Can anyone ELI5 what the issue is; I understand/assume larger Earth increases gravity so more for rocket to overcome, but why doesn't that also affect jet aeroplanes?
Or does it, it's just that this is space.SE so naturally they're asking about rockets specifically?
Planes get lift from the pressure difference between top and bottom of the plane.
A higher gravity planet pulls harder on air, increasing the pressure from any given mass over any given area, which IIRC doesn't affect this difference directly.
Indirectly, a higher density atmosphere (which is technically a different question to pressure; look at Venus for example), will lead to higher drag, needing more engine thrust to maintain any given speed. Lift depends on speed, but is easier to design around.
> but why doesn't that also affect jet aeroplanes?
Jet engines pull in air and expel it out the back, creating thrust. The energy to do so comes from fuel, but almost all of the reaction mass is air.
Rockets don't have this luxury; they must bring all the reaction mass with them. This causes a big problem of diminishing returns. Adding more fuel means you can burn longer, but also makes the rocket heavier so it doesn't accelerate as much with the same thrust.
The result is that the fuel required goes up exponentially with the desired delta-v, as expressed by the rocket equation .
Or does it, it's just that this is space.SE so naturally they're asking about rockets specifically?