What is a recovery system?

A recovery system is a feature of a rocket that allows it to come back to Earth with minimal damage.

Introduction

This guide is intended to serve as a starting point for water rocketeers wanting to add a recovery system to their water rocket. Having a sound understanding of what a recovery system is, being aware of counter-intuitive concepts and drawing on the experience of many water rocketeers will help in the design and construction of their own recovery systems.

This guide does not provide step-by-step instructions on how to attach a parachute to a water rocket. It provides the background information you will need in deciding which combination of recovery system features will suit your needs. At the end of the guide we provide links to examples of the various recovery systems developed by rocketeers over the years. And yes some of these included step by step procedures on how to attach a parachute to a water rocket.

We rate some of the recovery components in order to make it easier for the individual to decide which components they wish to use in their recovery systems. The ratings are purely subjective and change over time. These include:

Classes of recovery system

There are two main classes of recovery systems: passive and active. Passive recovery systems have no moving parts and are a part of the rocket design. Active recovery systems typically contain moving parts that activate at some point in flight to slow the rocket down.

Passive Recovery

In general passive recovery systems are simple, inexpensive and reliable, however, they are usually only suitable for lightweight rockets without fragile payloads.

Passive recovery systems include:

Recovery Technique	Advantages	Disadvantages
Lawndart The rocket goes straight up and comes down nose first at high speed.	- Simple - Lightweight - Inexpensive - Reliable	- High speed return - Only for small rockets - Dangerous - Unsuitable for fragile payloads
Backgliding / Backsliding The rocket is marginally stable. Rocket goes up straight but comes down sideways.	- Simple - Lightweight - Inexpensive - Reliable	- Limited rocket design - Not suitable for heavy rockets
Tumble The rocket is designed to be unstable. Generally used for small boosters in multi stage rockets.	- Simple - Lightweight - Inexpensive - Reliable	- Cannot fly by themselves - Only suitable for small boosters

Examples of Passive recovery systems:

Active Recovery

Active recovery systems offer flexible design that can be configured and adjusted for a variety of flight profiles and payloads. They are also typically used in large rockets. This, however, comes at the cost of complexity, expense and reduced reliability.

Examples of active recovery systems include: 

Recovery Technique	Advantages	Disadvantages
Parachute The rocket uses a parachute to increase drag to slow its descent.	- Lightweight - Compact - Suitable for large rockets	- Tangles - Problematic high speed deploys - Drift
Streamer The rocket uses a ribbon instead of a parachute to create drag.	- Lightweight - Compact - Low drift	- Suitable only for small rockets - Higher speed descent
Helicopter A set of blades are deployed that spin up the rocket up and the lift generated by the blades slows the rocket down.	- Compact - No tangles	- Complex - Moderately heavy - Unsuitable for descent footage from camera
Moving Cg The rocket redistributes its weight to cause the rocket to use backsliding or tumble recovery. Typically the weight would move towards the Cp.	- Compact	- Complex - Suitable for light rockets only
Moving Cp The rocket changes its Cp so that it uses tumble or back sliding recovery. Typically it would move towards the Cg.	- Lightweight	- Complex - Suitable for light rockets only
Retro The rocket fires a retro rocket (air/air & water) to slow itself just before impacting with the ground.	- Minimal drift - Minimal air time	- Heavy - Difficult timing - Unsuitable for larger rockets
Glide The rocket is equipped with wings that generate lift and the rocket glides to a soft landing.	- Long flight duration - Landing accuracy	- High drag on takeoff - Lift needs to change in flight - Complex
Separation The rocket separates into a number of parts each of which can use any other of the passive or active recovery systems.	- Lightweight	- Complex - Suitable for light rockets only
Balloon The rocket inflates a balloon to either increase drag or when combined with a lighter-than-air gas, produce lift.	- Potentially very long duration flight time	- Complex - Suitable for lightweight rockets only

Active Recovery Phases

While passive recovery is simple there are phases that typically take place in active recovery systems. 

Pre-flight stowage; Arming; Start; Monitoring; Activation; Deployment; and Post-flight maintenance.

1. Pre-flight stowage

This includes the folding of parachutes and placing them in their compartments, rolling up streamers, cleaning out chemical reservoirs, etc. These are tasks that need to be performed before each launch. This phase also includes the process of placing the rocket on the launch pad and how to safe the recovery system during this procedure.

2. Arming

This phase is usually performed just prior to launch and typically prior to pressurisation. This involves priming the recovery system for activation. The rocket remains in this state until launch. For example timers are initialised or wound up, chemical reactants are loaded, air flaps latched etc.

