Over Stability and Weather Cocking

[Dredger]

I understand how the stability of a rocket is calculated. The distance between the center of gravity (CG) vs. the center of pressure (CP) is measured in body tube width, or caliper. I gather 1-2 caliper is optimum for low powered rockets, with the CG being forward of the CP.

When a rocket has a 2+ caliper stability, it is considered overly stable. This can lead to weather cocking, or the rocket nosing into the wind during flight.

What I don't understand is conceptually how over stability causes weather cocking. You would think that having the CG and CP close together would make it easier for a lateral (side) wind to change the angle of attack, and thus make the rocket nose into the wind easier.

Does anyone have a good reference article I might read on over stability vs. weather cocking?

 

[shreadvector]

What you have been told is missing one massivly important fact: A rocket that is overstable will ONLY weathercock if it is moving very slowly as it leaves the launch rod relative to the crosswind.

If it leaves the rod fast, then weathercocking is very tiny.

TARC folks learned this and continue to learn this the hard way every year. Their simulations tell them their rockets are overstable (lots of weight in the front from those eggs and altimeter), so they cut the fins down to tiny little nothings. The result is that the tiny fins stubs are lost in the turbulant boundary layer on the surface fo the rocket and they cannot keep the rocket moving straight until the rocket deviates and they are then at a huge angle of attack in the air as the rocket is moving. This results in flights that 'wangle' all over the palce and a horrible altitude.

Anf it is "caliber" not "caliper".

 

[Exactimator]

https://exploration.grc.nasa.gov/education/rocket/rktcock.html

 

[bill_s]

When a rocket has a 2+ caliper stability, it is considered overly stable. This can lead to weather cocking, or the rocket nosing into the wind during flight.

What I don't understand is conceptually how over stability causes weather cocking. You would think that having the CG and CP close together would make it easier for a lateral (side) wind to change the angle of attack, and thus make the rocket nose into the wind easier.

If the rocket turns, the thrust turns with it. Inertia -- the CG -- is the only thing that wants to keep going in the initial direction. Essentially the small amount of "extra" weight in the nose, and the inertia it gets on the rod, guides the entire flight in a sort of chain reaction, it makes it go straight, so it keeps making it go straight.

With the CP and CG more spaced, this a larger lever arm that wind can use to turn the rocket. Eventually the rocket is moving sideways with the wind, so weathercocking stops, plus it's still accelerating.

The reason for needing more than a tiny stability margin is that often the CP moves forward in the case of air moving at an angle to the rocket (experiment in Open Rocket with "angle of attack" under Analysis>Component Analysis). This can make the rocket go temporarily unstable (anti-weathercocking), or if bad enough can set off a chain reaction of instability (cartwheels). As a rough rule of thumb, the rocket should be stable or not unstable for angles of attack up to 10 degrees, and have a best-case margin greater than the worst case anti-margin.