Stupid newbie question: Harness layout

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ActingLikeAKid

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I feel like this may have been asked already but my search skills are off today - I blame being sick.

On my scratch HPR build, I'm starting to think about my harness layout.


General specs:
About 1.75 lbs dry, twice that with a 38mm DMS in it.
About 4 and a half feet long, 54mm lower half, upper half is 38mm->29mm.


I know that some folks swear by the "5x the rocket length" rule for shock cords, but that seems excessive - that would be over 20 feet of cord. I'm not using dual deploy, just a 36" chute with a Chute Release.


I'm thinking about 10 feet from the top of the BT to the chute, then another 4 feet from the chute to the "nose" (I say "nose" because the rocket splits about in half - the upper section is about 27" long).


Then I started getting concerned about things getting tangled. Would it be simpler to just tie the chute to the eyebolt in the "nose"? This seems like it would reduce tangling but it also seems like this is something nobody does, so there's probably a good reason....
 

soopirV

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I think you're going to get as many responses as there are people to respond, so I'll start with my own $0.02 (which is worth about half that much!): If you're talking midbody separation with engine deployment, I would first calculate the volume to ensure that the motors you plan to fly contain enough of a charge to be effective (I've been burned...once!).
From there, there are things you can do to influence how long your cord needs to be- Z-folds seem to be the most common- it's a way of folding the cord back on itself 8-10 times, and holding the "bundle" with a wrap or two of masking tape. This is easily separated, but saps a good deal of ejection force from the system. Doing this with more tape will theoretically mean that you can use less cord. Always ground test tho!

I have no experience with a JLCR, so my advice is worth even LESS than $0.01 perhaps!
 

ActingLikeAKid

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I think you're going to get as many responses as there are people to respond, so I'll start with my own $0.02 (which is worth about half that much!): If you're talking midbody separation with engine deployment, I would first calculate the volume to ensure that the motors you plan to fly contain enough of a charge to be effective (I've been burned...once!).
It ought to be sufficient... I'm planning on AT DMS; the instructions say "about half the BP" for under 2.6". I've got 54mm for the lower BT, and what's not motor is going to be fairly full of chute, harness, and barf, so I'll go with about half the vial. Don't have a scale for BP (yet), so I'll eyeball & cross my fingers. Shouldn't be a problem knocking the laundry out :)

From there, there are things you can do to influence how long your cord needs to be- Z-folds seem to be the most common- it's a way of folding the cord back on itself 8-10 times, and holding the "bundle" with a wrap or two of masking tape. This is easily separated, but saps a good deal of ejection force from the system. Doing this with more tape will theoretically mean that you can use less cord. Always ground test tho!
The problem with ground testing is that I don't have BP, detonation electronics/charge canisters, or really a place to put the charge (I guess maybe I could see if a friend has a spent DMS, but that doesn't seem like the sort of think you'd just keep around. And then I'd still have to wire up a charge...)

Was planning on Z-folding....while I can't ground test, I think 10 feet of cord ought to be enough...eh, we'll see when it flies!
 
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soopirV

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I struggled with ground testing too, until I made the decision that BP is "dead" and bp substitutes are viable options. I bought a pound of Hodgon 777 from my local Sportsmans warehouse, and have used a surprising amount of it groundtesting (it's a lot of fun to make things go "bang"!). If you're using motor-eject, you don't HAVE to ground test- but I wouldn't trust AT's advice to "use about half" for anything- there are great calculators out there that will tell you how much you need. Gram scales are cheap! Much cheaper than a rocket you've spent hours finishing!
If you don't have a calculator (and now that I know someone who needs one, can I find the one I had years ago?? NO!), post your dimensions- inner diameter, length of pressurizable space, and any other requirements (like shear pins) and one of us will tell you if the AT reload you have in mind will have enough oomph to get your recovery gear out!
 

noffie79

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If you have an iPhone, there's a great app called Rocket Calculator. It gives quick and dirty sims, you can calculate vent hole sizes, ejection charges, chute sizes and decent rates, drift distance and quite a bit more. I use it a lot.
Rocket Calculator by Peter Stanley
https://appsto.re/us/XA44C.i
 

smugglervt

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Longer is always better than shorter.
 

blackbrandt

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Another advantage of stupid long recovery harnesses... if the rocket with a 50 foot harness lands in a 55 foot tree, you can still get it ;)
 

Handeman

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Another advantage of stupid long recovery harnesses... if the rocket with a 50 foot harness lands in a 55 foot tree, you can still get it ;)
Unless it gets draped across the top of the tree, which is what usually happens. Then you have to live with the disadvantages of too long of cord and not get any benefits.
 

ActingLikeAKid

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Thanks everyone for your help! Here's what I have: The mid-body separation does not require any shear pins.
The chamber (which will have a good chunk of volume taken up with harness, chute, etc) is about 15" long and 54mm (2.12") diameter.
According to the FAQ here, AT DMS has 1.4g of FFFFg in the charge...
https://www.apogeerockets.com/Rocke...ingle_Use/Aerotech_38mm_Motor_I500T-14A_HP_SU
If I'm using it right, Rocket Calculator says 0.7g of BP*, half the container, gives me around 25 psi, enough to break 2 shear pins that I don't have, and ought to be plenty to pop the NC and pull the laundry out ... yes?

It's too early, I haven't had enough coffee, and I'm sick. I blame all of these for the brain fog that's preventing me from dredging up 25-year-old Newtonian physics lessons and getting a reasonable answer on what sort of trajectory the nose would take...it's 273g. So 16 feet (10 to the chute, 6 to the nose) of nylon cord z-folded & wrapped...that ought to be plenty?

*-Based on FFFF; AT says FFFFg in their charges, but I'm guessing that there isn't a significant difference?
 
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