Aerial refuelling has been enthusiastically embraced by air forces around the world since the 1950s. Will we now see airliners refuelled the same way?
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“When I spent time on alert when the Cold War was on, I had to spend a week underground in a bunker,” says Chris Hoctor, former crew member of a United States Air Force refuelling tanker aircraft and author of Voices from an Old Warrior, recalling his time in the Strategic Air Command (SAC) in the 1980s.
“Our job was to get the B-52 bombers pretty far towards their targets. Then we had predesignated airfields to land on in Europe and Japan because we would be pretty low on fuel ourselves. The worse-case scenario was that we would have to give the bombers all of our fuel and then jump out.”
Tankers are fuel stations in the sky for bombers, transport aircraft, much smaller fighters, and now drones, allowing them to fly much farther than they would normally managed.
Despite its unglamorous reputation, in-flight refuelling has played a key role in crucial strategic missions, as Hoctor describes, as well as in the conflicts in the Middle East, famine relief and response to natural disasters. The history of the last 60 to 70 years would have looked very different without the capability it gives to the air forces of the world to extend the range of their planes.
The KC -46 is Boeing’s latest tanker design (Credit: Boeing)
In December 2016 the test flights of the first of 500 new tanker aircraft planned for the US Air Force (USAF) over the coming decades took place. The scale and ambition of this order shows the continued importance of aerial refuelling for the USAF. The planes on test at the moment look like conventional airliners, but the last part of this huge order is likely to look like nothing we have seen before. The tanker of the future is expected to look like a stealth bomber, have weapons of its own and – perhaps – fly and refuel without a human crew.
Aerial refuelling may be called air refuelling, in-flight refuelling and even tanking, but whatever you call it, it comes down to the same thing: transferring aviation fuel from one aircraft (the tanker) to another (the receiver) during flight. In the 1930s it was a way to extend the range of the great flying boats that flew across the Atlantic and connected the far-flung corners of the European empires. Today it is solely the preserve of the military – although there are a few voices calling for the in-flight refuelling of commercial airliners to be revived.
People have been trying to solve the problem of how to do in-flight refuelling almost since the birth of aviation itself
– James Kemmitt, Cobham Mission Systems
While accidents are rare, refuelling a plane 20,000ft (6,100m) up is inherently very dangerous. It has been called an “organised mid-air collision”, because the planes have to get as close as 75ft (22m) to each other. Sometimes the crew have to perform a tricky manoeuvre called tobogganing to refuel planes faster or slower than themselves.
Almost as soon as the first aircraft flew, there were wild ideas about refueling them while they still flew. “People have been trying to solve the problem of how to do in-flight refuelling almost since the birth of aviation itself,” says James Kemmitt, product management director at Cobham Mission Systems, who gave a recent presentation to the Royal Aeronautical Society. “Six years after the Wright brothers flew, a cartoon in Punch magazine in 1909 depicted an airship trying to winch a hose down to a plane below with the observation that the chief difficulty to be overcome in aviation is that of replenishing supplies of fuel while in the air.”
The breakthrough, when it came, was based on technology patented in 1921 by Russian-American aviator, inventor and air power advocate Alexander P de Seversky, whose company went on to build some of the most iconic aircraft of World War Two.
When history was finally made, in July 1923, it was surprisingly close to the Punch cartoon, except the airship had been replaced by another plane. Two slow-flying biplanes of the US Army Air Service (which later became the USAF) flew one above the other with a hose running from a handheld fuel tank to the fuel tank of another. Later, the same crew flew the same type of plane non-stop from the Canadian border down to Mexico by refuelling over Oregon and California.
While other countries, including Britain and France, were experimenting with the concept as well, daredevil pilots called “barnstormers” in the US wowed fairground crowds with demonstrations of this dangerous technique.
In the 1930s the focus shifted from experiments to making the process safer and designing a system that would be practical for day-to-day flying. The big step forward was the design of a spill-free refuelling nozzle. Then in 1935 British former air force pilot and pioneer of long-distance aviation Alan Cobham demonstrated his “gas station in the sky” concept. This used a practical, if complex, method, called the grappled line refuelling system, by which the pilot had to throw a grappling hook to catch the line. By the outbreak of war the system had been used to refuel 15 commercial flying boat flights across the Atlantic.
The cost of designing and building a plane just to be a tanker was prohibitive
Now it was the military who took the technology further. The RAF had equipped bombers such as the Lancaster with Cobham’s technology in order to join in the bombing of Japan in 1945 – but the war ended in the Pacific before the technique could be used. In 1949 the USAF flew a plane around the world for the first time using an adaption of this British technology. The 94-hour journey was flown by the Lucky Lady II, a B-50 Superfortress similar to the plane that had dropped the nuclear bombs on Japan four years earlier. By the early 1950s the Americans were deploying their first squadrons of tankers, and during the Korean War in-flight refuelling took place for the first time in a war zone; civil aviation was risky enough at the time without adding more risk to it.
