OT ballistics question for Mr. Raab

[Aim to maim]

Sorry to be off topic, but it occurs to me that with your NASA background and reloading expertise, you are unusually well qualified to answer this puzzle.

I watched the Discovery launch this PM and like all Americans, was thankful to see they made it safely to orbit. The network I was watching showed views from the main fuel tank camera, together with a text box showing velocity, altitude, etc.

At the time of external tank separation, the velocity appeared to read approximately 15,000 mph. I'm guessing main engine shutdown follows very shortly after the external tank is jettisoned, since the shuttle itself could carry little fuel and most of that is required for retrofire for return to earth.

I seem to recall that the velocity required for near-earth orbit is somewhere aound 17,500 mph. My question is, how could the shuttle continue to accelerate from 15,000 mph to 17,500 mph, even in a vacuum, once the source of acceleration (main engines) is shut down.

There are several assumptions here on my part and any one of them being incorrect could make the question invalid. Could you help me out here?