Ye Olde Rocket Forum

Go Back   Ye Olde Rocket Forum > Work Bench > Building Techniques
User Name
Password
Auctions Register FAQ Members List Calendar Today's Posts Search Mark Forums Read


Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 05-26-2006, 07:48 PM
chalinaty's Avatar
chalinaty chalinaty is offline
NAR 85414; NARTREK Silver
 
Join Date: May 2006
Location: Chatham, ON, Canada
Posts: 128
Question Swing Test

I have a group of high school students building model rockets from 9" BT-20 body tubes. They are using friction-fit 18mm motors (mostly B6's) and we are having trouble stabilizing our rockets. Is it reasonable to add 1/2 an ounce of weight to the balsa nose cone on this type of rocket with three 1.75" x 1.5" fins? This is the only way we can get them to fly forward when we use the swing test.
Thanks for any help.
Reply With Quote
  #2  
Old 05-26-2006, 08:28 PM
CPMcGraw's Avatar
CPMcGraw CPMcGraw is offline
BARCLONE Rocketry
 
Join Date: Mar 2005
Location: Mobile, Alabama
Posts: 5,357
Default

Quote:
Originally Posted by chalinaty
I have a group of high school students building model rockets from 9" BT-20 body tubes. They are using friction-fit 18mm motors (mostly B6's) and we are having trouble stabilizing our rockets. Is it reasonable to add 1/2 an ounce of weight to the balsa nose cone on this type of rocket with three 1.75" x 1.5" fins? This is the only way we can get them to fly forward when we use the swing test.
Thanks for any help.


Sounds like a fun class! Wish I had science classes like that way back when!

Here's something you may (or may not) not be aware of. Apogee has a demo version of their RockSim design-testing software available for free on their website. With this software running on your (Windows) computer, you can create a computer model of that rocket your class is working with, and actually "tweak" the design a little at a time until the stability margin reaches 1.00 or better (the magic number).

It's much better not to add any dead weight (like ballast), but to alter the design of the model until the numbers are right. You said they were limited to 9" BT-20 tubing. Here's something to look at: Rockets can be adjusted through their length to reach that magic number of 1 calibre stability, they can have their fins modified (made larger, or change the shape), or as you've noted, add weight to the nose. RockSim can do this for you without actually building the rocket until the numbers are right. Obviously, you want the students building the models, but you need something to work from as a reference datum.

In respect to your class design, I actually found a commercially available model with similar dimensions (Estes Hi Flyer) that was unstable with the C6. Ballast would have corrected the stability problem, but just changing the fin shape did the trick without ballast. I could only discover that using RockSim, before flying my stock kit. You can add weight, but it's far better to change other parameters first. Try lengthening that body tube if you can, before adding weight...
__________________
Craig McGraw

BARCLONE Rocketry -- http://barclone.rocketshoppe.com
BARCLONE Blogsite -- http://barclone.wordpress.com
BARCLONE Forum -- BARCLONE Forum

BARs helping BARs

SAM 0044
AMA 352635
Reply With Quote
  #3  
Old 05-26-2006, 09:05 PM
CPMcGraw's Avatar
CPMcGraw CPMcGraw is offline
BARCLONE Rocketry
 
Join Date: Mar 2005
Location: Mobile, Alabama
Posts: 5,357
Exclamation School Class Plan -- Hole Puncher

Quote:
Originally Posted by chalinaty
I have a group of high school students building model rockets from 9" BT-20 body tubes. They are using friction-fit 18mm motors (mostly B6's)


Here's an idea of what you can achieve with that 9" tube and the right shape in the fins. I used the very common 2.75" Ogive nose cone known as the BNC-20N, and 1/16" thick balsa sheet stock for the fins, together with your 9" BT-20.

Study the 2D drawing to see what else you need to put in the model, like a thrust block and a recovery system. I used a 10" 6-line parachute, which you can make from garbage bags to save money. Grab that demo version of RockSim and study the RKT file for additional information.

Notice the breathtaking performance numbers, all generated by RockSim:

Length: 12.27"
Fin Span: 2.87"
Diameter: 0.736" (BT-20)
Weight: 0.35 oz

There is no additional weight added to this rocket to achieve stability!

A8-5.......647'......Dv 5 FPS......Margin 1.47
B6-6......1190'......Dv 12 FPS.....Margin 1.32
C6-7......2076'......Dv 4 FPS......Margin 1.01

Note these altitude values again. These are true simulation values, and RockSim is very accurate in its predictions! You've got a missile with almost 2100 feet altitude capability in your students' hands! For reference, those Dv numbers represent how fast the rocket is moving at the moment the parachute is deployed. Dv means Velocity at Deployment.

If I can be of additional assistance, just holler out loud! (Preferably through this forum, of course. I monitor it all day long...)

Enjoy!
Attached Thumbnails
Click image for larger version

Name:  Hole Puncher 3D.jpg
Views: 153
Size:  30.8 KB  Click image for larger version

Name:  Hole Puncher 2D.jpg
Views: 124
Size:  27.3 KB  
Attached Files
File Type: rkt Hole Puncher.rkt (36.5 KB, 136 views)
__________________
Craig McGraw

BARCLONE Rocketry -- http://barclone.rocketshoppe.com
BARCLONE Blogsite -- http://barclone.wordpress.com
BARCLONE Forum -- BARCLONE Forum

BARs helping BARs

SAM 0044
AMA 352635

Last edited by CPMcGraw : 05-26-2006 at 10:15 PM. Reason: Updated 2D image; wrong motor shown
Reply With Quote
  #4  
Old 05-26-2006, 10:34 PM
CPMcGraw's Avatar
CPMcGraw CPMcGraw is offline
BARCLONE Rocketry
 
Join Date: Mar 2005
Location: Mobile, Alabama
Posts: 5,357
Default Other motors you might consider...

Quote:
Originally Posted by CPMcGraw
A8-5.......647'......Dv 5 FPS......Margin 1.47
B6-6......1190'......Dv 12 FPS.....Margin 1.32
C6-7......2076'......Dv 4 FPS......Margin 1.01


I'm going to add some new motor values to this list, namely the 13mm motors which require a few components to make them fit.

1/4A3-3T......140'......Dv 10 FPS......Margin 2.26
1/2A3-4T......345'......Dv 2 FPS.......Margin 2.16

I did not change the fin size or shape to achieve these numbers, only added those components needed to fit the motor. You might want to think about using these less-powerful motors if you have a small field to fly in, as I do. These motors allow your students to "Bring 'Em Back Alive"...
Attached Thumbnails
Click image for larger version

Name:  Hole Puncher 13 2D.jpg
Views: 125
Size:  27.4 KB  
Attached Files
File Type: rkt Hole Puncher 13.rkt (42.3 KB, 138 views)
__________________
Craig McGraw

BARCLONE Rocketry -- http://barclone.rocketshoppe.com
BARCLONE Blogsite -- http://barclone.wordpress.com
BARCLONE Forum -- BARCLONE Forum

BARs helping BARs

SAM 0044
AMA 352635
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump



All times are GMT -5. The time now is 08:02 AM.


Powered by: vBulletin Version 3.0.7
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Ye Olde Rocket Shoppe © 1998-2024