75mm Custom Build

NickT14870

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I've flown and built quite a few high power rocket kits mostly in the 38mm and a few 54mm range. All of these werw kit builds though coming from Loc,Wildman,Madcow,etc. Recently I've been wanting to get in custom rocket building aswell as building my first 75mm rocket. This is still the very early design phase for me but I'm looking to build something about 8ft tall and to exceed Mach. I've been researching information regarding fin design such as fin flutter aswell as some basic calculations for centering ring strength. This being my first experience into custom rocket building I was wondering if anyone had any good tips or information I might overlook. Thanks in advance
 

rfjustin

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I've flown and built quite a few high power rocket kits mostly in the 38mm and a few 54mm range. All of these werw kit builds though coming from Loc,Wildman,Madcow,etc. Recently I've been wanting to get in custom rocket building aswell as building my first 75mm rocket. This is still the very early design phase for me but I'm looking to build something about 8ft tall and to exceed Mach. I've been researching information regarding fin design such as fin flutter aswell as some basic calculations for centering ring strength. This being my first experience into custom rocket building I was wondering if anyone had any good tips or information I might overlook. Thanks in advance
I think it depends on what your goals are. Are you going for high(er) altitudes? On-board video? Flight testing redundant altimeters? Not that there has to be a goal, it just helps with understanding what your goals are for others to opine.

You will find out quickly that scratch building a rocket tends to be much more expensive than a lot of kits out there that have 75mm holes. Are you looking at fiberglass components or other options?

Best,
 

NickT14870

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I think it depends on what your goals are. Are you going for high(er) altitudes? On-board video? Flight testing redundant altimeters? Not that there has to be a goal, it just helps with understanding what your goals are for others to opine.

You will find out quickly that scratch building a rocket tends to be much more expensive than a lot of kits out there that have 75mm holes. Are you looking at fiberglass components or other options?

Best,
I would like to get into higher altitudes nothing crazy just above 15,000ft and eventually get into cameras. Right now I'm only a L2 cert so the goal would be to eventually get a L3 to go even higher. I also was looking at using fiberglass components particularly in the 4in category.
 

techrat

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While I have exactly zero experience in this area, what I have seen is that to exceed Mach, you generally need a minimum diameter design (therefore if your motor mount is 75mm, the airframe shouldn't be much wider, just enough for TTW fins), and the fins need to be the smallest and thinnest they can be without sacrificing strength or stability. Nosecone needs to be fairly pointy and the airframe long and skinny. Everything needs to be fiberglass, and the entire rocket assembled with 2-part, slow-cure epoxy. And that's basically all I got. Sorry.
 

waltr

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My only suggestions is to Sim any design with a variety of motors.
Made changes and sim some more.

As to altitude, long burn and slow gets higher than short burn and fast. Reason is lower velocity has lower drag.
Just a something to keep in mind when doing sims.
 

rfjustin

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I would like to get into higher altitudes nothing crazy just above 15,000ft and eventually get into cameras. Right now I'm only a L2 cert so the goal would be to eventually get a L3 to go even higher. I also was looking at using fiberglass components particularly in the 4in category.
My .02, start with a fiberglass 4" kit with a 75mm hole. Currently, this is one of the most cost effective options on the market today:
 

Handeman

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Sounds like you have 2 goals, fly 75mm motors and exceed Mach 1. Both are quit easy to achieve.

I have a 3" rocket with 54mm MMT that will break Mach and +11,000 ft. on 6 and 6XL Pro54 K motors. I'm building a 4" Go Devil with a 75mm MMT that will break Mach and 11,000 ft with a AT L1120W motor. I'll go even higher with 75mm M motors.

The real question is do you have a place to fly them? Being on the east coast, even though we have the waiver, the winds, ceiling, woods and wetlands all combine to preclude flying to those altitude on all but one or two days a year, if you're lucky.

If you're not looking to achieve both goals with one rocket, you could go with a larger diameter 75mm MMT rocket to fly the bigger motors to lower altitudes and a 38mm or 54mm minimum diameter for Mach busting and still keep the altitude lower.

Now if you have the waiver and recovery area to routinely fly in the 15K to 20K range, you might want to consider having a couple TAPs weigh in on the 75mm build so you can use it for a L3 cert flight later if you want.

Have fun which ever way you go.
 

3stoogesrocketry

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My .02, start with a fiberglass 4" kit with a 75mm hole. Currently, this is one of the most cost effective options on the market today:
Exactly this. Almost anything 4 inch with a 75mm hole will easily hit 15k on a smaller 6knsish load.
 

StreuB1

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Extreme Wildman as well as the Motoreater are both very good kits that will VERY EASILY achieve your goals and more. Any 4" FG rocket with a 3" hole will, as it was stated above. Put an Additive camera shroud on it from Andrew and you'll also get breathtaking video all the way up and all the way down. As well, these kits will be extremely robust and [likely] last a lifetime. My Extreme Wildman fell from 14,000' with no chute and I could have reflown it after about an hour of minor work.
 
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