This article describes a water rocket recovery system that will detect the highest point of the flight, separate the nose cone from the
rocket body and deploy a parachute that will bring the rocket safely down again.
This recovery system is a combination of an optical apogee detector and a high voltage solid state electromagnetic
electromagnetic system described here.
This previous electromagnetic deployment system was based on repulsion of 2 permanent magnets holding the nose cone in place
by discharging a high-voltage flash capacitor through a relay coil. The trigger was provided by a timer circuit
that was started by an inertial launch detector. The time could be set between 4-7 seconds, which was about the time needed
to reach apogee - with my rockets anyway.
Although this system proved to be very fool-proof and successful, there is no fixed relationship between the time between launch and
apogee. This is dependent on rocket shape (drag), launch pressure, rocket weight, reaction mass, etc... Have a look at some of the
excellent water rocket simulators on the internet to understand that there are a lot of parameters influencing maximum height = apogee.
Detection of the apogee height is possible by a number of known techniques:
- Intelligent altimeter: a barometric altimeter that detects it's max value can initiate chute deployment
- Magnetic: when the rocket tips over at apogee, detection of the earth's magnetic field will change polarity
I chose to develop something new that I had in mind for some time already: optical apogee detection.
The starting point is simple: it is safe to say that most launches take place outside.
Most launches are also pointed upward and most launches take place when the weather is nice.
This all leads to the following statment: an optical detector inside the rocket body
pointing up will "see" more light than an optical detector pointing down. The sun is a very bright light source and in
most cases the sky will be brighter than the earth. Exceptions: night launches, water, snow... etc. But let us assume that
in most cases the assumption about 2 photodetectors is true.
When the rocket reaches apogee and the center of gravity is located towards the nose, most rockets will tip over and
come down nose first, aka the infamous lawn-dart. This is typically the moment you start praying for your recovery system to function properly.
Also, just after apogee, the speed of the rocket is very low, so this is ideal to eject a chute with very little risk of being
ripped apart or shock cord breaking,....we've seen it all. One remark here: I always use a rubber band / bungee as a shock cord and
this has never broken yet.
One of the targets of this project was also to keep it as stupid and simple as possible (KISS).
I am convinced that you can build a much better optical apogee detector using a microcontroller interfacing an optical mouse sensor,
as the robotics people do, but the goal here was to keep it accessible for anyone with limited soldering and electronics skills.
So, sit back, relax, grab your favorite regional beer and let's have a look at the simple electronics needed for an optical apogee detection
+ deployment system.
Have a look at the Schematic diagram.
The 2 photodetectors are simply 2 photodiodes. I used the SFH203: Datasheet
A photodiode that is reverse biased will allow a current to flow through the diode that is in linear relationship
with the light falling onto the diode chip. The photodiode housing is mostly transparent, allowing visible light to
reach the embedded chip. So, the more light onto the chip, the more current flowing through the diode.
Since it is easier to compare 2 voltages than 2 currents, the current through the photodiodes is converted to voltage
with 2 resistors. Here, the resistor value is 2.2 MegaOhm, resulting in a voltage between 0 and 6 Volts depending of the
light intensity that is detected. Different photodiodes will result in different currents. The problem is that the
photo diodes have to detect a wide dynamic range of light input (eg a dark rainy day compared to a bright sunny day).
The use of 2 photodiodes that are compared relatively to each other makes the sensor insensitive to absolute light vaues.
It is also possible to use a photoresistor here instead of a photodiode.
One of the photodiodes will be mounted in the rocket looking up (towards the sky at launch). The other is looking down at launch.
This means normally that across the top resistor there will be a higher voltage than across the bottom resistor. The 2 resulting voltages
are now ready to be compared with - surprisingly - a voltage comparator. In this case I used a very common LM393:
Some applications with voltage comparators + optical sensors can be found here and
The LM393 is an 8 pin component with 2 identical voltage comparators on board. Here, only one is used.
The other one is tied to fixed voltages at the inputs to prevent it from oscillating.
At the negative input (pin 2) we have the photodiode looking up at launch generating a higher voltage than the photodiode at the positive
input (pin 3). Negative is higher than positive, this results logically in a low voltage at the output (pin 1).
When the output is low, the LED connected between the output pin and the power supply ( 12 V battery), will conduct and emit light. The green
LED is an indication that the apogee detector is ready and working - to be checked just before launch. I used a high-brightness LED
here which is clearly visible even from 10m away in broad daylight.
The voltage comparator and the photodiodes need of course an on-board power supply.
Here, I used a small 12V battery - type MN21, eg by Duracell:
There are better and even smaller batteries available in the RC world.
However, this battery is light weight and yet affordable.
It is not rechargeable and can only deliver a very small current due to its high internal impedance.
To switch the power supply on/off, a small power switch is added - bottom right of the schematic diagram.
So far, we have built an optical detector that will signal when the rocket tips over at apogee - the LED will go out -
but this won't stop it from continuing its high speed journey towards the ground.
