David C Isby lays out the different options available to those wanting to buy an air-to-air refueller
Air-to-air refuelling tankers in service today will be flying well into the 21st century incorporating improved versions of one or both of the two basic types of air-to-air refuelling systems, boom or drogue. Like armament is to fighter aircraft, they are the critical onboard systems that allow them to do their jobs. Reliability and interoperability with many different types of receiver aircraft throughout a wide operational envelope are the key criteria for air refuelling systems, but so are their rates of fuel transfer. While an operational tanker may seldom transfer fuel at its maximum rate, the ability to do so when needed is no less important than maximum speed is to a fighter.
The US Air Force-developed flying boom system offers a high rate of fuel transfer, up to some 6,000–8,000lb per minute (ppm) (2,761–3,650kg). It requires an operator on board the tanker to fly the boom, a telescoping rigid tube (containing a fuel line) with small wing-like aerofoils, into the receiver aircraft’s dorsal receptacle; originally a purely manual feat, modern tankers use a computer-assisted system. No other air arm expects its tankers routinely to deploy worldwide to carry out such a sustained high operational tempo as does the US Air Force, with its large numbers of thirsty receivers needing to operate in any weather or environment.
The more widely used hose-and-drogue system trails its drogue behind the tanker. When stabilised, the receiver aircraft flies its probe into the drogue. Fuel can be dispensed at a maximum rate of 5,000ppm (2,300kg) from a large centreline drogue system, but a lower rate of 2,500–2,800ppm (1,180–1,270kg) is more usual; most tactical aircraft’s plumbing cannot receive a greater fuel flow.
The US-built Boeing KC-46A Pegasus tanker and the Airbus Defence & Space A330 multi-role tanker transport (MRTT), the two major tanker designs coming off the production lines today, are designed to use both these systems regularly at the same time without having to be reconfigured. While these two tankers dominate world markets, there are several cheaper and, in most cases less capable, alternatives available.
Legacy tankers
The US Air Force currently operates 342 Boeing KC-135R and 54 KC-135T Stratotankers. It is currently planning to continue operating decreasing numbers of its KC-135s until they are replaced by the last of the still notional, yet to be defined KC-Z tankers, an event thought likely to take place around 2060. The US Air Force’s KC-135s have an airframe life of 60,000 flight hours and even the oldest are only halfway there.
A Stratotanker has a maximum fuel load of 203,000lb (46 tonnes), which yields some 161,000lb (30 tonnes) available for transfer – the tanker’s total ‘give’ – over the course of a four-hour mission. The KC-135 uses boom refuelling, though it can be modified to trail a drogue from 9ft (3m) of hose attached at the end of the boom. Officially designated the Boom Drogue Adapter, during the 1991 Gulf War this modification earned the nickname ‘Iron Maiden’ from unimpressed RAF and US Navy pilots who had to refuel from its rigid drogue. Of the about 400 US Air Force Boeing KC-135s, only 20 have been plumbed to use both the boom and underwing pod-mounted drogues on the same mission.
TANKERS AND AIR REFUELLING SYSTEMS MILITARY
Current US Air Force upgrades to the KC-135 do not directly improve its refuelling system. The first upgraded Block 45 versions were delivered in 2013. This programme, to be completed in 2026, provides the KC-135 with a new digital glass cockpit incorporating a new communications, navigation, surveillance/air traffic management (CNS/ ATM) suite to make the jet compliant for global airspace access. An upgraded autopilot makes the Block 45 more responsive during air refuelling. In a separate programme, the US Air Force is upgrading 1,400 of its KC-135’s CFM International CFM56-2 (F108) turbofan engines, increasing fuel efficiency and reducing operating costs.
Former US Air Force KC-135s have been procured by air forces worldwide. As more leave US Air Force service, replaced by the KC-46A, it is likely that others may follow. France still has the KC-135 versions it ordered in the 1950s, while Chile, Singapore and Turkey are currently flying ex-US Air Force KC-135s.
The 59 Boeing KC-10A Extenders – the US Air Force’s largest capacity tanker – are fitted with a centreline boom as well as a drogue and can refuel aircraft using either system on the same mission. Total onboard fuel capacity is 356,000lb (161 tonnes), about twice as much as a KC-135. KC- 10As can dispense 200,000lb of fuel 2,200 miles (3,520km) from their base. This year, an upgrade to make the KC-10A fieet CNS/ ATM compliant and upgrade their cockpits was completed. The US Air Force had been considering retiring the KC-10s in 2019–24, but the current tanker shortfall has put this on hold. The Royal Netherlands Air Force operates two similar KDC-10 tankers that were produced by converting two commercial DC-10 transports.
