Stability of Tall Thin Rockets

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?

The CP location changes with non-zero angle of attack and on high L/D designs it can move a lot. Programs like Rocksim and Openrocket can show this movement so that you don't have to rely on a rule of thumb.

Check the time when the rocket is exiting the rod/rail as that is when the angle of attack will be greatest. Be sure to simulate using the lowest thrust motor you plan to use with realistic winds.

Check out Apogee Peak of Flight Newsletter Issue #239 it has to do with stability of tall thin rockets.

rharshberger said:

Check out Apogee Peak of Flight Newsletter Issue #239 it has to do with stability of tall thin rockets.

Click to expand...

https://www.apogeerockets.com/education/downloads/Newsletter239.pdf

Thanks, I had issues trying to link the newsletter via my cell.

UhClem said:

The CP location changes with non-zero angle of attack and on high L/D designs it can move a lot. Programs like Rocksim and Openrocket can show this movement so that you don't have to rely on a rule of thumb.

Check the time when the rocket is exiting the rod/rail as that is when the angle of attack will be greatest. Be sure to simulate using the lowest thrust motor you plan to use with realistic winds.

Click to expand...

This ^

The 10% and 1 caliber rule comes from the idea that a 10:1 L/D was thought to be the ideal rocket shape.

Simulations eliminate rules of thumb or at least makes them more scientific and meaningful.

Now let's shake things up and ask: what if the long thin rocket has a transition? (reducing or standard) which "caliber" will you use Now Mr. Krabs!?

It's super-rocs and variable diameters that made me forego calibers in favor of the old sounding rocket stability margin of 8-15% of vehicle length. I aim for 12% and ignore diameters entirely.

The other big consideration for long, thin rockets is to use an extra-long coupler. Those guys tend to corkscrew going up if the BT isn't sufficiently strong at the junctions.

Bat-mite said:

The other big consideration for long, thin rockets is to use an extra-long coupler. Those guys tend to corkscrew going up if the BT isn't sufficiently strong at the junctions.

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Very true! Just like spaghetti, long rockets bow more than short rockets and the couplers will see greater bending moments at the joints

Nytrunner said:

Now let's shake things up and ask: what if the long thin rocket has a transition? (reducing or standard) which "caliber" will you use Now Mr. Krabs!?

It's super-rocs and variable diameters that made me forego calibers in favor of the old sounding rocket stability margin of 8-15% of vehicle length. I aim for 12% and ignore diameters entirely.

Click to expand...

Just put the transition at the CP ^_^

And long motors getting lighter sending the CG forward.
And the CP rushing forward above M2.3 trying to catch the CG and kill the whole rocket.

So many things to watch out for.

Well, the CP moves backwards at lower speeds so it has to go fairly fast to get back to the low velocity stability point! Anyway I don't have the data in front of me from many sims I've done, but IIRC below M3 that isn't really an issue. Flutter, body flex, 3 fins increasing likelyhood of coning compared to 4 fins, stuff like that can be issues. Shifting mass in a high performance rocket could be an issue. Moonburners, C and D slots, have shifting mass and can have asymmetric thrust. High spin rate precession could be an issue. Coning provides a non-zero angle of attack which shifts the CP forwards. Marginal can become unstable.

The CP doesn't move forwards all that fast anyway. For rockets which are almost entirely motor, or for hybrids, one should pay attention to CG shift. For flight > M3 one should pay attention to CP shift. For all rockets one should pay attention to CP shift as a function of angle of attack.

If one is working on a high performance rocket, one should run the sims with as accurate a model of the rocket as feasible. I like RasAero for this. Plots of CP vs mach number and angle of attack are useful for designing purposes. AKA how much fin do you need...

All IMHO of course.

Gerald

I haven't seen any evidence that the needed stability margin has any relationship to the diameter of the rocket. I think it's used because if you check your rocket out in the field for margin (you know the CP and just got the CG), and want to eyeball measure it, it's a lot easier to see the margin is one diameter than 10% of the length. That and many rockets are similar (which doesn't help you when yours is not).

UhClem said:

The CP location changes with non-zero angle of attack and on high L/D designs it can move a lot. Programs like Rocksim and Openrocket can show this movement so that you don't have to rely on a rule of thumb.

Check the time when the rocket is exiting the rod/rail as that is when the angle of attack will be greatest. Be sure to simulate using the lowest thrust motor you plan to use with realistic winds.

Click to expand...

In OR, it's a bit hard to find, it's under Component Analysis, just start changing the Angle of Attack slider. You should be able to get it up to 10 degrees before the stability goes negative. With short rockets, it's not unusual to find they are stable over wider angles even with margins as low as .5 calibers, but they can also turn faster.

I have flown successfully rockets that anti-weathercock, that is, become "unstable" for a predictable length of time -- in this case long MPRs approaching 1 pound on the Estes F15 motors. The actual flights agreed perfectly with the OR simulations. However these rockets were not long enough to be highly flexible.

Thanks for this post bill_s. I've started looking into this more seriously. I'm designing a two stage rocket at the moment and given the Princeton Team experience combined with this thread I believe I need to be more across this issue in my simulation.

So moving forward, in your above comments regarding using component analysis and changing AOA, would that be leveraging the "Extended Barrowman" feature set of OpenRocket? From the OR documentation it definitely seems that Galejs's correction/extension to Barrowman's original equations seem to be available. See below.


From https://openrocket.sourceforge.net/techdoc.pdf page 24.

I'm just trying to get my head around how to leverage these capabilities effectively in my own sim. Thanks to you (and everyone else in this thread) for bringing this issue up before I started seriously designing my two stage stack for next year!