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#142 - Doonside

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#1 to #140 (Updates)

 

HOW IT WORKS
The section explains the basic concepts behind water rocket features.

DetMech

Introduction

The DetMech mechanism was developed to automatically detach a rocket from its parachute line. It is designed to assist in recovering rockets snagged by their parachute in trees.

Total weight of prototype: 8 grams + 9 gram servo.

How It Works

The mechanism is based on a simple clamp that holds down a parachute cord. At the appropriate time, the clamp is released by a servo motor and in turn releases the cord. You may loose your parachute but you'll have a better chance at recovering your expensive rocket and payload.

The clamp arrangement shown here is made from thin aluminium sheet, but could well be made of plastic or wood. The grooves  in the top and bottom plates mesh together and help prevent the parachute cord from sliding within the clamp when the parachute opens. There can be a large lateral force on the clamp during parachute opening.

Aerodynamically it has a very low profile.

The bottom plate is attached mechanically to the collar with screws rather than glued due to the large forces involved. The main parachute line should also be fitted with a shock cord to further reduce stress on the clamp.

The little stops either side of the bottom plate prevent the top plate from moving side to side while the cord is pulling on it.

This mechanism is controlled by the second servo output on the flight computer. An activation time delay of around 20 minutes after takeoff allows the rocket to be normally recovered in most cases, and gives the recovery crew a chance to retrieve the rocket from an easily reached a tree.

Click on the photos below for a larger version.

Detail showing the different components that make up this prototype.
Clamp opened. The top clamp plate is also attached to the bottom plate mounting screw via a short length of Kevlar cord to prevent it from being lost. The Kevlar cord is entirely on the inside of the sleeve to prevent it getting tangled with the parachute cord.
The parachute cord is laid into the bottom groove. It is important that the cord has a strong knot on the end of it that will prevent the cord from sliding out of the clamp.
Closing the clamp pinches the cord in the groove. The string could have a section of heat-shrink tubing placed on it to help prevent it from being severed on the edge of the clamp.
The servo motor horn is then positioned to keep the clamp closed. The clamp can be made longer to provide more force to keep the parachute line secured.
A side view of the closed mechanism, showing the small drag profile.
A rocket with the DetMech mechanism located in the space between bottles. The DetMech servo is controlled by the flight computer in the nosecone. The flight computer also controls parachute deployment.

Videos

Prototype Ground Tests:

Limitations

  • The mechanism will not always ensure that a rocket is freed from a tree. The rocket may get caught by its fins in a branch, but  it should at least be more readily shaken out of the tree.
  • The rocket may be substantially high above the ground, and releasing the rocket may cause damage when it hits the ground. To help mitigate this issue, the controlling circuit can give a warning prior to dropping allowing a person to either catch the rocket, or stand clear.

Extensions

  • The next version will use a single servo motor to deploy the parachute as well as detach it, saving on weight and complexity. The dual purpose mode of a single servo would be based on Christian Thomsen's RC dual chute release mechanism.
  • The servo could be controlled externally by a remote control. Allowing the rocketeer to release the rocket when appropriate.
  • A variation on the mechanism could make it completely internal to the rocket body reducing drag.
  • Another alternative is to replace the servo motor with a water soluble paper or plastic (such as Polyvinyl alcohol). This could be wound around the clamp to hold it down, and when it rains or humidity in the air would cause it to dissolve and release the mechanism.

 

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