Here's how I design my interplanetary vehicles On top, is a 2 stage or apollo stlye vehicle. Underneath it is a transfer stage, I tend to use liquid engines more than NERVAS because better TWR=Less boredom Then, a "get into orbit" stage. This typically also has fuel left. The first stage is a large, usually 4x liquid booster feeding (could be asparagus, but meh, I got plenty of fuel), typically 2 km/s of delta-v I either go big or go small. No middle marker!