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I know that the F-16 (F-16I and Block 50/52/60 models) have conformal fuel tanks standard now on top of the wing/fuselage area. The Silent Eagle model of the F-15E proposed by Boeing for South Korea, and other F-15E customers had them added. The F-18E (Superhornet) has demoed a set recently.

But on older aircraft, how common were conformal fuel tanks? Or have they always been drop tanks before?

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    $\begingroup$ The conformal fuel tanks of the F-18 are for the next gen model, the "super duper" hornet. Along with conformal fuel tanks it has larger engines, internal stores, and some other tweaks. $\endgroup$ – Rhino Driver Apr 16 '15 at 17:04
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Technically speaking, conformal tanks don't take up hardpoints, so I'm not sure what we have today has an equivalent in older designs.

Otherwise, I'd say somewhat common; streamlined fuel tanks and other conformal packages have been used for quite a while in several different designs.

Some World War II Supermarine Spitfires used what was called a "slipper tank" - a streamlined package of up to 90 gallons (additional source).

The early 1960s-era English Electric Lightning used a 'conformal ventral store' that could hold either a fuel tank or rocket engine (?!). The Gloster Javelin used a similar set of "bosom tanks".

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  • $\begingroup$ Hardpoints were a concern on older designs, primarily smaller fighter-bombers. The P-51, for instance, could carry droptanks on its wing hardpoints when flying bomber escorts, or bombs for precision strike missions or CAS. The P-51 had the ability to fit a slipper tank as well if greater range or loiter time were needed and the hardpoints were unavailable. $\endgroup$ – KeithS May 11 '15 at 18:22
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    $\begingroup$ I've never seen a P-51 slipper tank, got a link? How did it fit with the gear and radiator all converging right there? $\endgroup$ – egid May 15 '15 at 15:19
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    $\begingroup$ IIRC the tanks actually fitted over the top of the fuselage like saddlebags behind the cockpit. But, I can't find a picture online. By the time the D variant was introduced the plane could carry a lot of gas without any special modification (like 480 gals with standard droptanks, which given the cleaner piston-engine design was plenty) $\endgroup$ – KeithS Jun 26 '15 at 21:41
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    $\begingroup$ Hm, never seen that. If you ever find a link share it! I've only seen the ridiculous huge ferry tanks that look like floats. $\endgroup$ – egid Jun 27 '15 at 15:24
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The Messerschmitt Bf-110 used a conformal fuel tank under the forward fuselage, colloquially known as "Dackelbauch" (Dachshund belly). The picture below should explain well why this name was chosen.

Bf-110 with conformal tank under the forward fuselage

They were introduced to allow planes stationed in Norway to attack targets in Northern England. When empty, their content would be a dangerous mixture of air and fuel vapor, so they were not very popular with their crews and only used briefly.

If you want, you could even call the center wing tank of some biplanes a conformal tank. However, conformal tanks are rather unusual.

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    $\begingroup$ If a center wing tank counts, wouldn't a normal wing tank? :) $\endgroup$ – egid May 15 '15 at 15:20
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They're not unheard of, as from your examples. Like any design, they have advantages and disadvantages.

Pros:

  • Typically more aerodynamic, and lighter, than a droptank of similar capacity.
  • Carried closer to the fuselage than hardpoint droptanks, reducing roll moment, increasing maneuverability.
  • Doesn't take up hardpoints desired for munitions (the primary reason the F-15E uses them is that it wouldn't have a useful range carrying 2000lb Mk-82s otherwise)
  • Typically allows more fuel storage than the centerline droptank (the only other location that isn't usually a munitions hardpoint)

Cons:

  • Aerodynamics are still negatively impacted
  • Cannot be jettisoned mid-flight for weight/drag advantage
  • When empty of liquid fuel, CFTs can still contain a combustible vapor which could explode given a hit that might otherwise do only superficial damage (most modern designs use a bladder to contain the fuel and minimize vapor release into the CFT shell)
  • Not all airframes tolerate CFTs of a meaningful size without significant changes in handling. The F-16I's performance is apparently unaffected by the additional 460 gallon-capacity CFTs, but the F-15E/I/SK, despite the airframe's air-superiority pedigree, is known to be at a disadvantage in a dogfight to the F-15C, the USAF's primary air-superiority variant.
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