Overview
The primary disadvantage with drop tanks is that they impose a drag penalty on the aircraft carrying them. External fuel tanks will also increase the moment of inertia, thereby reducing roll rates for air combat maneuvers. A rule of thumb is that only about half the capacity of a streamlined drop tank actually goes towards increasing the aircraft's overall range, the rest going to overcome the added drag and weight of the tank itself. Drag in this sense varies with the square of the aircraft's speed. The use of drop tanks also reduces the number of external hardpoints available for weapons, and increases the aircraft's radar signature, both problematic for modern tactical aircraft.
Usually the fuel in the drop tanks is consumed first, and only when all the fuel in the drop tanks has been used, the fuel selector is turned on the airplane's internal tanks.
Some modern combat aircraft use conformal fuel tanks (CFTs) instead of or in addition to conventional external fuel tanks. CFTs produce less drag and do not take up external hardpoints; however, some versions can only be removed on the ground.
The Matra JL-100 is a special hybrid drop tank and rocket pack; it combines a rocket launcher in front with 19 SNEB 68 mm (2.7 in) rockets and 250 litres (66 USgal) of fuel behind into one single aerodynamically-shaped pod for mounting on combat aircraft such as the Dassault Mirage IIIs and English Electric Lightnings.
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