Estes Pro Series and Shock Cord Length

[RickGr4]

Rocketry Friends,

This weekend I built eight Estes rockets including four Pro Series rockets. I expected the shock cords for the Pro Series rockets to be longer than LPR/E2X rockets but they are even longer than I expected. Usually when I build a rocket I make sure the shock cord is longer than the overall length of the rocket but the shock cords included with the Pro Series are much longer than that.

I have read in this forum that formulas exist for proper shock cord length. However I was hoping someone might provided me with a quick shortcut.

TIA,

Rick

 

[dhbarr]

It's all a bunch of handwaving & guesswork. If you're unsure use a little longer until it ground tests well.

 

[neil_w]

3x the length of the rocket is a rule of thumb often bandied about here. I tend to make my LPR shock cords somewhat shorter than that, usually closer to 2x length of the rocket, and have had no problems. In any case, most builders will always use a shock cord that is a *lot* longer than the rocket itself.

 

[CPUTommy]

I bought elastic material that's 20 feet long for 1 dollar, I use that on my MPR builds, being extra long is never a bad thing, being to short.. not good, I have a nose cone still flying, (shock cord broke and nose cone held chute open and up up and away it went, we all watched it get smaller and smaller and smaller until it was gone)

All my cords are extremely long now and I've never had that problem again.

 

[Nytrunner]

If you build the PSIIs stock, that piece of elastic is fine for length. But make sure you protect it from ejection gas more than adequately!

It's not as tough to the hot gas as Kevlar or tubular nylon. I 've been messing with how my PSIIs go together, and the shock cords get mounted further back, so I wrap the potions nearest to the motor in ducttape. Next couple of builds will have attempts at baffles to mitigate that a tad.

 

[Kallahan11]

The elastic is fine, longer is better unless your going for altitude records and want to be a weight weenie. the 3x rule is for kevlar and nylon as they do not stretch and need more time for the nose cone to decelerate to stop zippers.

 

[RickGr4]

Thanks for the advice friends.

I am taking the eight rockets I mentioned in my original post to the Tripoli-MN launch this weekend. In regards to the four Pro Series rockets (Mammoth, Ascender, Prowler and Trajector) I am going to leave the shock cords at the supplied length (roughly two times body length) unless I see a reason to do otherwise.

That being said, I have some rockets which I fly regularly that I am going to take a closer look at the shock cords. They might be a bit short.

 

[Raffy]

We have an Estes Patriot rocket in which I put a Kevlar shock cord that is about 2.5 feet long, or just a bit longer than 1 times the length of the rocket. It has done just fine on 3 flights so far. Most of you all probably know this, but I would strongly recommend against using the junky rubber shock cords that come with a lot of Estes rockets. They get brittle over time and will eventually break, often resulting in a nose cone that goes bye-bye and a rocket body that impales the ground. I use a spool of Kevlar kite twine that has 300 lb strength; it seems to work very well.

 

[neil_w]

We have an Estes Patriot rocket in which I put a Kevlar shock cord that is about 2.5 feet long, or just a bit longer than 1 times the length of the rocket.

That is rather short for a non-elastic shock cord. The concern would be the force of the "tug" exerted on the mount when the nose reaches the end of its travel. A longer shock cord will give the nose cone more time to decelerate, and hopefully not be moving too quickly by the time it reaches the end of its travel. That is one reason why HPR rockets, with non-stretchy cords, use very long ones.

Elastic cords will absorb some of the shock, so they can be a bit shorter.

Your 1x kevlar cord might work fine and last for a lot of flights, but it is likely putting excessive stress on the mount and the NC attachment.