SSTO'S

Thanks for the tips,
I managed to build a successful Ssto, but experienced some trouble during building (also rockets).
Sometimes a error message appears, informing that the setting is "airplane" but the craft seems to be a "rocket", even though it's clearly a Ssto.
Suddenly, all flying mechanics are glitchy and all the indicators in nav sphere are wrong, ussualy 90º off

I also find SSTOs very challenging to make. Some suggestions I have would be to start with your payload plus 10% and then add your tank and engine combination that gives you a delta-v of at least 6000 and twr over 1.1, this gives you enough fudge factor. If you want jet engines and wings then consider all their weight part of your dry mass.

Lastly I would recommend paying close attention to your engine's specific twr as well as their performance at both sea level and in vacuum.

@Bmcclory additionally the the rapier engine in ksp was basically made for SSTOs and the nuclear engine is completely overpowered, because it uses propellants with a density comparable to kerosene but with the isp of liquid hydrogen, causing its mass efficiency to skyrocket (pun intended).

Your landing wheel opens because your landing gear action group is ON by default when you load craft it automatically opens the leg,
You have to turn OFF landing gear action group from "edit action group" option, this option is available in properties of command pod or command chip