Truax has written about the Big Dumb Booster alternatives. I call my concept "The Proxmire", for Propane Oxygen Minimal Research.
Specific Impulse and specific density are tradeoffs. H2/O2 Turbopump is very high specific impulse, but very low specific density. Solid Fuels are the other way around. C2H6/O2 Pressurefed is pretty middling, has low cost, and is a lot simpler than either of the other two designs. No toxic waste output, no ultracold cryogenic technology, no incredibly high power density turbopump machinery, etc.