On January 31, 2023, the U.S. Air Force issued a Request for Information (RFI) asking the industry to provide a design for a new generation of tanker aircraft (NGAS), which is planned to be first flown around 2032 and put into service around 2040. The RFI requires technical details of in-flight refueling, the ability to operate in small airports and airstrips, good survivability in high-threat battlefield conditions, and in-flight refueling (for relay refueling) and in-flight refueling capabilities.
This will be the KC-Z program of the US Air Force, the KC-46 that is now being manufactured in series belongs to the KC-X program, and the KC-Y is a "transitional tanker" after the KC-46 to fill the shortage of KC-46 in numbers, and it is possible to fire again between the KC-46 and the Airbus A330MRTT, but not in large quantities. The KC-Z is the true next generation tanker.
The U.S. Air Force has the largest number of tankers in the world, the most experienced use of tankers, and also has the strongest dependence on tankers. The new thinking of the US Air Force tanker aircraft is worth paying attention to.
The tanker belongs to the sequence of combat aircraft, but it is probably the most inconspicuous of the combat aircraft, and at the same time indispensable. In an air war, range is very important. In World War II, the German Me-109 had excellent maneuverability and firepower, but the range and air time were difficult to fight even across the English Channel, and the inability to maintain useful air combat time over the British Isles made a large number of German bombers unable to effectively cover the "Battle of Britain". On the other hand, the American P-51 not only has excellent firepower and maneuverability, but also has enough range to fly from the British Isles to Romania, and the German mainland is not a problem, effectively covering the strategic bombing of the B-17.
In modern times, the Pacific Ocean is known for its "demon lies in distance", which makes the US Air Force and Navy aircraft fearful. The existing KC-135 and KC-10 tanker planes are old and can only be operated from Kadena in Okinawa and Andersen Air Force Base in Guam, making it difficult to ensure reliable sorties in wartime.
Theoretically, any airport that can take off and land large civil airliners can operate conventional tankers, but the tanker base cannot take off and land, and requires adequate fuel storage and suitable refueling facilities. Refueling a tanker with a refueling pipe at a civil aviation airport is like refueling a tanker truck with a refueling pipe at a neighborhood gas station, and the refueling rate is too low, which seriously affects the dispatch rate. The refueling aircraft converted from civil airliners are also very particular about the quality of the runway, and to put it mildly, the potholes and the quality of the temporary pit filling are not up to standard, which affects the operation, which also affects the wartime sortie rate. Tankers also have command and control, security and other issues.
The only U.S. Air Force tanker aircraft in the Asia-Pacific region that have sufficient fuel, refueling, damage management, command and control, and guard capabilities are dispatched from Kadena and Andersen. Kadena is now a "free gift", and Anderson's wartime survivability is also unreliable.
The KC-46 solves the old problem, but it still does not solve the problem of the dispatch base.
The bigger problem is battlefield survivability. These tankers, which are based on civil airliners, have good economy and range, but they have no stealth capability and lack self-defense capabilities, and can only retreat far away in the face of long-range air-to-air missiles. It is generally believed that American tankers need to be at least 1,200 kilometers away from China's coastline to operate safely, but Kadena is only more than 500 kilometers away from the coast of Jiangsu and Zhejiang. In other words, after taking off from Kadena, the American tanker needs to go 700 kilometers to the depths of the Pacific Ocean to start a safe enough refueling operation. This is patently absurd.
The US Navy is testing a new generation of MQ-25 Stingray carrier-based tankers, which will significantly reduce the pressure on partner refueling of the F-18E/F. According to reports, 1/3 of the F-18E/F's flight hours are spent on partner refueling in the operation time of the existing carrier-based air wing, which is very inefficient and substantially crowds out the combat use of the F-18E/F.
The US Air Force has also been working on stealth tankers for a long time, and Lockheed has even proposed a design for a tailless winged tanker. But the requirements for the KC-Z show the new conception of the US Air Force for tankers.
