Bottle Rocket B

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jma
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Re: Bottle Rocket B

Post by jma » February 12th, 2012, 4:22 pm

A-Ro wrote:
jma wrote: Does moving the fins toward the nose make the rocket unstable? Or will the rocket fly okay as long as the CP is ahead of the center of gravity?
First of all, the CP should definitely stay BEHIND the center of gravity. Otherwise, the rocket will not be stable at all and flip over almost immediately after launch. What you want ahead of the center of gravity is the CLA (center of lateral area) to make the rocket backslide without nosediving. So as long as your fins are in a position that keep the CP below the CG and the CG below the CLA, it should work, but only one position will work best for your rocket.
Thanks, A-Ro.
I meant to say CP behind CG.
It's a challenge to move the CG back and have it position in the middle of CP and CLA. The 1 litter bottle is very light and the nose cone is long and heavy therefore the CG is too close to the CLA and too far from the CP. I tried to move the fins forward to make the CG and the CP closer together, but it gave me mixed flight results. I got 1 good flight(16 seconds) and 2 bad flights (about 6-7 seconds, nosediving). I couldn't figure out what was wrong. The cone is 3' long, fins tip: 2cm, fins chord: 7cm, semispan: 7cm.

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Re: Bottle Rocket B

Post by A-Ro » February 13th, 2012, 3:50 pm

jma wrote:
A-Ro wrote:
jma wrote: Does moving the fins toward the nose make the rocket unstable? Or will the rocket fly okay as long as the CP is ahead of the center of gravity?
First of all, the CP should definitely stay BEHIND the center of gravity. Otherwise, the rocket will not be stable at all and flip over almost immediately after launch. What you want ahead of the center of gravity is the CLA (center of lateral area) to make the rocket backslide without nosediving. So as long as your fins are in a position that keep the CP below the CG and the CG below the CLA, it should work, but only one position will work best for your rocket.
Thanks, A-Ro.
I meant to say CP behind CG.
It's a challenge to move the CG back and have it position in the middle of CP and CLA. The 1 litter bottle is very light and the nose cone is long and heavy therefore the CG is too close to the CLA and too far from the CP. I tried to move the fins forward to make the CG and the CP closer together, but it gave me mixed flight results. I got 1 good flight(16 seconds) and 2 bad flights (about 6-7 seconds, nosediving). I couldn't figure out what was wrong. The cone is 3' long, fins tip: 2cm, fins chord: 7cm, semispan: 7cm.
Well, my first thought is just try to make the nose cone lighter. If you're using a paper cone, try to wrap it in the fewest possible layers that still works (it's a good idea to reinforce it toward the bottom more-that's where it could crumble the easiest). If you get this right, you should be able to make the fins much larger than you have them now, therefore making a greater overall lateral area so that it floats slower. If not a paper cone, the only other thing I can think of is to make the fins smaller. That makes the bottom half have less air resistance on the way back down, making it tilt forward less. This will result in less time, but that may be a necessary sacrifice to ensure consistency.
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Re: Bottle Rocket B

Post by jma » February 14th, 2012, 4:54 pm

A-Ro, I made the nose cone lighter and the fins larger as you recommended. I just launched the rocket, and got 16+ seconds every time. Thank you very much for your help.

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Re: Bottle Rocket B

Post by simlebottlerockets » February 15th, 2012, 2:10 pm

Would it be possible to in essence "Rifle" a bottle rocket and come out with positive results? My team would love to try it.

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Re: Bottle Rocket B

Post by chalker » February 15th, 2012, 7:02 pm

simlebottlerockets wrote:Would it be possible to in essence "Rifle" a bottle rocket and come out with positive results? My team would love to try it.
I assume what you really are asking is if angling the fins slightly to cause the rocket to naturally spin would help? Many thoughts are no, for 2 reasons:

1. The energy to induce the spin has to come from somewhere... and that somewhere is a reduced maximum height of the rocket.
2. Most rockets I've seen are way more impacted by slight winds than anything else. There isn't enough inertia in the bottle rockets to allow the spinning to help prevent wind from blowing it off course.

But feel free to try it and let us know if it does somehow help.

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Re: Bottle Rocket B

Post by jma » February 15th, 2012, 7:36 pm

Could the wind cause the backslider to nosedive? It just happened to mine today.

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Re: Bottle Rocket B

Post by chalker » February 15th, 2012, 8:02 pm

jma wrote:Could the wind cause the backslider to nosedive? It just happened to mine today.

That sounds more like a center of gravity issue.

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Re: Bottle Rocket B

Post by jma » February 15th, 2012, 8:53 pm

chalker wrote:
jma wrote:Could the wind cause the backslider to nosedive? It just happened to mine today.

That sounds more like a center of gravity issue.
I flew my rocket 3 times yesterday and the time results were all over 16 seconds. Today I replaced the fins (the old ones were bent), using the same materials and dimension, and launched again and yet the rockets nosedived. It was very windy so I don't know if the rocket wasn't built right or the wind caused it to nosedive.

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Re: Bottle Rocket B

Post by UnknownyMous » February 18th, 2012, 1:12 pm

Is there anything we need to do to the rocket? Like do we need to write our names on it, or draw a line to show how much water we want it to be filled?
Bottle Rocket and Microbe Mission for 2012
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Re: Bottle Rocket B

Post by chalker » February 18th, 2012, 7:21 pm

UnknownyMous wrote:Is there anything we need to do to the rocket? Like do we need to write our names on it, or draw a line to show how much water we want it to be filled?

Usually not, but individual event supervisors might have tournament specific requirements.

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