Modal analysis with acceleration loading in patran/nastran

Hi,
I want to do modal analysis for a rocket that is subjected to various accelerations (like 9g, 18g). I want to wonder if the modal analysis results are different for unloaded and loaded (with acceleration) cases. If they are different how can I solve this problem with acceleration loading?
I have read "inertia relief" procedure for static analysis. But for the modal analysis I have not encountered any information.
Can you help me?
Thanks a lot...