You don’t need to worry about bad weather obscuring your photos. NASA only launches in good weather, but the sky may be a bit overcast. Generally, space shuttle launches take place either during the day or in evening. You need to adjust your settings based on these conditions.
For day launches, you are going to be experiencing a lot of brightness from both the sun and the reflections off of the shuttle itself. Try setting your ISO to between 100 and 200, and take a few pictures before the launch to see which settings actually work better. For evening shots, try setting your camera at between 800 and 1600 ISO setting. Again, experiment with the shots before the shuttle takeoff.
Avoid using the flash. You are going to be far away anyway, and the flash won’t reach. Plus, you’ll probably just over-expose the shot since there is so much metal to bounce off of.
You also need to adjust your aperture settings. Generally, f/8, f/11 and f/16 are great settings for photographing the space shuttle.
Also use the manual focus for taking pictures of the space shuttle. Your auto focus is going to have trouble focusing on something so far away, and it may just focus on a nearer object that’s within the frame.
Use the focusing brackets on your camera to ensure that the space shuttle itself is within the two brackets. If not, your camera may have trouble focusing on the space shuttle over the smoke and flames.