On September 27, 2016, SpaceX finally revealed their Mars transportation architecture (https://www.youtube.com/watch?v=H7Uyfqi_TE8). It was a very exciting moment. Regular readers will know that I have engaged in idle speculation on the topic, and I was gratified to see I got the details mostly right, though their system is a lot larger (up to 450T cargo!) than what I initially had in mind. If you're interested, see my best guess from 2015: https://docs.google.com/docume
The architecture is designed around the principle of "cheaper is better" which almost always drives "simpler is better". Yes, it is possible to get more mass (maybe) with less fuel if there is an intermediate stage or multiple cores, but the most overlooked handle is the size of the rocket. Mars requires a developing a new super heavy lift rocket anyway, so it may as well be BIG! The SpaceX booster, with a nominal 550T to LEO capacity, fits the bill.
(Click to expand) Left panel: Historical data from robotic missions, showing Mars entry profiles. Parachute descent typically commences in the bottom left at around 500m/s. Central panel: Results from my ballistic motion simulation reproducing behaviour of previous landings, validating the code. Right panel: Entry profiles of several hypothetical future Mars vehicles, with Curiosity for reference. LDSD levels out a little higher (depending on total loading), while Red Dragon needs a significant mass offset to achieve enough lift to not hit the ground. The three curves marked ITA (Interplanetary Transportation Architecture) represent different lift parameters for the SpaceX ship. Horizontal flight represents banked turns to prevent multiple skips out of the atmosphere. Their high lift and high entry speed compensate for their high mass, and they don't get too close to the ground. Mars' highest mountains are >20,000m tall.
The CAD models look great, but clearly represent an early draft. The interior space of the crewed module is a bit spartan (needs bulkheads), while the oxygen feed lines to the 42 raptor engine cluster look a lot like a brain angiogram scan. Getting prop feed to 42 engines that are throttling and pogoing, across a giant thrust structure trampoline, while damping every instability and cavitation, sounds like a nightmare/worthy engineering challenge to me.
This was the most exciting part by far. The reusable architecture calls for single stage return from Mars. It's all very well to draw spaceships (spaceship!) all day long, but when the rubber hits the road, the system requires a monster engine, as well as fuel tanks with practically imaginary mass. That's a good place to start, and that's what SpaceX has been working on.
The SpaceX Mars plan is a compelling vision for moving lots of humans to Mars. A complete system will be much more detailed and probably a bit different, but importantly this lays a technical foundation and is a great starting point for future system discussions.