Elon Musk is selling a fairy tale

To show why this is so, I present this truncated chart of delta v's

What this shows is that it takes 7847 meters/sec velocity to get from the ground on Earth to Earth's orbit ( vice versa ).  Likewise, it takes 3502 meters per sec velocity to get from Mars orbit to the ground ( and vice versa ).  The two highlighted blocked areas are of particular interest.  Starting from Earth, it takes 16,540 meters/sec velocity ( or delta v )  to land on Mars.  In order to go to from Earth orbit to Mars orbit, it takes 5748 meters/sec.

You should think of these velocities ( or delta v, which means literally change in velocity) as mileage figures.  The greater the delta v, the greater the "mileage" is to get to a destination.  Now, another term that is thrown about in rocketry is ISP.  ISP is like the gas mileage.  It will tell you how much fuel you need for any delta v trip you want to make.

Thus, if you have a low ISP engine, it means you have a low MPG type engine.  If you want to go a long way, you are going to need a lot of fuel.  Thus, low ISP's and high delta v's are like low gas mileage engines being used for long trips.  That's gonna cost you some bucks in fuel.

Now look at this chart I constructed

Hopefully, this chart isn't too "busy".  I'll interpret it briefly.  I highlighted those parts of the chart that should be of interest to this discussion.  Number 1:  Musk wants to use a Methane / Lox engine with an ISP of around 380.  I highlighted this with a box in grey in the fifth line from the top of the chart going horizontal all the way across.  All you need to know here is that this line represents various combination of ISP vs Delta V's.  The Delta V is highlighed in a vertical column to the far right side of the chart.  Note the circled cell of 78.6%.  What this means is that a DIRECT trip straight from Earth orbit to Mars would require that 78.6% of the mass of the vehicle MUST BE FUEL.  This doesn't even allow for tanks and plumbing and whatnot.  Before you're done, you won't have any room for comforts and amenities .

People aren't going to pay millions to suffer or even die under these conditions.  What you need to do is to break up the trip into smaller segments.  That will make the trip bearable, and not only that, it will allow you to deliver more cargo to Mars, which will make the trip to the surface more bearable also.

I did this here, provided that the chart isn't too busy, you can see that if you used a nuclear thermal rocket from the LaGrange Point in order to get to Deimos, you need to use only about 20% of the mass for fuel.  That leaves substantial room for amenities.  You are going to need these in order to keep people healthy and happy on the long, long trip.

Bottom line is that Musk will not succeed with Methane/LOX engines without using refueling and Nuclear Thermal Rocket Engines.  He needs to have a lot of cargo delivered and set up BEFORE he tries to send a lot of people to Mars.  He needs the Nuclear Thermal Engines and the refueling depots so that he can make more room for people and cargo.  It won't work otherwise.

But Musk is a clever fellow.  He won't mention nuclear rockets just yet until he gets a more firmed up track record.  But he MUST do it eventually.