We have all experienced the frustration and agony when one of our rockets ,that took
several hours to build, lawndarted and broke up. Not to mention the risk of hitting
a human target with a falling rocket.
In the last year or so I have been looking into several mechanisms to deploy a
parachute as a safe recovery device.There is the obvious Tomy timer design, but being
an electronics engineer I wanted to have a timer that is more reliable and accurate.
This is a very simple just-do-it electronic deployment device that I designed.
The philosophy behind it is: carry as few electronic components as possible
in the nosecone for 2 obvious reasons:
1/ Sooner or later, this rocket with its sophisticated electronic deployment device
will end up dangling from a tree top. The less Euros/Dollars that get lost here, the better.
I could integrate a battery/microcontroller/servo solution here, but especially here in Belgium
there is a great risk of loosing your rocket sooner or later. I have had rockets that
lawndarted into someone's garden en never showed up again (mysteries...).
The idea is to retract a nosecone holding pin into the rocket body and to separate
the nosecone from the main rocket body with a rubber band on one side.
This releases the parachute that is packed in the nosecone.
This is very much the same setup as with a Tomy timer (either horizontal or
The basic functionality of this device is a pin-pulling solenoid, like they can
be found in automatic door-openers, dishwashers...
Mostly, these standard household solenoids are too bulky and heavy for our purpose.
This is a picture of a commercial light-weight solenoid.
They are quite expensive: over 10 Euros (and $). IMHO one of the challenges of the water rocket hobby
is to build the devices as cheap as possible, preferably from recycled parts.
Therefore, I used a solenoid from a 12V automotive car relay. The type of relay that
is used for the headlights e.g. They typically have a coil impedance of around 100 Ohms.
The lower the coil impedance the better: more current through the coil means a
bigger magnetic field and a bigger pulling force on the pin.
Some old door bells also have a solenoid and pin that can be salvaged for parachute
An electromagnetic pin puller uses the same theory and principles as a coilgun.
To learn more about coilguns:
Barry's coilgun design site
Greg's Coil Gun Page
Disassemble the headlight relay until you are left with the coil only. Normally,
there will be an iron core throughout the center of the coil. When removing this core,
be very careful not to damage the coil wiring.
You should be left with a hollow coil now.
Time to look for a magnetic pin with a diameter a little less than our coil center.
Use e.g. an iron nail or a steel bolt of which you cut the head off. Wrap some protective
tape around the coil wire to protect it. Now it is time to assemble the whole pin pulling
I use a small nylon (or wooden) block as the front guidance of the pin.
Add a bolt or tie wrap in the middle of the pin between front and coil so that the
pin is entirely inside the front block in one position and when the pin is in the
other position (protruding from the front block), the other end of the pin should be JUST
INSIDE the coil. Fix the front end block and the bolt with a drop of
CA glue (e.g. Loctite 406). I used a piece of experiment printed circuit board
for the pin puller base.
When you apply a mild 12V to the relay coil, not much will happen.
The idea is to apply a much higher voltage across the relay coil, resulting in a
much higher peak current that will "suck in" the pin.
I charge an electrolytical capacitor up to 300V and discharge it over the coil.
The pin really slams back into the coil!.
The idea is to charge this capacitor on the ground before launch and to discharge
it over the coil after a delay set by an electronic timer that starts at launch.
Schematic diagram of the "electronics" in the nosecone:
At the left of the diagram, there are 2 contact terminals that are available on the
outside of the nosecone. These terminals are used to charge the 150M/450V capacitor
up to 300V DC. The diode between the terminal and the cap serves 2 purposes:
1/ Avoid wrong polarization of the charging device, which would destroy the capacitor.
2/ Avoid a nasty shock when you touch the 2 outside contact points after the capacitor
has been charged. With the diode, no current can flow back through your body and
you can safely touch both terminals at the same time without getting blown out of
The "timer" is a simple RC combination: R = 2M2 potmeter + 2M2 fixed resistor and
C = 1 MicroFarad/400V. This leads to a timing constant between 4 and 10 seconds
(tolerance on R and C), which is about the timing range after launch for safe parachute
The C of the RC combination starts to charge through the 2 contact points on the
right of the diagram which are in fact a home made intertial switch. I use a simple
end-point detection switch salvaged from e.g. a CD ROM or floppy drive.
I add some (iron !)weight to the contact lever and place a small permanent magnet
about 3 mm underneath it. Due to the large g-forces at launch the weight will pull
the lever down and close the switch. Without the magnet, the switch would open up
again when the g-forces allow the lever to go up again. Now, the weight remains
attracted by the magnet and the switch remains closed. You have to pull the lever
back up into its starting position before every launch.
When the switch is closed, the 1M capacitor starts charging, from the big capacitor
which is already at 300V. When the voltage reaches 200V DC, the 200V surge protector
will conduct (the spark gap flashes over) and the gate of the thyristor will get triggered.
Instead of a surge protector, a small 200V neon lamp may be used or 2 x a 100V neon bulb
The thyristor shorts the coil over the big capacitor and there will be short peak current
of 300V / 100 Ohms = 3 Amps through the coil.
