Stability is a VERY complicated and thorny issue, because essentially it is a MOVING target that greatly depends on the rocket, the flight parameters, and the environmental parameters of the atmosphere through which the rocket is moving.
Static stability is the easiest concept to grasp and to define, because dynamic stability, that is to say, stability
in flight basically is constantly changing to some degree or other. Therefore static stability seeks to make these changes moot by defining the stability of the rocket "at rest", hopefully in a close approximation of
typical conditions the rocket will be exposed to in flight, and conditions which the rocket is unlikely to exceed in flight.
CG, center of gravity, is of course rather easily obtained by balancing the launch-ready rocket (without ignitor, since that is ejected at ignition and does not fly with the rocket (usually, HOPEFULLY!). CG gets a bit more complicated on "non-symmetrical rockets" like the space shuttle stack, multistage rockets which eject series stages or parallel stages in flight, rockets carrying off-center payloads (cameras, etc.) and the like. Even in a "typical" three-fin and nosecone (3FNC) rocket, CG moves SOME, as the propellant is burned in engine, the CG moves forward, since the nose weight (and hopefully the parachute weight and location) remains the same. Other engine types (liquid, predominantly, though hybrids are a different case too) have their CG locations change in different ways, usually aft, as the engine burns. BUT for 99% of hobby rocketry, it can be said the CG moves FORWARD as the engine burns.
CP, center of pressure, is a MUCH more dynamic and difficult to measure property. CP is affected by the design, fin size, span, shape, drag, the rocket's overall shape, size (Reynolds numbers come into play at certain size thresholds), angle of attack, weight distribution (moments of inertia, length, oscillation characteristics, etc.) wind speed and direction, flight speed, static stability margin, "apparent" angle of attack from wind, air density, air turbulence, and probably a hundred other factors. Some have extremely subtle effects that are barely measurable. Some have significant effects that can have OVERT effects, and some are subtle at times, and overt at others, and combine and compound with other effects to create greater CP changes than at other times, and sometimes certain factors can cancel each other out or work together beneficially to enhance stability. It's EXTREMELY complicated (that's a part of why rocket science is HARD
)
SO, how do we get a handle on it?? One way is to look at the 'worst case scenario'... if we design the rocket to be stable with the WORST conditions it's likely to ever see, we can be pretty darn sure it'll be stable under virtually every flight condition we're likely to come up against. Fortunately this is a rather simple matter. By determining the
center of lateral area we can determine the *rough* CP point, which, interestingly enough, is the point at which the CP would approximately be if the rocket were
ninety degrees to the airflow-- essentially 'flying sideways'... So if the rocket could stabilize from THAT, we can be pretty darn sure it's stable. This is called the
cardboard cutout method. Draw an exact outline of the rocket on cardboard, cut it out, and balance that "profile" on a ruler-- the balance point is the center of lateral area. Rocksim can be reset under the 'calculate stability by' box by clicking it and choosing "Cardboard cutout method" from the drop down menu.
Now this method, as you can tell, is VERY VERY conservative. It can lead to EXCESSIVE stability, where the rocket will gyrate and oscillate excessively, weathercock excessively in windy conditions, and other such undesirable effects that rob energy and therefore altitude. That's where the Barrowman Method comes in, which makes certain assumptions about the shape of the rocket and their aerodynamic affects on the rocket's flight. Those assumptions simplified the calculations back in the pre-computer era so that rocketeers with sufficient math skills who wanted to take the time and effort to crunch the numbers by hand could actually sit down and calculate the CP to greater accuracy than the cardboard cutout method would allow. This would allow greater precision and optimization of the rocket design. But it was a VERY labor-intensive process taking several hours at best, and STILL had quite a few limitations because of the ASSUMPTIONS made in creating and simplifying the mathematical formulas. For instance, the Barrowman Method couldn't handle but either 3 or 4 fins-- odd numbers of fins were out. Complicated shapes beyond the basic tubular rocket with radial symmetry (transitions, nosecones, tailcones, evenly spaced fins) were out-- so calculating CP with side pods, boosters on the sides, gliders, etc. was out. Basically, at the end of the day, you STILL had an APPROXIMATION.
The "Rocksim Method" used in the Rocksim program is based on the Barrowman Method and uses it's calculations as the basis of it's function, with some tweaks to the assumptions and some more complex mathematical modelling to get 'closer' to the "real" CP than the basic Barrowman Method allowed. Complex math isn't much of a problem anymore with the massive computing power available now (your PC is more powerful than the Apollo flight computers that got us to the MOON!). Still, there are underlying ASSUMPTIONS in the math models that may or may not accurately reflect or predict the CP location in every instance-- some parameters may be inaccurately modelled, some may be ignored, some may be 'over conservative' in the math models, etc.
Basically, to know the TRUE CP, you'd have to measure all the influences on the rocket at EVERY GIVEN MOMENT IN FLIGHT and plug those measurements into equations to determine the EXACT CP AT THAT PRECISE MOMENT, because as the parameters change from moment to moment, the CP changes as well.
Fortunately, the approximations we get from computer programs, hand crunched numbers, and even cardboard cutouts are enough 99% of the time. The real ART of stability determination is in contest rocketry, where balancing all those forces, and making certain design tradeoffs combine to either give you a winning design or leave you in everyone else's dust...
SO, summing up,
static stability is the mathematically calculated stability points (CG/CP relationship) of the rocket. As the rocket takes off and speed increases, the fins DO have more stabilizing effect. This is important to know in relation to launch guide length and motor selection, because the rocket needs to get sufficient speed before leaving the launcher for the fins to stabilize it. The static stability doesn't 'go away', but it DOES change (subtley we hope!) For heavy rockets, this means either A) longer launch guides, or B) a "bigger motor", either in a higher impulse class (switching from an "A" to a "B" motor, or choosing a motor with a higher 'peak thrust', say a A8 instead of an A3, which will accelerate the heavier rocket faster off the pad, reaching fin-stabilizing speed faster.
Hope this helps! OL JR
