I am relatively new to KSP, but have done some manned an unmaned missions to duna, the mun and minmus and build a small space station (Sandbox). The problem that I keep having is the trouble of choosing the right (rocket)engine. Eg: In the beginning I literally just used the Mammoth engine for the first stage, no matter the space craft.
Although I have learned a bit, some engines still seam very similar to me, or I don´t really know what engine to use for the job.
What are the radial mounted engines used for? Like Spider and Twitch
What first stage setup would you use for a) a crewed close distance mission to LKO or the moons of Kerbin b) a crewed mid distance mission eg to Duna c) a crewd long distance mission eg Eeloo?
The same a) b) and c):
What setup would you use for second and third stages?
And as a last question, which engines would you say have the same usecase, but one is just better than the other?