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