Recovery harness question

[davdue]

I've only built one HPR but I have always used tubular nylon for it and MPR. Why do people use Kevlar? It seems like it would very unforgiving with no stretch at all.

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[chadrog]

It does not burn.

[stantonjtroy]

Typically I use a length 3 to 4 times the rocket length. With this I get sufficent Aero/drag breaking of the parts that when the cord maxes out, there's very little momentum left. No zipper problems yet. (Knock Wood)

[troj]

I've only built one HPR but I have always used tubular nylon for it and MPR. Why do people use Kevlar? It seems like it would very unforgiving with no stretch at all.

Because you're not relying on stretch or elasticity in high power, and Kevlar doesn't burn, whereas nylon will melt/burn.

FWIW, nylon only stretches a few times, then it no longer has any stretch, either.

-Kevin

[davdue]

Because you're not relying on stretch or elasticity in high power, and Kevlar doesn't burn, whereas nylon will melt/burn.

FWIW, nylon only stretches a few times, then it no longer has any stretch, either.

-Kevin

I have never had a problem with nylon burning but I do always use a nomex blanket and dog barf. I also use 3-4 times the length of the rocket like stantonjtroy recommended.

[JoeG]

Because you're not relying on stretch or elasticity in high power, and Kevlar doesn't burn, whereas nylon will melt/burn.

FWIW, nylon only stretches a few times, then it no longer has any stretch, either.

-Kevin

+1 for Kevin

If you are depending on stretch in the cord your cord is not long enough. Personally, I use at least 5 times the length of the rocket as a rule. When using dual deploy both of my harnesses are that long. Harness is cheap compared to most of the consequences of a harness being too short. Also a very long recovery harness might allow you to reach at least one section of your rocket if it lands in a tree. Never mind how I know.

[jimbo]

+1 for Kevin

If you are depending on stretch in the cord your cord is not long enough. Personally, I use at least 5 times the length of the rocket as a rule. When using dual deploy both of my harnesses are that long. Harness is cheap compared to most of the consequences of a harness being too short. Also a very long recovery harness might allow you to reach at least one section of your rocket if it lands in a tree. Never mind how I know.

+2 for Kevin and Joe

Stretchy stuff is for underwear. :-) IMHO kevlar is better than TN simply because it is flame resistant. Length is your friend.... how many times have I seen a DD rocket pop the main at apogee because the drogue harness was way too short. The upper section of the rocket hits the end of the harness very hard because the harness is too short and doesn't allow the upper section to slow down. Rapid stop......nose cone keeps going and pulls the main out. Measure out what you think is enough, then add some more. I use a LOT of harness in the drogue section of my conventional DD rockets

Hey Kev, how's that big project coming along? Full "what" to "what" :-)

Jim Ballard
Tripoli West Texas
TRA 10883 L3

[blackbrandt]

I use 18 feet of Kevlar in my optima so I can grab it in the thorns easier.

[Disaster_Guy]

Totally agree, it is more about the length than material. Even on my short Gizmo I used 15' of harness. I use 1' climbing webbing for most of anything HPR because it is strong and cheap but I also use nomex or PBI protectors for the lower length.

[cwbullet]

Totally agree, it is more about the length than material. Even on my short Gizmo I used 15' of harness. I use 1' climbing webbing for most of anything HPR because it is strong and cheap but I also use nomex or PBI protectors for the lower length.

Ditto. I use the same.

[Handeman]

I have yet to see a kit that supplied enough shock cord to satisfy me. I always replace or add to what comes with the kit.

It is hard to get too much shock cord in a motor ejection rocket, but it is possible.

I've see a lot more DD rockets with too long of shock cord then too short. More is not always better. If your main deploys at apogee because of the shock of hitting the end of the shock cord, you're using too large of deployment charge. I think "Blow it Out or Blow it Up" is a just an excuse for being too lazy to properly design and test the deployment system. I have never seen a rocket come apart because of an inadvertent apogee deployment, but I have seen a 40 lb booster section free fall because the deployment system failed after the main deployed and the booster section free fell to the end of a too long of shock cord causing the shock loads to rip the anchors apart.

[cwbullet]

I always ground test and add a little extra to ensure the charge is large enough. Call it superstition or paranoia.

[jimbo]

I always ground test and add a little extra to ensure the charge is large enough. Call it superstition or paranoia.

I do the same Chuck. I add 15-20% for flight and it has worked very well for me.

[davdue]

Here is a pic of my L2 test flight on an AT I211. I used 9/16" X 15' TN from Top Flight. The complete recovery system (except hardware) is from Top Flight. I plan to do my L2 cert at AirFest on an AT J415

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[jimbo]

Good luck with your L2 flight Dave! That J415 is a great motor. I really wanted to go to Airfest this year but I will be getting ready to move about that time.