Gloire_bb has it right.
Military transports carry too much extraneous ballast and compromises to be competitive with a commercial airliner as the basis for a tanker. They have big, draggy fuselages for voluminous cargo, complex, heavy high lift devices on the wings for short runway ops and unnecessarily beefy landing gear for unpaved fields, none of which matters in the tanker role. Fuel is a fairly dense payload, so the big fuselage is just a drag penalty and forward basing a scarce high value asset such as a tanker is insanely risky while offering no benefit to its mission, so STOL and rugged landing gear amount to dead weight.
That there are numerous tankers based on military transports can invariably be traced back to three scenarios:
1) You have a small air force that simply doesn't have the budget for dedicated tankers, so the choice is between having the transport fleet perform both jobs or no tanker capability at all.
2) You are cut off from external sources of efficient long-range airliners, and the closest *domestic* approach to one is your biggest military airlifter.
3) You are in the luxurious position of having your fixed-wing tanker needs covered by efficient airliner-derived refuellers and want to complement them with a slow-moving platform for refuelling helos.
Forward basing (STOL & unpaved field capability) hasn't ever been a significant factor as far as I'm aware.
A large-scale example of 3) would be the USMC KC-130s (though their large tanker support isn't organic, rather than being supplied by the USAF), pretty much anywhere else it's a very small niche role though.
The Soviet-era Il-78 grew out of 2), at the time the only serious alternative to the Il-76 platform would have been the Il-86 medium range airliner but its famously lack-lustre payload/range was only very slightly better, so the race was decided by commonality with the freighter fleet. Come the 21st century however and Russia has a perfectly sound (if commercially unsuccessful) long-range airliner in the Il-96 which out-lifts and out-ranges even upgraded Il-76s with variants of the *same* engine (PS-90A) by such a margin that it's not even funny.
Yet they decided to use the Il-76 again in what is one of their biggest post Cold War procurement blunders (apart from funding too many Flanker derivatives, giving MiG endless money for uncompetitive Fulcrum variants instead of the innovative Skat UCAV and generally missing the UAV bandwagon). The reason, in a nutshell, was Ukraine - their shift to the West made the An-70 politically untenable as a Il-76 replacement in the cargo role, and with upgraded Il-76s once more assuming this task, commonality was touted as an argument for the same platform as a tanker. Too bad Il-76 final assembly wasn't actually domestic though (Tashkent, Uzbekistan) and had just collapsed, scuppering the PLAAF order (a few members round here probably recall). This meant they had to expensively move production to Russia and still aren't up to speed with it even as the clean-sheet Y-20 moves into mass production.
Long story short, China currently finds itself in the 2) predicament as well, but at least the Y-20 has fairly modern aerodynamics, unlike the 1960s Il-76. Once it gets WS-20 engines it'll do ok. Ideally they would have collaborated on these issues, with Russia contributing an even better wing design and PS-90A interim engines in exchange for a Y-20 production license and Il-96 tanker sales to China. The latter would go on to fit WS-20 once ready and Russia would soldier on with the PS-90A until the high-thrust PD-14M became available. I guess Ukrainian involvement in the Y-20 made that a non-starter though.
Military transports carry too much extraneous ballast and compromises to be competitive with a commercial airliner as the basis for a tanker. They have big, draggy fuselages for voluminous cargo, complex, heavy high lift devices on the wings for short runway ops and unnecessarily beefy landing gear for unpaved fields, none of which matters in the tanker role. Fuel is a fairly dense payload, so the big fuselage is just a drag penalty and forward basing a scarce high value asset such as a tanker is insanely risky while offering no benefit to its mission, so STOL and rugged landing gear amount to dead weight.
That there are numerous tankers based on military transports can invariably be traced back to three scenarios:
1) You have a small air force that simply doesn't have the budget for dedicated tankers, so the choice is between having the transport fleet perform both jobs or no tanker capability at all.
2) You are cut off from external sources of efficient long-range airliners, and the closest *domestic* approach to one is your biggest military airlifter.
3) You are in the luxurious position of having your fixed-wing tanker needs covered by efficient airliner-derived refuellers and want to complement them with a slow-moving platform for refuelling helos.
Forward basing (STOL & unpaved field capability) hasn't ever been a significant factor as far as I'm aware.
A large-scale example of 3) would be the USMC KC-130s (though their large tanker support isn't organic, rather than being supplied by the USAF), pretty much anywhere else it's a very small niche role though.
The Soviet-era Il-78 grew out of 2), at the time the only serious alternative to the Il-76 platform would have been the Il-86 medium range airliner but its famously lack-lustre payload/range was only very slightly better, so the race was decided by commonality with the freighter fleet. Come the 21st century however and Russia has a perfectly sound (if commercially unsuccessful) long-range airliner in the Il-96 which out-lifts and out-ranges even upgraded Il-76s with variants of the *same* engine (PS-90A) by such a margin that it's not even funny.
Yet they decided to use the Il-76 again in what is one of their biggest post Cold War procurement blunders (apart from funding too many Flanker derivatives, giving MiG endless money for uncompetitive Fulcrum variants instead of the innovative Skat UCAV and generally missing the UAV bandwagon). The reason, in a nutshell, was Ukraine - their shift to the West made the An-70 politically untenable as a Il-76 replacement in the cargo role, and with upgraded Il-76s once more assuming this task, commonality was touted as an argument for the same platform as a tanker. Too bad Il-76 final assembly wasn't actually domestic though (Tashkent, Uzbekistan) and had just collapsed, scuppering the PLAAF order (a few members round here probably recall). This meant they had to expensively move production to Russia and still aren't up to speed with it even as the clean-sheet Y-20 moves into mass production.
Long story short, China currently finds itself in the 2) predicament as well, but at least the Y-20 has fairly modern aerodynamics, unlike the 1960s Il-76. Once it gets WS-20 engines it'll do ok. Ideally they would have collaborated on these issues, with Russia contributing an even better wing design and PS-90A interim engines in exchange for a Y-20 production license and Il-96 tanker sales to China. The latter would go on to fit WS-20 once ready and Russia would soldier on with the PS-90A until the high-thrust PD-14M became available. I guess Ukrainian involvement in the Y-20 made that a non-starter though.
