Large stability margin with small fins?

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JRThro

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I'm not sure where this thread should go, so I hope the moderators will either move it or approve of its being located here.

I'll admit up front that I've never gone through the Barrowman stability calculations by hand, but here's my question:

What effect does the magnitude of the restoring force centered at the CP have on the stability margin?

I can imagine a relatively long rocket with small fins having a nice large stability margin but having a small restoring force centered at the CP. So even though it might be overstable as calculated by Barrowman, it might act more like a marginally or even neutrally stable rocket.

Does this make sense? Or am I missing something?

Thanks in advance.
 
One of the problems with small fins is they can stall at a high angle of attack.

So even if it shows that it's overstable they may act "unstable" in a real world situation.

Like rod whip sending it off and not being able to recover.
 
I haven't yet seen that happen, but I have seen something else. Small fins won't move the CP very far, so to keep the CG and CP apart by the right amount, the CG has to move, which means a longer rocket, more nose weight, or both of the above. Such rockets sometimes seem to become unstable at high velocity, causing them to zig-zag at altitude, then regain stability and continue on course until deployment.

See this thread for a discussion of this behaviour.

Basically, the small fins don't generate much restoring force, so the rocket swings some way off course before they can correct it. Then it swings back into line, only because of that nose weight, it has a fair amount of inertia, so it keeps going. And now it's off course in the other direction. Repeat... (In the above-referenced thread, Shreadvector posted an article e-mailed to him about this which explains it far better and in more detail than that.)
 
Thanks, guys!

The link to shreadvector's post with the contribution by the unnamed gentleman was very informative, as it answered my question directly.

You can indeed have a stable or overstable design with small fins that are less effective than they need to be for a good straight flight profile.
 
Originally posted by JRThro
I'm not sure where this thread should go, so I hope the moderators will either move it or approve of its being located here.

I'll admit up front that I've never gone through the Barrowman stability calculations by hand, but here's my question:

What effect does the magnitude of the restoring force centered at the CP have on the stability margin?

I can imagine a relatively long rocket with small fins having a nice large stability margin but having a small restoring force centered at the CP. So even though it might be overstable as calculated by Barrowman, it might act more like a marginally or even neutrally stable rocket.

Does this make sense? Or am I missing something?

Thanks in advance.

yes
you are only taking into consideration the static result(barrowman) ... add a puff of wind and that all changes dramaticaly.and becomes complicated very quickly.. if there was absolutly no wind and the rocket was built in perfect alignment it would go perfectly straight. but thats not the real world
(barrowman is just a starting point ,and not the whole picture )

*edit- this is just a basic, simplified, in a nutshell answer. there are many other factors that may be involved such as off center thrust. the post that was linked earlier is far more informative.(I just read it, very good info)
 
The referenced thread talked about dynamic instability due to pitch-roll coupling. That is a possibility if you have a spinning rocket, but the more likely case is marginal stability due to the CP shifting forward at high angle of attack.

Robert and Peter Alway wrote a great paper about back-sliding recovery in long rockets with small fins. The Alway paper shows how the CP moves forward as the AOA increases, resulting in marginal or negative stability margins at some AOA. The Barrowman margin calculation assumes zero angle of attack. The cardboard cutout method of stability calculation assumes 90 degrees AOA. As the AOA goes from 0 to 90 degrees, the CP gradually moves from the Barrowman point to the cardboard cutout point.

It is possible to have a rocket that is overstable according to Barrowman (zero AOA) but unstable in the presence of a wind gust (high AOA). Such a rocket may fly straight on 9 flights, then do some wild gyrations on the 10th fligh.

To determine if your rocket is at risk, calculate the CP two ways:
1) Barrowman (or RockSim): You want about 1-2 calibers of stability here.
2) Cardboard cutout method: You need >0 calibers here.

If you have positive Barrowman stability but negative cutout stability, you could experience some interesting flights.
 
The math aside basically any small fin rocket will have a trend
to "weather-cock". The longer the rocket and the smaller the
fins the more it will want to be affected by the wind.

Tube fin rockets are a good example since the fins are 1:1 in
size ratio to the body tube, instead of being 1 to 1.5.

