Is Polyester webbing Ok to use for Harness?

[Scottc99]

I am still in the planning phase of building a high power rocket. I am planning to build a LOC Fantom 438 EXL and estimate it will weigh in the 3-4 pound range. Most the posts I read on recovery harnesses use Kevlar or tubular nylon. I have not seen any threads that talk about Polyester webbing. On the Strapworks website 5/8" has a breaking strength of 2000lbs and melting point of 500 deg. While 1/2" tubular nylon has abreaking strength of 2000 lbs and melting point of 380 deg.

My question is: Is Polyester webbing a good choice for recovery harnesses, or are there other factors that make it less desirable such as fraying easily or weight or ?

Thanks for your input.

 

[blackjack2564]

My only experience with it;

Not very flexible, but rather "stiff". Hard to fold and place in airframe. Think of poly rope vs nylon...also very stiff comparatively speaking. That you can find in big box stores for comparison, with out having to buy.
Tubular nylon is soft and pliable.
Kevlar almost as TN.

This was based on only one type..... I had on hand.
I would definitely try a sample length before I committed to a bulk amount.

 

[Handeman]

My PML Callisto is the only rocket I have with nylon strap in the harness. I built it in 2003 and it used a 1" nylon/polyester? strap for the MMT to piston connection. After at least 60-75 flights it broke the tube on landing on a cold day. I checked the strap all the way to the MMT and it had some frizzies on it from the heat, but was still in very good shape. I put a coupler in the tube and fixed the BT and have been flying it without worry about the strap since.

I do agree with Jim, it is stiffer than the tubular stuff, but I haven't had any issues because of that.

 

[jazzviper1]

I would say NO.
Polyester is not as strong as nylon and less resistant to heat.

 

[dave carver]

I have wrapped the strap with duct tape down inside where it attaches, near the motor well ejection charge. You can see the scorching on the tape but it hasn't penetrated to the strap as far as I can tell. I had one small rocket , a 3"er that flew countless times with the favorite motor an AT I357-T. I didn't wrap the strap and it did eventually burn through. There's also Kevlar sleeves that are intended to cover a strap that would melt to protect it.

 

[jcato]

My question is: Is Polyester webbing a good choice for recovery harnesses, or are there other factors that make it less desirable such as fraying easily or weight or ?

I agree with Mike ('JazzViper') -- polyester (i.e. Dacron) is generally not as good as nylon - for any of various reasons, but the one that I most adhere to is the fact that it is of a 'higher modulus' (i.e. is less stretchy) and, thus, will put more shock loads on the rest of the vehicle. There will be those who remember how much I was 'down' on those glorified rubber bands (i.e. 'bungee') in years past and may think talking about 'stretchy' seems bizarre coming from me. I'm not down on 'stretchy', I'm down on 'weak and inappropriate' for recovery, which bungee is. At least nylon is a rational compromise - *some* stretch, but also high enough tenacity (i.e. strength) to handle the chaos that recovery deployment really is. By that logic, Dacron polyester is not indicated, as it is stiffer and a bad choice for 'shock loading'.

Remember this as well: If your anchorages are robust enough, it *can* handle shock loading -- that's just good design. Remember, the two pieces connected together with whatever 'bridle' you have are *both* "free bodies in space" (i.e. one end is NOT bolted down), and thus, any shock loading will accelerate BOTH parts in a way to dissipate that excess energy (provided anchorages and connecting material is of that level of strength). Don't get too carried away worrying about all that 'hard shock hitting the end'. Design your anchorages in a way that can handle those unforeseen and unplanned loads - use a material capable of those levels of energy and then don't worry.

Dacron *does* have higher resistance to UV radiation - which is why it is almost exclusively used for sails on sailboats (lots of time exposed to the sun). Rocketry recovery is much less exposed (although it surely is exposed during its function interval) - just have to watch for degradation of nylon components and replace when needed.

Tubular nylon is about as good a material for recovery harnesses and other rigging as one can find. Protect it from flame in the 'high heat' areas (Kevlar or Nomex sleeves, etc) - but use it's lower modulus to your advantage.

-- john.

 

[Scottc99]

Thanks guys. Great stuff. This is exactly what I was looking for. A couple "old salty " pros who have been there and done that. Needless to say I will be looking into tubular nylon or maybe Kevlar.

One other question, is there a formula for the length of the recovery harness. My plan is a LOC Fantom 438 EXL. Single deployment for my L1, but eventually dual deploy.

Thanks.

 

[Onebadhawk]

I would say NO.
Polyester is not as strong as nylon and less resistant to heat.

+1..
I believe poly is less heat resistant then nylon...
I am not understanding where your heat resistance figures came from
and why they seem to show the opposite...

Teddy

 

[bobkrech]

Polyesters and Polyimides are different types of polymers, but there are many different polyesters and polyimides.

For example, nylon is a polyimide, but there are literally hundreds of nylons.

Some nylons are not as heat resistant as the most common polyester, however most nylons have equal or more heat resistant than the most common polyester.

You need to read the fine print and check what the heat ratings are for each specific fabric. The key word is melting point. It will take a finite amount of time for a fabric to melt, but once the fabric melts, it has zero strength and the game is immediately over.

Bob

 

[Handeman]

One other question, is there a formula for the length of the recovery harness. My plan is a LOC Fantom 438 EXL. Single deployment for my L1, but eventually dual deploy.

Thanks.

I think you need to start a new thread on that one. You'll get a lot of opinions on that and very little agreement.

Personally, I think shock cords should be no more than 3 - 4 times the length of the rocket with the drogue side shorter. I also advocate a minimum amount of ejection charges so you don't stress the recovery harness. Others subscribe to the "blow it out or blow it up" philosophy and advocate long shock cords to compensate for the large ejection charges.

Its always an entertaining argument.

 

[Onebadhawk]

I think you need to start a new thread on that one. You'll get a lot of opinions on that and very little agreement.

Personally, I think shock cords should be no more than 3 - 4 times the length of the rocket with the drogue side shorter. I also advocate a minimum amount of ejection charges so you don't stress the recovery harness. Others subscribe to the "blow it out or blow it up" philosophy and advocate long shock cords to compensate for the large ejection charges.

Its always an entertaining argument.

That is exactly correct,,,
Too large of a charge and you'll need a band-aid for the fault,, an extra long harness....
And too short of a harness on the drogue side and you'll need another band-aid for a different fault,, heavier / more shear pins in the nose cone,,,
which will mean more energetic charge to blow the nose,, which will mean a longer harness for the nose / top / main........

Much easier to get everything right on the ground in the first place......

OK,,,
Now,,,
Everybody get ready,,,,
DUCK!!!!!!!!!!!

Teddy