>>11672270Probably propane, more efficient than kerosene, more dense than the lighter hydrocarbons, way easier to store even without cryogenic temperatures, self-pressurizing at room temperature, easy to ignite, doesn't produce significant coking, etc etc. If you're a big company with the resources and funding you can sub-cool your propane and oxygen and achieve a bulk density on par with kerosene at higher specific impulse, meaning much higher stage performance in terms of delta V AND thrust to weight ratio.
An 8000 kN sub-cooled propalox gas generator would be kino as fuck. Actually, forget the 8000 kN single engine, and instead mass produce ~2000 kN engines, since they'd probably get better thrust to weight ratio that way and possibly cheaper too. Cluster 4 of them and you have your virtual 8000 kN engine, cluster 7 in a slightly bigger footprint and you're getting 175% the thrust. Since these are optimized for boosters anyway it makes sense to go for the maximum thrust density per square meter of booster base area. Assuming each engine nozzle is 1.5 meters wide and you pack them in with a 25 cm gap between them, a cluster of 7 would have a maximum diameter from nozzle edge to opposite nozzle edge of 5 meters. That's the same as a Delta IV stage.
Building a liquid booster off of the dimensions of the Delta IV first stage, we're working with a cylinder ~40 meters tall and 5 meters wide. Given the bulk density of sub-cooled propalox is 1014 kg/m^3, and assuming a 90% volume usage (some gaseous volume is necessary in both tanks), that's 706,860 liters of propellant, which would have a mass of 716.756 tons, and would exert a weight force of ~7030 kN. This would be combined with the stage dry mass (assuming a ~10% mass fraction) to produce a total booster wet mass of 800 tons and a weight force of 7840 kN. The combined thrust of the seven engine cluster however is 14,000 kN, giving the booster a thrust to weight ratio of almost 1.8, making it VERY workable.
