TN vs. TK

I am about to start a scratch-build of a 2.25" rocket that I intend to fly on G through I motors. For shock cords I usually go with tubular Kevlar, but I'm thinking of going with tubular nylon for this one. I have not used this material before. I am looking at 2000 lb. test line (overkill?). The TN is 9/16" wide and sells for $0.45 per feet, and the TK is 1/4" wide and sells for $0.70/ft. Any thoughts on the pros and cons of each material? The TN is wider but it is flat, so that may ease the packing. It is also less expensive. I am impressed by the price:strength ratio of TN, but I am curious about the durability. Would I need to use a Nomex cord protector with it? Also, is it safe to anchor it to the mmt? As I mentioned, I am familiar with tubular Kevlar, although I have very little experience with such a high test strength variety.

The 2000 lb. test sounds a bit heavy duty even to me, but the next lowest-rated cord that I can find is 750 lb. TK. The 2000 lb. TN is the lightest test I can find in it (strong stuff!). As you can probably tell, I'm kind of interested in giving the nylon a try.

Oh, yeah, vital statistics: 53.7" long, 2.34" OD, probably about 22-24 oz. empty weight (RS says 20.5 oz.),and motor deploy. I had been thinking of making this my Level 1 cert. rocket, but now I think I may go with something a bit larger in diameter (another scratch-build) because for the cert. flight, I want the rocket to remain in sight. But I digress...

Anyway, thanks in advance for your thoughts/advice.

MK

Personally, between those two, I'd use the 1/4" TK. It packs really small, stands up to ejection charges better, and is overall a great material. I've also had good luck with the TN though, which is both cheaper and a bit stretchier (so you don't get quite as hard of a shock when the bits hit the end of the cord). It does tend to get toasted over time though.

You didn't say how long your shock cord is going to be. That would help to determine which way to go if it were me. A longer cord - say 20 feet or so, go with the nylon (and yes, I use a nomex cord protector when I use TN from the motor mount)... save about half the cost ($6.50 in your case).

You can always do both, and get the best of both worlds... take a 6' long piece of tubular kevlar (I use 11mm Bull Tape), anchoring one on each side of the motor mount, and bring the middle out the top of the booster. Put a swivel there, then use a clip to mount the nylon cord for the rest of the recovery line (and I put a nomex protector right below the swivel, so only the kevlar gets exposed to the gasses and hot particles).

Lots of ways to achieve safe recovery harnesses.

For that size and weight rocket, I would, and do, use the 1/8' TK. It's plenty strong, packs much smaller, and doesn't need the cord protector. I've been using the 1/8 TK for my L1 rocket for a couple of years now. It weights 3lbs 3oz w/o motors. It's only 2" ID so space is at a premium. The TK is perfect.

I didn't go up to the 9/16" TN until I flew my 8 lb. 4" dia. L2 rocket. That has lots of space for recovery. If I had to use something else, I'd use the 1/4" TK for that, although since it's dual deploy and the charges are calibrated pretty well, the 1/8" TK would probably work fine in that too.

9/16" TN will drive you crazy, trying to fit it in there. It's also terrible overkill.

Even the TK, to be honest, is horrible overkill.

BTW, I happen to have several hundred feet of TN in an appropriate size, if you want to go that route. If so, send me a PM and we'll figure out something we're both happy with.

-Kevin

Thanks to all for your responses. I have some 1/8" TK on hand that I will use. I have one follow-up question, though, and it's one that I have been meaning to ask for awhile. A good rule of thumb for shock cord length is anywhere from 3-5x the length of the airframe. I have been observing that for quite some time now. But I have always wondered, but never asked until now, was - what part of the airframe? Do you calculate it based on the overall length of the rocket, or do you base it on the length of airframe aft of the separation point? In rockets that have relatively short nose cones and that eject out of the top of the airframe, it's pretty obvious that you use the first standard. But what do you do with a rocket that has a long payload section or that separates in the middle?

My FSI Eos clone illustrates this dilemma. It is 42.5" long, and about half of that is a payload section that is located above a solid transition section. If I calculate the shock cord length by multiplying the overall length by 5, I get a shock cord that is 17', 8.5" long. The separation point is the base of the transition, and if I base the length on the length of the rocket below the separation point x 5, I get a shock cord length of 8', 9", or about half of the length that was derived using the first method. Which calculation would make more sense in that situation?

