PDA

View Full Version : Swing Test


chalinaty
05-26-2006, 07:48 PM
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.

CPMcGraw
05-26-2006, 08:28 PM
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...

CPMcGraw
05-26-2006, 09:05 PM
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!

CPMcGraw
05-26-2006, 10:34 PM
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"...:D