first scratch build... where to start??

[Jake]

Hey guys, I've been doing some research, and now have some Q's for ya. My first question is, when you go to cut out the fins for a rocket, is there a rule of thumb to use in making sure that the fins are big enough to give the rocket stable flight. I did some Googleing on the subject, but came up empty handed. If there are any articles about this, a link would be greatly appreciated
I've also done some research on how to find the CG and CP of a rocket, and I know that the CG needs to be in front of the CP. But all the info I've found, shows how to find them after the rocket is made. So I'm wondering if there is a way to ensure that a rocket will be stable before its made. Because I really don't want to get done building the rocket and then find out its unstable.
If I had Open rocket, I'm sure I could answer questions myself, but I'm having trouble getting it to run on my computer
Thanks for any help!
Jake

 

[Rex R]

I would suggest fins that are somewhere in this range (for a bt60 airframe) 2 - 2.5" in width(span).
rex

 

[cruiten]

...My first question is, when you go to cut out the fins for a rocket, is there a rule of thumb to use in making sure that the fins are big enough to give the rocket stable flight.

There is so much information on this subject that I don't understand how you could come up empty-handed. Here is very basic synopsis that I found at: https://www.info-central.org/?article=122:

Fins
Fin Dimensions
A fin that is 2 diameters of the airframe in root length and span and a chord length of about 1 diameter will be effective.

Fin Shape or Planform
The shape you see more than any other is called the clipped delta, and is known for its effectiveness. The clipped delta resembles a parallelogram, with the fin swept somewhat to the rear. The root and chord lines are near parallel, and the leading and trailing edges are near parallel. There are many, many shapes that will get the job done. Some look cooler to me than others. One of the most efficient fin designs looks like a simple rectangle attached to the tube.

Shaping the Fin
The leading edge of the fin should be rounded, the trailing edge shaped like a V. The chord edge should remain square.

Number of Fins
Three fins will almost always do the job. Four fins work too, but only marginally better as far as improving CP. Some have said that four fins reduce wind-induced spin.

You are correct that modeling/designing in a tool like OpenRocket will be very insightful.

...If I had Open rocket, I'm sure I could answer questions myself, but I'm having trouble getting it to run on my computer
Thanks for any help!

What is the "trouble" that you are encountering?

Here are some general guidelines to get OpenRocket to run on your computer:

• Make sure that you have a current version of Java installed.
• Make sure that you have downloaded the current version of OpenRocket. (At the time of this writing it is "OpenRocket-12.09.1.jar")
• On most Windows Operating Systems you can just click the OpenRocket "jar" file to start the program.

If you don't have a current version of Java on your computer then you can get it here: https://www.java.com/en/download/index.jsp

After you have downloaded and installed Java you can start OpenRocket in a variety of ways. Here are a couple of Windows-centric ways:

• Using Windows Explorer, navigate to the directory that contains the OpenRocket jar file and double-click the file
• Using the command prompt window you can navigate to where the OpenRocket jar file is and type the following: "java -jar OpenRocket-12.09.1.jar"
• Create a short-cut on your desktop. For me the short-cut's "Target" value is: "C:\Program Files (x86)\Java\jre7\bin\javaw.exe" -jar "C:\Users\cruiten\Rocketry\OpenRocket-12.09.1.jar" and the "Start in" value is "C:\Program Files (x86)\Java\jre7\bin" (Please note that the "javaw.exe" is not a typo, the javaw command is identical to java, except that with javaw there is no associated console window. I use javaw because I did not want a command prompt window to appear.)

Please be aware that the path names in my examples work on my computer, but you will have to make changes based on where Java is installed and where you keep the OpenRocket jar file.

Good luck,

Cor

 

[Jake]

I would suggest fins that are somewhere in this range (for a bt60 airframe) 2 - 2.5" in width(span).
rex

Awesome thanks rex! And did you just come to this conclusion by trial and error?
Jake

 

[Jake]

Hey crutine!
thanks for that link about the fins. I'm not sure either, I guess I'm was just searching/looking in the wrong places

The main trouble I'm having is that once it's downloaded, I'm not able to get it to open (yes I have the newest version of Java).
Just now I googled my question, and was redirected to a thread here. I have windows 8, so that may be the trouble...... Im gonna try and follow the steps that they mention taking there. HOLY cow.... So as I'm typing I re-installed open rocket, and all of a sudden it pops up!!!! Crikey I hate computers.... Sorry for putting ya through the loop, but now its working. waahoo! Now maybe I can stop bugging you guys
HAHA thanks again

 

