The Forward Reaction Control System (RCS) controls many aspects of the shuttles flight while in orbit. While on the ground there are covers over these thrusters to stop foreign objects falling in and potentially damaging the motors. In orbit the are open. The model has most of them moulded as if the covers are in place. Particularly the ones on the side.
The picture shows the shuttle out at the launch pad and it still has the covers. Pictures without the covers in place are not common since it’s presumably difficult to take pictures of them while in orbit.
This shows the details on the kit, with the flat, featureless areas where the thrusters should be. If this was a model of the shuttle stacked up then these would be fine with a little extra detailing.
They were drilled out and the area smoothed off with needle files and sandpaper.
The holes were backed with a big squash of Milliput and working slowly with the tip of a paintbrush the thruster bells were formed in the soft Milliput. Using water and a little alcohol you can produce a vert good finish. Also allow the Milliput to set up for a few hours before giving it a final polish while still slightly soft.
The thrusters on the nose were more detailed than those on the side but the oval ones at the front still looked too flat, so these were also removed.
Here they have been drilled out.
And filled in using the same Milliput technique described above.