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DD Recovery Harness Length
#1
I'm [slowly, as I'm studying a lot about it] getting my feet wet with dual deploy, so bear with me. In DD setup, is there a guideline to follow in regard to the length of the recovery harness for the drogue chute and the main? Do you want the drogue longer than the main, or vice versa? I also assume this will depend on the width of the harness as well, correct?

My question pertains specifically to nylon, but other types are welcomed. 

Thanks!
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#2
Rule of thumb is 4 to 5 times length of rocket. I usually do 4X for both sections and never have a problem.
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#3
I'd ask what you plan on doing for your dual-deploy. There's many ways to go about it...

Personally, I'm a fan of streamers at apogee, because... no one else really is, and tracking a 25' streamer on descent is real easy from the ground. As such, I tend for longer lower-upper harnesses. 5, 6, 7 times the complete body length.

If freebagging the main, you really don't need much length of harness. Just enough to give yourself some cushion... perhaps a body-length, maybe one and a half. In this case, the harness connecting the pilot to nosecone is about the NC length, as well.

I won't use nylon anymore. Had one catch an ember off an ejection charge and melt through, dropping the booster from about 1,500 feet on a rocket my son designed. Kevlar ever since.


Later!

--Coop
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#4
(11-25-2015, 05:00 PM)dixontj Wrote: Rule of thumb is 4 to 5 times length of rocket. I usually do 4X for both sections and never have a problem.

Thanks! That's what I came across on my search, but that logic seemed to be only about single deploy. I assumed it could be applied to DD, but wasn't sure.

(11-25-2015, 10:10 PM)Coop Wrote: I'd ask what you plan on doing for your dual-deploy.  There's many ways to go about it...

Personally, I'm a fan of streamers at apogee, because... no one else really is, and tracking a 25' streamer on descent is real easy from the ground.  As such, I tend for longer lower-upper harnesses. 5, 6, 7 times the complete body length.

If freebagging the main, you really don't need much length of harness.  Just enough to give yourself some cushion... perhaps a body-length, maybe one and a half.  In this case, the harness connecting the pilot to nosecone is about the NC length, as well.  

I won't use nylon anymore.  Had one catch an ember off an ejection charge and melt through, dropping the booster from about 1,500 feet on a rocket my son designed.  Kevlar ever since.


Later!

--Coop

I've got an Estes Partizon I modified for a 38mm MMT that I plan on DD. The goal is to put it up at about 2500'-3000' for testing. This is my learning rocket  Smile
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#5
I like the stogie shock cord to be nice and long (maybe hit that 5x length of rocket mark), but my main shockcord is always shorter.

On my first DD flight, testing it before I go for L2, my shock cords were the same length. The resulting ejection for the main shot the nose cone straight at the fin can. Post flight inspection showed a hole in the gel coat of the molded fiberglass nose cone. A little bit of bondo fixed it.

Ever since that flight, my shock cords have been nice and long conpared to the rocket, and my main has been maybe 1/2-3/4 the length of the drogue cord.

NAR# 98194
Level 1: CTI I-216, 3,043'
Level 2: CTI K-740, 5,999'

Personal altitude record: 12,400' CTI L395
2014 total impulse: 9,018.2 Ns (76% M)
2015 total impulse: 7,171.7 Ns (40% M)
2016 total impulse: 18,664.2 Ns (91% N)
2017 total impulse: 8,281.1 Ns (80% M)
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#6
there are a lot of thoughts on this...
I personally tell people to use a Looooooog cord for the apogee event, a short one can cause a main deploy at apogee. Best off Test it make sure that the charge does not have enough force cause the entire length of cord to be used by inertia. That Jerk at the end will pop off your nose cone.


Bill Clune L3 | NAR#88583
Been a launch Director
Founded a Club
MARS Club|SRC|MDRA - Forever
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#7
(11-26-2015, 09:13 AM)T34zac Wrote: I like the stogie shock cord to be nice and long (maybe hit that 5x length of rocket mark), but my main shockcord is always shorter.

On my first DD flight, testing it before I go for L2, my shock cords were the same length. The resulting ejection for the main shot the nose cone straight at the fin can. Post flight inspection showed a hole in the gel coat of the molded fiberglass nose cone. A little bit of bondo fixed it.

Ever since that flight, my shock cords have been nice and long conpared to the rocket, and my main has been maybe 1/2-3/4 the length of the drogue cord.

(11-26-2015, 10:09 AM)wclune Wrote: there are a lot of thoughts on this...
I personally tell people to use a Looooooog cord for the apogee event, a short one can cause a main deploy at apogee. Best off Test it make sure that the charge does not have enough force cause the entire length of cord to be used by inertia.  That Jerk at the end will pop off your nose cone.

That was my initial through as well. I will most definitely going to be testing it, too. 

Thanks for your input. Hope you guys are enjoying your Thanksgiving!
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#8
I used 4.5 times the body length as that was the amount of kevlar I had available and I use the motor ejection charge at apogee for the drogue.  The drogue is attached about 3/4 of the way up the kevlar from the body tube.  I attach the main, wrapped in a nomex blanket, to the nose cone and use redundant cable cutters to release.  I set the release at 300 feet and have had one successful recovery and no failures.  All of the electronics are in the nose cone.  I'm sure there are as many ways to do this as there are people trying it...but this worked for me.
Joe Southworth
NAR 98381
Southern Colorado Rocketeers (SCORE)
Level 1 Modified Initiator
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#9
It also depends heavily on the size of your BP charges. You want the charge to be able to push out the laundry, but if the charge is overzealous, then you need longer cords to keep it from snapping back.
John S.
NAR #96911
TRA #15253
MDRA
Level 1, 2014-Mar-15 -- Aerotech Sumo, H133BS
Level 2, 2014-Jun-21 -- Giant Leap Vertical Assault, J240RL
Level 3, 2016-03-12 -- MAC Performance Radial Flyer, M1101WH, 13,028 feet
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