Science Buddies: "Ask an Expert"

I am an 8th grader doing a project with this question: How does the center of gravity on a rocket (from nose to tail) affect how true and consistently the rocket flies? It would really help if these five questions could be answered. Thank you very much for your help.

What is the difference between inertial and gravitational mass?

As the center of mass moves toward the tail, is there a point where the rocket suddenly does not fly as true? Or is it linear?

What is the worst possible point for the center of gravity to be and why?

What is the best possible point for the center of gravity to be and why?

What happens to the rocket in the air when the center of gravity is very close to the tail?

Thanks again for your help.

What is the difference between inertial and gravitational mass?

Their definitions are different. Inertial mass is the mass determined by measuring its resistance to acceleration. Gravitational mass is the mass determined by measuring its weight at rest (or passive force) and using the gravitational acceleration constant to convert it to mass units. If you believe Albert Einstein ideas on this subject, he developed his general theory of relativity starting with the assumption that inertial mass and (passive) gravitational mass is identical and that no experiment will ever detect a difference between them (the weak version of the equivalence principle).

Your other questions are unanswerable in the absense of any control system approach.

The design of any rocket control system requires figuring out where to apply control forces. If you look at some of the more exotic air planes with forward control surfaces compared to older tail dragger designs, you can see that control/stability problems can be attached in very different ways.

In air, a control fin that is farther away from the center of gravity has its affect multiplied by the torque lever distance to the center of gravity. In a vaccum, any control jets also have their affect multiplied by similar torque distances but require counter forces to prevent having an effect in in yaw, pitch, and roll. In an air stream, there are naturally occuring counter forces from drag that are missing in a vaccum.