On that shock cord 2-3x BT length rule....

[MichaelRapp]

I know the general rule-of-thumb that a shock cord should generally be 2-3x the BT length, is just a rule of thumb, but does it mean the entire shock cord should be that or just the part that extends past the top of the BT?

 

[tim cubbedge]

My rule of thumb is 4-5 times the entire rocket length.

 

[mbeels]

I wondered the same thing.

My interpretation is that the effective shock cord length is just the part that extends past the body tube. My reasoning is that the purpose of the shock cord length is to allow the recovery gear to slow down, and it only starts slowing down once it leaves the rocket. (but yeah, it's a rough rule of thumb).

 

[Banzai88]

The larger the rocket, the heavier the parts, the less 'stretchy' the recovery harness is, the longer it should be. 4-5X is my general rule for HP/Kevlar. 2-3X is fine for rubber shock cord LP providing there's sufficient space.

I would rather lose a few dozen feet of achieved altitude carrying extra cargo weight in the form of a longer harness aloft than lose the whole rocket and/or create TWO uncontrolled falling objects for lack of a few extra feet to allow things work safely.

 

[58pan]

I thought the rule was "Still room in the rocket? Add more shock cord!"

 

[billdz]

"4-5X is my general rule for HP/Kevlar"
What's the rationale for a long shock cord? Is the concern keeping the parts of the rocket apart during decent? Preventing a back kick on the upper section? Or something else?

 

[jlabrasca]

"4-5X is my general rule for HP/Kevlar"
What's the rationale for a long shock cord? Is the concern keeping the parts of the rocket apart during decent? Or is it something else?

It is something else.

https://www.rocketryforum.com/threads/calculating-shock-cord-size-and-lengths.48638/#post-473720

 

[jderimig]

Dumb ROT. BT length has no relation to the ejection energy you need to dissipate.

 

[rharshberger]

Dumb ROT. BT length has no relation to the ejection energy you need to dissipate.

My recovery harnesses are in 5' increments, longest is 25' for each half of a 60lb 7.5" rocket.

 

[shreadvector]

Dumb ROT. BT length has no relation to the ejection energy you need to dissipate.

YES!

Big Daddy is short but still need lots of shock cord. (And it needs that beveled nose cone shoulder filled or covered....)

Mean Machine - 2-3 X length = INSANE.

And who knows where that thumb has been?

 

[58pan]

All kidding aside, this probably started with the Estes kits coming with the elastic shock cord about 18" long and with the fairly energetic BP ejection charge the nose cone would come snapping back and smack the front of the body tube. The solution was to add more shock cord. The 2 or 3 times length was just a general guide for the average size rocket. It also helped with zippering when the delay was off.

 

[mo2872]

My rule of thumb is 4-5 times the entire rocket length.

A little thread resurrection........in a dual-deploy setup, I assume you would split that cord length between the upper and lower halves of the rocket?

 

[tim cubbedge]

A little thread resurrection........in a dual-deploy setup, I assume you would split that cord length between the upper and lower halves of the rocket?

I usually keep the drogue a little longer than the main but each section is 4-5 times the rocket length, again a rule of thumb but many follow this rule.

 

[mo2872]

So for this particular case, I have a Composite Warehouse Ibex 54, that is 5’ in length, dual deploy. Using that rule of thumb, I’d need 40-50 total feet of harness?? 20-25’ between each section? Seems a bit long, but I‘m new to this DD thing, so what do I know.....

 

[Banzai88]

So for this particular case, I have a Composite Warehouse Ibex 54, that is 5’ in length, dual deploy. Using that rule of thumb, I’d need 40-50 total feet of harness?? 20-25’ between each section? Seems a bit long, but I‘m new to this DD thing, so what do I know.....

Yes, that's not uncommon

 

[tim cubbedge]

So for this particular case, I have a Composite Warehouse Ibex 54, that is 5’ in length, dual deploy. Using that rule of thumb, I’d need 40-50 total feet of harness?? 20-25’ between each section? Seems a bit long, but I‘m new to this DD thing, so what do I know.....

In your case, I’d probably have a 25’ drogue & 20’ main, more or less.

 

[3stoogesrocketry]

I always fly with a shock cord length longer then the tallest tree in the field.....

 

[teepot]

I always fly with a shock cord length longer then the tallest tree in the field.....

Now, that makes sense. But 40 or 50 feet of shock cord. That seems excessive.

 

[3stoogesrocketry]

Now, that makes sense. But 40 or 50 feet of shock cord. That seems excessive.

50 foot tree equals 30+ feet in each section . Shock cord is never a excessive when your booster is hanging just above your head. Also 3 inch and smaller rockets do not need to use 1/4"+ tubular kevlar . I use braided kevlar cord for anything under 3 inch.

 

[Banzai88]

Now, that makes sense. But 40 or 50 feet of shock cord. That seems excessive.

You must have small trees where you are. Along the Mid Atlantic region, we have 60+ feet tall pines EVERYWHERE.

 

[teepot]

Pahrump, Nv. = no trees