In-flight refuelling. One of NASA's Lockheed SR-71 aircraft approaching the boom of a KC-135 'tanker' in mid-air. When the SR-71 is close enough, an operator in the tanker aircraft can 'steer' the boom to the refuelling point (red and white square behind cockpit). Fuel can then flow from the tanker aircraft to the SR-71. This technique is used extensively throughout the world. A very large proportion of the fuel capacity of an aircraft is used in taking off, so this system is relatively fuel-efficient. In military service, it is used to extend the flight duration of combat aircraft such as patrolling fighters or bombers on long-range missions.