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Shock Cord Choice and Mounting

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gary7

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I have an MPR rocket: 4 x 58 inches, final wt will be ~ or < 3#, will fly on a G motor, 36 in chute. What is the best shock cord? Kevlar? Nylon Elastic? Other? How you would mount it? If the length = 3x the rockets length (I have seen that general rule somewhere), is that really enough? Pros & Cons of your choices? I know, lots of questions but like everyone, I don't want to zipper or crash this rocket I am putting lots of time and effort into.
 

ScrapDaddy

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here is A QRC

Kevlar:
Pros: 6x stronger than steel, extremely heat resistant Because of these qualities Kevlar shock cords generally outlast the rocket.
Cons: Somewhat Abrasive getting the Braided Variety may help. Since kevlar is very strong, it is stronger than your body tube, hence at very high deployment velocities(60fps-90fps) It MAY zipper your tube:blush:

Nylon Elastic: Pros:Flexible and soft , very unlikely to zipper because of this.
Cons: Not Very heat Resistant, i have a model that uses one and i put a healthy amount of wadding in and after 3 flights the cord has nearly burned thru. However there is a solution, Nomex Shock Cord Protecters, Madcow and CWD both sell them and CWD has Very Long versions, however this my get expensive .

Elastic: this is nearly ruled out in the MPR range there are no true notable advantages, and they do not last very long nor are very strong.

MOUNTING:

The Classic Estes Tri-fold mound: This Is Typically preferred to some in MPR because of the ease and simplicity of it, it is also very strong when drenched in wood glue, however the strength is only equal to the bond of the glue itself, and i recommend gorilla wood glue for this


Loc Style Mount: I have no info on this one yet

Centering ring Cord Mount: This is The standard method in apogee kits, cut a small notch in the centering ring that centers the Motor Mount to the Body Tube and slip the shock cord thru the notch and tie.
Pros: Very Strong
Cons, Can only be used with kevlar shock cords because of the proximity to the motor

Epoxy Clay Mount: This Is the simplest method simply take a glob of the epoxy clay and stick the shock cord in the middle of the glob
Pros: Strong As the epoxy itself
Main Con Is Weight


Hope this helps!:)
 

Handeman

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A couple of things. If you have the room, and can reach inside the BT, use a u-bolt in the upper centering ring to attach the shock cord to. You don't need a big heavy duty one, 1/8 should be plenty for MPR.

Tie your shock cord to a quick link. Use tape or a nomex sleave on the lower end of the shock cord. The quick link goes to the u-bolt. Do NOT use a quick link on the plastic loop on a nose cone. I've tied shock cords to those loops for years without issues. When I've seen people use quick links with the nose cones, they snap at the end of the deployment and the metal snaps the plastic loops.
 

jef955

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Could try Kevlar through the motror mount and tie a loop in it about 2" below the end of the body tube. Elastic from the end of the tube out of the model to the nose cone. Kevlar inside the tube takes the brunt of the heat (being much more heat resistant) and the elastic is kept away fromt the heat and does'nt zipper the body tube. Another option is light stainless steel fishing leader in place of kevlar - you can find those anywhere like Wal mart etc...I have had good luck with this system - tried it on about 6 rockets so far.
 

AndyC

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For MPR, I often glue a kevlar strap to the motor tube, that is long enough to reach out of the rocket a couple of feet (similar to the way PML suggests, as well as others). From there, I tie on some nylon. For a rocket this size, I'd use either 3/8" tubular or 1/2" flat strap (enough to get good separation of the rocket and avoid parts smacking together on the way down - for a rocket the size you mention about 15'). I'd avoid anything bigger to keep the weight down and still fly on a G; same goes for avoiding too much hardware - I usually only use a quiklink/swivel for attaching the chute. For the kevlar, I like the 1" strap Wilmans sells at $1/ft. You don't need the strength, but the wider stap makes zippers less likely. If you use smaller strap/cord/TN to exit the body tube, I like to put something on the cord to spread the force out and make zippers less likely. What works for me is the self-stick thick felt stuff they sell to put on the bottoms of chairs at hardware stores. Cut (2) 2" square pieces, and sandwich the shock cord in between them at the right spot to intersect the end of the body tube.
 