The rocket that I am building now isn't quite that extreme as an example, but it does feature a 14" long payload section topped by a 9.7" long nose cone. Basing the calculation on the overall length of the rocket will give me a significantly different figure than If I based the calculation on the length aft of the break. I am unsure of which calculation to use and I am looking for some advice on this final detail.

MK

I've always used the full length of the rocket. My L3 has ~4x the rocket length in drogue cord, and 2.5x in main (and that's of the full rocket length).

I too have always used the full body length.And here is just a tip, when you are crunching numbers, for shockcord length Paracute size ETC. I like to use the largest/safest number, "Better Safe than sorry"

So if one decides to use a Kevlar loop (see Giant Leap Rocketry), is there a simple formula for how much? I know some say the length should be 3-5 x the rocket length but what about the Kevlar strength or weight? In my case, is a 20 feet long x 9/16 Kevlar strap over kill for a 54" rocket weighing ~5.5 pounds?

Using ever longer cords is not always the best solution, though. Extremely long shock cords can be difficult to pack and difficult to deploy effectively, especially with smaller diameter rockets, and they add unnecessary bulk to the recovery system. My goal is to find the most efficient length for the cord. I understand the rationale for longer cords, especially when they are the non-stretching type, such as Kevlar. I have been using long shock cords in my scratch-builds for years and I have been lengthening the cords in kits that I have built when necessary. A "snap-back" is an extremely rare event for me, but when it has occurred, it was because the cord didn't fully deploy or fully extend after deployment. The cord was certainly long enough but its effective length was attentuated by being difficult to extract from the airframe or being so tightly bunched that it didn't completely pay out.

Chain-stitching or rubber-banding the cord doesn't always work in smaller diameter (under 1.5") rockets, because the nose cones (and usually empty payload sections) often have such low mass that they do not carry enough momentum to pull the stitched or bunched cord apart. Kevlar cord isn't particularly slick (in fact, it's downright abrasive), so a confined bunch of it has a fair amount of internal friction. One can reach a point beyond which making the cord any longer can be as harmful as making it too short. This is the reasoning behind my desire to find efficient lengths for my shock cords. I want to keep their lengths below that "coil mass" point. They should be just long enough to be effective, with a bit of an additional length as a safety margin. I haven't come up with a good formula for calculating that ideal length yet, but lately I have been experimenting with basing it on a multiple of the length of the airframe that it is deployed out of, rather than on a multiple of the entire rocket's length. I haven't done enough trials yet to determine if that method works any better, though.

The diameter of the rocket that I am currently building (ID=2.25") and the mass of the payload section will both be large enough that chain-stitching the shock cord will probabl;y work without any problems, so I'll go with the longer calculated amount for it. Thanks for your input.

MK

Exceedingly long cords presents another problem -- when the main deploys and inflates, but the cord isn't fully stretched out. In this case, the parachute stops in the air, while the rocket continues to free fall, until it hits the end of the cord. That sudden stop puts a tremendous shock load on the entire system.

I've seen separations happen at the point that happens.

The logic behind exceedingly long cords is usually because you don't want the cord getting jerked when it hits the end. My suggestion is that rather than going to an excessively long cord, reduce the size of the ejection charge.

But then, I don't much care for the "blow it up or blow it out" philosophy, either.

-Kevin

troj said:

Exceedingly long cords presents another problem -- when the main deploys and inflates, but the cord isn't fully stretched out. In this case, the parachute stops in the air, while the rocket continues to free fall, until it hits the end of the cord. That sudden stop puts a tremendous shock load on the entire system.

I've seen separations happen at the point that happens.

The logic behind exceedingly long cords is usually because you don't want the cord getting jerked when it hits the end. My suggestion is that rather than going to an excessively long cord, reduce the size of the ejection charge.

But then, I don't much care for the "blow it up or blow it out" philosophy, either.

-Kevin

Click to expand...

You explained my dislike of long cord very well. I've seen L3 cert flights fail because the cord seperated after the main opened and the rocket fell to the end of a long cord.

I use cords about 3 times the total length of the rocket. I also loop the cords together and tape them every 3 loops. Then the ejection has to tear the tape to stretch the shock cord, expending energy. Even if the ejection doesn't tear all the tape, the weight of the rocket under the main will usually complete the job.