Cant build good rockets?

[GoSlash27]

a few points:

 #1 The rocket needs to be designed from the top down, not the bottom up. You start from the payload, minimum t/w, and the DV budget and design the upper stage. Then you treat that as payload for the lifter and repeat. I do this using the reverse rocket equation in a spreadsheet, but you can do it with MechJeb.

 #2 High t/w saves DV, but low t/w saves mass and cost. 0.7 t/w is fine for an upper stage and 1.4 is plenty for a lower.

#3 SRBs are cheap, but heavy. I don't recommend using them in early career for orbital boosters.

My recommendation is to use the LV-909 for an upper stage and an LV-45 for the booster. Upper stage needs a t/w of 0.7 minimum and 1,700 m/sec DV. The lower stage needs a t/w of 1.2 minimum and 1,800 m/sec DV. If you follow this approach, you can orbit and safely recover up to 3 tonnes of payload without upgrading the starting facilities. 

Good luck!
-Slashy