is there such thing as a shock cord that is too long?

[outasight13]

as an experiment/joke, i put a 20 foot long 1/4 " elastic shock cord in my executioner. Is this too long, or is there no such thing really.
i always see pictures of HPR with REALLY long shock cords, so this should work, but it might be a little bouncy right?

 

[richalex2010]

i think the only time a shock cord is too long is when:
a. It won't fit in the body tube
b. the body hits the ground while the nose is still under chute and the shock cord hasn't come taut yet

In other words, no there is no such thing

Anyone? am I wrong? right?

-Jake


Oh yeah, I have a shock cord at least 20' long on my Stormcaster. Not too long, I have at least 2 flights on it with no damage.

 

[snuggles]

My EZI was equipped with a 50 foot(yes 50 foot) long TN line.My L1 flight was announced as a separation cuz it took SO long for things to pull taut. Needless to say, I did get my L1.
No there is no such thing as a shock cord that is too long.
I currently use yellow masons line for all my LPR and MPRs . At least 3 X the length of the BT. never had a cone snap back.
Works for me.
Mark T

 

[Ted Cochran]

Let's discuss this a bit.

I think that "longer is better" is mostly a misconception, especially when larger models are involved. It's like saying "more BP is better" or "more thrust is better."

--Longer chock cords do not "give the parts time to slow down," except for very light or very draggy models, or degenerate cases involving extremely long shock cords and rockets deploying well before apogee. After apogee, all parts of the rocket are always accelerating due to gravity. A really long shock cord just lets the part without the parachute attached accelerate (downward) to a higher speed before being brought to an abrupt stop at the end of the shock cord. This is not A Good Thing.

Further, for practical purposes, the relative velocity among the parts that is imparted by the ejection charge does not change appreciably before the end of the shock cord is reached (unless the cord is hundreds of feet long . The aerodynamic drag just isn't big enough.

--Longer shock cords can enable the parachute, if it is attached near the nose cone, to fully open and then be hit by the fin can, following in trail. This happens more often than you might think!

--Longer shock cords _significantly_ increase the odds that your rocket will end up in any downwind trees and powerlines you may have. Think of it as making the target (the tree) much bigger with respect to the net (the shockcord).

So, how long should the shock cord be? It usually only needs to be long enough to enable the parachute to get out and get opened. If you attach the parachute to the fin can, and not the nose cone, the shock cord can be fairly short--it has to be longer than the rocket, to keep the nose cone from clanging against it during descent. If the nose cone is quite heavy (e.g., if it is full of weight), you can use a longer shock cord to let the nose cone land first, and give the fin can time to slow down before it lands.

If the ejection charge is forceful enough to cause the nose cone to reach the end of the shock cord and bounce back, there is an argument for increasing the length of the shock cord to reduce the chance that the parts will collide on the bounceback--but there's a better argument for decreasing the ejection charge!

It's far more important that the shock cord be STRONG enough. Deployment forces, especially during early or late ejections, can be inconceivably high-- a ten pound rocket can generate 1000 pounds of force fairly easily--so everything you do to manage those forces and size the shock cord to handle what you might not be able to control will save you rockets in the long run.

That's my opinion, FWIW, but I'd be happy to hear other points of view!

--tc

 

[graylensman]

Originally posted by Ted Cochran


If the ejection charge is forceful enough to cause the nose cone to reach the end of the shock cord and bounce back, there is an argument for increasing the length of the shock cord to reduce the chance that the parts will collide on the bounceback--but there's a better argument for decreasing the ejection charge!

--tc

Well, I fly only low power stuff - so decreasing the ejection charge really isn't an option. on the other hand, I make my cords roughly 2x or 3x the length of the body tube, and this generally handles the problem.

 

[cjl]

I usually use 3-5x the length of the rocket (on DD, I use 3-4x the length for each, usu. a little shorter on the main). 20' should be fine on an executioner, it won't be a problem (as long as it fits in the bt)

 

[JRThro]

Originally posted by graylensman
Well, I fly only low power stuff - so decreasing the ejection charge really isn't an option. on the other hand, I make my cords roughly 2x or 3x the length of the body tube, and this generally handles the problem.

That's just what I was going to say, that I really can't reduce the ejection charge since I only use 13 mm and 18 mm BP motors. And the length of unstretched shock cord that extends outside the body of my rockets is generally about 2x the body length.

 

[aksarben10]

You know I actually saw a cord that was too long once. It was a HPR rocket, some of you may have seen it as it belongs to a Kloudbuster member and is known as the 'Toast" Rocket since the fins are shaped like toast and maybe for the number of times it has crashed and been rebuilt.

Anyway, I do not know how long the cord was but the rocket was set for dual deployment the when the main popped on the way down the rocket body pummel to the ground. The cord was to long and the main never inflated as there was never any weight on it since the cord lenght allowed the tail section to reach the ground.

The 'Toast' Rocket has since been rebuilt and flown many many more times. It is a great rocket, I once saw it do a tail slide and a flip backwards before deploying, it never fails to amaze.

Scott Pearson

Originally posted by richalex2010
i think the only time a shock cord is too long is when:
a. It won't fit in the body tube
b. the body hits the ground while the nose is still under chute and the shock cord hasn't come taut yet

In other words, no there is no such thing

Anyone? am I wrong? right?

-Jake


Oh yeah, I have a shock cord at least 20' long on my Stormcaster. Not too long, I have at least 2 flights on it with no damage.

 

[Silverleaf]

So, how long should the shock cord be?

IMHO:

IF the nosecone can clear the fins with the shock cord/ kevlar fully extended, thats long enough in my book.

In most cases (not all) that simple rule of thumb will prevent damage to the fins and nosecone (from each other) as they pass by upon ejection.