What happens when dynamic stability exceeds 2.0?

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AliHabes

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Hey there,

I have this 271 cm long and 15 cm wide rocket weighs about 17.5 kg which reaches Mach1 and 3170 meters apogee. Its static stability margin is 1.38 and dynamic is 3.25, but dynamic margin looks too stable to me. I wonder what the consequences would be when the dynamic stability margin exceeds 2.0.

Thank you all.
 
That is far too much rocket for a person asking this type of question in this way.
That is far too much snark for a person who thinks that "Amateur Professional" actually means anything.

To the best of my knowledge and recollection, and I'm pretty sure I'm right here, there is no such thing as "dynamic stability margin." If there were, and if it could be as easily interpreted the static margin, then we'd all be using it all the time. When you state that the dynamic stability margin is 3.25, to what exactly are you referring?

Dynamic stability is a complicated subject, with several different but interrelated figures of merit, and no simple answer for what constitutes a good design. To gain a better understanding of it, you might start with a series of articles that Tim Van Milligan wrote in Apogee's (that is, his) newsletter, starting with issue 192. Tim also covers the subject in his book; that will give you the same information and may in fact be reprints of the newsletter articles. (Some of the book is put together that way, but I don't remember if the dynamics section is.) The book, obviously, has a lot of other good information as well; I'm not here to shill for Tim, but it is a book worth having on your shelf.

There are, of course, other sources for the same information. After you've perused one of them, please come back with a more specific question, if you still feel the need to.
 
A classic historical example of aerodynamic instability, when the relative winds exceed the flutter speed:
https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)
similar things can happen to wings, tails, and fins, if they are not stiff enough for the flight speed conditions.

The consequences from too large of a stability margin, from the standpoint of dynamic flutter? Well I guess the rocket is built heavier/costlier than really necessary, so you fly only to a lower altitude, or require a higher impulse engine than otherwise? Not sure what prompted the original question.
 
I stand by my initial post.

If flinging 40 lbs at 750mph, being confused about stability margins is both surprising and concerning.
 
A classic historical example of aerodynamic instability, when the relative winds exceed the flutter speed:
https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)
similar things can happen to wings, tails, and fins, if they are not stiff enough for the flight speed conditions.

The consequences from too large of a stability margin, from the standpoint of dynamic flutter? Well I guess the rocket is built heavier/costlier than really necessary, so you fly only to a lower altitude, or require a higher impulse engine than otherwise? Not sure what prompted the original question.
Fin flutter is, of course, important, but it's not what dynamic stability is about. Natural frequency (of the rocket, not just he fins) and damping coefficient, etc. are something else entirely.

If flinging 40 lbs at 750mph, being confused about stability margins is both surprising and concerning.
Yes it is. So try doing something useful about it.
 
I get that you want to help someone who is badly out of their depth and I just want them to stop and back up before it goes badly.

I'd point out that unless there's a fair bit of base drag 1.38 static is likely insufficient through the transonic; this person is probably not in the US; it is their first post; and a rough calc on a motor that would do this is firmly in L3-to-EX territory.
 
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The effect of base drag on stability is discussed well in another Peak of Flight article in two parts, here and here. The effect is generally not important unless the rocket is short for its diameter (or wide for its length). That's in part because the effect's magnitude depends to a great degree on the rocket's diameter, while CP and CG locations both depend mostly on the length, so if the diameter isn't a fair fraction of the length it will be insignificant. I am also thrilled to have just learned from the Galejs paper above that a tall skinny rocket needs a much larger static margin than a "normal" one, so in such a rocket the small contribution from base drag is doubly insignificant.
 
Fin flutter is, of course, important, but it's not what dynamic stability is about. Natural frequency (of the rocket, not just he fins) and damping coefficient, etc. are something else entirely....