GrossApproximator

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For MPR, I often glue a kevlar strap to the motor tube, that is long enough to reach out of the rocket a couple of feet (similar to the way PML suggests, as well as others).

Is it bad to just glue the nylon strap right to the motor tube instead of using the kevlar strap? I've heard from several people that nylon won't be harmed by the ejection charge since the charge's heat doesn't last long.
 

Handeman

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You will hear a lot of opinions on this one. I have a PML Callisto with a heavy 3/4" wide nylon strap glued to the MMT and to the piston. It's getting gray and stiff from the ejection charges, but I'm not worried about it yet and it has 30+ flights on it.

Kevlar would definately hold up longer, but.....
 

cjl

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Is it bad to just glue the nylon strap right to the motor tube instead of using the kevlar strap? I've heard from several people that nylon won't be harmed by the ejection charge since the charge's heat doesn't last long.
The nylon will get toasted by the charge, but only after quite a few flights.
 

DeWain

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Sorry, Gary7, I'm not answering your question, but rather am asking another one.

I am relatively new to MPR and HPR, so I don't yet have a feel for the correct sizes for shock cords. Do folks here have a good rule of thumb for the correct size or Kevlar and tubular/flat Nylon for different size rockets. I have an L1 rocket with a 9/16 flat Nylon (with the lower section protected by a K&S protector). I am thinking of using a Kevlar/tubular Nylon combo for rockets as small as D-powered rockets. What are good sizes for D-powered rockets? E? F?
 
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ScrapDaddy

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ok your question could mean 2 things so I'll answer the 2 possibilitys

possible answer 1, the shock cord should be 2-3X your complete rocket length for nylon and Kevlar shock cords

possible answer 2, use 100 LB test for rockets powered A-D use 300 LB test for D-g and use 1500+ for H-onward
 

cjl

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ok your question could mean 2 things so I'll answer the 2 possibilitys

possible answer 1, the shock cord should be 2-3X your complete rocket length for nylon and Kevlar shock cords
Possibly even longer, depending on the rocket. That's a good start though.

possible answer 2, use 100 LB test for rockets powered A-D use 300 LB test for D-g and use 1500+ for H-onward
Again, a good start, though it heavily depends on the rocket. I also tend to go a bit heavier on Kevlar cords, simply because extremely thin Kevlar can cause zippers very easily.
 

DeWain

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ok your question could mean 2 things so I'll answer the 2 possibilitys

possible answer 1, the shock cord should be 2-3X your complete rocket length for nylon and Kevlar shock cords

possible answer 2, use 100 LB test for rockets powered A-D use 300 LB test for D-g and use 1500+ for H-onward
Thanks Scrap Daddy. Sorry that I wasn't clear... I meant number 2... the width/dia of the flat/tubular Nylon.
 

ScrapDaddy

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Possibly even longer, depending on the rocket. That's a good start though.

Heh...... close but no cigar :D

Again, a good start, though it heavily depends on the rocket. I also tend to go a bit heavier on Kevlar cords, simply because extremely thin Kevlar can cause zippers very easily.
Heh...... close but no cigar :D
 

Handeman

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There are a number of factors that can come into play when choosing a shock cord.
  • Weight of the rocket
  • Weight of recovery system
  • Space available
  • Motor eject or electronic
  • Anchor point
  • Performance vs. sport flying
I've been using 1/8 tubular Kevlar for my 3lb dual deploy rocket. With electronic control of the ejection, you are much less likely to have a high speed deployment and a resulting zipper. This allows the smaller Kevlar to be used. The rocket also has relatively small bays for recovery so the 1/8" tubular Kevlar is much easier to pack then the 9/16" nylon I had tried originally.

All anchor points are not the same. You can use a quick link with an eye bolt or c-clamp type anchor. If you are using the plastic loop on a nose cone you must tie the cord to the loop. A quick link will quickly break the loop off the nose cone. Flat nylon will actually work better and spread the load along the plastic loop better then thin Kevlar.

If weight is a factor in a performance rocket, then the thinnest Kevlar is probably the best way to go. Again, proper motor ejection timing or electronic ejection is the best way to prevent a zipper.

