Everything changed back in 1966. James Barrowman, while working at NASA as an aerospace engineer, developed a series of equations that enabled us to estimate the CP of rockets. Once the CP is estimated it is relatively easy to determine if most rockets are stable. He went on to publish this in a now famous reports put out by Centuri called TIR-30 and TIR-33 ( both of these are available at JIMZ Rocket Plans – TIR-30 – TIR-33 -PDFs). There are many computer programs out today that make use of his equations to help design and simulate flight of your rockets. One of the most popular is RockSim available from Apogee (Since this was originally written in 2006 there has been the wonderful addition of Open Rocket). So when you see your rocket climb straight as an arrow instead of turning toward you be thankful to James Barrowman.