Rocket Stability Question? Open Rocket

[Scott Chase]

I was watching a video on the tube on how to build a rocket in open rocket software. There was a part in the video where the teacher said that when building the designer should keep the stability as close to 2 as possible with the motor loaded. Bellow is the design I came up with to make the stability as close two 2 as I could. The problem is that the video did not say what the distance between CP and CG should be.

My question is with the stability at 2 are my CP and CG far enough apart? I've done some simulations and the rocket appears stable to me. Can you please set me straight concerning this topic? Plots are below!
Thank you
Brian Johnson

 

[lakeroadster]

Stability of 2 means the distance between the cp and cg is 2 times the body diameter.

2 is very conservative for a rocket that is LPR and a standard configuration like the one you have shown.

 

[Scott Chase]

Stability of 2 means the distance between the cp and cg is 2 times the body diameter.

2 is very conservative for a rocket that is LPR and a standard configuration like the one you have shown.
[/QUOTE
Thanks for the info
Thank you,
Brian

 

[Steve Shannon]

Brian,
The general rule of thumb is that the Center of Gravity (CG) should be at least one caliber (body diameter) ahead of the Center of Pressure (CP). In truth, as long as the CG is truly ahead of the CP, aerodynamic forces will cause the rocket to be stable. The reason most try for at least 1 caliber is because the CP is just an estimate. It cannot be easily determined like the CG. So, having a margin of at least 1 allows a little tolerance in case the estimate is incorrect. Also, CP location can change as your rocket flies faster. The CP in a supersonic rocket can actually move forward, so again, having a safety margin helps ensure the rocket stays stable.

 

[jqavins]

There's more to it than that. The CP will move forward when the rocket is at non-zero angle of attack due to body lift. The Barrowman equations, and the somewhat improved equations used by RockSIm and (I assume) Open Rocket don't count body lift, so their CP positions are reasonably accurate only when the AoA is zero. And when the AoA is zero is also the only time that the stability margin doesn't really matter.

Peak of Flight #470 has a reprint of a paper by an MIT Lincoln Labs staffer that goes into this in some depth. The upshot is that for the magnitude of flight perturbation one should reasonably be prepared for, and with a pretty much "normal" shaped rocket, one should be prepared for the CP to migrate as much as one body diameter (caliber) so a static margin of somewhat more than that is necessary. By "normal" we mean that the length to diameter ratio is in the ballpark of 10:1.

The CP migration depends more on the length of the rocket than its diameter. 1 caliber movement is about the same as 10% of the body length in the same "normal" case. Short rockets like the Big Daddy don't need as much static margin (three quarters of a caliber is probably OK, maybe even one half) while very long rockets like the Mean Machine need a lot more (10 calibers is the author's recommendation, if memory serves).

That's the reason that some people prefer to look at CG-CP separation relative to length rather than diameter. So instead of looking for 1 to 2 (or to 3, or whatever you consider too high) calibers, they say one should look for 10%-20% (or to 30%, yatta yatta) of the total length. Which would seem to make sense, not that I'm fully signed on.

 

[Scott Chase]

There's more to it than that. The CP will move forward when the rocket is at non-zero angle of attack due to body lift. The Barrowman equations, and the somewhat improved equations used by RockSIm and (I assume) Open Rocket don't count body lift, so their CP positions are reasonably accurate only when the AoA is zero. And when the AoA is zero is also the only time that the stability margin doesn't really matter.

Peak of Flight #470 has a reprint of a paper by an MIT Lincoln Labs staffer that goes into this in some depth. The upshot is that for the magnitude of flight perturbation one should reasonably be prepared for, and with a pretty much "normal" shaped rocket, one should be prepared for the CP to migrate as much as one body diameter (caliber) so a static margin of somewhat more than that is necessary. By "normal" we mean that the length to diameter ratio is in the ballpark of 10:1.

The CP migration depends more on the length of the rocket than its diameter. 1 caliber movement is about the same as 10% of the body length in the same "normal" case. Short rockets like the Big Daddy don't need as much static margin (three quarters of a caliber is probably OK, maybe even one half) while very long rockets like the Mean Machine need a lot more (10 calibers is the author's recommendation, if memory serves).

That's the reason that some people prefer to look at CG-CP separation relative to length rather than diameter. So instead of looking for 1 to 2 (or to 3, or whatever you consider too high) calibers, they say one should look for 10%-20% (or to 30%, yatta yatta) of the total length. Which would seem to make sense, not that I'm fully signed on.

So am I OK with the stability of 2 with the Estes Ascender Above.

 

[rocketgeek101]

So am I OK with the stability of 2 with the Estes Ascender Above.

Not jqavins, but yes you should be fine in this instance. The Estes Ascender is a pretty typical design (not particularly tall or short) and ~2 cal should be about right.

 

[jqavins]

Yes. Sorry for the side track into theory. I totally and completely concur that your rocket is fine at 2 calibers.