The 2016 Spacex Mars Colonization plan has been published online

Spacex is estimating they wil be able to achieve $140,000 per ton for the trips to Mars. If a person plus their luggage is less than that, taking into account food consumption and life support, the cost of moving to Mars could ultimately drop below $100,000.

Cost will be brought down 5 million percent with
* fully reusable rocket
* orbital refueling
* Propellent production on Mars
* CH4 / O2 DEEP-CRYO Methalox fuel

The Spacex ITS (Interplanetary Transport Systme) rocket booster is really a scaled-up version of the Falcon 9 booster. There are a lot of similarities, such as the grid fins and clustering a lot of engines at the base. The big differences are that the primary structure is an advanced form of carbon fiber as opposed to aluminum lithium, we use autogenous pressurization, and we get rid of the helium and the nitrogen.

Spcex has been able to optimize the propellant needed for boost back and landing to get it down to about 7% of the lift-off propellant load. With some optimization, maybe we can get it down to about 6%.

Spacex is now getting quite comfortable with the accuracy of the landing of rockets. With the addition of maneuvering thrusters, they think they can actually put the booster right back on the launch stand. Then, those fins at the base are essentially centering features to take out any minor position mismatch at the launch site.

The Raptor engine is going to be the highest chamber pressure engine of any kind ever built, and probably the highest thrust-to-weight. It is a full-flow staged combustion engine, which maximizes the theoretical momentum that you can get out of a given source fuel and oxidizer. We subcool the oxygen and methane to densify it. Compared with when used close to their boiling points in most rockets, in our case, we load the propellants close to their freezing point. That can result in a density improvement of around 10%–12%, which makes an enormous difference in the actual result of the rocket. It gets rid of any cavitation risk for the turbo pumps, and it makes it easier to feed a high-pressure turbo pump if you have very cold propellant.

One of the keys here, though, is the vacuum version of the Raptor having a 382-second ISP. This is critical to the whole Mars mission and we are confident we can get to that number or at least within a few seconds of that number, ultimately maybe even exceeding it slightly.

Over time, there were would be many spaceships. You would ultimately have upwards of 1,000 or more spaceships waiting in orbit. Hence, the Mars Colonial fleet would depart en masse.

It makes sense to load the spaceships into orbit because you have got 2 years to do so, and then you can make frequent use of the booster and the tanker to get really heavy reuse out of those. With the spaceship, you get less reuse because you have to consider how long it is going to last—maybe 30 years, which might be perhaps 12–15 flights of the spaceship at most. Therefore, you really want to maximize the cargo of the spaceship and reuse the booster and the tanker as much as possible. Hence, the ship goes to Mars, gets replenished, and then returns to Earth.

This ship will be relatively small compared with the Mars interplanetary ships of the future. However, it needs to fit 100 people or thereabouts in the pressurized section, carry the luggage and all of the unpressurized cargo to build propellant plants, and to build everything from iron foundries to pizza joints to you name it—we need to carry a lot of cargo.

The threshold for a self-sustaining city on Mars or a civilization would be a million people. If you can only go every 2 years and if you have 100 people per ship, that is 10,000 trips. Therefore, at least 100 people per trip is the right order of magnitude, and we may end up expanding the crew section and ultimately taking more like 200 or more people per flight in order to reduce the cost per person.

However, 10,000 flights is a lot of flights, so ultimately you would really want in the order of 1,000 ships. It would take a while to build up to 1,000 ships. How long it would take to reach that million-person threshold, from the point at which the first ship goes to Mars would probably be somewhere between 20 and 50 total Mars rendezvous—so it would take 40–100 years to achieve a fully self-sustaining civilization on Mars.