How efficient is iron smelting ?
As rorshach said, they do have throttleable boosters now, but the ones on the shuttle are not MANUALLY throttleable. They do, however, neck down in certain areas to provide less thrust as they pass through maximum dynamic pressure (about 750psi). After this point, the area of the fuel being burned widens and the thrust picks up again. It is technically throttling, but it occurs at predetermined times and cannot be adjusted. At the same time, the shuttle's main engines are throttling up. This is what Commander Scobee was talking about when he said "throttle up".