I downloaded a Solidworks model of the space shuttle, but I need to remove the engines so the back surface is flat. When I try to delete the entries and fill in the groove, I am then unable to mirror the parts, and it tells me the sketch is under-defined.

Here is the download link GrabCAD - CAD library

I would like the remove the engines, and fully attach the rudder to the surface. I was able to connect it but it still said undefined.

Please help! Thanks