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Cone Rocket Stability

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GrossApproximator

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I understand a thing or two about stability in normal rockets, but I'm lost when it comes to cone rockets. I'm using RockSim 9, and whenever I design a cone rocket, it seems nearly impossible to get the 1+ calibre stability that normal rockets have. Do cone rockets have different stability requirements than normal (3FNC etc) rockets? If so, what are they?
 

The EGE

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You are correct, getting a cone rocket to have 1-caliber stability is well-nigh impossible. With any cone, the CP is going to be 1/3 up from the bottom*. As long as the CG is above that mark, it'll fly straight, even with a small margin of stability.

* With wide cones, base drag pushes the CP aft, to the point where any saucer is inherently stable under thrust even though they're not statically stable.
 

GrossApproximator

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You are correct, getting a cone rocket to have 1-caliber stability is well-nigh impossible. With any cone, the CP is going to be 1/3 up from the bottom*. As long as the CG is above that mark, it'll fly straight, even with a small margin of stability.

* With wide cones, base drag pushes the CP aft, to the point where any saucer is inherently stable under thrust even though they're not statically stable.
Cool. I didn't know that. :)
 

MarkII

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As EGE said, base drag moves the Center of Pressure rearward. In fact, in some models it is not on the rocket itself but is behind it. On the Art Applewhite Rockets website, Art has published diagrams of the CG/CP relationship in his 24mm and 38mm Delta Saucers with various motors. In every diagram, the CP is behind the saucer. These are for saucers, not cones, but the phenomenon is probably similar for them. Apparently RockSim was not able to correctly calculate the CPs, because Art used wind tunnel testing to determine them. With the CP placed in its correct location in relationship to the Center of Mass/Center of Gravity, one can easily see that these saucers are stable.

I think that there might be another factor that is related, though: the stability of short, fat rockets and long, thin rockets. Saucers and cone rockets fit into the group. I am not up to speed on all of the specifics behind this, but apparently it is more accurate to calculate the stability margin as a percentage of the rocket's length for very short or very long rockets, rather than as multiples of the rocket's diameter. According to what I have read, the gold standard in that case would be 10% of the rocket's length; the CG/CP separation distance should be at least 10% of the rocket's length. In the diagrams at Art's site, one can see that this alternative standard is also easily met.

I can't outline the math behind all of this for the simple reason that I don't know any of it. (My education was notably light on math.) These are just the heuristics that I have picked up.

Mark K.
 

Ironnerd88

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:clap:Great stuff!

I have been looking for this for a while. Thanks, rocketguy (et al)!

I have done a little work with cones (shorter that those in the article). The cones I fly are about 7" long by 4" wide at the base (based very closely on the Centuri X-24 Bug, but without the fins, and round instead of triangular bulkhead). Swing checks show they are stable with the CG at (or slightly behind) the calculated CP. I have not FLOWN one in that configuration yet, but I am thinking of giving it a try.

Might be cool to stage from a C6 to an A10T :), or use the empty space for an altimeter / keychain camera.
 
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