It's been my flight experience with tube fin and other rockets
with small fins and longer lengths that they need greater thurst
or speed at take off to overcome the wind enduced instability. I
have flown my BT-80 tube fin rocket with a I200 in 16 mph winds
but only twice with BJ motors under 5 mph winds. A BJ motor on
my tube fin rocket would put the flight at horizontal shortly after
take off in winds over 10-15 mph.

Just a thought.

William
 
My experience from direct observation is the opposite (although I'm sure others have much more observational experience who may disagree). I've only seen wind increase the stability of rockets, hence "over stable" rockets weathercocking. It's not instability that causes weathercocking, but the introduction of a horizontal angle of attack. The more overstable the rocket, the greater the effect of the angle of attack. Ever notice how AMRAAMs tend not to weathercock as much as rockets without canard fins?

Best example of this was an NCR Lance that I saw go up on a relatively windy day. The builder had not put the noseweight into the rocket, so it *should* have been unstable. It was the only flight of the day that didn't weathercock severely, and it even arced slightly *with* the wind on the way up. I believe that on a calm day it would have begun skywriting almost immediately off the rod.
 
Originally posted by WillCarney
The math aside basically any small fin rocket will have a trend
to "weather-cock". The longer the rocket and the smaller the
fins the more it will want to be affected by the wind.
Ummmm, generally LARGE-finned rockets are the ones that weather-cock. At low speeds early in the flight, there is not much forward airflow to maintain stability, so the sideways wind will impart a strong torque on large fins.

If anything, tube fins should be LESS affected by wind than a comparable number of flat fins. Each tube fin can be modeled as 3 small fins whose semi-span is the same as the tube diameter. A typical 6-tube rocket acts like it has 18 narrow fins. Converting those 18 narrow fins into 6 long fins should give the same amount of restoring force but have much more lateral area and be more susceptible to weather-cocking.

One thing about tube-fins compared to flat fins is that at high angles of attack, they no longer act like fins. Instead of flowing cleanly through the tube, the air bouncing around the inside walls builds up a back-pressure that blocks the flow. The longer and narrower the tube fins, the worse the blockage becomes.

The combination of these two effects implies that wind alone should not be able to cause much weather-cocking, but that wind near takeoff will render the tube fins less effective and make the rocket more susceptible to other disturbances like asymmetric thrust or rod whip.
 
Originally posted by BobCox
Converting those 18 narrow fins into 6 long fins should give the same amount of restoring force but have much more lateral area and be more susceptible to weather-cocking.
Everything in your post makes sense to me except this part. If the fins have the same amount of (total) restoring force, shouldn't the rocket's susceptibility to weathercocking also be the same?
 
I should have worded that differently. They have the same total restoring force at small angles of attack. The airflow coming straight from the front can interact with all of the fins. Air coming from the side is blocked by the body and the adjacent fins. A rocket with lots of narrow fins (which is sort-of what you get with tube fins) will not have much fin area exposed to the sideways wind.
 
Could the original question be related to the cases posed by Galejs in his analysis of the stability of short, fat rockets? In that analysis, he described how short, fat rockets don't need a 1.0 or greater stability margin because their stability is not affected much at all by the angle of attack of the rocket. In proving his point, he shows the case of a very long rocket (with relatively small fins) that initially has a high stability margin, but the stability margin is greatly reduced by the angle of attack.

He references the extremely disproportionate effect of body lift on a rocket that is highly stable with small fins.

Here's the link to Galej's report:

https://projetosulfos.if.sc.usp.br/artigos/sentinel39-galejs.pdf

EDIT: After re-reading the original question and the notes in Galej's paper, I'm sure I hit the nail on the head with this reference. :cool:
 
Another data point is my four stage Rheinbote. I've flown the upper two stages alone, and in that case the upper stage is perfectly stable. Due to the length of the boosters, I have to use a length of Quickmatch to ignite each subsequent stage, and on one occasion the Quickmatch wasn't quick enough, there was a delay in upper stage ignition, so the rocket slowed down, weathercocked, then sent the upper stage towards some nearby trees. Fortunately it only reached the nearest one and ended up low enough that it could be recovered. The point is, that upper stage was stable even in wind.

When the full stack was added, it was a different matter. I never saw what happened myself because I lost sight of the upper stage after the three boosters had done their work, but other, keener eyed observers, reported that the upper stage zig-zagged.

Which means the upper stage is less stable at high velocity than at low velocity, even when there is lower wind during the high velocity flight.
 
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