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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

Simple Altitude-Based Deployment

Discussion about deployment systems including altimeters, timers, air speed flaps, servo systems, and chemical reactions.
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SaskAlex
WRA2 Member
WRA2 Member
Posts: 238
Joined: Tue Apr 07, 2009 2:36 pm

Simple Altitude-Based Deployment

Post by SaskAlex » Thu Jun 13, 2013 12:27 pm

Here's something I started a long time ago and only recently finished up and tested out. This is a highly effective and dead simple electronic deployment system. The only downside is cost, as it requires an altimeter with deployment capabilities. If you are already looking for a logging altimeter, though, it's not that bad. I use the Perfectflite StratoLogger, which you can buy online for $72.

The StratoLogger's ejection ports give a one second connection to it's power supply (4-12V, IIRC) at ejection. One is set to fire at apogee and the other fires on descent at a user-defined altitude. These are meant to fire e-matches in a pyro rocket, but I have them set up to turn a servo motor. I took the servo apart, removed the control circuitry, and soldered on some extension leads to the motor's existing leads. It's been a while since I did this, and there might have also been a mechanical stop which prevented the servo horn from over-rotating. If there was, I removed it. So it's not really a servo anymore, just a geared down DC motor. I run the altimeter on a 2 cell Lipo, giving about 8V at full charge, and I'll probably recharge it when it hits 7.3V. That's a little high for this 4.8-6V servo, so I put a 20 ohm resistor in series with the servo motor. During testing the servo was drawing around 100mA when unloaded at about 5V. This resistor gives a voltage drop of about 2V, so the motor only receives about 5.3-6V. When the altimeter fires, the servo turns through several complete rotations, which is more than enough to release a rubber band.

Mechanically, the deployment system is similar to the tomy timer release I used earlier- http://www.wra2.org/forum/viewtopic.php ... ease#p8207, but with the tomy timer replaced by the servo motor.
DSCF2887 (563x640).jpg
On this side of the payload you can see the altimeter, lipo battery, power switch, and my MD80.
DSCF2888 (587x640).jpg
Here you can see the axle and hook I've added to the servo horn. This a fairly important feature, which I'll discuss after the last picture.
DSCF2889 (480x640).jpg
DSCF2890 (386x640).jpg
Since the axle is supported right near the copper "hook", very little of the pull from the elastic is actually exerted on the servo. Since the rubber band goes around the hook so close to the axis of rotation, it exerts very little torque on the motor. This way there is enough friction in the un-powered servo motor to retain a fairly strong pull from the rubber band. The motor requires no holding current. It only draws current for the 1 second that the altimeter fires. When it does turn, it's not fighting the rubber band, just releasing it, so you don't have to worry much about how much torque the motor can deliver. This setup allows for a small servo, and a small battery. This lipo should last hundreds of launches before it needs recharging (unless perfectflite has understated the current that the altimeter draws when it's beeping out the locating signal). It might actually be better just to power this with some small button or coin cell batteries.

I put this through three test launches the other day, and the release mechanism worked flawlessly. This system has several advantages over time based deployment. First of all, the altimeter detects launch for you, so you don't need to do that manually to start a timer. Secondly, if you have it set to fire at apogee, you don't need to predict flight time. Just turn it on and launch. Thirdly, it can easily be adapted to dual deployment or a high speed low altitude deployment (one of my current goals).

Let me know what you think.
Alex
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Tony
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Posts: 61
Joined: Sat Feb 16, 2013 3:39 am

Re: Simple Altitude-Based Deployment

Post by Tony » Thu Jun 13, 2013 2:45 pm

I like it!!


"Houston, we've had a problem..."

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Jamie5335
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Posts: 89
Joined: Sun Jun 02, 2013 1:15 pm

Re: Simple Altitude-Based Deployment

Post by Jamie5335 » Sat Dec 21, 2013 1:40 pm

WOW!! That looks great!

Forgive me- this was posted a while ago, but did it work??

