I can't remember what I did before but I know there is a way to make the missile launcher instant load and it just fires ... what is the console command to do this?

There is a Legendary type called "Never Ending".

This makes it so the magazine size is effectively infinite - you never need to reload at all because the magazine contains your entire ammo pool.

The odds of getting a Legendary Missle Launcher is already low, so doubly-so especially with this prefix. But if you are on the PC, you can use a mod like http://www.nexusmods.com/fallout4/mods/4042/ to apply one, or use the console to attach the mod directly (LeighPouse has made a http://www.nexusmods.com/fallout4/mods/943/? to make that easier if you don't want to use *.esp).

...And yes, it also works on the Fat Man. ( ?~ ?? ?° )

If you just want to do this in general, and don't care about running cheats all the time, God Mod will make all shots not consume ammo at all. The reload isn't INSTANT, but you won't need to do it, because one missile will last indefinitely. Just tgm to turn it on. (You will also not take damage, rads, all settlement crafting is free and requires no perks, never overencumbered, infinite AP... might be some other things I'm forgetting, too.)

The amod console command will add a mod to the item you have targeted in the console, but I don't know if it adds a random mod, or if you have to give it the ID of the mod you want to add. If it does need an ID, I don't know how to find the ID for the Unending Legendary Mod that would do what you want.