I drove from Western Canada to watch STS 128 launch last August and I elected to leave my cameras in the vehicle and just live the moment. When I arrived on site and saw the array of equipment set up at the public launch viewing site I was glad I did. One guy had a 600mm Canon set up on the beach and I asked him if I could take a peek through his viewfinder. My suspicions were confirmed. Even with that level of hardware, the image was unsatisfying given the subject distance. I took 8X50 binoculars and I estimate that the vehicle image in the binoculars was about five to eight shuttles high. You're a long way away.
The exhaust flame is extremely bright. Jonathan's advice to underexpose two stops from your basic sunlight exposure is good. I'd say at least two stops. Once the vehicle leaves the immediate vicinity of the launch structure, it's all over. Get your shot during the first few seconds of launch, otherwise, there's no sense of scale to the image.
Sound, however, suffers little from the distance. It will rattle your ribcage.
Have fun. I drove 13,000kms and spent two weeks of my life for a one minute show. Totally choked with emotion, I called my wife when it was over. "Call me back when you're coherent", she said.
All in all a superb ROI. : )