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Veracity
12-17-2007, 10:18 AM
I just read that certain companies recommend that a shock cord should be at least three times as long as the rocket itself.
Wow.

Thoughts?

Rocketflyer
12-17-2007, 10:52 AM
I just read that certain companies recommend that a shock cord should be at least three times as long as the rocket itself.
Wow.

Thoughts?

Makes sense. My minimum on LPR is 1& 1/2 times, with 2x's the norm.

Shreadvector
12-17-2007, 10:56 AM
I just read that certain companies recommend that a shock cord should be at least three times as long as the rocket itself.
Wow.

Thoughts?

My thoughts are that this "rule" is useless. How can you use this "rule" for a Fat Boy and a Phoenix Missile which are both BT-80 based with WAY different tube lengths and overall mass and different motors with different ejection force?

The shock cord needs to be long enough for the rocket you are flying to absorb the shock of the nose cone ejecting (with the ejection charge supplied by the motor you are flying) and not stretch beyond it's elastic limit and break or snap back violently.

moonzero2
12-17-2007, 12:16 PM
It never hurts to have it too long, but can be destructive if it's too short. Error on the long side. The recommendations above are good ones. Have fun flying!

mperdue
12-17-2007, 01:37 PM
It never hurts to have it too long, but can be destructive if it's too short. Error on the long side. The recommendations above are good ones. Have fun flying!

It can certainly hurt to have it too long. The extremely long shock cords on the rocket that landed on the power lines at NSL 2007 caused a BIG problem by bridging all three phases of power and dropping electrical service to 2000+ homes. The cord only needs to be long enough to absorb the shock of normal deployment. A little common sense goes a long way.

Mario

moonzero2
12-17-2007, 02:58 PM
As recommended above of 2 to 3 times the body tube length.
A little common sense goes a long way.
I agree.
And if flying around power lines, then erroring on the short side sounds good. :D

tbzep
12-17-2007, 03:01 PM
About the shortest I put in a small scratch built rocket is 4 ft. Small as in bt-20 to bt-50 size rockets. I'll go shorter only if I can't fit the recovery system in the rocket. Bigger rockets get longer cords in them, but I usually don't go over 6ft with LPR model rockets. I've used big honking straps in HPR. :)

I agree with Fred that the rule of thumb isn't a very good rule. A Mean Machine would need an 18-20 foot long cord with that rule! That would look pretty goofy with the older style nosecone ejected version. Then the Fatboy he mentioned would only get one a couple of feet long. The ejection charge would be milder in the MM than the Fatboy because it has to fill a much larger volume.

After my big shotgun ejections with A10's and A8's and several D12's that don't even blow all the clay cap off and don't even knock all the wadding out, I can't say that motor size should have anything at all to do with deciding on shock cord length. Prepare your rocket as if every ejection will be supersized and use as long a cord as is practical. Like Mario said, a little common sense goes a long way.

ghrocketman
12-17-2007, 03:13 PM
That must have been a real HOOT at NSL 2007....I would have liked to have seen that.
Barring obstacles other than such power lines in the area, there is no such thing as a shock cord that is too long as long as the ejection charge can eject it with the Chute.

Thruster
12-17-2007, 03:59 PM
I fly B and C sized Rockets so what should I use as a shock cord, I have just got #100 and #400 Kevlar Thread. Is that good for the size rockets I have?

ghrocketman
12-17-2007, 10:38 PM
400# kevlar is way overkill for shock cord leader on any sort of LPR or MPR.
100# is PLENTY

mojo1986
12-18-2007, 04:17 PM
Shock cords don't have to be all that long.............you just need to use two! I always use a separate shock cord and parachute for the body tube and the nose cone. Works great, with no destructive snap back. And I've never had a shock cord break using this system either..............it seems that this type of deployment puts less stress on them.

Joe

Mark II
12-18-2007, 08:51 PM
About the shortest I put in a small scratch built rocket is 4 ft. Small as in bt-20 to bt-50 size rockets. I'll go shorter only if I can't fit the recovery system in the rocket. Bigger rockets get longer cords in them, but I usually don't go over 6ft with LPR model rockets. I've used big honking straps in HPR. :)
That's just a tad long for me. I generally start with 3x the airframe length, and then adjust up or down from there, based on the rocket's girth, weight and motors that I expect to put in it. A very long shock cord acts a bit like a streamer, and it can do unexpected things to your recovery drift distance.

Lately, I have been using a 3-part shock cord. The first part is the Kevlar leader inside the airframe. To that I attach a piece of elastic with a length of 0.5x (or occasionally 1x) the length of the airframe. (I adjust this based on the "stretchy-ness" of the elastic.) Finally, I attach a length of non-stretch shock cord, like paracord, that is 1.5x to 2.5x the length of the elastic. The idea is that the short length of elastic stretches to absorb the ejection shock, while the longer length of paracord gives the model room to decelerate from the stretch without incurring snap-back damage from the nose cone or payload section.

Mark K.