3. Start

Start occurs when some event during or just prior to the flight takes place that causes the recovery system to enter the monitoring phase. A start can occur in different phases of flight. Some common ones include:

Mechanism	Notes / Examples	Rating
Launch Detect	This event happens when the rocket starts to accelerate. G-switches, small air flaps, or strings attached to launchers all are examples of launch detect triggers.
Small air flap	A small flap is typically primed in a perpendicular position to the main axis of the rocket. As the rocket accelerates on takeoff the air pressure on the small flap causes it to drop down against the body of the rocket causing the recovery mechanism to enter the monitoring phase. Examples: 1 , 2 , 6 , 38 ,  44 , 51 , 60 ,  98 ,
Small weight	A small weight is suspended on the rocket that prevents the recovery mechanism from starting. The inertia of the hanging weight is enough to activate the system when the rocket is launched. Sometimes the weight remains on the rocket and sometimes it is mounted externally and falls off. Examples:  82 , 88 , 5 ,
String attached to launcher	A string is typically attached to the launcher or an object on the ground so that when the rocket launches the string activates the mechanism usually by releasing something on the recovery mechanism. Examples: 8 ,  9 , 48 ,  53 ,  65 , 97 ,
G-switch	Typically an electronic component that senses acceleration and causes electrical contacts to open or close. Examples:  66 , 67 , 69 , 70 , 90 , 91 , 101 , 103 ,
Accelerometer	An electronic sensor that can measure the amount of acceleration and output that for further processing. The system can detect positive acceleration on launch. Examples:  49 ,
Pre-burnout detect	This event can happen prior to the rocket running out of thrust. For example internal pressure change.
Accelerometer	An electronic sensor that can measure the amount of acceleration and output that for further processing. Because acceleration can be monitored throughout the entire flight, when a certain acceleration value or an integration of the acceleration data is achieved the system can activate other components.
Pressure based	A pressure sensor/pressure switch whether mechanical or electronic can determine the internal pressure of the pressure chamber and can generate a signal when the internal pressure drops to some predetermined value above atmospheric pressure.
Burnout detect	This event happens when the rocket starts slowing down when it stops producing thrust.
Pressure based	A pressure sensor/pressure switch whether mechanical or electronic can determine the internal pressure of the pressure chamber and can generate a signal when the internal pressure drops to atmospheric pressure. Examples: 14 , 18 ,
-ve G activation	A mechanical, electronic or chemical switch that activates as the rocket undergoes negative acceleration at burnout. An example of this is an inverted mercury switch. Examples:  80 , 94 , 95 , 107
Remote	A remote signal is sent to the rocket at the same time as the launch
RF remote	NOTE: This is not the same as remotely activating the parachute. (See below)

4. Monitoring

After the system is started the recovery system enters the monitoring phase. Monitoring involves the recovery system monitoring some parameter(s) for change or exceed some preset threshold value. When this happens the recovery system enters the activation phase. Monitoring can be electronic, chemical or purely mechanical. There are numerous trade-offs between the various monitoring approaches. These mechanisms typically fall into one or more of the following groups:

Monitoring mechanisms initiate the deployment sequence.

