Recovery Harness Sizing (diameter/strength)

[wonderboy]

I'm finishing up the rocket build for my L3 attempt. I've built a LOC VII. I've built relatively heavy (glassed airframe, full length coupler, redundant electronics, etc) and expect my dry weight to be around 25-30 pounds. I purchased a pair of 1/4" tubular Kevlar recovery harnesses from LOC when I ordered the kit. These ones (1/4" 25 Foot): https://locprecision.com/collections/model-rocket-recovery/products/tubular-kevlar-shock-cord

I'm second guessing the diameter/strength of my choice and am wondering if I should upsize. The 1/4" LOC harness says its good to 1900 pounds. I've found these harnesses on Wildman's site, which are significantly stronger: https://wildmanrocketry.com/collections/onebadhawk-harnesses/products/obh-011

These 1/2" harnesses are rated at 7200 pounds.

Assuming the rocket is on the heavy side at 30 pounds, the 1/4" Kevlar is good to ~63g decel, and the 1/2" Kevlar is good to ~240g.

I do have a case of over-analyzing everything right now, but this is one that is simple enough to upgrade. What are people's opinions on the strength required?

 

[ep29030]

My rule for the recovery train (shock cord thru parachute): design for 50-100X the rocket’s mass at time of flight. Over 100X is excessive; under 50X will result in shred for flights gone bad. That is based on Knacke’s Parachute Manual, where there are extreme examples that generated 50-70x g loads on recovery system. As to length, I go with 4-5X length of the rocket. If I am using Kevlar cord, I Z-fold the cord and put several rubber bands around the Z-fold. As they are broken, they reduce the jerk on the cord when it fully stretches.

 

[troj]

The reality is that at 1900lbs, the harness likely isn't the weak point in the system. It's probably an anchor point, or induced stress due to a knot. Maybe even the parachute, depending on what it is and how it's constructed.

One of the reasons I like wider is it's just a bit more surface area for if it hits the edge of the tube hard. Minor additional resistance to starting to zipper.

If you use appropriately sized ejection charges and a bagged main with a pilot chute, the shock load on the harnesses shouldn't be near that high. There are, of course, no guarantees.

-Kevin

 

[Handeman]

What @troj said. Kevin is right, the shock cord probably isn't the weak point. Also, along with the right size ejection charges, especially at apogee, there is another thing you need to watch for. You need the right size drogue. If your rocket gets into a nose/payload down position and drags the fincan after it. The main can be ejected downward, open and almost stop in the air before the payload section hits the end of the upper shock cord. The worse part, other then a part fouling the main, is the fin can continuing to fall past the main chute, payload section, and hitting the end of the apogee shock cord very hard. The longer the cords, the larger the speed differences and the higher the shock loads. I usually see this several times each launch and things almost always stay together and the flier is congratulated for a nice flight. I think it was just dodging a bullet they probably didn't even know was coming, but I think I'm in the minority.

I did see one level 3 that was using 100 ft. of seat belt webbing as an apogee shock cord. The whole rocket was over 100 lbs. The main and payload section stopped first and the fin can fell past them and never even slowed down when it hit the end of that shock cord. It just ripped the anchor point out of 3/4" aircraft plywood and dropped 1000 ft. ballistic.

A recovery system isn't just the components, it how they all work together for the whole recovery from apogee to ground. Every component and how it operates affects every other component and it's operation.

 

[Wrightme43]

I have learned alot from Ted @Onebadhawk he will help you too. Write him a email and explain what you want to have happen, and he can create exactly what you need. His prices are more than fair, and include a tremendous amount of experience he learned the hard way.

You have much more experience than I do. I mean no disrespect, and I only want you to succeed!

Steve

 

[RokitFlyr]

To answer your question, go with the 1/2".

 

[bobbyg23]

Just curious what your taps say about it? They should have gone over that when you submitted your paperwork.

 

[lr64]

If you really want to know, make up dummies of each fastening point and see how much tension it takes to yank the ring out of each point. Add an allowance for knots and wear and tear on the cord. There is no point making it any stronger than that.

I still wonder why so many people have gone to Kevlar. I just poked around Matweb, looking at Kevlar 49 and Nylon 66 fibers. Let's say we size our shock cords by yield strength. An elastic modulus of 16,300 kpsi, and a yield strength of 435 kpsi means that the elongation at yield is 2.7 percent. For nylon 66, average elongation at yield is given as 6.8 percent. So our nylon cord will have absorbed about 2.5 times as much energy at yield. For a given amount of energy to be absorbed, the force will be only 40 percent as much. The numbers I'm getting are a little funny, though, since the elongation at break is given as 2.4 percent. If that's true, then the Kevlar is absorbing even less energy. In a really bad situation, the nylon will stretch 27 percent before breaking. If the elongation at break numbers are right, that's over 10 times as much energy absorbed.

