Shifting of the center of gravity of a hybrid motor

[MetricRocketeer]

Hi TRF colleagues,

I was going through the Advanced Certification Study Guide of the Tripoli Rocketry Association.

I don't understand the answer to the following question: "What typically happens to a marginally stable rocket with a hybrid motor during the thrusting phase?"

Here is the answer: "The rocket may become less stable."

And here is further explanation: "As the CG of the hybrid motor shifts aft, so does the CG of the rocket, which may result in an unstable flight."

I have no experience with hybrid motors. Nevertheless, I don't understand why or how the center of gravity of a hybrid motor shifts.

Could someone please explain why that happens. Also, does that happen with black-powder or reloadable motors?

Thank you.

Stanley

 

[OverTheTop]

Think about what happens to the liquid N2O in the tank as it is consumed during the burn. Where does the liquid end up and where does the gas ullage end up during boost? Please consider . It might help to think about just the motor and where the motor CG is shifting to during boost.

This question gets quite a few people, but with a little consideration it can be determined.

 

[MetricRocketeer]

Thank you, @OverTheTop. That makes sense.

 

[dr wogz]

Also think of where the fuel is put in an airplane. you will see that the fuel tanks are on (or about ) the CG of the airplane, for exactly the same reason: so that as the fuel is consumed the CG stays (relatively) the same..

 

[OverTheTop]

Also think of where the fuel is put in an airplane. you will see that the fuel tanks are on (or about ) the CG of the airplane, for exactly the same reason: so that as the fuel is consumed the CG stays (relatively) the same..

Or in the case of the Concorde they pumped fuel forward and aft to balance the plane as the CG shifts between supersonic and subsonic flight.