Conformal fuel tank

Conformal fuel tanks (CFTs) are additional fuel tanks fitted closely to the profile of an aircraft that extend either the range or "time on station" of the aircraft. CFTs have a reduced aerodynamic penalty compared to external drop tanks, and do not significantly increase an aircraft's radar cross-section. Another advantage of CFTs provide is that they do not occupy ordnance hardpoints like drop tanks, allowing the aircraft to carry its full payload.

Conformal fuel tanks have the disadvantage that, unlike drop tanks, they cannot be discarded in flight, because they are plumbed into the aircraft and so can only be removed on the ground. As a result, they will impose a slight drag-penalty and minor weight gain on the aircraft even when the tanks are empty, without any benefit. They can also impose g-load limits, although not always an absolute issue, the CFTs on the F-15E actually allow the same maneuverability without g-limitations.

Distended internal tanks are fuel tanks that create a bulge from the fuselage or are mounted flush with the fuselage.

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Wikipedia