Not to mention the "prehistoric air refueling rhapsody" before World War II, the earliest practical air refueling was the KC-97 that Boeing used for the B-47 bomber. The B-47 was a first-generation jet bomber with high speed, but amazing fuel consumption. The KC-97 was modified from the B-29 bomber, and its speed, range, and fuel load were barely enough to meet the needs. However, when the B-52 appeared, Boeing newly developed the KC-135, which not only kept up with the speed of the B-52, but also perfected the hard refueling to facilitate large-flow air fuel transportation. The last KC-135 rolled off the assembly line in 1965, and the fleet was being replaced by the KC-46, but progress was slower, and the KC-135 remains the workhorse of American tanker aircraft to this day. The KC-135 later evolved into the Boeing 707, creating Boeing's civil throne, which is another story.
From the very beginning, the US Air Force tanker was in the service of bombers, and they needed to transfer a large amount of fuel and a high transfer rate, so they locked on to large aircraft from the beginning. Air Force fighter jets were refueled in the air later, but because of path dependence, hard refueling was also adopted.
Generally speaking, the United States and Europe are the former, and China and Russia are the latter, but the KC-130 and Airbus A400M of the US Marine Corps are also modified from military transport aircraft. The former has good economy and has a large proportion of the take-off weight of the fuel on board; the latter has the ability to operate a field airfield and is easy to install military standard equipment (such as telecommunications, self-defense electronic warfare, etc.), and each has its own advantages. But both routes start with large aircraft.
It is impossible for the US Navy's carrier-based tankers to use large aircraft, but for carrier-based aircraft, it is also heavy. Another difference: carrier-based aircraft use soft refueling, which is simple and lightweight, but the fuel transfer rate is relatively limited.
For many years, the typical way for US Air Force fighters to dispatch is that the fighters are fully loaded with ammunition and reduced fuel to ensure that the total weight is not overweight, which makes it difficult to take off. But immediately after takeoff, refuel in the air and go to the battlefield when it is fully fueled. On the edge of the battlefield, another tanker aircraft is waiting on tour. In actual combat, fuel consumption is much faster than ammunition consumption due to seeking opportunities and standby. When the fighter is running out of fuel, it exits the battle, joins up with the tanker, and returns to the battle after filling up the tanker. This greatly saves the round-trip time and increases the effective number of fighters on the battlefield.
How to fight in wartime, how to practice in peacetime. Due to the high dependence on tankers, the insufficient number and good rate of KC-135 at one time forced the US Air Force to postpone and cancel the "Red Flag" exercises.
But this model does not work for an adversary like China. If the tanker planes have to stay 1,200 kilometers away from China's coastline, it will be difficult for heavy-duty US fighters to reach the "point to point" of China's coastline. It's not going to fight.
Even at a distance of 1200 km, the J-20 with the PL-15 makes the tanker a turkey to be hunted, and the KC-46 with limited self-defense electronic warfare capabilities are all survivable. In different simulations of warfare, the consequences of the downing of American tanker planes were severe, often causing several American fighters on the battlefield to crash into the sea because of running out of fuel. This is unacceptable for the US Air Force and Navy.
Tanker stealth is an idea, but the cost of large stealth aircraft is ridiculously high, and the B-21 is smaller than the B-2, not only because of technological progress, but also to control costs. Electronic systems can be miniaturized, but bomb load and range cannot be miniaturized. The B-21 is required to shrink only in terms of these "hard indicators" in order to reduce weight and control costs.
Another idea is decentralization, with smaller tankers deployed in a distributed manner at the front and relay refueling. Refuel the forward tanker with a large tanker in the low-risk rear to maintain higher efficiency, and relay the refueling with a small tanker in the high-risk front to maintain higher survivability. Because the target is small and relatively easy to achieve stealth, the survivability of the battlefield is greatly improved, and it can be deployed in the front, increasing the effective time of US fighters in the air. This is where the KC-Z's in-flight refueling requirements come from.
Another idea, of course, is unmanned. The long-endurance technology of UAVs is very mature, and unmanned tankers can patrol the front line for a long time, much longer than manned tankers can last, which is the key to docking refueling and sustained existence. UAV refueling technology has already been demonstrated with MQ-25 to reach a sufficiently high level of technological maturity. A tanker may be one of the best applications for the long-endurance characteristics of UAVs.
