That's 1.2^3, or 70% more... not even close. Also remember that making the rocket bigger also increases the dry mass, which further increases the fuel requireme

That's 1.2^3, or 70% more... not even close. Also remember that making the rocket bigger also increases the dry mass, which further increases the fuel requirements. The difference in ISP between a liquid-fueled rocket (say 450s for LH2/LO2, 350s for RP1/LO2) and solid propellant (250s) makes a huge difference to your prop fraction. To get, say 8km/s delta-v, just plug it into the rocket equation: LH2/LO2: 6.1 initial/final mass. RP1/LO2: 10.3 solids: 26 Staging can help this a bit, but the basic fact remains. It's very difficult to get useful payload to orbit with solids.