What does over stable do to a rocket

I just downloaded the demo version of Rocksim and all my designs are over
stable by a caliber or two or three. What does this really mean.
snipped-for-privacy@aol.com
Reply to
TRMaurer
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I said it more effectively and far more succinctly.
That one read like a Bruce Kelly tome.
Jerry
Reply to
Jerry Irvine
lots deleted
It sure can be too stable for practical applications. A rocket like the Mean Machine that is way overstable wobbles as it ascends because it is slow to react to external forces and when it does, it overcorrects. This action causes the rocket to fly inefficiently.
Mark Simpson NAR 71503 Level II God Bless our peacekeepers
Reply to
Mark Simpson
I've seen this in action on my Shadow Composites' Shockvalue, also long and thin. Originally I was overconservative and put in way too much noseweight so it was way overstable. It went straight up, but with a lot of spiral that robbed altitude (and worse, tricked the altimeter into deploying late). After a few of flights like this I figured it out and reduced the noseweight to bring the stability margin back to something reasonable. Now it flies dead straight, no spiral. Nothing like a controlled experiment to verify theory. Kind of cool, actually. Rocket science in action.
As Mark says, overstable basically equals underdamped. When any kind of transient gets the rocket gets pointed off the flight path, it overcorrects. In two dimensions, it would oscillate, in three dimensions, it spirals.
The other problem with overstablity which has also been mentioned, is that it increases weathercocking.
2-3 calibers is fine. I'd rather be a little conservative and go with 1.5 or 2 calibers than try to push the limit at 1.1 caliber, because there are a lot of approximations in the math.
Somewhere around 5-6 calibers is starting to get too much and you get a spiral.
-- Erik Ebert, L3 TRA# 09105 NAR# 79868
Reply to
Erik Ebert
I had a video of one of my 'overstable' rockets analyzed by someone involved in a bit of aerodynamic forces and here was the response.......
"It seems to me that the motion it was exhibiting is a classic example of the phenomenon of "pitch-roll coupling". Basically it is an effect similar to a gyroscope or a wobbling bicycle wheel. The rolling of the rocket along its main axis gives it a lot of angular momentum in that direction. Then a little bit of residual lift from the combination of wings and canted fins produces a slight pitching moment (this is probably all but impossible to completely eliminate). However, just like a gyro or a bike wheel, the spinning rocket is deflected sideways to the direction it is trying to pitch, due to the angular momentum it has. The result is a corkscrewing motion. So I would repeat my suggestion that if you can eliminate , or at least minimize, the rolling of the rocket, it should automatically cure the pitch-roll problem, since it won't have angular momentum any more."
After that under thrust the rocket exhibited a 'coning' effect. End result? 20 oz of nose weight, the pitch-roll coupling is minimized and the coning has all but disappeared.
Chuck
Reply to
Chuck Rudy
Gary wrote in news:3F45593C.5080307@below:
Let's look at the dynamics. "Overstable" simply means the moment arm for the restoring force is longer than "necessary". What that means is that less of angle of attack is necessary to generate sufficent restoring force to act to reduce the angle of attack. It says, by itself, *NOTHING* about wobbling, spiraling, weathercocking, etc.. These are all consequences of other dynamic considerations. Pitch/roll coupling has already been mentioned. Another consideration is the distribution of mass in the model, which will affect its angular inertia, i.e., how it responds to forces that rotate it around its CG. If all the mass is at the CG, it will respond (accelerate, i.e., increase its angular velocity around the CG) faster than if the mass is mostly at the two extreme ends of the model. This also affects how fast it will *decelerate* after it passes though zero angle of attack, which is what actually determines how much the model will "overcorrect".
len.
Reply to
Leonard Fehskens
Dudes...I thought it meant move to the cow pasture and get away from the horses...
Reply to
Hippiestew
Caused by C-slot motors.
Reply to
Jerry Irvine
Your visual description only applies to the example of the mass being far away from the cg.
If the mass is distributed (unlike an egglofter with an egg on one end and a heavy motor on the other), it will not build up much angular momentum and it will correct-damp quickly and fly straight.
