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
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
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
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.
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!
--
Jerry Irvine, Box 1242, Claremont, California 91711 USA
Opinion, the whole thing. <mail to: snipped-for-privacy@gte.net>
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).
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.
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
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.]
--
Gary "Waiting on ROCKSIM for Linux" Bolles
NAR 82636
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.
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