Shock Cord Mounting

I am building a 8-10 lb. rocketry for my L2 and I was curious about shock cord attachment options. I was either considering running Kevlar all the way down and around the motor mount or using a loop of Kevlar epoxied to the upper section of body tube similar to what is in LOC kits. I think running to the motor mount is stronger, but you cannot really inspect for issues. I like that the LOC style mount is easily inspected and can be replaced if needed. What are everyone's thoughts? Any suggestions for strong yet inspectable mounts?

One other question. Does anyone have any tips for gluing bulkheads for an E-bay together without gluing my threaded centering rod to the bulkheads?

Thanks,

Chris

Another method is installing a SS u-bolt on your top CR.If the CR is far down in the airframe or a small diameter you can still fish a SC through the u-bolt with a string and nut.Tie the nut to one end of string and drop it down through the u-bolt the other end to loop in SC.Pull the loop through u-bolt and insert opposite end of SC through the loop.Then pull that end until the SC is cinched down on the u-bolt.

As for gluing the BPs just use an eyebolt...run a nut down onto the eyebolt,slip on a washer,then the two BPs to be glued,another washer then a nut and tighten down.Once its set remove the nut and the eyebolt screws right out...the key is don't use so much glue/epoxy that it squirts out everywhere when you tighten down the nut((s) if your using two all threads for the bay x2 the above).

That is what I was figuring, I guess I'll just need to take it easy on the epoxy on the bulkheads.

The rocket will be 3" diameter with a 54mm motor mount, so there isn't going to be enough room to put a u-bolt on the motor mount.

A stout kevlar loop coming out of the fore CR --in essence, a flexible u-bolt that you may make as long as you require for your hand size-- would be sufficient, I'd think...



Later!

--Coop

Another method I use when there isn't room for U-bolts is stainless steel cable. Crimp a loop into each end of cable ( I use aluminum crimps as they are lighter) and epoxy the loops onto the MM in the same manner you would with Kevlar. I don't have a pic of the cable attached to the MM, but here is an attempt at shooting a pic of the cable down inside the BT, and another pic of the cable. On my last build I taped the exposed cable loop with vinyl electrical tape to add a layer of corrosion resistance to the stainless.

So, run from the motor mount to just below the tube opening. That sounds like a good option. Do you ever have issue with the cable interfering with the chute or rest of the shock cord? What cable do you buy, there are tons of options at McMaster?

If you have a longer loop you will need to be careful how you pack. A burrito style wrap in a nomex blanket close to the diameter of the BT will keep it from getting under the loop. Just make sure that your loop is long enough so it can be moved to the side if running a longer motors.

The cable in my pics was ordered from McMaster. Here is the order info I used:

5 8908T12 Type 316 Stainless Steel Wire Rope, 1 X 19 Strand, 1/8" Diameter, 1780# Break Strength, 10' L, 0.85 Per Foot, 8.50

I used a SS wire for my L1 DD rocket. Never again. It might work for larger diameters, 4"+, but mine was 2" ID and I had to slot a dowel and use that to twist and push the wire down into the BT. A real PIA. The wire is NOT flexible.

I've used Kevlar loops attached to MMT and loops attached the BT like LOC says. As for strength, I'm not sure there really is any difference. If you have a deployment that tears the BT apart because you used the LOC method, I suspect the rocket was trashed before the SC anchor gave way.

I prefer the Kevlar loop attached to the MMT and run around the outside edge of the upper CR. That keeps the SC further away from any motor ejections. Not having the cord attached in the LOC method, although strong enough, leaves a smooth BT for easier chute deployment.

Now if you happen to forget to put the cord on the MMT before you assemble it, don't ask, the LOC method works just fine.

