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38mm minimum diameter build questions
#1
Hi, This is going to be my first scratch build and being minimum diameter i have a few concerns about the design
I have put the OpenRocket file of what I am thinking of building as an attachment, it will be about 110cm (43") long with dual deployment, Phenolic tubing and an Aeropack minimum diameter retainer with CTI casings.

1. Can fibreglass (for fins) be sanded underwater to avoid health problems from dust?

2. Without using tip to tip reinforcement will these fins stay on with just a fillet; and will fin flutter be a problem?

3. For electronics I will be using a raven but will also get a tracker and don't want a radio licence, would the BRB 900 (http://bigredbee.com/brb900.htm) be a good choice?

4. How much shock cord should I use in each section?

5. Would this fail for any other reason that I can't think of or do you have any other suggestions?

Thanks


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#2
I don't think the fin shape you have is very efficient for the speed showing in your sim, and may flutter.  I would change the fin shape a little, to have the leading edge more swept back, and the trailing edge with little or no sweep.    I would also change the fin height to be as least as tall as the body diameter.  

You should also check stability margin through all speeds, by running a simulation and plotting the stability margin.  Stability margin will increase with speed through about mach 1.3, then start to decrease.  Make sure stability is at least 1 throughout the powered portion of flight.

 You can get away with only large fillets no no tip-tip reinforcement.  You might want to make the fins a little thicker (stiffer) like 3/32" instead of 1/16".  That would virtually eliminate the risk of flutter with decent size fillets, and a different fin shape.

The BRB 900 should work just fine on any altitude you can reach on a 38 mmm motor.
Lastly, fiberglass fins can be sanded under water to control dust, But I would just buy a dusk mask instead.

Shock cord, you'll get varying opinion on, but I usuall youse about 20 to 25 feet


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#3
Thanks for the very helpful response. I will make those fun adjustments now
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#4
I was going to post essentially what Mark already said - use thicker FG (3/32), skip the tip to tip reinforcement, and use a mask. (that's easier than sanding underwater!).
BRB makes good, affordable, reliable products. Another option to consider is to borrow a tracker form a fellow flier, who is licensed. Our club has a half dozen or so we loan for use on launch days, that are under the license of one of our on-site members. If he can't attend the launch, we can quickly reprogram using a callsign from a licensed member attending that day's launch...
As to shock cord length, like Mark said you'll get a variety of opinions. Most of us use longer lengths than necessary... 3 - 5 times the length of the airframe. Be sure to ground test whatever length you use, to ensure when you deploy you get a solid separation, without needing to eject to the full limit of the cord's length.
Greg
Greg Young - L3
TRA 00234
NAR 42065
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