Turbofan tankers
Converted airliners operating as tankers are widespread. These include Canada’s current CC-150 Polaris tankers which were formerly Airbus A310 airliners. Israel’s Bedek and Israeli Aircraft Industries (IAI) have carried out more than 60 tanker conversions from Boeing 707 and 767 airliners; these are in service with Israel and air arms worldwide. A typical KC-707 tanker conversion has two drogue-equipped pods, each with a 3,300ppm (1,500kg) flow rate, capable of carrying out refuelling at 1,500–35,000ft and 200–350kts indicated airspeed (KIAS). The Israeli Air Force continues to rely on its KC-707s – it recently bought an airliner for spares – and is making modifications to allow them to refuel Israel’s Lockheed Martin F-35i Joint Strike Fighters.
Israeli KC-767 conversions have over91 tonnes (200,000lb) fuel capacity; many use an IAI-developed version of the boom system, with an enhanced fly-by-wire version now being offered to export customers. The IAI-developed advanced ARP-3 wingmounted drogue pod system has been exported on a number of converted airliners, including a KC-767 conversion for Colombia.
In addition to 707s and 767s, Israel has converted other types for tanker operations, including Lockheed Martin C-130 Hercules transports and six Soviet-era Ilyushin Il-78MKI Midas for India. Israel has offered India the KC-767-300ER MRTT, an upgraded version of the platform it supplied the Colombian Air Force, as a lowerprocurement cost (reportedly a quarter of the price) alternative to the Airbus A330 MRTT preferred by the Indian Air Force.
The Brazilian Embraer KC-390 first fiew in 2015. It is designed to be a multimission aircraft, using two Cobham 912E underwing drogue pods – designed specifically for the KC-390 – and internal auxiliary fuel tanks to refuel rotary-wing and fixed-wing aircraft. It has a fuel capacity of 23.4 tonnes (51,500lb) and is capable of being refuelled in the air itself. Some of the 28 KC-390s ordered by Brazil will be configured as tankers; deliveries are planned to start in 2018.
Russia and China are investing in upgrading their tanker capabilities. To replace Russia’s current Soviet-era Il- 78M Midas tankers, its United Aircraft Corporation is developing a tanker version of the Il-76MD-90A transport, the Il-476. The new jet is fitted with four fourth-generation PS-90A-76 turbofan bypass engines and equipped with three UPAZ-1M drogue refuelling units, each with a maximum transfer weight of 5,200ppm (2,400kg). It will have a maximum transferable fuel load of 110 tonnes (242,500lb). The programme has been delayed, reportedly by a requirement to design out all airframe and refuelling system components produced in Ukraine. In addition to a requirement for 30 aircraft for the Russian Air & Space Force, the machine is being offered for export, especially to countries operating combat aircraft built in countries comprising the former Soviet Bloc.
Turboprop tankers
Tanker versions of the Lockheed Martin C-130J Super Hercules include the US Marine Corps’ KC-130J, US Air Force Air Combat Command’s HC-130J Combat King II and Air Force Special Operation Command’s MC-130J Commando II. Their primary mission is refuelling helicopters, but they can also refuel probe-equipped fixedwing aircraft. These are normally equipped with two 2,000ppm (910kg) Cobham 901E underwing drogue units and have a maximum fuel load of 57,000lb (26 tonnes). They can refuel from sea level to 29,000ft and be fitted with either a low-speed drogue for helicopters (105–130 KIAS) or a highspeed one (185–210 KIAS). A KC-130J can offload 57,000lb of fuel at 500 nautical miles (925km) radius. The helicopter refuelling mission is challenging. The speed difference between the tanker and the helicopter means that hoses of the tanker can easily get caught in the rotors of the helicopter. The KC-130J has been exported to Kuwait (three) and Saudi Arabia (five) and France (two, primarily to refuel special operations forces helicopters). Six Italian C-130s are capable of using roll-on/roll-off (ro-ro) refuelling equipment.
The Airbus Defence and Space A400M Atlas was designed from the outset as a multimission aircraft with tanker capabilities. With a fuel capacity of some 37 tonnes (81,500lb), or 45 tonnes (99,000lb) with extra internal tanks, all except the RAF’s versions are delivered with hardpoints and plumbing for three drogue refuelling systems, one 4,000ppm (1,800kg) pallet-mounted centreline system and two 2,600ppm (1,200kg) underwing pods. The major A400M partners – France, Germany and Spain – originally planned on each procuring nine or ten sets of refuelling equipment. The new Cobham 908E underwing pod system with an improved coupling system was flight tested on an A400M tanker in 2015, when it refuelled a Spanish F/A-18 Horner. Like larger turbofan-powered tankers, the A400M is limited in its ability to refuel helicopters. The A400M can receive fuel at 2,900ppm (1,800kg).