Public Missiles Limited kits have the simplest and lightest system, in my experience. You cut or file a flat in the inside of the front centering ring just wide enough and deep enough for the shockcord, and when you epoxy the centering ring to the motor mount, you do so over the shock cord attachment; so the cord goes under the CR, and is then epoxied down flat to the MMT. You can use tape to squeeze it down flush to the MMT behind the CR. The two disadvantages are that the shock cord is not replaceable (but unless your rocket is very very wide, almost all are not replaceable) (also, if you take good precautions (nomex sleeve and/or kevlar cord), the BP damage will simply not weaken the cord until long after something else fails), and that you have to remember to glue it in when you're still putting the motor mount together. =p

CCotner said:

Public Missiles Limited kits have the simplest and lightest system, in my experience. You cut or file a flat in the inside of the front centering ring just wide enough and deep enough for the shockcord, and when you epoxy the centering ring to the motor mount, you do so over the shock cord attachment; so the cord goes under the CR, and is then epoxied down flat to the MMT. You can use tape to squeeze it down flush to the MMT behind the CR. The two disadvantages are that the shock cord is not replaceable (but unless your rocket is very very wide, almost all are not replaceable) (also, if you take good precautions (nomex sleeve and/or kevlar cord), the BP damage will simply not weaken the cord until long after something else fails), and that you have to remember to glue it in when you're still putting the motor mount together. =p

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I've got a PM Callisto I built in 2003 that must have at least 50 flights on it. The shock cord has held up fine so far. Since it is nylon, I'm starting to have some concerns about it. It's a piston system and one of the key things I do is make sure I reduce the charge when flying HPR motors. It only needs about 0.50 grams of BP for a reliable ejection. My opinion is that less BP = longer life for the SC.

These are all good ideas. Along the lines of post assembly options, I've got a rocket that has been glassed and is overall very sturdy, but currently has the LOC style shock cord mount. It seems like that is the weakest portion of the rocket. I am considering doing a retrofit to a modification of the steel cable idea by drilling in from the bottom through the centering rings, threading in the cable, and then crimping a loop in the top. I am currently planning on using the 1/16" braided cable, but haven't quite decided how to do the attachment at the back end. It seems reasonable to put the cable through a washer, crimp the cable to have the washer secured to it, and then epoxy the washer and a portion of the cable to the rear end. The rocket weights in at about 3.2lb without engine. Does anyone have any suggestions for anything cleaner/better?

Thanks.

If the motor mount is 54mm then get the tapped forward closure so you can use a forged eyebolt for the shock cord mount, and you can replace the shock cord when needed.

Kehoes23 said:

If the motor mount is 54mm then get the tapped forward closure so you can use a forged eyebolt for the shock cord mount, and you can replace the shock cord when needed.

Click to expand...

That's what I do, also.

Ahh, That is a great idea. It does have a 54mm MMT, I sometimes like to launch it on 38mm as well(but looks like there is a tapped one for it too, even an open tapped closure from Apogee). I currently only have the CTI 38mm casing, which doesn't have the nice tapped forward closure option. Guess that's a good reason to buy and fly Aerotech in this one.

CCotner said:

Public Missiles Limited kits have the simplest and lightest system, in my experience. You cut or file a flat in the inside of the front centering ring just wide enough and deep enough for the shockcord, and when you epoxy the centering ring to the motor mount, you do so over the shock cord attachment; so the cord goes under the CR, and is then epoxied down flat to the MMT. You can use tape to squeeze it down flush to the MMT behind the CR. The two disadvantages are that the shock cord is not replaceable (but unless your rocket is very very wide, almost all are not replaceable) (also, if you take good precautions (nomex sleeve and/or kevlar cord), the BP damage will simply not weaken the cord until long after something else fails), and that you have to remember to glue it in when you're still putting the motor mount together. =p

Click to expand...

+1 I attach about 5' of tubular Kevlar to the MM under the upper CR, with a sewed in welded SS d-ring, to attach the remainder of the shock cord and cover it with a nomex sleeve. I've use this method on a HyperLOC 300 and a 54mm scratch build with no signs of shock cord damage.