To get you started on your UFO, here is a little tutorial on creating those aperture type doors :)
Create a 12 sided disc, make sure it's centered at the origin and bevel a few times, delete the disc once you have enough segments.
Select all the inner points and rotate as shown. Make sure your mouse cursor is at the origin and rotate as shown.
The reason I use the Multishift tool is to move the points slightly so they don't all share the same locations in space. Otherwise, when you merge points later on you'll have a mess on your hands.
Delete the the unwanted polygons and then stretch the selected points to create a nice tip for the blades.
Extrude the mesh to add thickness and edit the end points as shown, it should be easier if each set of blades were on a different layer....and you're done. Tthe rest of the UFO is up to you :)