The 'S' rank is the best rank you can earn in any mission. Do not bother unlocking other aircraft until you get the F22 Raptor. The F22 Raptor is the best all-around fighter in the game, with incredibly useful special weapons you can equip to make all missions much easier. It is recommended to proceed through the central Aircraft Tree path and unlock the F22 Raptor as early as possible. Mission 19 is long, but gives a lot of MRP for completing it. Missions that give a lot of MRP include the Fleet Destruction and Farbanti missions. If you do not like playing multiplayer to earn MRP, you can replay campaign missions in Free Play mode. It only takes a few minutes, and you do not even need to be good at the game. Even if you are the worst player in a game, you will still earn approximately 70k per completion. Multiplayer is the quickest way to earn MRP. Successfully complete the game on any difficulty to unlock extra skins in Campaign mode. Its weapons cost 200,000 MRP each, for a grand total of 2,400,000 MRP.
To purchase it, you need 2,000,000 MRP and to have unlocked either the YF-23 Black Widow II, F-22A Raptor, or Su-57. Successfully complete Campaign mode on any difficulty to unlock the X-02S Strike Wyvern under the Aircraft Tree.