Stability measurement as a caliber or as a percentage

[MetricRocketeer]

Hi TRF colleagues,

In vNARCON 2023's excellent presentation on OpenRocket, I learned that you can choose to have stability shown as a caliber or as a percentage. I had not known that before.

So, showing stability as a caliber — this is what I have always used — the formula of course is (CP - CG) / body-tube diamter. Thus, for one rocket that I have drawn in OR, the CP is 130 cm, the CG is 114 cm, and the body-tube diameter is 10.2 cm. Hence, the caliber is shown as 1.62. (Incidentally, the result should be 1.57 — so, what causes that discrepancy?) However, that is not my main question.

Here is my main question. If I set my preference to show stability as a percentage, the result is 9.42%. But what does that mean, please — 9.42% of what? How is that number generated?

Thank you.

Stanley

 

[Neutronium95]

That is the percentage of the rocket length. It tends to work better for both fat and skinny rockets. I usually try to keep it within 8-18%.

 

[neil_w]

Consider that for a canonical 10:1 ratio rocket, 10% of length = 1 caliber. So that’s sort of the “center point”.

 

[Buckeye]

Hi TRF colleagues,

In vNARCON 2023's excellent presentation on OpenRocket, I learned that you can choose to have stability shown as a caliber or as a percentage. I had not known that before.

So, showing stability as a caliber — this is what I have always used — the formula of course is (CP - CG) / body-tube diamter. Thus, for one rocket that I have drawn in OR, the CP is 130 cm, the CG is 114 cm, and the body-tube diameter is 10.2 cm. Hence, the caliber is shown as 1.62. (Incidentally, the result should be 1.57 — so, what causes that discrepancy?) However, that is not my main question.

Here is my main question. If I set my preference to show stability as a percentage, the result is 9.42%. But what does that mean, please — 9.42% of what? How is that number generated?

Thank you.

Stanley

(CP-CG)/rocket length

 

[MetricRocketeer]

Consider that for a canonical 10:1 ratio rocket, 10% of length = 1 caliber. So that’s sort of the “center point”.

What does "canonical" mean here?

 

[MetricRocketeer]

Hi @Neutronium95, @neil_w, and @Buckeye, as well as anyone else,

Thank you for your informative responses.

Now to the discrepancies, please.

CP = 130 cm
CG = 114 cm
Length = 175 cm
Maximum diameter = 10.2 cm

So, why doesn't the caliber equal 1.57 instead of the indicated 1.62?

And why doesn't the percentage equal 9.14% instead of the indicated 9.42%?

Thank you.

Stanley

 

[SolarYellow]

Consider that for a canonical 10:1 ratio rocket, 10% of length = 1 caliber. So that’s sort of the “center point”.

I've read that 8-15 percent of overall length is an old rule of thumb that was accepted for sounding rockets. It's possible I have the paper or manual that comes from saved in PDF and just haven't read it yet.

To me, just considering the physics of what's happening, it makes a lot more sense to use the overall length as the denominator than to use the diameter of the BT. There has been a lot of digital ink (and real ink before that, I'm sure) spilled about how you can get away with less than a caliber on short, fat rockets and long, skinny rockets need more. That's because the rule of thumb of "caliber" picked the wrong variable to use as the denominator. Use a more logical variable in the denominator, and you likely end up with a more effective rule that's applicable to a broader range of situations with less hand waving and fudge factor required.

 

[JoePfeiffer]

CP = 130 cm
CG = 114 cm
Length = 175 cm
Maximum diameter = 10.2 cm

So, why doesn't the caliber equal 1.57 instead of the indicated 1.62?

And why doesn't the percentage equal 9.14% instead of the indicated 9.42%?

That looks like a bit much to be rounding error... as always, please post the .ork so we can look at it.

 

[MetricRocketeer]

Hi @JoePfeiffer and everyone else,

Thank your for your reply.

This data data is from an Apogee Peregrine, but I am getting these discrepancies from every rocket.

Stanley

 

[neil_w]

It's a rounding issue. Actual diameter is 10.16 cm. CG is 113.79 cm, and CP is 130.25 cm.

I don't know why metric values aren't shown with more precision, we can take a look at that.