choice of reference system, are described. It is demonstrated how the MFP-axis formulation introducessingularities in the EOM. The singularities are identiŽ ed, and the effect of their removal is shown in a numerical application. The conclusion is that the body-axis formulation generally leads to a more accurate solution. As part of the current investigation, equations were derived for the steady-state limits of common motion variables. These limits are not affected by the variousmodeling techniquesdiscussedhere.