This causes the pin to slam back into the coil. As a thryristor, I use an SGS-Thomson
TYN1004. It is capable of withstanding a 1000V off-state voltage and 60A peak current -
4A continuous. Here is a connection diagram of this thyristor:
Close up of the inertial switch + weight + magnet and diode,
capacitor and potmeter:
Completed pin-puller unit:
Now we can build the pin pulling unit into a nosecone: one side of the nosecone
is pulled down with a rubber band and the other side is held by our protruding pin.
This picture shows the pin puller unit fixed onto a plywood base.
At the bottom of the unit, there is a bottle cap to screw the nosecone onto a rocket.
The blue part of the nosecone is the chute container that comes off. The red and black lead
go to 2 contact points, these are 2 screws that are also used to fix the plywood
base in the nosecone.
However, we still need a charging device to charge the main capacitor to 300V.
Preferably something that is light weight and can be used in the field.1/ The electrolytic flash capacitor.
The answer to this is simple: (the guts of) a disposable camera with flash unit.
Disposable cameras with flash unit are widely availabe and cost around 10 Euros.
They have a Xenon flash bulb, a normal 36 exposure film roll, a 1.5V AA Cell battery
and an electronic 1.5 to 300V up converter to charge the flash capacitor.
The flash capacitor is the main electrolytic capacitor of our pin puller. The rest
of the electronics, together with a simple 1.5 V battery will be built into a small
housing that can be used to charge the capacitor inside our rocket nosecone.
This way, we don't have to carry any batteries as a payload. Besides, the flash
capacitor is quite small compared to a similar off-the-shelf electrolytic capacitor
with the same capacitance/voltage.
Normally, when you bring in a disposable camera for development, the film roll is
taken out and the rest is thrown away. A real shame, so try to get hold of such an
empty housing (with the electronics, battery,etc still in ) at a 1 hour photo processing
2 items need to be salvaged from the camera:
2/ The 1.5 to 300V DC inverter
Some more info on how to disassemble a disposable camera:
Kodak Max by le Magicien
Flash tube circuits
Night launch flasher
The only advice I can give when dissecting a disposable camera is:
before touching any electronics inside the camera, discharge the flash capacitor
over a 1 kilo Ohm resistor and remove the battery. This way, the capacitor will not
charge again to 300 V. If you don't, be prepared for a couple of nasty shocks!!
The internal electronics of the camera may look like this:
Disconnect the flash capacitor (the large black tube on the top right of the picture).
Use this capacitor on-board the rocket for the pin puller.
Put the rest of the electronics into a nice housing.
Attach a 1.5V AA cell battery and two pushbuttons as in the diagram:
I also added a small meter. This is actually a small field strength indicator from
a portable radio. Place a large resistor (range:2 - 10 MOhm) in series with the
meter and connect it to the output of the charger. When you buy a new meter especially
for this project, ask for something in the range of 300V / 10 MOhm = 30 micro Amps.
The less current the meter draws, the better.
The thing that is clipsed at the top of the charger box is a small tool with on
one end a pin - to put the intertial switch back into the open position through
a hole in the nosecone - and the other side is a small screwdriver blade - to adjust
the potmeter of the timing circuit - again through another hole in the nosecone.
Why 2 pushbuttons?
One is placed over the contacts of the original pushbutton of the camera to start
the flash charging and the other one in series with the 300V output.
The reason is simple: safety.
You need both hands to push on both pushbuttons simultaneously, so normally you do
not have any hands left to touch any of the terminals. Also, when you release both
pushbuttons, there is no high voltage coming out of the charging unit anymore.
Attach 2 alligator clips to the charging terminals: a green or black one for the minus
terminal and a red one for the plus terminal. These will be clipped onto the charging
contact points on the nosecone before launch.
This results in a very compact charging device, that looks quite impressive
in the field. It looks very professional when you attach this magic electronics
box to your rocket before launch!
Checklist before launch:1/ Pack the chute inside the nosecone.
2/ Place the nosecone on the rocket onto the electronics compartment.
3/ Pull the pin through a lip of the nosecone to hold it in place while it is
being pulled off by a rubber band at the opposite side.
4/ Fill the rocket 1/3 with water.
5/ Place the rocket on the launch tube.
6/ Make sure that the inertial switch is in the "open" position.
7/ Charge the pin puller capacitor with your 300V charger.
8/ Pressurize the rocket.
10/ Around apogee, after about 5 seconds, the nosecone holding pin will be retracted
into the electronics compartment and the nosecone + parachute will be released.
Some pictures of the first tests - click to see a larger image:
Looking closely at the pictures you will see that the rocket consists of:1/ A 1.5 liter Spa mineral water pressure vessel.
2/ The top of another bottle as fixation point for the nosecone.
3/ A plywood base for the pin-puller electronics.
4/ The lid of a choco jar as parachute platform.
5/ A nosecone of another water bottle, with the neck replaced by a Styrodur
insulation foam point. On the inside: foam rubber to help the nosecone
being pushed off.
While you are soldering all this stuff on a piece of experimenting printed circuit board,
do not forget to enjoy your favourite regional beer !!
The author assumes no liability for any incidental, consequential or other
liability from the use of this information.
All risks and damages, incidential or otherwise,
arising from the use or misuse of the information contained herein
are entirely the responsibility of the user!