynamic |
| purposes in a high power rocket is to protect | | | | surfaces to control the terminal velocity. |
| the parachute from hot ejection charge gases | | | | Since the aerodynamic requirements of |
| and particles, and for orderly deployment. A | | | | vertical flight and gliding flight are |
| common material for deployment bags is Nomex | | | | usually mutually exclusive, there needs to be |
| cloth, which is fire resistant. | | | | some sort of mass shift to allow transition |
| | | | between vertical flight and gliding flight. |
| Orderly deployment means that the parachute's | | | | In addition, since the a glider and a rocket |
| suspension lines are fully extended, and the | | | | are optimized in mutually exclusive ways, all |
| harness is tight before the parachute | | | | gliding rockets represent a compromise |
| inflates. This reduces the opening shock | | | | between these two competing requirements. |
| forces. A large parachute opening force can | | | | Even very large rockets can be glided down. |
| tear a rocket or recovery system apart. | | | | Many folks use radio control to fly their |
| | | | gliding recovery rockets. |
| A pilot parachute (a small parachute) may be | | | | |
| used to pull the deployment bag off the | | | | And this is not necessarily all. You could |
| parachute. Some deployment bags are designed | | | | deploy a lighter than air balloon that slows |
| to serve as its own pilot chute. | | | | the rocket's descent. You could have huge |
| | | | air-brakes deploy from the rocket body. It |
| In all cases the aim of the recovery system | | | | depends on what you want. |
| is to slow the rocket down. All recovery | | | | |
| systems decrease the terminal velocity in | | | | Which one is best? It all depends on the |
| some way, either through the properties of | | | | rocket and what you're trying to do with it. |
| aerodynamic drag or aerodynamic lift. It | | | | For anything over a few ounces, though, |
| might be possible to use buoyancy as well, | | | | parachute recovery is pretty much the |
| but I've never seen it done. | | | | baseline. They are the most efficient for |
| | | | their weight and bulk. |