The basic challenge of designing the F-22 was to pack stealth, supercruise, highly integrated avionics, and agility into an airplane with an operating range that bettered the F-15, the aircraft it was to replace:
“One problem we typically face when trying to stuff everything inside an airplane is that everything wants to be at the center of gravity,” Hardy explains. “The weapons want to be at the center of gravity so that when they drop, the airplane doesn’t change its stability modes. The main landing gear wants to be right behind the center of gravity so the airplane doesn’t fall on its tail and so it can rotate fairly easily for takeoffs. The fuel volume wants to be at the center of gravity, so the center of gravity doesn’t shift as the fuel tanks empty. Having the center of gravity move as fuel burns reduces stability and control. We also had to hide the engine face for stealth reasons. So, these huge ducts had to run right through the middle of real estate that we wanted to use for everything else. The design complexities result in specialized groups of engineers arguing for space in the airplane. That was the basic situation from 1986 through 1988.”
And as the plane when fully fuel and then in the air continues a mission, that center of gravity changes. MiG-21 when expending 2/3 of fuel can be unconditionally unstable. Lots of crashes.