Mechanism	Notes / Examples	Rating
Timer Based
Mechanical timer   (Tomy Timer)	A purely mechanical timer takes a certain amount of time after the start to activate the deployment sequence. Examples:5 , 15 , 41 , 2 , 44 , 48 ,  54 ,  59 , 61 , 28 , 65 , 81 , 84 , 87 , 88 , 89 , 97 , 105
Electronic timer	An electronic version of the mechanical timer.   Examples:9 , 13 , 24 , 42 , 11 , 43 ,  23 , 53 , 66 , 67 , 69 , 72 , 90 , 101 , 103 ,
Chemical	Once started, a chemical reaction can take a certain amount of time to activate the deployment mechanism. Examples: 4 , 16 , 17 , 80 , 94 , 95 ,
Pneumatic/hydraulic	Such as a small balloon deflating at a certain rate, or a water reservoir emptying at a certain rate. Examples: 3 ,  75 ,
Environment sensing
Speed/Air flap	An absolute air speed based sensor that relies on the airspeed to fall below a certain threshold. Examples: 39 , 20 ,  40 , 51 , 56 , 60 ,  82 ,  85 , 98 ,
Accelerometer	The rocket's acceleration is monitored either mechanically or electronically to detect apogee. Examples:  49 ,
Drag based   (Gravity Based)	This is sometimes referred to as "gravity based" deployment for the wrong reasons (see Counter-intuitive section) The sensor can measure the difference in force of a body in free fall without drag (inside the rocket) and drag exerted on the rocket due to air resistance. Examples:  29 , 72 , 79 , 100 , 107
Barometric	The rocket can measure its altitude based on relative or absolute atmospheric pressure. These are usually electronic sensors. These were originally developed for pyro rockets. Examples: 12 , 73 , 91 ,
Magnetic	This sensor uses the Earth's magnetic field to determine when the rocket has tipped over at apogee. Examples: 92 , 93 ,
Light measurement	This sensor measures the change in light difference between earth and sky, triggering when the two reverse as the rocket pitches over at apogee. Examples:  70 , 71 ,
Air speed sensing	Measures the speed of air rushing past the rocket through propellers or other sensors extended into the air stream. This can be used to measure relative or absolute air speed.
Active Ranging, IR /   sonar / laser etc	The rocket can measure its altitude by emitting a signal and reflecting the signal off the ground.
GPS	Use of the GPS network to measure altitude.
Sound sensing of air   moving past rocket	A microphone can measure the noise the air makes as it rushes past the rocket. As the rocket slows down near apogee the measured volume decreases and starts increasing again as the rocket accelerates towards the ground. This change in volume can be used to activate the deployment mechanism.
Sound using Doppler	A constant sound source emitted from the ground can be detected by the rocket and a change in the Doppler effect can be detected by the rocket as it passes through apogee.
NOAA / NSA	Nosecone-off-at-apogee. The nosecone is loosely attached to the rocket with a parachute or streamer under it. Under ideal conditions it is suppose to fall off as it approaches apogee deploying the parachute. Examples: 77 ,  102 ,
Remote activation
RF	A radio signal is sent to the rocket to activate the recovery system. Other than the typical remote model remote control, this includes mobile phones, CBs, etc. Examples: 10 , 12 , 19 , 50 , 58 , 62 , 68 , 104 ,
Visible / IR	An IR or visible light signal is flashed at the rocket from the ground to activate the recovery system.
Tether	A tether is connected to the rocket from the ground to activate the recovery system when it reaches a certain altitude.
Audio	An audio signal is sent from the ground to the rocket to activate the recovery system.

5. Activation

Activation is the recovery system’s response to when the monitored parameter(s) exceeds the preset value. This most commonly is the act of releasing a latching mechanism.

Mechanism	Notes / Examples	Rating
Rubber band	The rubber band is typically wrapped around the body, nosecone or parachute door holding it in place until it is released. Examples:  29 , 39 , 40 , 41 , 2 , 44 ,  54 , 60 ,  79 , 81 , 82 , 84 , 87 , 88 , 89 , 97 , 98 , 105, 107
RC Servo motor	An RC servo motor is usually activated by onboard electronics to release the recovery system by pulling a pin or similar. Examples: 11 , 42 , 7 , 9 , 43 , 26 , 27 , 50 , 53 , 58 , 62 ,  10 , 19 , 68 , 71 , 90 , 91 , 93 , 101 , 103 , 104 ,
Solenoid	A solenoid is used to activate the recovery system via onboard electronics usually in the form of a pulse or a discharge from a capacitor. The solenoid activates a pin similar to a servo motor. Examples: 67 , 13 , 22 , 47 , 22 , 69 , 70 ,
Mechanical armature	This refers to all mechanical arrangements that involve levers, strings and the like. Examples:  3 , 48 ,  51 ,  56 , 61 , 94 , 99 , 100 ,
Nitinol wire	Also known as muscle wire. This wire contracts when electricity is passed through it. It is typically used to pull a pin to release the recovery system. Examples:  49 ,
Pneumatic	Pressurised air is used in combination with a piston, balloon or bellows arrangement to activate the latch on the recovery system. Examples: 3 ,  16 ,  17 , 64 , 72 , 75 , 95 ,
Pyrotechnic	A pyrotecnic charge is ignited, or a wire is heated to activate the recovery system. Examples: 23 , 72 , 91 , 92 , 93 ,

6. Deployment

Deployment is the actual mechanical operation of the recovery system in slowing down the rocket. This also includes things like opening doors, pushing parachutes out with springs, releasing helicopter blades, moving weights, separating the fuselage etc.