Smaller tankers also mean that there is only a limited amount of fuel that can be transferred. This may still be enough for fighters, but not enough for bombers. This can be solved by refueling multiple relays along the way, rather than filling up all at once. This, of course, has increased the number of refueling operations, but with the help of automatic control and artificial intelligence, aerial refueling operations have changed from high-risk operations in the early years to routine operations now, and are more likely to become low-difficulty operations in the future.
In other words, the difference between a new fuel dispenser and a traditional fuel dispenser may be between a charging pile and a gas station. Gas stations are built in strategic locations, and passing vehicles stop to refuel and fill up at one time, but the number of gas stations is relatively small and the distribution is large. Charging piles are scattered all over the place, but they can only charge one car at a time, and it will take a long time. However, if you drive all the way and charge all the way, it is not a big problem if it is not full, as long as there is a charging pile in front of you to continue charging.
Smaller tankers meant the use of small airfields and field airfields, and the US Air Force finally realized the problem of the vulnerability of tanker bases. This is consistent with the general idea of dividing troops and concentrating fire.
The latest RFI does not specify the pneumatic configuration, and each manufacturer is free to choose according to the requirements. The tailless wing will certainly be one of the candidates, but not necessarily the most favorable candidate. The bomber uses a tailless flying wing because it has the highest aerodynamic efficiency, all airframe structures are used to generate lift, and the weight of the ineffective structures is reduced to a minimum. The tailless wing also has the best omnidirectional stealth capability, which is conducive to penetration and safe return. However, tailless flying wings are more demanding in aerodynamic design and structural manufacturing, and the cost is high.
The aerodynamic configuration of the Boeing MQ-25 Stingray has a lot of thought for stealth, but it still avoids tailless flying wings. The slender wing is conducive to a high lift-to-drag ratio, the shallow V-shaped tail reduces the problem of yaw control of the tailless wing, and the wide and flat central body not only provides a large internal volume of the aircraft, but also helps to learn from the mature design and manufacturing technology of the barrel-wing configuration. The "drop-in" air intake is the biggest technical risk, but for tankers that do not emphasize maneuverability, the risk is manageable.
Even if the new generation of tankers has stealth requirements, this is still fundamentally different from the penetration of bombers or the air supremacy of fighters. Deployed in the front, the tanker still does not need to approach to the distance of hand-to-hand combat. Distance is always the most stealthy comrade-in-arms. Bombers and fighters cannot choose where to fight, but tankers can choose where to conduct refueling operations, so as long as there is moderate stealth, it is enough.
In addition, the most emphasis on the fighter is forward stealth. When it comes to exposing the sides and back for a long time, it's time to accelerate out of the fight. Bombers place more emphasis on omnidirectional stealth, but forward direction is still the most important. This is not the case with tankers, where the lateral direction is perhaps the most exposed and stable direction for many times. The side radar reflection characteristics of the fuselage are important, but the forward reflection characteristics of the wing are less important.
The slender and small-swept wing forms a strong and stable reflection in a small angle range in the forward direction, but according to the appropriate trajectory planning of the direction of the battlefield electromagnetic threat, such a highly reflective state can be achieved in an instant, and it can "hide in the city" in the very high battlefield electromagnetic environmental noise, and destroy the detection and locking of the opponent's radar. Tailless wings also have long, straight edges, but they don't face the direction of travel.
The Chinese Air Force is also vigorously developing tanker forces. The H-6 is transitional, and the Y-20 is the first generation of real large tankers of the Chinese Air Force, but the Chinese Air Force also has a need for a new generation of tankers.
Chinese combat aircraft need to pass through the first island chain and enter the western Pacific, not only facing the interception of hostile air forces in the first island chain, but also the support of Chinese tanker aircraft crossing the first island chain. The new generation of stealth, miniaturized and unmanned tanker aircraft like the KC-Z is also important to the Chinese Air Force, and the KC-Z's new thinking from aircraft type, base to air operation has strong reference significance.