First Open Attempt look right?

[El Cheapo]

This is my....well...second open rocket attempt. It's a 4" Screamin' Green Meanie. I'm using a 2.6" stuffer tube, cold air baffle, open ended nose cone to store the laundry. Stability is about .50 with motors.

 

[DJDeadParrot]

Stability is about .50 with motors.

More nose weight, and it should fly right.

 

[o1d_dude]

I'm not 100% convinced that Open Rocket handles tube fins effectively.

Just a thought.

 

[El Cheapo]

Yes, a couple ounces will bring to the stability margin but not sure it needs to be one caliper since its so short and stubby. I was more or less asking about the design aspects.

 

[Rex R]

current version of open rocket does not do tube fins. there is a way to kinda work around this...but it is rather involved.
rex

 

[tsai]

Simple way to simulate tube fins is to put 3x rectangular fins with a root edge the length equal to the length of the tube fin, and a height equal to the diameter of the tube.

Why does this work? The actual surface area of the tube fin is circumference x length. Circumference is pi*diameter, or roughly 3*diameter.

So, 3*diameter*length.

Surface area of the rectangular fins is height*length. Use 3 fins for each tube, and we get 3*height*length.

In the case of the Screamin' Green Meanie, there's 3 tube fins, so you will need a total of 9 rectangular fins to get a rough approximation of the real CP.

If you do this, I believe you will find the the CP for this rocket is somewhere around 2-4" above the tubes.

I'm guessing stability is not going to be an issue.

Cheers,
- Ken

 

[El Cheapo]

Does the nine fins include the three regular fins or would it be twelve total? I'll give it a whirl tomorrow. Either way, I don't think stability will be an issue either which is why the 50% didn't concern me. More than likely it'll fly on 29mm reloads more often than clusters. Much less expensive than three E9's.

 

[tsai]

Does the nine fins include the three regular fins or would it be twelve total? I'll give it a whirl tomorrow. Either way, I don't think stability will be an issue either which is why the 50% didn't concern me. More than likely it'll fly on 29mm reloads more often than clusters. Much less expensive than three E9's.

Yes, 12 total. 9 to represent the tubes, and the normal 3 main fins.

 

[El Cheapo]

Finally got around to altering the file to reflect the tube fins as suggested by Ken, a.k.a. "tsai". It really did the trick as far as demonstrating stability. Well, at least I hope it did. I had to do three separate sets of three in order to get the spacing correct. Not only did it raise the stability margin but I was actually able to decrease the amount of nose weight by half an ounce. The screen shot demonstrates 4 x E9-8's loaded with 2.5oz nose weight.

It looks like 3 x E9-8 and 4 x D12-7 is about optimal for BP motors.

 

[accooper]

OK, how do you get open rocket to draw the tube fins?

Andrew
I am a BIG FAN of tube fins.

 

[Rex R]

I can think of two ways to do it. 1) oversize launch lugs, 2)add an 'inner tube' and adjust the the radial position. hth
rex

 

[accooper]

never thought of that thanks!

Andrew