The Bell-Boeing developed V-22 Air Refueling System (VARS) prototype tanker kit was tested on its MV-22 Osprey tiltrotor in August 2013. Adopted as a procurement programme by the US Marine Corps, VARS is scheduled to achieve initial operational capability (three deployable systems in one squadron) in 2018. VARS provides Ospreys with a ro-ro refuelling capability including two additional fuel tanks with 4,000lb (1.8 tonnes) of fuel and a Cobham drogue system. Using the plumbing already built-in for fuel ferrying and rapid ground refuelling, Ospreys will be able to refuel Lockheed Martin F-35Bs (at 220 kts) and other Ospreys and helicopters (at 110 kts) while serving as a recovery tanker on board US Navy amphibious warfare ships that cannot accommodate fixed-wing aircraft. The VARS will be upgraded to a 10,000lb (4.5 tonne) fuel capacity, starting in 2019.
Airbus is developing a comparable ro-ro drogue-based refuelling system for its CN235 (6 tonnes/13,200lb fuel capacity) and C295 (9 tonnes/19,840lb) twin-turboprop transport aircraft, intended primarily to refuel helicopters used for special operations and rescue missions.
Buddy-tankers
Tactical aircraft buddy-tanking capability requires appropriately scaled refuelling systems. The idea was first introduced in the 1950s, giving carrier-based US Navy A-4 Skyhawks the ability to fly strike missions against the Soviet Union. Selfcontained, usually with their own ram-air turbine for power, these systems require only low-voltage electrical connections and a hardpoint plumbed for access to the aircraft’s fuel system.
US Navy and Marine Corps F/A-18s routinely use these systems for carrier operations, logging 30,000 hours a year, fleet-wide, in tanker missions that often amount to a quarter of the sorties generated by deployed squadrons. Hornet/Super Hornet tankers are configured with a Cobham 31-301 drogue system mounted in a modified centreline fuel tank with a 1,370ppm (620kg) transfer rate. Capable of refuelling at 500–25,000ft and at 180-300 kts, an F/A-18E can carry a fuel load of up to 24,000lb (11 tonnes) in what the navy calls the ‘five wet’ configuration – five external fuel tanks with which the tanker can drag a flight of four Super Hornets 1,000 nautical miles (1,850km) from their carrier.
Germany and Italy with their Tornado strike aircraft and France with its Rafale swingrole fighters are among the NATO nations to equip and train its forces for buddy-tanking.
Future of air refuelling technology
The most immediate issue in the future of air refuelling is the resolution of remaining issues in the development of the systems that will equip the emerging generation of tankers. While there is no reason to believe the problems will not all be resolved – with time, money and testing – the refuelling equipment associated with the KC-46A (including delays to the Cobham RP- 910E-75 Wing Aerial Refueling Pods, the A330 MRTT (including the Airbus Military Aerial Refuelling Boom System (ARBS) for receptacle-equipped receiver aircraft), the A400M (especially limitations in its intended capability to refuel helicopters) and the Israeli KC-767 conversion (including the boom system) have all encountered well-publicised problems during development that have added costs overruns and time delays.
Unmanned air vehicles (UAVs) have the potential both to operate as tankers and be air refuelled themselves. The Northrop Grumman X-47B Unmanned Combat Air System Demonstrator demonstrated its air refuelling capability in April 2015 when it autonomously flew up to a tanker and manoeuvred its probe into the basket. The US Navy is planning to make air refuelling the primary mission of its first carrier-based UAV, the MQ-25 Stingray Carrier Based Air Refuelling System. In 2017, the Navy awarded risk-reduction contracts to four firms – Lockheed Martin, Boeing, General Atomics and Northrop Grumman – to develop competing designs for the Stingray. The programme, which has a $5 billion cost ceiling, is expected to issue a request for proposals before the end of 2017 and select one of the four designs in 2018.
The ability of a tanker to survive in contested airspace is increasingly important. The 324 nautical mile-range (600km) surface-to-air missile is likely to be a threat by 2030. On the ground, forwarddeployed tanker bases – and the fuel storage they rely on – are likely to attract increasingly accurate ballistic and cruise missiles. The US Air Force is considering emphasising survivability in its next new clean-sheet-design tanker, the KC-Z, projected to enter service around 2035. The gap between the KC-46A and the KC-Z will be filled by procurement of an interim type, designated KC-Y and likely to be an improved KC-46 design with survivability enhancements. The KC-Z may use a stealthy blended wing design and have extensive electronic warfare capabilities to give it greater range, survivability, and penetrating capabilities to go into a contested environment.