As a recovery system, I used the same electromagnetic repulsion mechanism that I have used very successfully in
my previous recovery system.
A magnet is attached to the iron core of a coil from a small 12V relay. 2 of these magnets hold the nosecone in place.
When a large current flows through the coil, a magnetic field with opposite polarity is generated and the magnet is fired from
the core of the coil. Depending on the force of the magnet and the current + number of windings of the coil, this can be quite
spectacular. To store the energy for the current through the coil, a flash capacitor salvaged from a disposable camera is used.
The flash capacitor is charged prior to launch to a respectable 300V, using a home-made high-voltage (HV) charger, as described
in this page. You can use the entire disposable camera as it is: take out the big flash capacitor
to use inside the rocket and use the rest as the charger: connect a red wire to the PCB of the camera where the + pin of the capacitor
was and a black or green wire to the - pin on the PCB. Add 2 alligator clips to the end of these wires and you have your personal
WARNING: You may get shocked by the charger and/or flash capacitor ! You may even die from this shock or worse: wet your pants and therefore,
the usual disclaimers are in place here: I am in no way responsible for what you are doing. Just be careful while dismantling flash cameras and
handling high-voltage components ! If you don't feel confident about what you are doing, sit back and have a regional beer instead.
The flash capacitor is charged via the 1N4007 diode. This diode has 2 functions:
There are 2 contact points to attach the charger to: a + terminal going via the diode to the + pin of the capacitor and a - terminal
connected to ground. As we will see further on in the construction details, both terminals are also used for fixation of the
PCB inside the rocket head section.
- Assure the right polarity when charging the capacitor - in case you swap the red and black alligator clip from your charger
- Make sure you don't get shocked when touching accidently the charging terminal on the capacitor just before launch
Now that we have the detection electronics and the charged capacitor, all we have to do is to provide an interface between
the low voltage optical detection circuit and the high voltage charge. This is accomplished with the transistor IRF740: Datasheet
This is a 400V power MOSFET. The minimal treshold gate voltage for the device to conduct is between 2 and 4 Volt. The output of our comparator
is 0 V when the rocket is pointing up and 12 V (or whatever the battery voltage is) when the rocket is pointing down. So, we can connect the output of the comparator
directly to the gate of this transistor.
The 2 coils are connected in parallel here. I use 2 coils to have 2 fixation points for the nose cone - with 2 magnets. You may consider
using only one coil and one magnet, depending on your mechanical setup of the nose cone separation. The 1N4007 diode across the coils is to
protect the MOSFET from overvoltage spikes. There is already a protection diode inside the transistor housing, so you may consider omitting
this diode. I soldered a diode between the 2 legs of each coil, just to hold them mechanically in place.
The relay coils are very delicate devices and whatever you move or touch may break the coil wire.
This concludes the description of the electronics of this recovery system. Have another beer. Up to the construction details.
These are the construction steps I followed to build a prototype. I'm not a mechanical genius, so feel free to do things differently.
I didn't have a Dremel tool either.
The way I interface the electronics compartment to the rocket is with a bottle cap, since all my rockets have a bottle neck on top.
Before firing up the soldering iron, we need to prepare 2 relay coils first.
Please take care while handling the relays not to damage the coil wiring !
- Take a relatively small 12 V relay. Remember that the size and weight of the coil is also important when it's going to be airborne soon.
- Remove the plastic housing. Careful!
- Cut the plastic around the solenoid. Careful!
- Remove the coil.
- Wrap a shrink sleeve or some adhesive insulation tape around the coil wires.
- Check if your coil has survived all these brutal manipulations: take an ohm meter and measure between the 2 terminals: something like
100-500 ohms is OK. Several million Ohms is not OK: you have cut the wire somehow. Throw away this coil and dismantle another relay.
Construction of the optical sensor:
The 2 photodetectors are mounted on a triangular piece of prototyping PCB (printed circuit board).
The photodiodes are mounted in a piece of black plastic tubing so that they are only sensitive to light falling into the front opening
of this tube. I simply cut 2 pieces - about 20 mm - of a black felt pen. One tube is soldered + glued on one side looking up and the other tube
on the other side of the PCB looking down (both have a 45 degree offset). I also wrapped the photodiods in a strip of insulation tape, first of all to match the
diameter of the black tubes and secondly to make sure that no stray light is falling onto the back of the photodiode. As the housing
is completely transparent, any incoming light from the back of the diode could lead to false results.
The result should look something like this:
Putting all this junk together now:
- Start off by cutting a circular piece of test PCB that matches the inside diameter of the bottle you intend to use as the
electronics compartment. The 1.5 liter bottles I use have an inner diameter of about 10 cm.
- Mark the spot in the middle of this PCB where a bottle cap will be mounted. In this space there is no room for components.
- Locate some space for the big components: the battery and the flash capacitor. Solder + glue them to the board. Add an extra cable
tie for fixation. Remember that these components are at the bottom side of the PCB during launch and will be exposed to over 100Gs.
