Technically, you wouldn't. Though it was never done, the shuttle could have taken the external tank with it into orbit. There were ideas to use the tank for stuff like space stations, but nothing ever came of them.If it were instead refuelled on-orbit and used as a drop tank, with an additional tank in the payload bay, you'd get something like: Isp = 450 s (SSME) Orbiter only: m0 = 69 tonnes m1 = 94 tonnes dV = 1300 m/s External tank: m0 = 120 tonnes m1 = 856 tonnes dV = 8600 m/s From a pure delta-V perspective, that could make a trip to lunar orbit and back. Indeed it might be possible to use the payload bay for a lander instead of extra fuel. Of course there are other complications. The SSME's would need modifying to be restartable, and perhaps increased gimballing to propel the shuttle straight. I'm not sure how much of a problem fuel boiloff during the trip would be. But worst of all, how are you going to get over 700 tonnes of fuel up there to refill the external tank!