Why are launch escape towers usually jettisoned fairly early in the flight? What if a problem went wrong during an upper stage burn and the escape tower had already been jettisoned? I know that carrying a heavy escape tower for a majority of the flight will certainly degrade the orbital capabilities of the rocket, but isn't crew safety more important?
Escape towers get jettisoned once they're no longer necessary, usually when the spacecraft can abort under its own power or using an intact upper stage. Using the Apollo abort modes as an example:
- The pad abort (before launch) and modes IA/IB/IC (between launch and escape tower jettison) would use the escape tower.
- Mode II (after escape tower jettison, early in the second stage burn) would use the Service Module's engine to pull away from the rest of the rocket.
- Mode III (later in the second stage burn) would use the third stage (and Service Module engine, if necessary) to abort to Earth orbit.
- Mode IV (during the third stage burn) would use the Service Module engine to abort to Earth orbit.