Not sure how to sim. . .

[Sandy H.]

I'm running a sim on an upscale and would like to properly model it. The attached pic shows the rocket in question.

I insert a nosecone and then the bodytube, but after that I'm lost. I want to apply external centering rings and a larger body tube after the smaller one. I'm not sure how to do it. I tried using a coupler or an inner tube, but the new larger diameter does not show up on the rocket specs, so I assume the CP will be a mile off.

Any help is appreciated.

Sandy.

 

[Zetoyoc]

I was curious about this myself after reading your post. I tried making the external centering rings by calling it a ring fin. It worked great. I just had to enter the correct thicknesses to make it solid. .

I do not know what sim you are using. I used rocsim 9 and it seemed to work. Also if you do not want to build it yourself I found a rocsim file on rocket review and used the scale feature. that also seemed to work.


-Z

 

[Pantherjon]

Rocksim? You need to put the parts in kind of backwards as far as the tubes are concerned..Start with the larger tube then add the smaller tube as an 'inside tube' and then make a not 'inside tube' the same size diameter as the smaller tube so the nosecone will go with it(make this tube like 0.0001" in length)..You should then be able to put the centering rings on the smaller tube and they will show externally..Not sure tho if RS will take them into account as far drag is concerned in simulations...

 

[gpoehlein]

Personally, I'd break the body tube into segments. The first segment runs from the nose cone to the first external centering ring. Instead of putting in a centering ring next, make a body tube that is as long as the centering ring is wide, with outer and inner diameters to match the centering ring around the body tube. Next segment is the span between centering rings and is regular body tube. Another segment like the first centering ring and so forth.

As an example, let's create one out of BT-20, with three AR2050s installed at four, five and six inches from the front. AR2050s are 1/4" (.25") thick and OD about .945". The Body tube segments would be:

Segment 1 Length 4", OD .736", ID .710"
Segment 2 Length .25", OD .945", ID .710"
Segment 3 Length .75", OD .736", ID .710"
Segment 4 Length .25", OD .945", ID .710"
Segment 5 Length .75", OD .736", ID .710"
Segment 6 Length .25", OD .945", ID .710"
Segment 7 Length .75", OD .736", ID .710" Or whatever length to the next transition, larger tube or whatever.

Note that the inside diameter stays .710" Because all the elements are paper, they should sim fairly accurately, but I'd weigh final rocket, measure the CG with recovery and no motor, and enter those in the file. This should get your CP reasonably accurate as well. Also note that you couldn't do this in real life, but Rocsim doesn't care whether each item is physically attached to the item in front of it or not - it just assumes it is. This is also a good way to sim a body tube inside another at some point in the length of the model - say a BT-20 going all the way from the nose to the motor mount, with a few inches of BT-50 on the outside and a transition (paper wrap) from the BT-20 to BT-50. Break the BT-20 at the transition, with the BT-20 that is inside the transition and BT-50 as inner tube.

 

[Sandy H.]

Wow, I guess one critical piece of information would have been the software. . . Openrocket. . .

I tried a few more things after posting (transitions etc) and the best solution seems to be what gpoehlein suggests. The CP seems to move around a bit when each component is added. Whenever I tried to do things with the inner tube or centering ring, it appears to ignore them from the CP standpoint.

I'll keep playing around with it. I think I will download the rocksim file as well to import it into openrocket and see what happens.

Thanks for the direction!

Sandy.

 

[Dr. Quigley]

Just be aware that inner tubes and centering rings do not count towards drag calculations, so altitude and such may be a bit off. Good luck on your build! Be sure to post a build thread!