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