[Rex R]

more or less. when I was 'designing' my dual deploy bird, I started with G. Stine's formula for clipped delta fins and they were huge (and the cp was way far back), trimmed them a bit and called it good. then started measuring fins on scale rockets and found that on 3 fin birds they were about 125% (1.25x) the body diameter, tried that in OR looked great, when I made them narrower (around 1x dia.) the cp started moving forward rapidly, so I concluded that 1.25 x the dia. would work and provide a safety margin for a normal length rocket. short and stubby birds will need more fin area(which is where design software is a big help).
rex

 

[Jake]

more or less. when I was 'designing' my dual deploy bird, I started with G. Stine's formula for clipped delta fins and they were huge (and the cp was way far back), trimmed them a bit and called it good. then started measuring fins on scale rockets and found that on 3 fin birds they were about 125% (1.25x) the body diameter, tried that in OR looked great, when I made them narrower (around 1x dia.) the cp started moving forward rapidly, so I concluded that 1.25 x the dia. would work and provide a safety margin for a normal length rocket. short and stubby birds will need more fin area(which is where design software is a big help).
rex

I see, good food for thought Thanks for the info
Jake

 

[Jake]

So, Like I said, I finally got Open Rocket working (BTW, thanks jpummil for the tuturial, it was very helpful!!)
And Last night I used it to design the rocket I'm panning on creating.
What do you guys think?View attachment Turbo dominator.pdf
Jake

 

[qquake2k]

It kind of reminds me of a downscale of the LOC I-ROC. I've always liked that design.

https://shop.locprecision.com/product.sc?productId=138&categoryId=16

 

[Rex R]

jake; if you would be so kind, could you change the stability units to 'cal' as opposed to inches(preferences/units/stability) that way I don't have to whip out the calculator(1st thing in my morning) to see if the design is stable(it should be). thanks
rex

 

[jpummil]

So, Like I said, I finally got Open Rocket working (BTW, thanks jpummil for the tuturial, it was very helpful!!)
And Last night I used it to design the rocket I'm panning on creating.
What do you guys think?View attachment 125384
Jake

Nice looking bird, Jake! Glad my tutorial was of some use :wink:

One word of warning...once you START scratch building and designing with Rocksim or OR, you WILL overwhelm yourself with a multitude of cool "what if" designs...try and pace yourself to enjoy each one from conceptualization to first flight. MUCH easier on the wallet :grin:

 

[Cl(VII)]

One word of warning...once you START scratch building and designing with Rocksim or OR, you WILL overwhelm yourself with a multitude of cool "what if" designs...try and pace yourself to enjoy each one from conceptualization to first flight. MUCH easier on the wallet :grin:

Second! So many (theoretical) rockets, so little time/budget/flight chances.

 

[Jake]

It kind of reminds me of a downscale of the LOC I-ROC. I've always liked that design.

https://shop.locprecision.com/product.sc?productId=138&categoryId=16

Yeah I guess it does.... really down sized though
Jake

 

[Jake]

jake; if you would be so kind, could you change the stability units to 'cal' as opposed to inches(preferences/units/stability) that way I don't have to whip out the calculator(1st thing in my morning) to see if the design is stable(it should be). thanks
rex

Will do, working on it now.
Jake

 

[Jake]

Nice looking bird, Jake! Glad my tutorial was of some use :wink:

One word of warning...once you START scratch building and designing with Rocksim or OR, you WILL overwhelm yourself with a multitude of cool "what if" designs...try and pace yourself to enjoy each one from conceptualization to first flight. MUCH easier on the wallet :grin:

Thanks for the warning
I think I know what you mean.... I already have another idea.... I'm just thankful that I just got a job. Maybe my wallet wont hurt as much

 

[qquake2k]

Thanks for the warning
I think I know what you mean.... I already have another idea.... I'm just thankful that I just got a job. Maybe my wallet wont hurt as much

Oh man, it's all over now. You're hooked on scratch building. They'll keep getting bigger and bigger, too.

 

[Jake]

jake; if you would be so kind, could you change the stability units to 'cal' as opposed to inches(preferences/units/stability) that way I don't have to whip out the calculator(1st thing in my morning) to see if the design is stable(it should be). thanks
rex

Ok... SO I took a look... And I'm not sure where I'm can change that setting.... How do you usually do it?
Also, at the bottom of the screen on OR, this is coming up: 'Warning: Discontinuity in rocket body diameter'.
I've checked all the measurements, and I'm not sure why its saying that. Have any of you ever had this happen?
Thanks, Jake

 

[Jake]

Oh man, it's all over now. You're hooked on scratch building. They'll keep getting bigger and bigger, too.

HaHa I sure hope so!!

 

[Scotty Dog]

Oh man, it's all over now. You're hooked on scratch building. They'll keep getting bigger and bigger, too.

HaHa I sure hope so!!

Ya know,thats not a bad thing.

 

[qquake2k]

Ya know,thats not a bad thing.

