What should I build next

@SamTheFox that’s a good idea

create an interplanetary lander,
or make your own version of apollo 11

@MaximumDeltaV
Great idea!
I did the same to teach myself the game. I made a rocket for each location I wanted to get to with relative delta v for each mission.
Sometimes it was just a probe fly by or a rover landing with a cage but eventually made its way to a full out colonization concept for cylero.
It was definitely great training for every aspect you will run into for this game.

So from what I can gather of your posts you seem to be a new player. I would recommend a challenge I am doing for myself to help learn the most efficient flight paths and rockets. Feel free to change this to make it easier, harder or whatever, but what I am doing is building a first stage reusable rocket that when stacked has an ingame price tag of less than or equal to 5 million dollars excluding other infrastructure. It also has to get a 1,500 kg payload into low orbit. (I made my payload a disposable camera satellite feel free to do whatever you want with yours.) I recommend using this to find your style of rocket building, favorite engines, common sub assemblies etc.