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3' or 6' launch rod.

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Senior Space Cadet

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I'm just finishing a BT-50 (24mm) build that I designed with the intension of it going pretty high. I'm going to put a Quest C12-8 motor in it.
It has the smallest fins I've ever put on a rocket this size. Theoretically, they are enough, but I'm wondering about the length of the launch rod I should use.
In 3/16, I have both 3' and a 6' launch rods. There will be friction between the lugs and rod for less distance with the 3', but the rocket will leave the 6' rod at a higher speed, so might fly straighter.
Just wondering what some of you more experienced rocketeers think?
 

Nytrunner

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How fast is it exiting the rod for the 3'?

(you can change the rail/rod length in OpenRocket, go to edit simulation)
((Also remember you only have guidance from the lug to the rod end, so subtract any rod length that's below the lug))
 

rharshberger

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One problem with small diameter long rods is rod whip is worse with longer rods. 3/16 kind of have a sweet spot between 3' and 4' any longer and the get "whippy". Use a 5' 10mm Makerbeam rail and Micro rail buttons instead ( www.rail-buttons.com ).
 

Senior Space Cadet

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One problem with small diameter long rods is rod whip is worse with longer rods. 3/16 kind of have a sweet spot between 3' and 4' any longer and the get "whippy". Use a 5' 10mm Makerbeam rail and Micro rail buttons instead ( www.rail-buttons.com ).
Too late for that now. Rocket is built.
I've been putting off making a rail launcher because of the cost and difficulty, but I'll have to do it eventually..
I have a 6ft. 1/4" rod too. Could of gone with that, but it's a pretty small rocket.
 

CalebJ

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Cut your longer 3/16" rod to 4 feet.
 

dpower

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Agree with others, a 4' long 3/16" rod is about right. If more length is needed for stable flight, I go with 1/4" lugs, and a 5' long 1/4" rod. If 5' isn't enough for stable flight, a rail is needed in my opinion.

This is my rule-of-thumb, for rod/rail size vs distance needed for stability:

3' - 1/8" rod
4' - 3/16" rod
5' - 1/4" rod
6' - 1010 rail

Of course, this doesn't always work, for example flying a small rocket on an E30 may only need 2', but the initial impulse can flex a 1/8" rod significantly.

I also use a mini rail sometimes, but in retrospect, I should have purchased a 5' one, the 4' one isn't always long enough.
 

Handeman

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This is my rule-of-thumb, for rod/rail size vs distance needed for stability:

3' - 1/8" rod
4' - 3/16" rod
5' - 1/4" rod
6' - 1010 rail
I agree with those sizes and that is pretty much the way our club does it with our pads. The only other rule we have is, NO rockets with G motors or large can be launched from rods. We also supply 1010 rails at our LPR pads because most TARC teams are buttons only now even with E & F motors.

We used to allow large rods on the HPR pads for the rare folks that had old rockets with 3/8", 1/2", or 3/4" launch lugs. No longer, it is rails only for HPR.
 

JohnCoker

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We used to allow large rods on the HPR pads for the rare folks that had old rockets with 3/8", 1/2", or 3/4" launch lugs. No longer, it is rails only for HPR.
Talk of big launch lugs reminds me of a bad idea of my own: trying to launch a 110lb rocket with a 1in rod: Big Kahuna fail. (Had to come back the following year with a rail.)
 
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