Drop tanks are mostly used in military aviation in order to increase the range of aircraft.
The usually cylindrical, almost like bombs shaped tanks are generally used on exterior load carriers under the fuselage, the wings or wing tips and can be dropped in flight. There are also aircraft types which are specifically adapted to the shape of aircraft auxiliary tanks (conformal fuel tanks ) use (such as the F- 15C).
However, the droppable auxiliary tanks can not be attached to any external load station and in any configuration, on the one hand needs to be connected to the internal fuel system, and secondly, the center of gravity at each filling level must remain within the permissible limits.
The fuel supply of the aircraft is switched so that the first fuel is transferred from the auxiliary tanks to the internal fuel tank. Like any external load influence external tanks by their air resistance, the speed and the weight and limited structural strength, the agility of an airplane. In addition, they increase the radar cross section. Therefore, they can be solved if required ( for example, air combat, emergency, etc.). For this they are usually separated by explosive bolts.
A rule of thumb is that only about half the contents of a drop tanks to increase the range available. The remaining fuel is consumed to overcome the higher drag of the aircraft with ejector tank.