#1019 How do planes refuel in the air?

How do planes refuel in the air? Either by using a long hose or a rigid tube to connect the fuel tanker with the plane being refueled. There are two systems of refueling a plane in the air. These are the probe-and-drogue system and the flying boom system. They’ve come a long way from the very first refueling attempt. In 1921, a biplane was refueled by wing walking. A passenger in a following plane got on the wing carrying a can of fuel, walked along the wing, poured the fuel into the tank of the first plane, and then got back in their own plane. That is not really a realistic method with modern planes. Let’s look at them in turn and their advantages and disadvantages

The probe-and-drogue system uses a very long hose that is released behind the tanker plane. The hose is generally about 30 meters long and about 6 cm in diameter. On the end of the hose is the drogue, which is a soft mesh, similar in shape to a badminton shuttlecock. The drogue is very aerodynamic and its purpose is to keep the hose as straight and level as possible. There is a lot of turbulence behind the tanker plane and there is a bow wave in front of the following plane, which means a hose without a drone would be uncatchable. The plane to be refueled has a probe, which is a stiff arm on the nose of the aircraft. These are usually retractable. The refueling valve is on the end of this probe. The tanker plane flies at a specific speed and as straight as possible. The plane to be refueled comes up behind and matches the tanker plane’s speed. By increasing their speed by about 2 knots, which is walking speed, they edge closer to the drogue and push their probe into it. If the connection is good, the hose snaps to the valve and fuel starts to flow. A drogue-and-probe system can pump fuel at a rate of about 2,000 liters a minute. The F-16 Fighting Falcon uses about 3,000 liters of fuel an hour and can hold about 7,000 liters, so it could be refueled in about 3 minutes.

The flying boom system uses a rigid tube to do the refueling. The boom is able to telescope out and it can be moved around. There is an operator in the tanker plane who can move it to make the connection with the plane being refueled. The boom has airfoils on it and the boom operator can angle these to move the boom around. When the trailing aircraft is close enough, the boom operator releases the boom and directs it into the refueling valve and then the fuel starts to flow automatically. The flying boom’s rigid tube allows it to pump more fuel than the softer hose and it can pump 3,800 liters a minute.

So, which system is best? They are both good, but for different situations. Flying boom systems are meant to refuel large bombers. These planes don’t have the maneuverability to be able to catch a drogue. With the flying boom system, they can just get in position and the operator does the rest. Bombers also need more fuel and the flying boom system can refuel them more quickly. You can’t have more than one flying boom on a tanker plane, but you can’t fit more than one bomber behind a tanker plane anyway, so that is not a problem. You can fit up to three drogue-and-probe systems on a tanker plane and fighters are smaller, so three can be refueled at the same time, which makes it faster. Fighters are also more easily buffeted by the turbulence coming off the tanker plane, so a soft hose is easier to move around with. A drogue-and-probe system can also be used to refuel helicopters as well, whereas their rotors would make it impossible to insert a flying boom. Some planes carry both systems, but most just carry one and a plane adapted for one system cannot use another. This can make it difficult when forces from multiple countries are trying to refuel.

There are different types of refueling tanker, but one example is the American KC-135 Stratotanker. It can fly for 2,500 km when fully loaded with fuel and it can carry 87,000 liters of fuel for refueling. Being able to refuel in the air allows planes to fly for much longer than they would normally be able to. It also allows them to take off with more weight because they don’t need to have as much fuel when they take off as they can top up when they are airborne. And this is what I learned today.

Photo by Pixabay: https://www.pexels.com/photo/gray-fighter-drone-above-white-clouds-87057/

Sources

https://www.af.mil/About-Us/Fact-Sheets/Display/Article/1529736/kc-135-stratotanker