My Tubular Treat model is overly stable and it does not wiggle at all during boost. It can't. It is so stable that if it tried to deviate from straight flight, the restoring force is great enough to push it's ass back down and keep the nose pointed forward.
Maybe we need to discuss what happens to a stable model that gets hit by a gust as it's flying through the air at high speed?
My opinion: you need adequate velocity off the rod (or rail) and more stable is good since it will help in the real world where fins are not always aligned and where nozzles are imperfect or erode causing asymmetrical thrust.
-Fred Shecter NAR 20117
-- ""Remove "zorch" from address (2 places) to reply.
Reply to
Fred Shecter
It is "mass-flow" thus "inertia matters".
This is no more obvious than a motor used in a Monocopter. Even on a wimpy endburner the mass-flow is forced by centrifical force against the outside edge of the casing and erodes the case and nozzle irregualrly. Especially scary on an H5. The casing has to be double thick just to cope with it :)
Jerry
Pardon the tech post!
Reply to
Jerry Irvine
They added 300 words to the dictionary this year or so. I suspect we are not likely to see this one added :)
Jerry
Reply to
Jerry Irvine
Did they add "Horribilize" or "Horribilizer" or "Horribilization" ?
-Fred Shecter NAR 20117
-- ""Remove "zorch" from address (2 places) to reply.
Reply to
Fred Shecter
If they did it is perfect for some of the more reactionary rmr posters.
Reply to
Jerry Irvine
good since
I think everyone is of track with the dynamics answers. The answer to to the question is that Rocksim was authored by a programmer who wants to help you design rockets. So he programmed in a bothersome "help message" that gerates lots of messages on RMR, but does little else. Just ignore it.
Alan
Reply to
Alan Jones
is good since
I agree in principle, but ROCKSIM has become one of the basic tools of the trade, even for newbies. I have seen (in class) the anxiety caused by the "over stable" warnings that the program generates, especially with little info on what "over stable" might translate into during a typical flight. The ability to include a "cardboard cut-out" stability guesstimate (which should be obsolete, IMHO) adds to their confusion. And the first question they ask is, "Why?"
Perhaps we can provide Apogee with some constructive feedback (and reduce the RMR messages) by soliciting ideas on how to "fix" this issue. For instance, I might recommend a context sensitive Help function that could pop-up an info screen on "stability" if ROCKSIM finds a design to be outside of its thresholds. Could be as simple as inserting a link to related help topics in any "error" message.
Don't get me wrong; I think ROCKSIM is a fundamental advance in model rocketry. But we shouldn't automate basic concepts out of the design process. Or ignore them.
[Note: We use an old demo version of ROCKSIM off the Apogee site. I don't use Windows at home (and can't afford retail ROCKSIM at school), so I don't know if the newer versions address this issue or not.]
Reply to
Gary
I thought I had qualified my response enough to make it obvious that "over-stability" does not imply any particular flight characteristic at all, only that an over-stable rocket is more subject to perturbations ("other dynamic considerations") than a "stable" rocket and that a typical "example" of such a response to perturbation may be a wobble or spiral (under-damped harmonic motion). However, I will submit that an over-stable model will, indeed, exhibit more weathercock than a similar model with "stable" ratings.
I usually recommend only fin size or body length changes to make ROCKSIM happy. The "apparent" stability criteria which ROCKSIM presents to the user (in the form of Cp and Cg indicators and relationships) tracks intuitively with body length and fin area changes and the Cp-Cg "moment arm" metaphor of fin lift stability. Certainly, many other factors affect dynamic stability, but they are not as visible in the ROCKSIM interface and new users may not even be aware of their presence or impact on flight characteristics (velocity issues or rotational inertia, for instance).
Reply to
Gary
Gary wrote in news:3F47E6A4.3070901@below:
What's worse is if your design has .99 caliber stability margin RockSim says it's "unstable"; if it has 1.01 it's "stable". While I appreciate the intent, the approach is too simplistic. In RockSim 7, this is almost ironic, with the CP now being calculated as a function of roll azimuth. Surely if someone understands this concept they can understand how to interpret the CG/CP margin.
len.
Reply to
Leonard Fehskens

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