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Basic Training - Friction Vs. Drag Separation
#1
If you have not read the definitive guide to calculating drag separation (No, Bill, that is not the distance between Rocky Horror and Priscilla, Queen of the Desert), I suggest you check it out here:

http://marsclub.org//index.php?option=co...&Itemid=50

It leaves one hanging in the middle though when it comes to the application of friction to counter the drag separation forces.  I'm curious what other members do.  What type of tape do you use?  Or has everyone moved on to nylon shear pins?  I'm going to avoid the shear pin issue for this build and stick with tape.  I've always had good success with masking tape on low impulse builds - just wondering if there is a better/preferred material for HP builds.

Thanks,

Dale
Dale Stoyer - L3
NAR #91256
TRA #13499
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#2
Friction fitting to avoid drag separation is usually not a problem because the forces are not that high.  Its generally the nosecone separation during the sometimes violent apogee deployment that you are trying to avoid.

Steve Gerber (a Dumas Brother) at NYPOWER showed me his friction fit on his nosecone on his L3 rocket.  Its on SO tight that once its pushed on it literally can not be removed by hand.  BTW, the Dumas Bros mostly eschew shear pins.

So one method is to get that friction fit REALLY tight and then ground test to determine the amount of ejection charge to safely get it off.  If you were going to do this scientifically you would test a series of charges and determine a) the amount that barely gets it off, and b) the amount that destroys the section.  Then you use the midpoint of a and b.
John Derimiggio
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#3
Awesome plan, wonderfully scientific, terribly expensive!

Dale
Dale Stoyer - L3
NAR #91256
TRA #13499
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#4
I never used shear pins until my L3 because I figured that's what I was supposed to do.  I never had any failures with friction fit either, but it's sometimes a pain in the ass getting the fit just right as the temperture changes (especially with quantum tube) or some of the tape rolls back on itself or you don't tear it right, etc.  The biggest advantage for me was knowing that when I (easily) slid the rocket together and put the pins in I didn't have to adjust anything - it was going to take the same force (about 65 lbs) to pop it apart that it took during ground testing the week before.
http://www.rocketmaterials.org/datastore.../index.php

I still friction fit everything else (size up to 5.5", impulse up to full K) but will probably use shear pins if I ever finish anything over 4" dia again.  I use any masking tape easily found during prep but prefer the blue stuff as it doesn't become impossible to remove the following season.

Just come out in August for UYIW[sup]TM[/sup] and check out all the rockets.  Most flyers love to show off their handywork.

Jeff
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#5
Very good reason to consider shear pins - consistent performance.  I'll probably try to postpone my sheer pin experience until my next rocket.  This time out I'd rather err on the side of apogee deployment rather than risk the catastrophic lawn dart.  A question, though, for those that use shear pins:  What do you use to ensure, well, shearing?  I had toyed with the idea of 'hardening' the shear ports by backing them with thin pieces of copper (left over from a heat sink experiment).  How does anyone using shear pins keep the shear port from getting sloppy after numerous deployments?  Am I wasting my time even worrying about that? 

Dale
Dale Stoyer - L3
NAR #91256
TRA #13499
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#6
The Clune Method for calculating drag separation. add blue anti-drag adhesive to the point where part A meets part B, pick it up by part c (the nose cone). expose the assembly to excessive gravitational forces (shake the heck out of it) if it separates add more blue anti drag adhesive.
Then, the most important part testing. insert explosive powder charges into there appropriate assembly's then at 3AM on any given Sunday take the unit to the nearest test facility (your mother in-laws back yard will do or maybe a neighbor who hates your dog or wife) and expose the ematch to a 9volt DC power source. if the nose cone poops off your good to GO

365 days a year, 6 left for flying.
Bill Clune L2 | NAR#88583
MARS Club|SRC|METRA|MDRA - Forever
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#7
Alright, I'll add my 2cents in here.

Frition fitting has it's advantages because it's cheap, but so is shear pins so I guess that is a null point.

The problem with friction fitting with tape is it is susecptable to tempature changes. Mainly in that a plastic nose cone will shrink faster in cold temps than your paper/phelonic/fiberglass airframes will. Therefore making the possibility of the nose cone coming off at apogee much more likely. Add into that is the fact that the most 'G' forces that the rocket experiences is when the drogue charge seperates the rocket in half and the two ends of the rockets SNAP when reaching the ends of the harness. So if the upper atmosphere is colder than ground level, (August through the winter), that nice tight friction fit you had on the ground is looser than Sh*t at apogee. IE, your nose cone just falls off at the snap. Can make for a looooong walk doesn't it?

With shear pins this isn't an issue since if you ground tested then you know that will take the main charge going off to seperate the nose cone and not a sudden snap. I have never had a nose cone come off at apogee since I started using shear pins. (FYI: McMaster Carr, a bag of 100 2/56 nylon screws about $5, plus a couple bucks for shipping, delivered to you in a couple days, will last at least two or three flying seasons easily). And don't worry about the hole getting sloppy, I will spread a thin coat of epoxy inside the air frame and onto the coupler and then sand them until I get a nice easy to slide apart fit and I still have a good shear pin hole to date.

I will always use shear pins on everything 4" and above, and my personal opinon on those who still use friction fit at this stage is that they are just very damm lucky so far.
I just wanna fly ROCKETS!!

Dutch
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#8
(08-12-2010, 12:26 PM)Mike Dutch link Wrote: and my personal opinon on those who still use friction fit at this stage is that they are just very damm lucky so far.

Or just VERY good. ;D
John Derimiggio
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#9
Quote:if the nose cone poops
I hope my nose cone never poops... ;D
David Haas
NAR #13780
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#10
(08-12-2010, 12:26 PM)Mike Dutch link Wrote: So if the upper atmosphere is colder than ground level, (August through the winter), that nice tight friction fit you had on the ground is looser than Sh*t at apogee. IE, your nose cone just falls off at the snap. Can make for a looooong walk doesn't it?
ALL excellent points (especially yours, Dave).  I'm just looking at it differently.  Some folks have reccomended that I just use motor ejection of the main at apogee for my L1 flight.  I'm going to try dual deploy because I think that will be a long walk.  If the nosecone comes off at apogee, it will not be dramatically different than motor ejection.

All in how you look at it.

Dale
Dale Stoyer - L3
NAR #91256
TRA #13499
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