Another factor with RMS motors is how much powder you put in the ejection charge. If you tend to use the whole supplied charge and the rocket has a small volume for recovery storage, you will have to have a stronger/longer shock cord.

One thing that can mitigate the strength of the ejection charge or speed of ejection is to loop your shock cord into 3" or 4" loops and wrap tape around every 3 or 4 loops. This way the multiple layers of tape have to be torn as the shock cord deploys, using up energy and reducing the shock on the cord itself.
 

MarkII

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DeWain, regarding your last question about shock cords for D-G rockets- that happens to be right where I am now (and have been for over a year) in my rocket building. Handeman provided a very thoughtful answer, but for D-G (MPR) rockets, the picture is a bit simpler. Recovery system deployment will always be single event, motor-initiated ejection at apogee, for instance. For all of mine, I have been using the same method that jef955 outlined above, except that I end the Kevlar or steel section very close to and just below the lip of the tube. For the Kevlat, I really like this microbraid tubular Kevlar, which has a 100 lb. rating. (It's the thinner cord in the picture.) I just think it's the greatest; it is not at all stiff, it doesn't unravel or fray very easily (unlike twisted Kevlar cord) and it is very thin and lightweight. In fact, it is so thin that you might not at first think that it could be sufficient, but then remember that it is Kevlar with the hundred-pound test weight. I obtained mine from Pratt Hobbies, but you can also get it at Commonwealth Displays. Sometimes I use stainless steel fishing leader instead. There's a good forum thread ongoing right now about it using steel cable. But if you go with cable for this type of mount, do not use use picture hanging wire. The stuff is nasty to work with, kinks easily, quickly develops "memory" and is too stiff - all characteristics that are exactly the opposite of what you want. It may work with other mounting methods, but not this one.

For the rest of the shock cord I have been using 1/4" to 3/8" braided flat elastic, which you can get cheap in the fabrics section of Walmart. For the larger rockets in this size and weight range, I have been using a three-part shock cord: Kevlar or steel leader for the first part, a short (1'-2') of flat elastic, and then a long section of polyester-blend paracord (a k a shade cord or Venetian blind cord). You can get it at Joanne's Fabrics, and I have also seen it in the camping section of Walmart and in the hardware section of K-Mart. Just make sure that you get the synthetic cord and not the stuff made from cotton. The reason that I use two types of cord for the "outside" section of the shock cord is to give it a little bit of stretch, but not too much. This is especially helpful if the rocket has a large payload section and you want to recover both sections together. The short section of elastic just above the tube lip provides the shock cord with some "give," yet it will not elongate that much and cause a snap-back. I have used this arrangement on my FSI Eos and Hercules clones, both of which have large payload sections.

Mark K.
 
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Neutron95

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Something that I would recommend is para-cord. It is nylon (I think) and extremely versatile. Just my 2 cents.
 

Micromeister

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I have an MPR rocket: 4 x 58 inches, final wt will be ~ or < 3#, will fly on a G motor, 36 in chute. What is the best shock cord? Kevlar? Nylon Elastic? Other? How you would mount it? If the length = 3x the rockets length (I have seen that general rule somewhere), is that really enough? Pros & Cons of your choices? I know, lots of questions but like everyone, I don't want to zipper or crash this rocket I am putting lots of time and effort into.
Gary:
Whenever I design or upgrade a MPR model I always use a motor mount Shockcord attachment method. with the shockcord routed to the inside outer edge of the forward centering ring. this allows the mount to be embedded in the glue or epoxy used to attach the motormount to the airframe. the most important part is that this also moves the Shockline or shockcord anchor to the furthest point away from the ejection charge flames and gases.

On MPR's I really prefer to use either a Stainless steel fishing leader or 7x19 strand Stainless Wire rope Shockline anchor that will allow our Kevlar/elastic to be further removed for the heat by another 8 or 12 inches. Knotting the Kevlar line to the stainless loop and placing a 1" square masking tape Anti-zipper tab on the line about 1/4" inside the forward body tube.
This little Masking tape trick has eliminated zippering from my Estes style BT-80 and BT-101 size Upscale models.

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