High Power - Harness Length

[troj]

I've got a little research project I'm finishing the noodling on, and I know there are different camps on shock cord/harness length. With that, I'm a bit curious on what folks do.

Assuming a 3" fiberglass airframe, where the total rocket length is 50", what length kevlar harness would you use? What's your "rule of thumb"?

To clarify (a friend asked) - if it's relevant to your answer, assume single deploy at apogee, using electronics.

-Kevin

 

[robopup]

If I were flying tight packed min diameter, which I usually am, probably about 25 or 30ft of 1/8" kevlar depending on what I was doing. I would also z fold the entire length with folds every ~3 in. It's always a good feeling getting a rocket back which has a few z folds unbroken.

 

[roytyson]

I've got a little research project I'm finishing the noodling on, and I know there are different camps on shock cord/harness length. With that, I'm a bit curious on what folks do.

Assuming a 3" fiberglass airframe, where the total rocket length is 50", what length kevlar harness would you use? What's your "rule of thumb"?

To clarify (a friend asked) - if it's relevant to your answer, assume single deploy at apogee, using electronics.

-Kevin

Not sure what you qualify as a harness. My assumption is a harness inside the body tube would be to just extend just below the length of the body tube, and if you can get exact enough, the connection point to the harness/shock cord would be at the top of the tube. This way if there is damage to the cord, it can be replaced vs trying to fix a harness. If your harness comes to the outside of the tube, then damage to the harness is much harder to fix.

If we are talking shock cord length, all I can stuff inside. "more thrust and longer shock cord is always the answer" is French for "min 30 feet".

 

[troj]

Not sure what you qualify as a harness. My assumption is a harness inside the body tube would be to just extend just below the length of the body tube, and if you can get exact enough, the connection point to the harness/shock cord would be at the top of the tube. This way if there is damage to the cord, it can be replaced vs trying to fix a harness. If your harness comes to the outside of the tube, then damage to the harness is much harder to fix.

If we are talking shock cord length, all I can stuff inside. "more thrust and longer shock cord is always the answer" is French for "min 30 feet".

What you're referring to as the shock cord is what I'm referring to.

So you'd shove 100' in there, if it would fit?

-Kevin

 

[3stoogesrocketry]

Kevin , being from the northeast, trees are always a concern. I will always try to get at least 10 to 1 length on my cordage. I would put 50 feet in your 5 foot rocket. In fact, i have three separate 4 inch rockers that use a 48 foot drouge cord each.

 

[tfish]

15 feet

roughly 4 times the length of the rocket..


Tony

 

[ep29030]

I like to do 4-5X length of the rocket, so I would use about 20ft. I Z-fold the Kevlar and put rubber bands around the folded cord to bleed off some of the snap/shock at the end of the stretch. I've been using 1500# Kevlar mule tape lately, as I bought a large length of it a couple of years ago. No issues.

 

[roytyson]

What you're referring to as the shock cord is what I'm referring to.

So you'd shove 100' in there, if it would fit?

-Kevin

Well, Ogre had 40' on the drogue and 60' on the main, so close... I think you seen my level of dedication to stuff that all in where it didn't fit!
I just enjoy a long shock cord. Everyone gives me a hard time when they tell me the rocket lands in one county and the nose cone in the next.

 

[Jmhepworth]

I like to say “long harnesses are your friend.” I go 5 feet of harness for every foot of air frame for both the drogue and the main. I only depart from that when I can’t make it fit, like in the booster of a two stage when the interstage couple takes up a lot of space.

 

[Rocketjunkie]

Apogee, about 50' if it will fit. Main only needs to be long enough to get the chute(s) away from the rest of the rocket. I use around 10-15'. Have 1/4", 3/8", 1/2", and 1" depending on the size of the rocket. Prefer nylon. Kevlar has only about 1/3rd the extension before break so puts 3x the load on your attachment points. Kevlar will zipper fiberglass tubes where nylon won't but may be cut. Nominal flights either works but the long arcing flights or an altimeter malfunction is when the difference shows up.

 

[T-Rex]

I tend to go with "Looks about right" which seems to work out to about 4x the length of the rocket in most cases. Sometimes I'll go 5x if the voices in my head think it's a good idea.
I guess I also factor in the material; kevlar gets a bit longer than nylon.

 

[jderimig]

I do about 15-20ft REGARDLESS of the length of the rocket. I then z-fold and use masking tape at the folds to disspate energy. Then tune the ejection charge based on how may z-folds I rip.

 

[Daddyisabar]

The longer the cooler. I marvel at the magnificent crochet jobs and the careful packing some of the old dudes have done at the launch. That's a really long cord you got in yer WAC CORPORAL!

 

[Glasspack]

Nice post Kevin ....

I certainly dont have as much experience as some of you,
but I have always thought
Tims words from Apogee
were ok .....
"You really cant have to
long of a shock cord"

Provided of course it fits....

I like the Z-fold with masking tape on mine