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enter image description here

For instance, this KC-46. Is this a converted Boeing 767? Or its own design?

Same with this Rivet Joint.

enter image description here

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    $\begingroup$ Because of the cost of designing, producing and then maintaining another aircraft type, it's just easier to start with the existing aircraft project, especially one produced in large numbers already. $\endgroup$
    – Mark
    Aug 26, 2022 at 11:29
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    $\begingroup$ Some tankers (DC-9s that I know of, possibly others) only use the 'standard' fuel tanks and don't have any additional capacity built in, as this is far easier from a certification point of view. $\endgroup$
    – Frog
    Aug 29, 2022 at 6:01
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    $\begingroup$ It depends on what you mean by converted. The design is converted from an existing model but most of the time they do not take a pre-built plane and physically change it. $\endgroup$
    – Max Power
    Oct 1, 2022 at 9:45

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The Airbus A330 MRTT (Multi Role Tanker Transport) is actually built on the same assembly line as the regular passenger A330s in Toulouse and then flown to Spain for modification:

The A330 MRTTs are based on Airbus’ popular A330 widebody passenger airliner, with the aircraft produced on the company’s commercial airplane final assembly line in Toulouse, France. Once their initial built-up is complete, they are flown to Airbus’ military aircraft facility in Getafe, Spain to be transformed with hardware and systems for their dual roles as an air-to-air refuelling platform and an airlifter for troops and cargo. [...]

During the conversion, Airbus teams install some 16,000 types of new components and approximately 450 new electrical harnesses (for a total cabling length of more than 50 km.), as well as 6,000 brackets and 1,700 connectors.

(airbus.com - Airbus transforms A330s into Multi Role Tanker Transports through an optimised conversion process)

The cabin can be configured in different ways, similar to how a passenger A330 could be configured based on the customer's need. The cargo hold is also available with the same ULD containers as the passenger variant:

Serving three missions with one capable platform, the A330 MRTT can carry up to 111 tonnes of fuel in the aerial refuelling mission – the highest capacity of any tanker aircraft. [...]

Requiring no reconfiguration, the A330 MRTT also serves as an efficient transport aircraft, carrying passengers on the main deck and cargo in the lower lobe. Seating capacity is up to 300 passengers on the main deck, which also can be arranged in a medical evacuation configuration that accommodates 40 stretchers and 20 seats for medical staff, along with 100 passengers. The lower lobe – which accommodates up to 37 tonnes of cargo, can be loaded with 27 LD3 containers or eight military pallets.

(airbus.com - Multirole Tanker - Defence History)

Such a mixed configuration might look something like this:
A330 MRTT
(airbus.com - A330 MRTT)

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  • $\begingroup$ That "mixed configuration" image at the end must refuel other aircraft from the MRTT's own wing tanks. I don't see anything in the cutaway that looks like extra fuel tanks... $\endgroup$
    – FreeMan
    Aug 26, 2022 at 11:10
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    $\begingroup$ @FreeMan I don't think any of them have extra fuel tanks. The normal fuel capacity of wing and center tanks is probably enough. Related: Do tanker aircraft distribute their own fuel? $\endgroup$
    – Bianfable
    Aug 26, 2022 at 11:31
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    $\begingroup$ You know, I probably read that answer when it was posted and 5 or 6 times since, but it just didn't stick. Thanks for the reminder. $\endgroup$
    – FreeMan
    Aug 26, 2022 at 11:55
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The tankers are "based on", not a regular passenger jet "converted to".

The same engines and basic body, but the whole internals are different.

The Wiki page for the KC-46 outlines this https://en.wikipedia.org/wiki/Boeing_KC-46_Pegasus

Some, but not all, of the other platforms are ex-airliners. https://www.air-and-space.com/Boeing%20707%20Military.htm

"EC-18B, 81-0894 at Vandenberg Air Force Base on June 9, 1990. It carries Boeing construction number 19583 and was delivered to American Airlines as 707-323C, N8403 on November 28, 1967. The Air Force acquired it on July 15, 1982. "

The E-3B AWACS platforms were purpose built by Boeing.

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  • $\begingroup$ Would this be the same for the Eg18 Growler? $\endgroup$
    – Boeing787
    Oct 5, 2022 at 2:07
  • $\begingroup$ The fact that none of the C-135s show up on the military 707 page indicates they are totally separate designs. $\endgroup$ Aug 5, 2023 at 9:18
  • $\begingroup$ I don’t believe an EA-18G Growler is used for tanker duties. The F/A-18E/F aircraft could perform tanker duties but used an external drogue pack on the centerline hardpoint for that. The fuel system is internally plumbed to allow gas to be exchanged that way. $\endgroup$ Aug 6, 2023 at 17:14
  • $\begingroup$ not necessarily. For example the Dutch Air Force used to operate several KDC-10As, which were converted DC-10-30s purchased from Martinair when they retired the type. Other countries operate converted 707s and no doubt others as well. $\endgroup$
    – jwenting
    Aug 8, 2023 at 5:02
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One counter-example : KC-390 (Embraer) doesn't seem based on an existing airframe, but designed concurrently as tanker and/or military transport from the ground up.

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I agree with WPNSGuy that most tankers are derivatives of transports, sometimes also bombers. But there are exceptions for sure.

The Dash 80 prototype led to the commercial 707 and the military KC-135 tanker. Both planes shared the basic design of the Dash 80 but were very different airplanes, neither one being a derivative of the other. One great difference was in the width and length of the fuselage. Airlines wanted the 707 fuselage to be 4 inches (2.5 centimeters) wider than the tanker’s.....

Source: https://www.boeing.com/history/products/707.page

So KC-135 and 707 are parallel but separate designs. In fact, military version of 707 is designated as C-137 or C-18 instead of C-135 and Boeing's internal designation of KC-135 would be Model 717

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    $\begingroup$ And in fact, the KC-135 carried the Boeing designation of “717” — certainly different than the Boeing 717 of today. I wish I had a picture of it, but the info plate as you ascend the crew ladder of the KC-135 states that: “Boeing 717”. $\endgroup$ Aug 30, 2022 at 21:34
  • $\begingroup$ 4"=2.5cm? Maybe... $\endgroup$
    – RetiredATC
    Aug 6, 2023 at 18:21
  • $\begingroup$ @RetiredATC 4" is about 11cm $\endgroup$
    – jwenting
    Aug 8, 2023 at 5:02
  • $\begingroup$ @jwenting I was referring to that conversation in the answer above. $\endgroup$
    – RetiredATC
    Aug 8, 2023 at 16:09

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