Recovery harness advice/opinions...

[threelapfancy]

Hi all. Looking for thoughts/opinions/advice on this topic. I am currently building several more HPR to add to my collection, but I will just use 1 example for the purpose of this post. Rocket is a 6" diameter, 10' tall using all Hawk Mountain tubing. Centering rings are home made Baltic Birch. Rocket will be in the 25-30 pound range when completed. Standard dual deploy configuration with an altimeter bay used as a coupler.

So I figure I should use 40-50 feet or harness for the main and drouge. Lately I have been looking at the specs for harnesses, eyebolts, U-bolts, quick links, etc and it seems that all of these have less strength than the harness, no matter if I use Kevlar or climbing webbing.

Kevlar is much more expensive that tubular nylon, and stronger as well as being flame retardant, but I am correct is saying that using a 1" tubular nylon that has a rating of 4200 pounds is going to be a stronger piece than lets say the quick link that is only rated for 1350 pounds? In theory the weakest link is where the failure point could be???

If this is the correct assumption than really the only advantage to using Kevlar would be the flame retardant properties? I have also seen some disadvantages to Kevlar as well, like it does not stretch for example, and of course the cost.

Thanks, all opinions welcome!
Steve

 

[blackjack2564]

On my 6in and larger projects, I have always used 1in TN. There is no ejection charge to worry about, only the altimeter deployment. On that end I simply wrap tape around the 2 ft closest to the eyebolt.

For my harness attachment, 2- 5/16 forged eyebolts and quick links in the 1200-1500lb range. Remember links and eyebolt are rated at the working load, actual load can be 3-5 times more.

 

[Handeman]

Can't help much with the 1", but my advice would be, if your main chute opens very quickly, you might want to shorten the harness on that end. The longer the harness, the further everything can fall and the more likely you'll have a lot of slack in the harness when the main opens. This will cause a significant shock load on the harness when everything hits the end of the harness.

 

[JDcluster]

What size quick links do you plan to use?
I use either 5/16 or 3/8" or even go as far as real climbers locking Carabiners.

I always tape the TN in a zig-zag pattern for most of it's length.

I usually run a 2 strap Y configuration on the booster, it helps to split the weight & prevents possible zippering.


JD

 

[hardinlw]

I went through this with my L3 project planning. The harness was rated at a "test" load of something on the order of 4000 pounds, but that is the load at which a brand new piece of harness with no knots or other stress concentrators failed. That is not an allowable working load which might be more like 1000 pounds. On the other hand, the eyebolts, etc., are rated for the working load which is many times the yield (failure) load. I concluded that 1" nylon harness and 3/8" forged eyenuts with 3/8" high strength threaded rod running through the electronics bay was good for 1000 pounds working load for each path. I used a Y-harness with 2 rods running through the electronics bay.

 

[Rocketjunkie]

Use a long cord (I use 50 feet) for the apogee event. This allows the paert of the rocket to slow down from the ejection event and reduces the jerk when the cord becomes fully extended. Long length is even more important with Kevlar. 10 feet is enough for the main. I much prefer tubular nylon as it will stretch about 15% before breaking while Kevlar will stretch less than 3%. Because of this, the loads on the mounts will be much higher with Kevlar. This also makes zippering more likely. Although Kevlar may have a higher tensile strength (for the same size as nylon), under sudden loads, the energy to break is about the same.

 

[rocketsmith]

Can't help much with the 1", but my advice would be, if your main chute opens very quickly, you might want to shorten the harness on that end. The longer the harness, the further everything can fall and the more likely you'll have a lot of slack in the harness when the main opens. This will cause a significant shock load on the harness when everything hits the end of the harness.

The more slack you have the greater the capacity to stretch and dissipate the forces (at least with tupular nylon, I never use kevlar; no stretch). Also, A longer harness allows the pieces to slow down and dissipate ejection energy before coming to their maximum extension.

 

[slogfilet]

One thing many people consider when choosing Kevlar vs tubular nylon is weight and size. Kevlar is going to be thinner and lighter for a given strength compared to tubular nylon.

For example, I used 9/16" nylon in my 4" LOC IV, but 1/8" Kevlar in my 2.26" Vulcanite. Either material is strong enough for either rocket, but weight and packing size was a big factor.

Since weight is not really an issue in your build, and you're working with a large diamter, tubular nylon may fit the bill. It's cheaper, has significantly more elasticity than Kevlar and is thicker, which helps reduce the danger of zippering.

If you use nylon, I'd definitely suggest a Nomex sheath to protect it from ejection gases. Check your shock cord after each launch; tubular nylon is cheap enough that replacing it isn't much of an issue.

 

[barstoolmike]

I remember reading a while back that your harness rating should be 20 times the wieght of the rocket.

50lb rocket x 20 = 1000 lb - All hardware pieces should be rated at 1000+ lbs

Also anytime Tubular nylon & Kevlar is looped over an item like a Quick link or D ring or Eyebolt, the load rating is cut in half.

So 4000 lb rated Tubular nylon is now down to 2000 lb.

Just what I remember reading.
Mike

 

[Handeman]

The more slack you have the greater the capacity to stretch and dissipate the forces (at least with tupular nylon, I never use kevlar; no stretch). Also, A longer harness allows the pieces to slow down and dissipate ejection energy before coming to their maximum extension.

That is exactly correct for the drogue end. If the main chute opens while the two halves are side by side, or the end with the main (upper) is below the other half (fin can), then the chute will slow the upper half, and the fin can continues to fall. If the cord is long enough, the half with the chute will come to almost a stop because the chute was sized for both halves, not just one. The fin can, which is usually the heavier half, falls and snaps its full weight on the recovery harness against a relatively motionless upper half.

With a shorter harness, the lower half will usually fall to the end of the harness before the upper half is slowed completely by the chute. This reduces the relative speed between the two halves when the harness reaches full extension, thus reducing the shock on the system.

A slow opening main chute also helps reduce the shock on the system.

It comes down to a compromise between cord length/shock when the rocket opens and cord length/shock when the main opens. Since I can control the sizes of the ejections charges and thus the shock at opening, I usually use shorter harnesses to reduce any shock from the main opening.

BTW, at the last launch, I watched a L3 attempt fail because the upper half fell below the fin can after drogue-less apogee deployment and when the main opened, the fin can fell twice the length of the harness before it hit the end of the slack. The attachment point on the fin can failed and the fin can fell free while the upper (lighter then anticipated) floated off.

For motor ejection (which wasn't what the thread was asking about), having the longer harness is definitely a benefit because you can't control exactly when the apogee deployment occurs. I would strongly recommend a slow opening main chute just to let the harness stretch out without shock before the main fully opens since you don't know if it's going to open at apogee or not.