The only concept that used _only_ drop tanks was the Starclipper, a Space Shuttle phase A design from Lockheed. It looked a little like the X-33, a large triangular vehicle with the V-shaped tank surrounding it. It was not even given a Phase B by NASA because (pardon me while I roll on the ground laughing) because the tank was expendable and so it wasn't FULLY reusable, which was a NASA requiremnt for the Shuttle! Of course Rockwell bid a flyback first stage, then switched to the ET and SRBs when it became obvious there was no money to develop the flyback booster.