Even more capable is the final variant of the venerable 747 program, the 747-8F, which could carry a substantially larger payload of 314,500 pounds to the target altitude. The problem with planning the fleet based on this super-jumbo is that the last 747-8 was delivered in January 2023 and the production line has closed39. Consequently, obtaining new aircraft from Boeing would require a line restart roughly a decade from now, and the chances of that are remote. Just 107 747-8Fs were delivered, along with 48 in passenger configuration40. The polar SAI mission would require more than 70 such aircraft, which is to say most of the production freighter fleet and roughly half the total fleet. The likelihood that so large a proportion of the delivered fleet could be acquired on the secondhand market is low, and passenger aircraft converted to the tanker mission would have marginally reduced payloads relative to the production freighters. Nonetheless, some substantial proportion of a polar SAI fleet perhaps could be assembled via used 747-8s. This raises the prospect of a mixed SAI fleet, consisting of some repurposed 747-8s and perhaps some similarly repurposed mid-life 777s, operating alongside a core of factory- delivered 777 Special Tankers. Freight operators are opportunistic in their fleet acquisitions and therefore customarily have a mix of production freighters and repurposed second-hand aircraft in their fleets. An SAI fleet might be similarly composed. Nonetheless, for the purpose of the discussion hereafter, we will assume an all 777 Special Tanker fleet is newly acquired from Boeing, noting that in fact some portion of it might be otherwise acquired.