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Trenman

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Got a new mpr kit but it's missing shock cord. All I have is a yellow thin rope like cord and the parachute. Is an elastic shock
cord not supposed to be used? Perhaps I am supposed to buy one? If so would a local hobby shop have one?
 

JDcluster

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No, it's Kevlar TM.
It's fire resistant, does not stretch much, but allot stronger than elastic cord.


JD
 

MarkII

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Check the kit's instructions for a parts list, and then make sure that you have all of the parts that were supposed to have been included. If anything is missing, contact the company that made the kit through their website or customer service phone number. Unless the kit was sold "as is," they must make good on the promised components. But as JD indicated, not all mid-power rockets feature elastic shock cords. Low-power kits almost always have elastic shock cords, high power kits never do; with mid-power kits, it's hit or miss: some do, some don't.

Why the difference? Because elastic (or lengths of airplane rubber), for several reasons, is not an ideal material for shock cords. It is fragile, easily melted and it stores energy (meaning that if it is stretched and released, it snaps back). Early on, high power fliers realized this. They soon switched to using very long lengths of very durable material, like tubular nylon or tubular Kevlar, instead of elastic cord. The new materials have very little stretch. They traded the energy-absorbing feature of elastic (with its downside - snap back) for length. Basically, they packed in very long shock cords, and when their recovery systems deploy, the energy is absorbed and dissipated by simply having the parts extend for long distances, allowing air resistance and momentum loss to slow them down, rather than arresting them with stretchy material.

Gradually this concept has been getting absorbed into the MPR range of kits as well, but it is not a universal feature of them yet. It has not been adopted very much in LPR yet for a simple reason: very long lengths of shock cord take up space and can have significant mass. With high power rockets, these aren't major concerns, because the diameters of these rockets can usually easily accomodate these large bundles of cord, and the motors have plenty of thrust. Low power rockets, on the other hand, usually have narrow airframes, which make packing long recovery harnesses a real challenge, and the extra weight can sometimes significantly affect the rocket's performance. MPR, which straddles these two ranges, often splits the difference with a combination that features lesser or greater percentages of each type of material. So it would not be surprising if your new kit has a Kevlar shock cord and no elastic. One key indicator is the length of cord that they gave you. If it is 3 or more times the length of the finished rocket, then it is likely that it is meant to be used as the entire shock cord. Read through the kit assembly instructions and check the parts list to be sure.

MarkII
 
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Trenman

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Its the talon 3. The cord is very long, am I supposed to use it all?
 

MarkII

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Its the talon 3. The cord is very long, am I supposed to use it all?
In all likelihood, yes. Is this described in the kit instructions?

There are ways to bundle up the cord neatly (with small rubber bands, tape or a technique called chain stitching) so that it is easier to pack, stays neat inside the recovery bay, and deploys tangle-free in a controlled way. Someone with experience in these techniques will probably respond, or else you can search the forum for info about them.

MarkII
 

Trenman

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Chain stitching. I'll have to do a search thanks!
 
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