	Notes / Examples	Rating
Parachutes	By far the most common recovery system. The parachute is either deployed in-line with the rocket or uses side deployment. Separated below into two separate sections.
Side deployment	The parachute is ejected out the side of the rocket. Usually some kind of spring loaded mechanism helps push the parachute out. Examples:  9 , 41 , 42 , 3 , 2 , 43 , 44 , 53 , 54 , 56 , 61 , 63 , 12 , 65 , 81 , 84 , 87 , 88 , 89 , 95 , 97 , 98 , 103 , 107
Inline deployment	The parachute is ejected along the axis of the rocket. Usually the nosecone separates from the main body of the rocket and the parachute falls out. Sometimes the parachute is also ejected from inside of the rocket using a spring loaded mechanism. Examples:  28 ,  29 , 39 , 40 , 7 , 11 , 16 , 17 , 22 , 23 , 26 , 27 , 49 , 50 , 10 , 64 , 19 , 25 , 67 , 68 , 69 , 70 , 71 , 72 , 77 , 79 , 80 , 82 , 102 ,
Streamer	A streamer is typically deployed in very similar ways to parachutes. Examples: 96 ,
Wings / Glider	Wings are typically fixed to the rocket and may either have active remote control or passive control of the air surfaces. Examples: 48 , 57 , 58 , 59 , 60 , 62 , 104 ,
Helicopter	The helicopter blades are usually folded against the body and spring loaded to open when deployed. The entire rocket usually spins on the way down. Examples:  51 , 85 ,
Changing CG	Changing the Center of Gravity (CG) usually involves moving a weight on the rocket, or discarding a weight from the rocket in order to affect its flight stability.
Changing CP	Changing the Center of Pressure (CP) usually involves moving air surfaces in order to affect the rockets flight stability. Examples: 94 , 100 ,
Retro rocket	The rocket contains a small air/water rocket pointed in the opposite direction of travel that provides thrust in order to slow the rocket down. This is usually activated in very close proximity to the ground. Examples: 99 ,
Balloon	A balloon can be inflated from an internally stored pressure chamber in order to increase the rocket's drag or when used with lighter than air gas to slow the rocket via increased buoyancy.

7. Post flight maintenance

After landing this phase includes tasks such as turning off powered electronics to conserve batteries, replacing or fixing components that may have been lost, damaged or consumed.

There are numerous designs for each of the above phases. Some designs are popular while others are experimental or theoretical. Typically a rocketeer will choose a particular design aspect for each of the above phases. Obviously not every combination makes sense, but there are many valid combinations that will work.

Counter-intuitive concepts

Following are a number of concepts that many rocketeers base their first recovery systems on only to be disappointed with less than ideal results. Sometimes these methods appear to work, but usually due to luck rather than based on good design. This does not mean the concepts should not be experimented with, but in general attempts are abandoned.

Assumed rocket behaviour at apogee

Relying on the rocket to go straight up and start to come down backwards at apogee.  This technique is often considered for "catching" the air under nosecone flaps in order to remove the nosecone. Another form of this approach is draping the parachute over the nosecone, hoping that as the rocket comes down backwards the parachute will open.

In real life this happens very rarely and generally all rockets fly in an arc, keeping a positive airflow over the rocket at all times.

Deployment at burnout

In this technique the recovery system is deployed when the rocket stops producing thrust. The aspect that is overlooked in this instance is the long coast phase to apogee that follows burnout. The rocket generally experiences the highest velocity at burnout so deploying a recovery system at this point is not recommended. Parachutes may be ripped from the rocket.

Gravity based deployment

This is the most common first design that rocketeers attempt because it looks deceptively simple and works well while testing it on the ground. The general incarnation of this design is a form of "hanging weight" attached to the latching mechanism. It is incorrectly assumed that the hanging weight will keep pointing down for the entire flight. When the rocket pitches over at apogee the weight is supposed to turn in relation to the rocket to activate the latch.

In practice there are two problems with this technique.

1. When the rocket stops producing thrust soon after takeoff, drag on the rocket will induce a substantial -ve acceleration on the rocket, and the hanging weight will want to hang upwards, deploying the parachute. This is the most common failure mode of these systems. Sometimes it may take a second or two for a parachute to fully deploy if it is released at burnout and depending on how the parachute is packed, the system may appear to work, but for the wrong reasons.

2. After the initial -ve Gs of the burnout the rocket and all internal components are essentially in free fall. (weightless) This free fall state continues from burn out through apogee all the way to the ground.  The only force acting on the system is drag and vibration from buffeting. Gravity does not magically only act on the hanging weight at apogee. The relative force between the rocket body (due to drag) and the weight near apogee is extremely small. What ends up happening is that there will be little movement between the weight and the rocket body.

Variations on this theme include a mercury tilt switch.

Recovery System Examples

The following links provide examples of water rocket recovery systems developed over the years by many water rocket enthusiasts. This list continues to evolve as more sources are found. Some of these references point to websites that may require registration, however, the registration is free.

The examples presented here are not necessarily by the original developers of the recovery mechanisms, some of the original designs are no longer available on the net while in other cases it is difficult to determine who developed a technique first.

Forum/YouTube nicknames are shown in italics. If you are the owner of the resources linked to below and would like to have your name or link corrected please fill in the form below.

If you discover broken links please let us know and we will attempt to fix them.

Passive Recovery
 

Active Recovery
 

Other recovery system indexes:

Good overview of various recovery systems by Paul Grosse:  http://www.paul.grosse.is-a-geek.com/h2orrecindex.htm