Musk goes for methane-burning reusable rockets as step to colonise Mars - Hyperbola

flightglobal.com

Excerpts:

• This plan is to use reusable rockets and along with Mars landing and ascent craft to take mankind to Mars within 15 years. And to do it Musk announced that liquid oxygen (Lox) and Methane would be SpaceX's principal propellants of choice.
• "We are going to do methane." Musk announced as he described his future plans for reusable launch vehicles including those designed to take astronauts to Mars within 15 years, "The energy cost of methane is the lowest and it has a slight Isp (Specific Impulse) advantage over Kerosene," said Musk adding, "And it does not have the pain in the ass factor that hydrogen has". 
• The new Raptor upper stage engine is likely to be only the first engine in a series of lox/methane engines. Larger engines will be derived from this. For all his arguments noting the advantages of having lots of smaller engine for engine-out redundancy, it is known that Musk has long wanted to have a larger sized engine that the current Merlin 1
• Musk envisions using a reusable heavy lift launch vehicle using a multiple of these newly developed large lox/methane engines on its first stage.
• Musk noted that the trick to economic viability was to add the new parts which allow reusability (thermal protection systems, landing legs, new engines etc,) without increasing the structural weight too much so that such a launch vehicle can still carry a useful payload. Musk stated that the payload on a very efficient conventional expendable rocket might be, at best about 4%, and suggested that 2% might be a more realistic figure for a reusable two-stage launch design. 

Comment:  So, now that Musk has ditched the hydrogen fuel way, he's going to use methane.  In order to determine what that does with respect to the amount fuel needed, I ran the numbers through the spreadsheet.

Significant dropoff in isp from hydrogen to methane --- 381 to 299

 If he uses the raptor engine all around, he will have extra fuel available for what he wants to do, but will it be enough?  This explains what Zimmerman was worried about and what I was missing.