Poor mounting will result in components being left behind on the launch pad, followed by the familiar lawn dart.
- Locate 2 opposite positions at the edge of the PCB for the coils. Drill 2 holes that match the diameter of the top part of the
relay coils. I soldered the coil to the PCB by cutting off a little of the iron flange along the relay coil and using the top part
of this flange to fixate it to the board. Again: be very careful with any manipulation you do on the relay coils !
A Dremel tool may come in handy here.
- Solder 3 or 4 fixation points at the edge of the PCB. 2 of them will also serve as contact terminals for charging the capacitor.
- Mount the smaller parts on the PCB where there is space left : R's, C's, comparator,etc..
- Solder the photodetector PCB perpendicular to the main PCB. The photodiode tubes should be located as close to the edge
- Your whole contraption should look something like this:
- Make sure that all loose wires and components are properly fixated with some glue / silicone. Everything that can break off will
eventually after a couple of launches.
- Testing time: switch on the 12V battery. For best results: go outside in the daylight. Tests under
fluorescent lamps may not be reliable. The LED should be ON when you keep the board with the components at the bottom side
(rocket going up) and the LED should switch OFF when you turn the board over with the components on top (rocket going down).
- Time to get your hands on some small but powerful magnets.
I bought a set of small yet very powerful magnets that are normally used to post papers on a magnetic board.
The magnets are embedded in some sort of fancy plastic housing. You can leave them like that, or do what I did: wrap a magnet inside a
piece of cloth, grab your favorite hammer and smash it. Throw away the plastic bits and pieces and try to separate the magnet from your hammer head.
Remember: the more powerful your magnet is, the bigger the repulsive force when you run high current through the coils.
To attach the magnets to the nose cone, I embedded the round magnets into a small LEGO block that I glued to the PET nose cone:
Again: feel free to apply more sophisticated mechanical skills here.
Alternative uses for voltage comparator 2.
The LM393 contains 2 identical comparators in the same package. So far, we only used one of them to compare the 2 voltages
from the optical detectors. The second comparator is tied to ground or 12V.
Some useful applications for the second comparator could be:
- Low battery detection: One of the input pins is connected to a reference voltage, eg a 3.3V zener diode.
The other input pin is connected to the raw battery voltage divided by 2 with 2 identical resistors.
When the battery voltage drops below 6.6V, the polarity of the comparator will change and you may connect a red LED or a
piezo buzzer to the output as an indication that it is time to swap the 12V battery.
- Add a delay after the first comparator trips. Adding a second RC delay after the first comparator output becomes
high can delay deployment after apogee detection. The main reason why you would want to do this: the chute will deploy later and
the rocket will not drift as far as when it's deployed at apogee. It saves your breath running for miles to recover it. The main
disadvantage of this system is introducing a fixed timer delay to a variable system: independent of the rocket's height at apogee,
the delay will be the same. This may worst case result in the rocket reaching ground level before the fixed timer has finished.
The same disadvantage as when using a fixed delay after launch to deploy the chute.
- A location beeper / oscillator to facialiate retrieval of your rocket in a corn field.
- High voltage charged indicator. An indication that the flash capacitor has been charged.
To make this work you need a high-impedance path from the flash capacitor to one of the inputs of the comparator.
- Others? Use your imagination... Have a look at some application notes for the LM393.
The chute is attached to the bolt eye + ring in the middle of the PCB with some rubber bands. At the bottom side of the PCB,
the same threaded bolt eye holds the bottle cap in place. Wrap the shroud lines + shock cord + rubber bands around the chute and
put it on top of the PCB, under the nose cone.
Inside the nose cone, I glued some foam rubber, eg from an old mattress. It pushes down on the folded parachute and helps in
pushing off the nose cone once the magnets lose their grip.
- Use Light Dependent Resistors (LDRs) instead of photo diodes. They may be easier to obtain.
The resistor in series with the LDR has to match the LDR value in dark condition. Since there are no absolute values in this design -
the 2 inputs are relative to each other - the resistor value of the LDR is not really important. As long as the value is not too low
and loads the battery too much. Using LDRs, the same construction details are important: the LDR should see only light falling into
the front end of the tube!
- Reduce weight by using only one coil on one side of the nose cone and a miniature hinge on the other side. Or even better: use one coil
and one magnet to keep a lid closed at the side of the nose section for horizontal chute deployment
- This last paragraph was written months after finishing construction of the "optogee" project.
We finally made it to a field to launch the system on top of a couple of my most reliable
- The nosecone is spraypainted bright orange. The optical sensors are located in the
transparent part below the nosecone.
- We launched it 8 times successfully that evening.
- When trying to make a video of it, I found out (afterwards) that
you have to start filming during countdown, way before the actual launch.
The result on low resolution is
You can see the parachute deployment and the landing in a corn field.
It took us 15 minutes to locate it. See "location beeper" a little higher on this page.