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- May 29, 2017
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Hopefully this forum can help me understand something.
There are two rules of thumb for rocket stability:
1. CG should be 1-2 calibers ahead of the CP.
2. CG should be about 10% of the rocket length ahead of the CP.
For many rockets, these two general rules will align fairly closely, but since one is a function of rocket diameter and the other is a function of length, they start to diverge significantly on short-wide rockets and tall-thin rockets. I'm in the design process of a tall-thin rocket right now and so I've been researching stability to determine which is the better rule to follow in this instance. The rocket's length to diameter ratio is 29:1 in its current iteration in OpenRocket, and stability is at 2 calibers, but this is only about 7% of the overall length.
When I try to think about it logically, the force required on the CP to pivot the rocket around the CG is going to be lesser as the CG gets farther ahead of the CP. That's just basic physics, lever-arms and such. So to me, it seems maintaining the 1-2 calibers of stability makes more sense than trying to aim for 10%. Aiming for 10% seems like it would unnecessarily increase the sensitivity of the rocket to weathercocking.
What confuses me, however, is that during an internet search, I read that somebody recommended as much as 8-9 calibers of stability on tall-thin rockets . This is exactly the opposite of the conclusion I came to. I don't know if there was subtext to that conversation, but since many thin rockets are minimum-diameter mach busters, perhaps they were taking into account the CP during trans-sonic flight, which my rocket will not accomplish.
So what says you? Is there any reason I'm missing that would make me want to chase after stability greater than 2 calibers on a tall-thin rocket, or should I be staying around 1.5-2?
There are two rules of thumb for rocket stability:
1. CG should be 1-2 calibers ahead of the CP.
2. CG should be about 10% of the rocket length ahead of the CP.
For many rockets, these two general rules will align fairly closely, but since one is a function of rocket diameter and the other is a function of length, they start to diverge significantly on short-wide rockets and tall-thin rockets. I'm in the design process of a tall-thin rocket right now and so I've been researching stability to determine which is the better rule to follow in this instance. The rocket's length to diameter ratio is 29:1 in its current iteration in OpenRocket, and stability is at 2 calibers, but this is only about 7% of the overall length.
When I try to think about it logically, the force required on the CP to pivot the rocket around the CG is going to be lesser as the CG gets farther ahead of the CP. That's just basic physics, lever-arms and such. So to me, it seems maintaining the 1-2 calibers of stability makes more sense than trying to aim for 10%. Aiming for 10% seems like it would unnecessarily increase the sensitivity of the rocket to weathercocking.
What confuses me, however, is that during an internet search, I read that somebody recommended as much as 8-9 calibers of stability on tall-thin rockets . This is exactly the opposite of the conclusion I came to. I don't know if there was subtext to that conversation, but since many thin rockets are minimum-diameter mach busters, perhaps they were taking into account the CP during trans-sonic flight, which my rocket will not accomplish.
So what says you? Is there any reason I'm missing that would make me want to chase after stability greater than 2 calibers on a tall-thin rocket, or should I be staying around 1.5-2?