The United States has devoted billions of dollars to building stealth fighters, stealth bombers, stealth cruise missiles and stealth spy drones. Surely a stealth tanker for refueling aircraft midflight would be an extravagance too much?

However, the concept of a stealth tanker is not as absurd you’d think for one simple reason: the Pentagon’s F-35 and F-22 stealth fighters, which it has made the lynchpin of its twenty-first century air warfare strategy, simply can’t fly far enough.

At first glance, the F-35’s six to eight-hundred-mile range doesn’t seem bad compared to conventional fighters like the Super Hornet or F-16. But those non-stealth designs can carry fuel in in underwing tanks into combat—meanwhile, an F-35 can’t carry those extra lumps of metal under its wings if it wants to preserve its miniscule radar cross-section.

Another problem with the short range of stealth and non-stealth fighters alike is the need to deploy them airbases or aircraft carriers well within range of an adversary’s ballistic and cruise missiles. Conflicts ranging from World War II to Afghanistan have shown that advanced fighters are never more vulnerable than when they are caught on the ground (or a carrier deck). It is virtually a given that in the event of a great power conflict, a terrifying missile barrage would rain down on forward airbases; and just how many airframes would emerge intact from that hail of death is anybody’s guess.

Fortunately, all U.S. jet fighters can be refueled mid-air. But though the modified airliners serving as tankers would strive to stay far away from hostile fighters, they are increasingly at risk to being shot down by very-long range air-to-air missiles like the Russian R-37, which can hit airliner-type targets from 250 miles away. The small numbers of stealth aircraft fielded by Russia or China would also likely concentrate on slipping past fighter screens to destroy the tankers and radar planes supporting them. After all, knock down the lumbering tankers, and you may also effectively strand a bunch of fighters over the Pacific without the fuel needed to return to base.

The dilemma is much worse for stealth fighters attempting to penetrate enemy airspace, as the F-35 is designed to do. Contemporary surface-to-air-missiles like the S-400 can already strike less agile aircraft (again, think tankers) up to 250 miles distant using 40N6 missiles. This means conventional tankers simply will have to loiter hundreds of miles back from defended airspace—and even there, will be visible on radar and vulnerable to attack by enemy fighters.

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A tanker with a reduced radar cross section, therefore could neatly address both problems, without having to be every bit as low-observable as a stealth fighter.

Enter the KC-Z

Currently, the Air Force is procuring 179 new KC-46A Pegasus tankers based on the Boeing 767. As it progressively retires its aging fleet of 400 KC-135 and KC-10 tankers, the Air Mobility Command originally planned to phase in another relatively conventional tanker called the KC-Y starting in the 2024s, before finally pursuing a KC-Z stealth tanker.

However, in 2016 General Carlton Everthart told Defense News the Pentagon may scrub the KC-Y in favor of procuring additional upgraded KC-46s and phasing in the KC-Z stealth tankers sooner—though by ‘sooner,’ think procurement beginning in 2035.

Already, there are multiple proposals as to what a KC-Z might look like—and they’re all bizarre enough to resemble the Quinjets from The Avengers movies.

In June 2018, the Air Force Research Lab, based in Wright-Patterson Air Force Base in Ohio, floated this angular and very bizarre-looking ‘Advanced Aerial Refueling’ concept model at the AIAA forum.

Meanwhile, Lockheed has its own stealth tanker concept model dubbed the ‘Advanced Tanker Concept.’ (photos here and here) that looked ready for filming in Star Wars: Episode IX. Earlier in 2018, the major defense manufacturer lost a competition to produce an MQ-25 carrier-based aerial refueling drone; Lockheed’s proposed flying-wing design emphasized stealth more than other entrants in the competition. The firm has also proposed embedding the stealth tanker’s high-bypass turbofans on the upper surface of the wings, like on the B-2, for cross-section reducing purposes.

However, the proposed designs aren’t pure flying wing, but instead take their cues from the Air Force’s expressed interest in a ‘Blended Wing Body’-type tanker. Rather than crossing a tubular fuselage against the wings, a BWB aircraft seamlessly merges the wings into the fuselage, resulting in a triangular shape. These are also known as ‘Hybrid Wing Body’ designs, as they are not a pure ‘flying wing’ because of the size of retention of a fuselage and tail fins.

A flying wing’s curved wing surfaces are very efficient at generating lift, and its ‘infinite plane’ naturally lend themselves to low radar cross sections as they lack hard, radar-reflective angles. However, tanker aircraft are routinely called upon to do double duty as cargo jets, so a stealth tanker may still need to have a bulged cargo compartment and bay door to serve as a fully capable substitute—the ‘C” in KC-Z stands for ‘Cargo’ after all. Those wouldn’t gel well with a pure flying wing design, which is why extant concepts have been hybrids.

An upside of a stealthy cargo plane is that it could be used for inserting Special Forces operators behind enemy lines—a capability the Special Operations branch has discretely studied for decades—or delivering critical supplies to forward outposts located under the anti-access umbrella of an adversary’s long-range anti-aircraft missiles. Nonetheless, a cargo-carrying stealth tanker design would simply not be as stealthy as a pure flying-wing designed only for aerial refueling.

Another challenge to making an affordable stealth tanker concerns the fact that stealth fighters and bomber achieve their low cross-section partly by incorporating radar-absorbent material (RAM) coatings or panels. However, RAM application significantly increases the operating costs and maintenance requirements of small stealth fighters. Presumably, that cost would be far greater spread out across a huge tanker that needs to fly thousands more hours every years, so a more cost-efficient form of RAM is surely necessary to avoid the $135 to $169,000 per flight hour operating costs of a B-2 stealth bomber.

The Air Force has additional ideas for making its future tankers more survival, including incorporating active protection systems to shoot down incoming missiles—yes, possibly with lasers. Another concept, however, would involve using next-generation radar jammers that employ a cognitive intelligence system to automatically adjust frequencies to keep up with frequency-agile radars. Such jammers could obscure or even misrepresent the position of an aircraft on radar. The Pentagon also would like its next-generation tankers to feature more autonomation to reduce the number of necessary crew and speed up the refueling process.

However, the Air Mobility Command has also expressed openness to a radically different approach to a stealthy KC-Z—taking a page from the Navy’s MQ-25, and deploying small, stealthy unmanned autonomous vehicle.

Stealthy drone tankers might fit with a ‘distributed’ refueling strategy in which multiple drones draw fuel from a large, conventional tanker ‘mothership’ and then zip forward into to provide refueling for stealth fighter in contested airspace. However, such a chain-refueling scheme could crash catastrophically should the un-stealthy mothership tanker be targeted by adversaries. Thus a ‘system of systems’ could also proposed mixing multiple ‘tiers’ of stealthy and non-stealthy tankers.

It’s worth noting there may be simpler, less expensive solution: weaning the Pentagon from its dependency on short-range jets, perhaps by relying more on long-range B-21 stealth bombers or future sixth-generation Penetrating Counter-Air fighter, making greater use of stand-off missiles, or introducing long-range unmanned UCAV stealth drones.

Sébastien Roblin holds a master’s degree in conflict resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring.