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It i s an underhung tank, but it is dropped after use, so it is a drop tank.
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The MiG-29 has a ferry range of 1,500 km without external fuel tanks, and 2,100 km with external tanks.[20] The internal fuel capacity of the original MiG-29B is 4,365 litres distributed between six internal fuel tanks, four in the fuselage and one in each wing. For longer flights, this can be supplemented by a 1,500-litre (330 Imp gal, 395 US gal) centreline drop tank and, on later production batches, two 1,150-litre (253 Imp gal, 300 US gal) underwing drop tanks. In addition, a small number have been fitted with port-side inflight refueling probes, allowing much longer flight times by using a probe-and-drogue system.
https://en.wikipedia.org/wiki/Mikoyan_MiG-29

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Note added at 9 hrs (2018-03-08 13:03:56 GMT)
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In aviation, a drop tank (external tank, wing tank, or belly tank) is used to describe auxiliary fuel tanks externally carried by aircraft. A drop tank is expendable and often jettisonable. External tanks are commonplace on modern military aircraft and occasionally found in civilian ones, although the latter are less likely to be discarded except in the event of emergency.

https://en.wikipedia.org/wiki/Drop_tank