Is the space shuttle able for space travel?

Answer 1

The answer is no. The reason is because the Space Shuttle Main Engines (SSMEs) draw fuel from the orange external tank (ET). Fuel from the ET is drained by the the time the shuttle reaches orbit 8 minutes after launch. The ET is then jettisoned where it burns up upon re-entry. LOX and LH2 fuel on board the orbiter itself that is meant for the SSMEs is only enough to provide the required de-orbit burn to slow the orbiter enough for re-entry.

Change in orbital altitudes are achieved though two smaller secondary engines called "orbital maneuvering system engines" (OMS) However, these are not nearly powerful enough to increase the orbiter's velocity to escape earth orbit.

In the days of Apollo, the Saturn V brought put the astronauts in to a very low "parking orbit" which gave them time to plan the trans-lunar injection (TLI, the rocket burn to speed up the spacecraft so it can escape earth orbit and be put on a course to the moon). After 2.5 orbits, the J-2 engine of the Saturn V's S-IVB (third stage) is fired for six minutes, and accelerates the spacecraft from 17,000 MPH to 25,000 MPH, allowing it to escape earth's gravity and be put on a course for the moon.