Optimal diameter for a model rocket

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Hello,

Pretty new to rocketry here and I was wondering how to choose the optimal rocket diameter base on the rocket engine. In our first rocket we used an Aerotech J425R motor and had the rocket diameter at 4". Is there a formula for calculating the needed rocket diameter?

Thank you in advance!
 

UhClem

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Is there a formula for calculating the needed rocket diameter?
You first have to decide what metric you want to optimize and you haven't even hinted at what that might be. Depending on what that is, there may or may not be something resembling a formula. Most likely not.
 

RocketScientistAustralia

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There are 2 types of rocket body. Ones you can get your hand in. All others.
Other than that there is the general 50x body dia guideline. Around that length tubes tend to act like straws. When they get loaded you tend to get buckling and then it breaks. In a rocket this all happens quickly. Careful design can avoid this.
What is optimal depends on what you are designing for. Camera launch platform? Altitude? Max velocity?
In special effects we used to say the optimal prop, cost nothing to make, lasted for exactly the length of filming and fell apart just as you got to the bin.

Obviously your parameters will be different. But you haven't said what they are.
 
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Wayco

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The J425R is a 38mm motor. If you were optimizing for maximum altitude, you would build a minimum diameter rocket with a 38mm body tube. A 4" body tube is at the other end of the scale, great for lower and slower flights, and yes, you can get your hand in it.

Your certified mentor should help you with all that information.
And of course, we are all hoping you have one of these..... Otherwise, you might have to rely on advice from people you don't know on the internet. o_O
 

Bat-mite

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So, what everyone is saying is that there is no formula, because the desired end determines the means. You obviously need a motor of enough thrust to lift your rocket and get it up to optimal aerodynamic speeds. You need a motor mount tube that is the right size for your motor. But beyond that, what do you want to do? Others have listed those examples.

There seems to be a rough standard for kit manufacturers that they commonly make the body tube two sizes of tubing larger than the MMT. So, 29mm MMT => 54mm BT; 38mm MMT => 75mm BT, etc. But those are random and not mandated.

This book may help you: https://www.amazon.com/Handbook-Mod...00K5WDJQ,1457182920,1412058104&srpt=ABIS_BOOK
 

RocketTree

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Since the J motor you mentioned is a high power motor, and your are new to the hobby, I would first suggest starting out with ‘model rocketry’ as the thread title suggests.

From there, you will be able to understand the body diameter vs motor sizing and design limitations, which could then be adapted to high power rocketry. A good example of this is flying a C6 motor in a BT80 rocket, versus the same motor in any smaller diameter body tube.

A 4 inch diameter rocket is a bit “fat” for a 38mm J motor, while a 2.6 inch would be more sleek. Minimum diameter would be exactly that. Totally up to you!
 

dr wogz

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Since the J motor you mentioned is a high power motor, and your are new to the hobby, I would first suggest starting out with ‘model rocketry’ as the thread title suggests.

From there, you will be able to understand the body diameter vs motor sizing and design limitations, which could then be adapted to high power rocketry. A good example of this is flying a C6 motor in a BT80 rocket, versus the same motor in any smaller diameter body tube.

A 4 inch diameter rocket is a bit “fat” for a 38mm J motor, while a 2.6 inch would be more sleek. Minimum diameter would be exactly that. Totally up to you!
While very sound advice, and one we all do encourage, "Student team" events [University / college] usually toss the 'student team' into the "High Power" end of things with little to no time to 'learn' in the classic sense..

events such as:
Launch Canada
Nasa SLI (Student Launch Initiative)
Spaceport America Cup
etc..

All these typically are a 4"-6" rocket launching on a L or M to 10Kft / 30Kft.. and this is the first time these "kids" do 'rocketry'.. And we do get a few "international" teams looking for advice!

And sometimes it's a class / group exercise (in Open Rocket or RS..): given a specific rocket motor, design a rocket to meet 'X' criteria.. "50 points to the one with the highest / lowest X.."



As others have said: what is the goal?
 

Wayco

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I think this is one of those threads where a beginner asks a general question, gets lots of relevant and irrelevant answers, but never comes back....
Maybe I'm wrong?
:dontknow:
 

Troy3003

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I tend to build the rocket I am interested in first, and then find a suitable motor(s) to fit the finished product using either Rocksim or OpenRocket. Depends on your motor limitations whether this approach would work for you.
 

Titan II

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I think this is one of those threads where a beginner asks a general question, gets lots of relevant and irrelevant answers, but never comes back....
Maybe I'm wrong?
:dontknow:
You are right in many cases. In this case:

The OP has posted on the forum 8 times starting in May.
The OP is part of a club at the Hebrew University of Jerusalem.
He is currently viewing fin design tips.
 

RocketScientistAustralia

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You are right in many cases. In this case:

The OP has posted on the forum 8 times starting in May.
The OP is part of a club at the Hebrew University of Jerusalem.
He is currently viewing fin design tips.
So we're assisting a rocketeer in Jerusalem, that we're unsure whether they have a mentor and would seem to have gone straight to a J motor.
What could possibly go wrong....
 

Troy3003

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Maybe you can look at it as assisting a guy interested in a hobby that we all love. Since we are making assumptions, I'll assume from the info in his first post that he is in a group, or team, etc. that enjoys rocketry and he is just trying to learn basics.
 

FredA

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different places, different rules..
Worldwide, motor vendors are still supposed to not sell certified motors beyond a person's cert regardless of location, are they not?

Clearly the cert exception is indeed an exception to the above - but doesn't seem to apply here.
 

Troy3003

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He didn't say he purchased the motor, he was flying with someone else according to his post. People seem to jump to their own conclusions here . We have no clue where he was or who he was with, or who's rocket it actually was. Probably not his rocket or motor if you just read the original post again and the questions he was asking.
 
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