“Alan Cobham was a bit of a barnstormer himself,” says Kemmitt. “He sold equipment to the RAF so they could bomb Japan. When they didn’t need it, he bought it back from the RAF on the cheap and then resold it to the US Air Force. The Americans were amazed that he was able to supply them so quickly.”
Countries like China and Australia are busily developing their ability to refuel planes in the air
In the 1950s the design of the tankers themselves improved. Boeing launched the world’s first aerial fuelling tanker (based on the same WW2 heavy bomber as Lucky Lady II) and then the world’s first jet-powered aerial tanker aircraft (based on the Boeing 367 civilian airliner). The cost of designing and building a plane just to be a tanker was prohibitive, particularly when bombers and airliners already had the muscle to lift heavy loads.
Today, most planes that refuel in the air use one of two systems: probe-and-drogue and flying boom. Probe-and-drogue is an improvement on Cobham’s earlier system, and is instantly recognisable as the descendant of those early trials with biplanes back in the 1920s. The tanker drops a flexible hose that trails out behind it. On the end it has a basket, or drogue, which meets the probe on the plane below; at its end is a valve that opens only when it fully connects with the probe.
The flying boom, developed by Boeing for the Strategic Air Command, is a long rigid tube with small aerofoils (wings) that allow an operator on the tanker plane to guide it down to make contact with the receptacle on the receiving aircraft.
In the future, in-flight refuelling is set to become even more important. Now, countries like China and Australia are busily developing their ability to refuel planes in the air. More research is going into the refuelling of helicopters, which is tricky and dangerous because 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. However, since the Vietnam War the United States has pioneered its use – in the first instance to rescue air crew who have been shot down.
The growing use of special forces in conflicts around the world has meant that its use is growing. The French air force uses helicopter refuelling, and others, including the British, are thinking of following suit.
And experiments with the air refuelling of drones have already began – and of drones refuelling other drones. In April 2015 the US Navy’s self-flying X47-B autonomously flew up to a human-flown tanker and manoeuvred its probe into the basket. In China earlier this year one rather small drone refuelled another – although both were still under human control.
If people want to save fuel and don’t want to expand airports then commercial aerial refuelling is the only way
– Raj Nangia, aeronautical engineer
Aeronautical engineer Raj Nangia has even been campaigning for commercial aerial refuelling for the last two decades. “It is an idea that won’t go away. If people want to save fuel and don’t want to expand airports then commercial aerial refuelling is the only way.”
Nangia’s research laid the foundations for the recent £2.7m European Union project Recreate (Research for a Cruiser Enabled Air Transport Environment) that examined the idea. Aerial refuelling meant that planes could be smaller and lighter because they didn’t need to carry so much fuel into the air. Using 200-seat airliners and a fleet of air tankers could mean a 15% fuel saving over 5,000 miles, including the fuel used by the tankers themselves. Testing in an aircraft simulator with airline pilots showed that commercial refuelling could take place safely using automated refuelling technology. It could also mean that smaller airports could begin to offer international flights.(
“Nasa, Airbus, they all like it,” says Nangia. “Who is going to make the first move is always difficult.”
The 500 tankers that will be delivered to the USAF over the next decades will heavily shape the future of the technology. “Boeing has been in aerial refuelling for the last 65 years and we have built over 2000 tankers,” says Mike Hafer, the global sales and marketing manager of Boeing’s new design, the KC-46. “The US Air Force want a fleet of 500 tankers and our KC-46 will provide a third of that. It can be refuelled itself by another tanker [only a handful of tankers in the current fleet can be] and it can switch between probe-and-drogue and boom. It was also built from day one to be a combat aircraft. It has night vision lighting. It has sophisticated defences to defend itself against missiles and to warn pilots that it has been detected by radar.”
While the second new tanker aircraft, the KC-Y, will probably be similar to the KC-46, the final plane, the KC-Z, is likely to be revolutionary: it will be stealth. “Everyone is building stealth fighters and bombers. It’s no good then if your refuelling aircraft is not stealth,” says James Kemmitt. “It looks like the KC-Z will be stealthy, operate closer to the front line and be protected. New airframes are expensive, so it will probably use an existing one from the new stealth bomber.”
But further into the future of aerial refuelling it is much harder to predict.
“Will there be a time when we no longer need aerial refuelling?” says Hafer. “Well, the need will be there for at least 30 years. It is like asking someone after World War One what the future is.”
“What will come after this? By the time it comes to replace this new generation of tankers we may not need tankers anymore,” adds Kemmitt.