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Dark Shadow Deployment

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Flight Log Updates

#190 - Polaron G3

#189 - Casual Flights

#188 - Skittles Part #2

#187 - Skittles Part #1

#186 - Level 1 HPR

#185 - Liquids in Zero-G

#184 - More Axion G6

#183 - Axion G6

#182 - Casual Flights

#181 - Acoustic Apogee 2

#180 - Light Shadow

#179 - Stratologger

#178 - Acoustic Apogee 1

#177 - Reefing Chutes

#176 - 10 Years

#175 - NSWRA Events

#174 - Mullaley Launch

#173 - Oobleck Rocket

#172 - Coming Soon

#171 - Measuring Altitude

#170 - How Much Water?

#169 - Windy

#168 - Casual Flights 2

#167 - Casual Flights

#166 - Dark Shadow II

#165 - Liquid Density 2

#164 - Liquid Density 1

#163 - Channel 7 News

#162 - Axion and Polaron

#161 - Fog and Boom

#1 to #160 (Updates)

 

Dark Shadow - Parachute Deployment Mechanism

Introduction

For Dark Shadow we wanted to make a lighter deployment mechanism than what Shadow used. One way to make it lighter was to reduce the overall length meaning there was less fiberglass in the body tube. This also meant ditching the piston mechanism and so we went with rubber bands to eject the parachute. For the expected high accelerations we again avoided going with a side deployment and went with the in-line approach. 

For full Dark Shadow build details please see: Dark Shadow Build Log - Part 1 and Dark Shadow Build Log - Part 2

Here is a video that describes the design and operation of the deployment mechanism

Nosecone

In order to make the deployment mechanism as compact as possible we wanted to use up as much of the internal space as possible. So since the nosecone was mostly empty we put the camera in it. This allowed us to reduce the overall length of the payload bay even further. The nosecone shell is removable to allow access to the 808 #16 V3 camera. The camera is just held in place with a foam block with a piece of tape over the top. The camera lens partially pokes out through the shell for minimum drag and clear view. A pair of access holes allows us to operate the camera with the nosecone shell assembled.


Camera is housed in a foam block.
 

Tape pulls foam sides together to
grip the camera

Nosecone shell
 

Grapple Arms

The grapple arms serve two purposes. They provide something for the rubber bands to pull against while pushing the nosecone out of the body tube. They also help pull the parachute out of the payload bay. The grapple arms are hinged so that when the nosecone is ejected they are free to swing open and let the parachute fall out from between them. The parachute fits just between the grapple arms so that when inside the body tube they can't open outwards and the parachute prevents them from closing inwards.

The bottom of the grapple arms overlap so that you only need to hold down one of them. A loop of wire is attached to one of the arms that hooks over the servo horn and holds down the entire nosecone.

This design was chosen to overcome the need for the nosecone to be friction fitted into the body tube. By being positively retained by the servomotor, there is no need for any friction between the nosecone and body tube. With smaller friction the ejection force can also be lower and why just two rubber bands are needed. This arrangement also eliminates the possibility of the nosecone drag separating at burnout.


Wire loop used to hold down nosecone

Nosecone with parachute between grapple arms

Rubber bands on the inside of the
body tube

Wire holds the rubber bands in place
 

Two rubber bands are used to eject the parachute. They are simply attached to the body tube through a hole in the wall and secured on the other side with a loop of wire. This allows the rubber bands to be easily switched to adjust the ejection force or replaced if they should break.

Electronics Package

The electronics package frame consists of a central fiberglass tube with two centering rings at either end. The centering rings are made of a balsa-fiberglass sandwich. A PVC ring is glued to one of the centering rings and has 6 threaded holes to allow the whole package to be attached to the body tube.

The 9 gram servo motor is attached to the inside of the fiberglass tube with the servo horn poking out to the outside.

Around the circumference of the fiberglass tube is a set of removable pockets made from PET plastic. Each of the pockets contains a separate electronic device. This allows us to remove components easily to be reused on other rockets. The bottom of the pockets are open so that the electronic device can sit against the lower centering ring. This provides support for device against the G forces.

This circular arrangement was chosen to allow us easy external access to the device buttons and their displays.

The Servo Timer II, servo motor and the zLog altimeter are all powered by the two 100mA 20C LiPos. We chose to again use a screw switch to switch the power on to the system as it is compact and gives a secure connection under high G loads. The AltimeterOne used its own separate battery for redundancy.

The Servo Timer II was re-programmed to give a configurable time delay from 5 to 17 seconds.


PET pockets for electronics

Top view

The electronics fits snugly inside

Battery pack

Pressure Chamber Attachment

The entire payload bay is attached to the pressure chamber via a PVC ring that is glued to the top of the pressure chamber. Shadow used a similar method. The PVC ring is the same size as used for the payload bay body tube mandrel so it is a nice fit. We used the PVC pipe because it is easy to tap holes into the plastic. The entire payload bay is attached with 8 x M3 countersunk screws.


Shock cord attachment

Removing pin

Shock cord loop

Electronics package

Parachute and Shock Cord

Dark Shadow uses a 36" Aerocon parachute. The damaged nylon shroud lines that came with the parachute were replaced with polyester ribbons as that is all we could find. Their size and strength are comparable to the nylon ones. We also ran the shroud lines from opposite sides of the parachute and then created a loop in the middle using heat shrink tubing.

The shock cord attaches to the top of the pressure chamber so that should the payload section break off, everything stays connected. The shock cord is attached via a removable pin to the top of the pressure chamber so we can easily separate the nosecone from the rest of the rocket. The shock cord is made from 3mm braided nylon cord. It has a short length of heat-shrink tubing over a section near the edge of the body tube to protect it from getting cut during deployment.


36" Aerocon Parachute

Damaged shroud lines
   

Shock cord loops through nosecone

New shroud lines

Heat shrink tubing loop

Diagrams

Following are internal detail diagrams of the deployment mechanism.

Dark Shadow Build Log - Part 1

Dark Shadow Build Log - Part 2

 

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