Flights of over thirty hours are now common, as B-52s and B-2s often fly half way around the world to hit targets. Smart bombs have made this sort of long distance mission a more attractive proposition. AWACS and J-STARS radar aircraft also stay up over a day at a time. Even the navy is using air force tankers to enable their carrier aircraft to fly missions lasting eight hours or more. The navy also has over two hundred P-3 patrol aircraft that fly missions lasting eight hours or more. Crews of the P-3 figured out how to hang twenty dollar net hammocks from parts of the aircraft to allow everyone to get some rest on those 12 hour flights. The two bunks in the rear of the aircraft were never enough.
For decades, the air force and navy have been buying better seats for the people who have to sit up there for long periods. The seat manufacturers used a lot of trial and error to improve their products. But it wasnt enough. So stretching exercises were developed, that could be performed in the restricted confines of the aircraft. Where space permitted, on the larger aircraft, a bunk was provided, where crew could stretch out and have a nap. On the B-2 bomber, which only has a two man crew, but a fairly large open space behind their seats, it was found that a ten dollar Wal-Mart beach lounger would fit. The two pilots took turns napping on that during round trip flights from the U.S. to Afghanistan.
But its not been enough. So now the air force has a wired (with 900 sensors) seat that will show them where the pressure is when some sits down for long periods. With that knowledge, small tweaks can be made to the seats. Actually, there may be custom cushions for crewmembers on long distance aircraft.
Meanwhile, the pilots and crews have found other ways to beat the problem. Electronic games, particularly Game Boy, have proved very popular, especially among fighter crews. These things may not deal directly with the discomfort, but they do help you to forget it for a while.
Theres a downside to in-flight refueling; aircrew spending too much time sitting down. This causes fatigue, discomfort and other bad things that lead to accidents, and people getting killed (or not killed, in the case of the enemy.) It's not just the aerial tanker crews that are having fatigue problems. Warplanes of all kinds are spending more time in the air, and this is causing problems with tired crews.