Actually calculation of the natural frequencies and damping/amplification of those modes are precisely one of the mathematical formulations for analyzing flutter. Flutter is but one example of aeroelastic dynamic instability. I just was giving one particular example of a massive structure thought to be built strong enough at the time, yet in a high wind condition crumbled to the ground, or water in this case. Yes the entire rocket is impacted, but typically the little flexible components get ripped off first and those failure modes are the most critical. Who cares if the fuselage mode gets amplified at Mach 3, when the fins have already ripped off at Mach 1? Pretty interesting subject, but not sure how to really help the original poster with their question, which is not very clear.

https://en.wikipedia.org/wiki/Aeroelasticity
 
Yup, at this point the OP seems to have disappeared rather than clarifying. One hopes he's gome off to read some reliable source on the subject.
 
And then there is this old paper about long neck rockets: https://argoshpr.ch/joomla1/articles/pdf/sentinel39-galejs.pdf
Which suggests a higher margin is needed on longer rockets. Base drag? Yeah if it's there, it'll add stability but I sure as heck don't know how to calculate it. Kurt

That is a really good article and explains really well how the Barrowman Eqs don't represent the extremes of short and long rockets very well. I wonder how the basic cardboard cutout method of estimating Cp location would compare in that same study.
 
Pretty interesting subject, but not sure how to really help the original poster with their question, which is not very clear.

My question is this: I know that static margin should be somewhere between 1 and 2. But when the fuel burns CG will elevate and so does the margin. At apogee(or when the fuel completely runs out, or where the rocket is the lightest) what would be the required distance between CG and CP. I know it may vary a lot when lots of factors are considered, but could you suggest any source that I could read and design the rocket as reliable as possible based on the further knowledge I get from the source?

Yup, at this point the OP seems to have disappeared rather than clarifying. One hopes he's gome off to read some reliable source on the subject.

Haha I've read more than you can hope but I need many more.


Thanks for all the answers by the way, they helped in different ways.
 
I still don't really understand your concern. The margin can't be less than 1. Burning fuel will, of course, move the CG forward, so your margin will get larger. No stability issues there. Is that all you are concerned about?

You should probably be more worried about fin flutter and dynamic CP as the rocket goes transsonic.

I'm with dhbarr on this one. Have you designed and flown many smaller, subsonic rockets before? And are you certified with an organization like NAR, TRA or CAR? This is a potentially dangerous project.
 
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Run stability vs mach plots in OR or Rocksim, along with Cp/CG.
Should answer your inquiry.
Remember this: once you begin flying at faster speeds approaching M-2.5 .....M-3 you encounter "mach jump" the phenomena where CP jumps forward dramatically.
Then you do require a margin near or more than 3.
This is from a N-5800 where starting with stability of 3.75 it's reduced to .75 at motor burnout.
Anything less would have been catastrophic.


Capture2.JPG
 
Hey there,

I have this 271 cm long and 15 cm wide rocket weighs about 17.5 kg which reaches Mach1 and 3170 meters apogee. Its static stability margin is 1.38 and dynamic is 3.25, but dynamic margin looks too stable to me. I wonder what the consequences would be when the dynamic stability margin exceeds 2.0.

Thank you all.

somewhere around 88 MPH you’re going to run into an issue when the rocket jumps back in time to October 21st 2015.
 
My question is this: I know that static margin should be somewhere between 1 and 2. But when the fuel burns CG will elevate and so does the margin. At apogee(or when the fuel completely runs out, or where the rocket is the lightest) what would be the required distance between CG and CP. I know it may vary a lot when lots of factors are considered, but could you suggest any source that I could read and design the rocket as reliable as possible based on the further knowledge I get from the source?
OK, so let me see if I've got this straight. When you wrote in the OP about dynamic stability exceeding two, you were referring to the change in margin during the flight? It's an understandable mistake, but that's not what "dynamic stability" means. The margin I think you're talking about is still called the staic margin.

So you want to know what the problem is with a static margin greater than 2, right? For lower speed rockets, it's just weathercocking. And weathercocking is mainly a problem when the rocket is going slowly, because that's when the effective wind angle of attack can be large. So if the margin goes up past 2 while your zooming along it's no problem.

Now, that's for subsonic flight. In transsonic and supersonic flight some things can be different, and I don't know enough to go into it. From what blackjack2564 wrote above, it would seem that if you're going supersonic you should be glad of that forward CG motion giving you higher margin.
 
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