This probably means that the forces on the rings, etc. are going to be far less. At least when two masses come to the limits of the cord. It's probably a bit more complicated with one mass and a parachute, but I wonder if that's really the failure mode. If an e-bay and a heavy nose cone are at one end of the cord, and the rest of the rocket is at the other end, then the forces are likely to be much higher than if the whole rocket is at one end and a light nose cone is at the other.

I wonder if the seat belt webbing on the level 3 rocket that Handeman saw would have ripped out the anchor point if it had been stripped to 1/3 of its width, softening the forces. Even at 1/3 the width, it should have been good for 1,000 to 2,000 lbs before breaking. A quick search on the net suggests that seat belt webbing is polyester, but nylon webbing would have more stretch and soften the shock.

For a given strength, Kevlar is going to be considerably lighter than nylon. However, it's likely stronger hardware will be necessary, and zippers will be more likely unless you reinforce the end of the tube.

Kevlar 49's service temperature is given as 149 to 177 C. I couldn't find that for nylon, but the average melting temperature was given as 244 C.

One thing I learned is that there are several flavors of Kevlar, and MANY of nylon, depending on which flavor and just how its made. Kevlar 29 is supposed to absorb somewhat more strain energy than the 49, and Kevlar 149 has a far higher service temperature. There are too many flavors and preparations of nylon to shake a stick at.

Another factor I didn't even look into was the effect of different weaves, etc.

 

[wonderboy]

Just curious what your taps say about it?

My TAPs are fine with the original plan to use the LOC 1/4" Kevlar harness. As I mentioned in my original post, I just wanted to get additional opinions and viewpoints on the topic. Based on what I'm reading here so far, it seems to me that going with the heavier harness buys more margin but isn't critical and it is more important to follow proper rigging techniques and proper layout.

I appreciate people's input, thank you all!

 

[lr64]

If the heavier harness isn't necessary, it just makes something ELSE more likely to break.

 

[troj]

I still wonder why so many people have gone to Kevlar.

That one is easy - heat tolerance. If you expose nylon webbing to ejection charges, it degrades. It doesn't matter with Kevlar.

With nylon, I wouldn't buy any generic nylon found online, as you don't know what it really is - as you said, there are different types. If you're going to use nylon, buy the stuff with ID threads and buy it from a reputable source. That way, you know what it's rated at.

-Kevin

 

[b.wieting]

If you really want to know, make up dummies of each fastening point and see how much tension it takes to yank the ring out of each point. Add an allowance for knots and wear and tear on the cord. There is no point making it any stronger than that.

I still wonder why so many people have gone to Kevlar. I just poked around Matweb, looking at Kevlar 49 and Nylon 66 fibers. Let's say we size our shock cords by yield strength. An elastic modulus of 16,300 kpsi, and a yield strength of 435 kpsi means that the elongation at yield is 2.7 percent. For nylon 66, average elongation at yield is given as 6.8 percent. So our nylon cord will have absorbed about 2.5 times as much energy at yield. For a given amount of energy to be absorbed, the force will be only 40 percent as much. The numbers I'm getting are a little funny, though, since the elongation at break is given as 2.4 percent. If that's true, then the Kevlar is absorbing even less energy. In a really bad situation, the nylon will stretch 27 percent before breaking. If the elongation at break numbers are right, that's over 10 times as much energy absorbed.

This probably means that the forces on the rings, etc. are going to be far less. At least when two masses come to the limits of the cord. It's probably a bit more complicated with one mass and a parachute, but I wonder if that's really the failure mode. If an e-bay and a heavy nose cone are at one end of the cord, and the rest of the rocket is at the other end, then the forces are likely to be much higher than if the whole rocket is at one end and a light nose cone is at the other.

I wonder if the seat belt webbing on the level 3 rocket that Handeman saw would have ripped out the anchor point if it had been stripped to 1/3 of its width, softening the forces. Even at 1/3 the width, it should have been good for 1,000 to 2,000 lbs before breaking. A quick search on the net suggests that seat belt webbing is polyester, but nylon webbing would have more stretch and soften the shock.

For a given strength, Kevlar is going to be considerably lighter than nylon. However, it's likely stronger hardware will be necessary, and zippers will be more likely unless you reinforce the end of the tube.

