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  #1  
Old 05-26-2019, 12:34 PM
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Rktman Rktman is offline
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Default A case of overstability?

Not sure which forum this belongs in, but this one looks like the most appropriate and the one most likely full of very knowledgeable builders. I'm trying to diagnose the problem with this oddroc design which just won't launch straight. It leaves the rod okay but after 10 or 15 feet it starts to go off-vertical and/or corkscrewing in big loops, and only kind of straightens out close to apogee. The CG is certainly far enough forward to be stable and I even added lead weights to the nose cone but nothing changed.

Is it possible it's overstable and every slight gust of wind will make it veer off at a tangent? It's currently sitting near the top of a 50' tree but I'd like to build a replacement that'll work like it should and won't pose a hazard to myself or any unnoticed spectators.
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Old 05-26-2019, 03:18 PM
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Have you tried different engines in it ?
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Old 05-26-2019, 04:19 PM
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My guess is alignment of guidance surfaces is your problem.
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Old 05-26-2019, 04:31 PM
frognbuff frognbuff is offline
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I believe our CP is too far forward. Since you don't want to remove the forward "box," (I'm fairly certain it would fly well without it), add at least 3" of body tube (6" would be better) and try again. Keep those nose weights too.
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Old 05-26-2019, 07:20 PM
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Quote:
Originally Posted by frognbuff
I believe our CP is too far forward. Since you don't want to remove the forward "box," (I'm fairly certain it would fly well without it), add at least 3" of body tube (6" would be better) and try again. Keep those nose weights too.


I agree. Effectively, there are 'fins' for 3/4 the length of the rocket. The CP is more than likely way, way forward.

A contributing factor could possibly be some turbulence created by the motor exhaust 'shock waves' being bounced around inside the aft 'box' area, though this effect may be rather minor and would cease to be an issue after motor burnout.

But frognbuff's suggestion of extending the bodytube (quickie payload section addition) would probably solve the problem. Would certainly be easy to test.

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Old 05-26-2019, 10:04 PM
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At best this is a marginally stable design, a quick RockSim simulation (attached) based on the photos you provided gives a stability margin of only 0.75 calibers with a D12-5 motor installed. It will take at least 1/2 ounce of nose weight to bring the stability margin up to 1.05 calibers for a stable flight. 290 feet above ground level (AGL) on a C11-3 and 748 feet AGL on a D12-5 with the added nose weight. Due to the large fin surface area, I would not fly this design in winds above 7mph. For a better simulation you need to provide me with measurements and other design details. As already mentioned lengthening the main body tube by about 3 inches will also bring the stability margin to the same levels as adding nose weight.

Aerodynamically, if you did a wind tunnel simulation of this design, I would guess that the lower set of fins are in the turbulent zone generated by the upper set of fins, thus they would have no effect on the the stability of the rocket! Taking this into account gives an unstable situation of -0.92 calibers with the D12-5 motor and requires about 6 ounces of added nose weight for stability at 1.01 calibers or lengthening the main body tube by 17 inches for the same margin. The performance is greatly reduced to 81 feet AGL for the C11-3 and 263 feet AGL for the D12-5 with the added nose weight, lengthening the main body tube more than doubles these attained altitudes.
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Old 05-27-2019, 08:22 AM
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Quote:
Originally Posted by ghrocketman
Have you tried different engines in it ?


Yup, everything from a D12 to an AT E15. Same results every time.
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Old 05-27-2019, 08:54 AM
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Quote:
Originally Posted by teflonrocketry1
At best this is a marginally stable design, a quick RockSim simulation (attached) based on the photos you provided gives a stability margin of only 0.75 calibers with a D12-5 motor installed. It will take at least 1/2 ounce of nose weight to bring the stability margin up to 1.05 calibers for a stable flight. 290 feet above ground level (AGL) on a C11-3 and 748 feet AGL on a D12-5 with the added nose weight. Due to the large fin surface area, I would not fly this design in winds above 7mph. For a better simulation you need to provide me with measurements and other design details. As already mentioned lengthening the main body tube by about 3 inches will also bring the stability margin to the same levels as adding nose weight.

Aerodynamically, if you did a wind tunnel simulation of this design, I would guess that the lower set of fins are in the turbulent zone generated by the upper set of fins, thus they would have no effect on the the stability of the rocket! Taking this into account gives an unstable situation of -0.92 calibers with the D12-5 motor and requires about 6 ounces of added nose weight for stability at 1.01 calibers or lengthening the main body tube by 17 inches for the same margin. The performance is greatly reduced to 81 feet AGL for the C11-3 and 263 feet AGL for the D12-5 with the added nose weight, lengthening the main body tube more than doubles these attained altitudes.


I believe you and Earl and Frognbuf are correct that it's unstable even though I assumed it was because it was modeled after a lot of tractor type rockets where the motor and weight is set far forward. It ended up heavy because I added 1.2 oz of lead weight to the nose thinking it would solve the problem. I had hoped to keep the BT short to keep the "Box Kite" look but it seems I'll have to give up on that.

BTW Teflon, that sim you did is amazingly accurate given you were working just from the pics I attached. Even your filename (Cubix) happened to be the same as what I settled on calling it. Attached my working sketches w/ dimensions below.
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  #9  
Old 05-27-2019, 07:51 PM
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The aft box is not as effective a ring fin because it is in the wake of the front box. If you make the aft box slightly larger, it will probably work as intended, like a ring fin. Not sure how much bigger...
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Old 05-27-2019, 10:43 PM
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Quote:
Originally Posted by Rktman
BTW Teflon, that sim you did is amazingly accurate given you were working just from the pics I attached. Even your filename (Cubix) happened to be the same as what I settled on calling it. Attached my working sketches w/ dimensions below.


The pictures in your post are named Cubix. I have created simulations from pictures hundreds of times, one of the reasons my avatar is the #1 certified RockSim user. The drawing you posted doesn't seem to be accurate; what is meant by #20 tubing? I used BT-70 for the main body. I based the measurements I made on the fact the picture shows a C11-3 motor installed (with igniter!). What is the distance of the upper fins from one end of the body tube? If you remove the upper box fins, (should be an easy fix) you get a cool looking tractor design that will fly stable see attached files. A box kite does not fly with its box panels at the angle of attack you used in your design, it fly's with the wind hitting the panels close to a right angle and from their corners.
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