However, besides the test firings of the new Raptor engine that is needed for the Mars transport, Spacex has also built a full sized carbon composite fuel tank.
The Interplanetary Transport system can launch 550 tons to low earth orbit which is nearly four times as much as the Saturn V.
The ITS spaceship will be nearly 50 meters tall and a maximum diameter of 17 meters.
Instead of leaving for Mars at 4.5km/s it will accelerate to 6 km per second. ITS uses six Raptor engines optimized for the vacuum of space. This will reduce the journey to Mars from six months to about 80 days in an optimal launch window. After launching and being fueled on orbit, the ITS could deliver 450 tons to the surface of Mars.
Not refilling in orbit would require a 3-stage vehicle at 5-10x the size and cost
Spreading the required lift capacity across multiple launches substantially reduces development costs and compresses schedule
Combined with reusability, refilling makes performance shortfalls an incremental rather than exponential cost increase
Allows reusability of the ship and enables people to return to Earth easily
Leverages resources readily available on Mars
Bringing return propellant requires approximately 5 times as much mass departing Earth
SOURCE – Spacex