Altitude detection is probably the best way of detecting when to deploy the chute, that or the uMAD which uses the Earth's magnetic field to detect launch!

Good Luck, I look forward to the results!

Jamie B


JSB Rocketry
Website: www.jsbrocketry.webs.com
Youtube: https://www.youtube.com/channel/UCczlD-uBjlYdJyBFEfuCEbg/feed

Jamie Bignell,
jamie.s.bignell@gmail.com
Somerset UK

"The important thing is to know how to take all things quietly" -Michael Faraday.

Lecce Water rockets
WRA2 Member
WRA2 Member
Posts: 6
Joined: Fri Apr 01, 2016 2:10 pm

Re: Simple Altitude-Based Deployment

Post by Lecce Water rockets » Mon May 28, 2018 4:03 am

SaskAlex wrote:
Thu Jun 13, 2013 12:27 pm
Here's something I started a long time ago and only recently finished up and tested out. This is a highly effective and dead simple electronic deployment system. The only downside is cost, as it requires an altimeter with deployment capabilities. If you are already looking for a logging altimeter, though, it's not that bad. I use the Perfectflite StratoLogger, which you can buy online for $72.

The StratoLogger's ejection ports give a one second connection to it's power supply (4-12V, IIRC) at ejection. One is set to fire at apogee and the other fires on descent at a user-defined altitude. These are meant to fire e-matches in a pyro rocket, but I have them set up to turn a servo motor. I took the servo apart, removed the control circuitry, and soldered on some extension leads to the motor's existing leads. It's been a while since I did this, and there might have also been a mechanical stop which prevented the servo horn from over-rotating. If there was, I removed it. So it's not really a servo anymore, just a geared down DC motor. I run the altimeter on a 2 cell Lipo, giving about 8V at full charge, and I'll probably recharge it when it hits 7.3V. That's a little high for this 4.8-6V servo, so I put a 20 ohm resistor in series with the servo motor. During testing the servo was drawing around 100mA when unloaded at about 5V. This resistor gives a voltage drop of about 2V, so the motor only receives about 5.3-6V. When the altimeter fires, the servo turns through several complete rotations, which is more than enough to release a rubber band.

Mechanically, the deployment system is similar to the tomy timer release I used earlier- http://www.wra2.org/forum/viewtopic.php ... ease#p8207, but with the tomy timer replaced by the servo motor.
DSCF2887 (563x640).jpg On this side of the payload you can see the altimeter, lipo battery, power switch, and my MD80.

DSCF2888 (587x640).jpg Here you can see the axle and hook I've added to the servo horn. This a fairly important feature, which I'll discuss after the last picture.

DSCF2889 (480x640).jpg
DSCF2890 (386x640).jpgSince the axle is supported right near the copper "hook", very little of the pull from the elastic is actually exerted on the servo. Since the rubber band goes around the hook so close to the axis of rotation, it exerts very little torque on the motor. This way there is enough friction in the un-powered servo motor to retain a fairly strong pull from the rubber band. The motor requires no holding current. It only draws current for the 1 second that the altimeter fires. When it does turn, it's not fighting the rubber band, just releasing it, so you don't have to worry much about how much torque the motor can deliver. This setup allows for a small servo, and a small battery. This lipo should last hundreds of launches before it needs recharging (unless perfectflite has understated the current that the altimeter draws when it's beeping out the locating signal). It might actually be better just to power this with some small button or coin cell batteries.

I put this through three test launches the other day, and the release mechanism worked flawlessly. This system has several advantages over time based deployment. First of all, the altimeter detects launch for you, so you don't need to do that manually to start a timer. Secondly, if you have it set to fire at apogee, you don't need to predict flight time. Just turn it on and launch. Thirdly, it can easily be adapted to dual deployment or a high speed low altitude deployment (one of my current goals).

Let me know what you think.
Alex
Hi. It's a nice system! But...is it possible to know how you decided to connect all the cables? Thanks!



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