Maybe not, but finding places to put them all can be difficult! Fortunately, I'm not married. I have rockets in my living room, bedroom, computer room, kitchen, garage...

 

[Jake]

Maybe not, but finding places to put them all can be difficult! Fortunately, I'm not married. I have rockets in my living room, bedroom, computer room, kitchen, garage...

It stinks.. Im already having that problem!!!!!!!!!!! Im gonna start hanging them from my bedroom ceiling....
Jake

 

[qquake2k]

It stinks.. Im already having that problem!!!!!!!!!!! Im gonna start hanging them from my bedroom ceiling....
Jake

Some of mine are big enough, that if I hung them from a ceiling, my head would hit the fins. It's a sickness...

 

[Jake]

Some of mine are big enough, that if I hung them from a ceiling, my head would hit the fins. It's a sickness...

a healthy disease I like to call it

 

[qquake2k]

a healthy disease I like to call it

I don't know about healthy, but it sure is fun! And to think I never planned on getting my Level 1, let alone Level 2. It kind of snuck up on me!

 

[Rex R]

lets see if a picture is worth multiple words(or in this case multiple pic.s) stability setting is a drop down menu with IIRC in, mm, cm and cal any changes you make take effect the next time you start the program. the discontinuity warning usually means that the nose cone base dia. doesn't match the body tube's dia., our usual reply is, 'post the ork file and we'll see what's up'...please post the....
rex

 

[Jake]

lets see if a picture is worth multiple words(or in this case multiple pic.s) stability setting is a drop down menu with IIRC in, mm, cm and cal any changes you make take effect the next time you start the program. the discontinuity warning usually means that the nose cone base dia. doesn't match the body tube's dia., our usual reply is, 'post the ork file and we'll see what's up'...please post the....
rex

Ok cool. Here it is with the stability switched to cal.View attachment The dominator.pdf
'Post the ork file'...... Ok, so I'm probably about to ask another stupid question to which the answer is sitting right under my nose. But what is an ork file?? I've searched the internet and have come up empty handed.
Jake

 

[Rex R]

and I'll follow with another question, you have saved your work, yes? when you saved your design OR saves the file as default 'yourdesign.ork'(or whatever you named it), you might need to let windows search for '.ork', right click open file location to find out where it is on your computer. once you know where to find it, you can then upload a copy here.
rex

 

[Jake]

and I'll follow with another question, you have saved your work, yes? when you saved your design OR saves the file as default 'yourdesign.ork'(or whatever you named it), you might need to let windows search for '.ork', right click open file location to find out where it is on your computer. once you know where to find it, you can then upload a copy here.
rex

HAHAHA I'm really kicking myself right now... 'hey Jake, look next to the saved file of you rocket and see that is says orc file!!!!' Here ya go.View attachment Turbo dominator.ork
I re-checked the dimensions, and you are correct. The nose cone base Dia. doesn't match the BT diameter. Will have to change that tomorrow.

 

[Rex R]

well don't kick yourself to hard, this is new to you and computers have a way of making folks feel like dummies . I wonder do you have the nose cone on hand? or would you be willing to accept an alternative balsa nose? reason I ask is I couldn't find a nose to match on the two websites I checked...so I went with a close match from semroc. I made a few small changes (including the semroc nose cone) mainly material changes and a few name changes. two notable changes 1) a c6-7 doesn't really work(delay is to long) changed to c6-5 2)parachute downsized to 12". edited file below.
rex

View attachment Turbo dominator_edited.ork

 

[Jake]

well don't kick yourself to hard, this is new to you and computers have a way of making folks feel like dummies . I wonder do you have the nose cone on hand? or would you be willing to accept an alternative balsa nose? reason I ask is I couldn't find a nose to match on the two websites I checked...so I went with a close match from semroc. I made a few small changes (including the semroc nose cone) mainly material changes and a few name changes. two notable changes 1) a c6-7 doesn't really work(delay is to long) changed to c6-5 2)parachute downsized to 12". edited file below.
rex

Yes I have it here. And here is the website which I used for dimensions. honestly, I really did allot of guessing about the cone. It was given to me so I dont know where It came from to get the correct dimensions, so I used the dimensions of another NC60 cone on another website ASSuming they were all the same..... So what im saying is I really dont know what the exact dimensions are for my cone. And I also didn't know what to label it as far as material is concerned. It's a plastic nose cone so I went with PVC I think because it was the closest to plastic I could see. (sending email now to see if the guy knows anymore about it).
I also see the other changes you made which I didn't notice. Like the engine block, and I only had one big launch lug on there, witch you changed to two smaller ones. Thanks will pay more attention to that stuff next time. And I also changed the motor to a c6-3 (I just went with a c for starters, because I knew I had a c of some kind, but I just checked and I have c6-3s)
Jake