Okay, I've done my Googleing and I know that if I ask a specific question
here I usually get an answer. So here goes. I am looking for a
paper/report/discussion on how spin stability works. Not a heavily laden
math treatment of it. Just some basics. I need to present this to 4th
Is it the gyro effect (conservation of angular momentum) that is at work? Is
it something else?
Although I am looking for a basic treatment I would not mind a little in
depth on the optimum spin rate and if there is a minimum fin size for
"Topics in Advanced Model Rocketry", Chapter 2 by Gordon Mandell. He
calls it roll stabilization, by the way. His is not a trivial
explanation, but it can be understood without any math (although he does
refer to some very significant equations to establish his assertions.)
I'll try to summarize, but you should go find a copy of this book.
Spin stabilization doesn't induce positive stability in a statically
unstable rocket--the rocket will still be unstable. Instead, there are a
few things at work that may keep an unstable rocket nonetheless flying
- Spinning suppresses the growth rate of instability. The proof of this
requires looking at the equations. For model rockets, with a flight
duration of a few seconds, a fast enough roll rate may be enough to keep
the instability from affecting the flight path.
- Spinning creates a high angular momentum, and subsequent additional
angular disturbances add only a small change to the momentum vector. The
initial size of the yaw or pitch rotation resulting from a disturbance
will be less for a spinning rocket than a non-spinning rocket. (It's
still possible to rotate a gyro off axis, it just takes a lot more force
than if it wasn't spinning.) This is sort of the gyro effect you asked
about, although "conservation of angular momentum" is misleading.
Angular momentum can be changed through a force on the rocket/gyro.
- Spinning turns all disturbances due to imperfections in the rocket
into "sinusoidal forcing at the roll frequency". In other words, a
crimped body tube, for instance, will not cause an arcing trajectory if
the rocket is spinning. However you will instead get yaw-roll coupling,
resulting in a "corkscrew" flight.
The magnitude of these effects depends on the roll rate--the faster the
spin the less the effect of disturbances on a rocket. In other words, a
statically unstable rocket that isn't spinning fast enough won't fly
straight. For example, the yaw-roll coupling I mentioned above could
grow large enough to send a slowly spinning rocket off course.
(Unfortunately, with yaw-roll coupling a very high roll frequency, while
helping to keep the rocket on course, could tear the rocket apart.)
(replace "spambait" with "merlinus" to respond directly to me)
Let me know if you didn't get the file I sent. Copyuright isn't an issue. We
don't adhere to NAR guideleines. (Badges?! We don't need no stinking
badges!) We have done spin-stabilized rockets, though, for many years.
On Mon, 14 Aug 2006 00:53:51 -0400, "Darrell D. Mobley"
Thanks, I was able to get it from that site, and it has not propagated
through ABMR yet. This report is well written, although it is padded
out with a lot of historical material. It shows that deflected spin
tabs do decrease altitude ("paddle wheel effect"?), at least for the
rocket tested which had no need to be spun.
There is a great deal of literature available on this topic. I'm
reluctant to suggest papers that may not be readily available to you,
or that may not be appropriate for your 4th graders.
Spin stability works best when the spinning is done about the
principle inertial axis, the one with the largest moment of inertia.
Thus, spin stability words best with saucer like rockets, and some
well designed space probes. You can spin a more typical rocket about
the roll axis, and it will appear stable for a time but eventually it
will try to transfer the angular momentum to spinning about the
principle axis, e.g. an end over end flat spin.
Perhaps the best know case is the Explorer satellite launched on the
Juno/Redsone vehicle. You can see the bucket full of Recruit motors
and satellite spinning atop the nose before launch. This is actuality
a two stage satellite boost package. The first stage has a rather
large moment of inertia, while the final satellite stage has a small
rocket like moment of inertia. To make matters worse, the satellite
had flexible whip antennae that helped transfer the spin from the roll
axis to the principle axis much quicker than expected.
Within the atmosphere, spinning is used quite effectively on bullets
and artillery shells. The spinning is not strictly stable, but
projectiles are fairly rigid, symmetrical, have a higher relative
moment of roll inertial compared to sounding rockets. The flight
times tend to be low, before the rounds begin to tumble, but it
greatly improves range and accuracy.
For unguided rockets, fins are preferred for static stability. Even
so, spin is desired to minimize dispersion. There can be some
confusion calling these rockets spin stabilized. Generally, you want
the highest roll rate at launch, to even out thrust misalignments, and
somewhat less spin rate at max q. This can be sometimes be achieved
with a spinning launch table, or a riffled launch tube. Sometimes,
small rocket spin motors are used. Most commonly, the fins are simply
canted to roll the rocket.
Dispersion is not as sexy a topic as altitude, but the tighter you
control flight dispersion, the higher altitudes you can fly to within
a given launch site, be it White Sands, or the school yard.
Using canted fins (or spin tabs) is difficult because you don't get
the roll rate where you want it the most. So, you tend to want to use
maximum amount of fin cant to spin up the rocket as quickly as
possible. This can also result in an increase in drag. There is
usually a maximum roll rate that will drive the rocket unstable, so
you want to stay well below that. There is also a resonant roll-yaw
coupling roll rate that should be avoided. Typically, sounding
rockets are designed to fly through the resonant coupling rate
quickly, before bad things start to happens. Many sounding rockets
are spin balanced, and have fin cant angles tuned for particular
For sport rockets, the roll you get through typical assembly, say a
fin misaligned 0.5 deg. is probably good. Some rockets with some
asymmetry, or clustering and staging, should have a bit more
deliberate roll rate built in.
The easy answer is duh. The fin size required for static stability
will have ample "roll power" to spin a typical rocket, and you need
not increase fin size just to induce roll. There will be very little
lag between the steady state roll rate and the dynamic roll rate as
The optimum spin rate, or optimum fin cant angle, depends on your
choice of optimality and your particular rocket. In many cases the
optimal fin cant angle is zero with fins sized for one caliber static
margin.l However, I've done some studies that suggests the two
calibers of stability is optimal for a dual egglofter. You might look
at an old Estes Cineroc carrier and decide that those barn door fins
are way too big. After designing your "higher performance" Cineroc
vehicle, and reviewing the resulting film, you will understand the
reason for the "oversized" fins, damping.
Alan Jones wrote:
> ... There is
> usually a maximum roll rate that will drive the rocket unstable
This is interesting, and doesn't seem to agree with Mandell's analysis
of spin stabilization. From what I read, the higher the roll rate the
less the rocket will be perturbed from a straight flight path.
(replace "spambait" with "merlinus" to respond directly to me)
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