Kevlar 49's service temperature is given as 149 to 177 C. I couldn't find that for nylon, but the average melting temperature was given as 244 C.

One thing I learned is that there are several flavors of Kevlar, and MANY of nylon, depending on which flavor and just how its made. Kevlar 29 is supposed to absorb somewhat more strain energy than the 49, and Kevlar 149 has a far higher service temperature. There are too many flavors and preparations of nylon to shake a stick at.

Another factor I didn't even look into was the effect of different weaves, etc.

It strikes me that this situation is much like tow straps vs recovery straps in 4-wheeling. ARB recommends different straps based on gross vehicle weight, aiming to get the right amount of elastic stretch to limit the peak force on recovery points like tow hooks.

 

[Ronz Rocketz]

That one is easy - heat tolerance. If you expose nylon webbing to ejection charges, it degrades. It doesn't matter with Kevlar.

With nylon, I wouldn't buy any generic nylon found online, as you don't know what it really is - as you said, there are different types. If you're going to use nylon, buy the stuff with ID threads and buy it from a reputable source. That way, you know what it's rated at.

-Kevin

It's funny. I was wondering why anyone uses Nylon. I was just meeting with a high school rocketry class and they had an intricate 3D printed motor mount that looped Nylon twice through the can. I steered them toward 400lb Kevlar instead.

 

[KenECoyote]

It's funny. I was wondering why anyone uses Nylon. I was just meeting with a high school rocketry class and they had an intricate 3D printed motor mount that looped Nylon twice through the can. I steered them toward 400lb Kevlar instead.

Loc Precision provides a pretty nice nylon Y-harness with their large kits (including the Skinwalker which many consider a "very impressive" rocket) and the kits include quick link attachments to u-bolts.

The nylon has stretch and generally lower cost while the Kevlar has heat resistance and greater strength for the same size/smaller packing.

 

[Capt. Eric]

I had a nylon cord that came with the kit (PML Bull puppy 2.2) burn through on the 1st flight. Fortunately it didn't result in much damage. Much prefer to use Kevlar now for its heat tolerance. On the rare occasion I use nylon, it's oversized, and I shield it with metal duct tape near the mmt and keep the rest above the dog barf. But I still don't like it very much.

 

[troj]

I've seen folks have success in wrapping nylon in duct tape to protect it from the ejection charge - I'm not a fan of this, because of the gooey mess that the adhesive makes. The readily available nomex sleeves are a much cleaner approach, in my opinion.

That said, I rarely use nylon.

-Kevin

 

[Capt. Eric]

I've seen folks have success in wrapping nylon in duct tape to protect it from the ejection charge - I'm not a fan of this, because of the gooey mess that the adhesive makes. The readily available nomex sleeves are a much cleaner approach, in my opinion.

That said, I rarely use nylon.

-Kevin

I hear you. The metal stuff doesn't get all gooey like the plastic fabric crap. It is a little stiff though, so you need some room in the body length. Wouldn't work well in a Big Daddy.

 

[SolarYellow]

What about a few feet of Kevlar for the flame resistance and then nylon for most of the harness for the stretchiness?

 

[Tractionengines]

What about a few feet of Kevlar for the flame resistance and then nylon for most of the harness for the stretchiness?

Yep, but you still need to address 2 things.

NO KNOTS. They weaken the cord too much.

Sizing still needs to be selected to give balance.

 

[lr64]

Yep, but you still need to address 2 things.

NO KNOTS. They weaken the cord too much.

Sizing still needs to be selected to give balance.

How is a knot to splice worse than a knot at the end?

Balance? Why would this matter? The stretch certainly wouldn't be balanced.

 

[troj]

NO KNOTS. They weaken the cord too much.

Use the correct knots and size accordingly and you're fine.

Climbing knots are your friend.

 

[Tractionengines]

How is a knot to splice worse than a knot at the end?

I don't tie a knot to join the kevlar to the nylon (usually). Splice an eye into one, then slice an eye into the other thru the first one. Done... no knot, no quick link, no extra weight, etc. Only requirement is that both cords must be a splicable material.

Balance? Why would this matter? The stretch certainly wouldn't be balanced.

Misunderstanding in my very short reply. I meant balanced in that rated capacity should match, and fit in with the overall recovery system. ie 1/4" tubular kevlar at 1900# could be paired with 3/8" tubular nylon at 1100# (or) with 5/8 tubular nylon at 2200#. The 1100# will have more stretch at the same load (shock absorption), and smaller size than the 2200#, but is it enough capacity.

If you use the same "sized" materials, the nylon will be MUCH lower rating than the kevlar.