On Tuesday, 9/27/2016, Elon Musk addressed an audience of about two thousand people to outline his “Mars Colonial Transportation System” which had been in the works for years. But more importantly, he made a compelling argument about the attributes required by any system that seeks to make a radical improvement in cost of space transportation: Full Reusability, Refilling in Orbit, Propellant Production On Mars, and Right Propellant.
Full reusablity is a must in any system that has high capital costs. Everything from bicycles to air travel would be cost prohibitive without re-usability. Can you imagine if every time you rode your $100 bike some place you had to build a new one? How many bike trips could you take? Air travel is even a better example. When you ride a Boeing 737 some place, you are riding a $90M piece of equipment. It holds about 180 people, which means that the cost would be $500,000 per person if it was not re-usable. But right now you can use that plane from LA to Las Vegas for only $43 per person. That is because the fuel only costs $10 per person. And the plane can be use with very little maintenance on trip after trip after trip. Full reusability is key to reducing the costs of space travel..
Refilling in orbit means that the space craft that goes to Mars can be launched with nearly empty tanks and then multiple filling trips can be made to fill the tanks. This reduces the size of the system and allows for the materials required to go to Mars to be split across multiple launches.
Being able to produce propellant on Mars (or at other destinations) is key to reducing the cost. It takes fuel to carry the fuel to return. Being able to produce fuel at Mars, and use that fuel to get the space craft back to earth will be key.
In order to produce fuel on Mars, you need to make sure that you are using the right propellant. The F9 rocket uses kerosene which is produced from oil. And as best we know, there is no oil on Mars. But methane is another choice. Methane can be produced with CO2 and H2. And since the atmosphere of Mars is 97% carbon dioxide and there is significant amounts of water ice, which can be broken into H2 and O2 using electrolysis, it would be possible to produce methane on Mars. SpaceX has also developed and recently tested the Raptor engine which is a methane burning rocket.
With the four characteristics of Full Reusability, Refilling in Orbit, Propellant Production On Mars, and Right Propellant, it would be possible to reduce the cost of space travel to a level that would open up the solar system to more exploration and colonization.