What does the string test for model rockets actually prove?

[Selectron]

"String Test" for Rockets

This doesn't slot easily into any of the Physics Forum's categories but I guess it is kinda aerospace engineering...

And my uni's space society we make small model rockets, just simple things out of paper and card and a small motor. On the sheet of instructions we have for making the rockets it says that to be suitable for launching the rockets must pass the "string test" - tie a piece of string around the rocket at the position of its centre of gravity and whirl it around your head in a circle - the rocket is fine if it flies tangentially to the circle. We were discussing this and couldn't work out exactly what this test was supposed to prove - and surely by virtue of the string being tied around the centre of gravity the rocket will always fly horizontally, tangential to the circle. Do you have any ideas about what the purpose of the test is?

 

[D H]

The purpose of the test is to determine whether the center of drag is below (i.e., closer to the engine) than is the center of mass. The rocket will be unstable if the center of drag (aka center of resistance) is above the center of mass. You want your rocket to be stable, so you need the center of drag below the center of mass. The string test is an easy way to determine if this is the case.