>>11603141See, it's dummies like you that obfuscate space flight mechanics for everyone.
Fast interplanetary transfers will remain science fiction until we develop direct-fusion propulsion systems that are high thrust and light weight, which won't be for a LONG time if it's even possible.
If we define a fast transfer to Mars as being anything less than or equal to 1 month of travel time, you need to leave Earth's sphere of influence moving at around 30 km/s, or about 60 km/s relative to the Sun, and you'll get to Mars in 31 days, at which point you need to capture at Mars, which will be a minimum delta V of ~36 km/s. The vehicle's total delta V budget, assuming no other burns except for the escape and capture burns, will need to be about 68 km/s (another 2-ish km/s needed to actually leave Earth, before accelerating to cruising speed).
68km/s is absolutely infeasible to get out of a single stage vehicle. To give you an idea, if you use hydrolox engines with 475 Isp (a generous Isp figure), and you imagine a total vehicle mass of ten million tons, to achieve 68 km/s of delta V your dry mass needs to be 4 tons. 4000 kilograms of tanks, engines, and payload, with 9,999,996 tons of propellant. This is not possible to build, even if you were using frozen oxygen and frozen hydrogen as structural material in places or throughout.
Okay, so just for argument's sake, say you want to limit the mass ratio to 80% propellant, how good does your Isp need to be? 0.2 mass ratio, delta V budget of 68 k/s, you need 4350 Isp in order to make that happen. That's on the order of what you get out of a decent ion drive, so possible to cram into a vehicle with modern technology, but there are two problems. First, off, this giant vehicle would need either a giant ion engine or a giant cluster of ion engines, and a giant power supply to match. Second, it's still going to accelerate extremely slowly, so no fast transfers for you.
