Earth-Mars Transportation System

This is the best use for the massive SLS rocket being built.  Rather than one off missions to the surface of the planet, install bits and pieces of an infrastructure that will support regular missions.

Each part of the infrastructure will be specialized to accomplish one part of the mission.  For example, there would be a mission to Low Earth Orbit.  This pretty much exists already.  The next leg would be to the proposed Gateway at EML-2.  You'd devise a specialized craft for that mission.  From the Gateway, there is only a delta-v of 2.17 km/sec to Deimos.  From Deimos, it takes only .7 delta v to get to the surface.  Likewise, there'd be a specialized craft that went from the Gateway to Deimos.  And so on.

To get back to Deimos from Mars' surface will take more than just .7 km/sec using aerobraking, but the rest of the trip back to Earth is just back tracking through the delta-v's.  Thus, to get from Deimos to EML-2 would require only 2.17 km/sec.  But there may be a problem.  To get from Mars's surface to Deimos would take 5.9 km/sec, according to this mission table map.

Well, I don't like that part, so let's just go from the surface to Mars' orbit, then on to Deimos.  That would change things somewhat on the inbound, but you could use aerobraking and refueling to achieve that.  That would be the Deimos' base's mission.  Then aerobraking on the way down could still be employed, and refueling on the Mars' surface for the trip back up to orbit.  Deimos could serve as a base for the refueling of the Mars' surface trips and the trips back to Earth.  By the way, to get to Mars orbit from its surface requires 3.5 km/sec delta-v.

That's the point.  Specialized missions for each piece of equipment and each base.

But more than that.  The heaviest part of the equipment is the lander.  It would be best to take that to Mars' orbit and keeping it there, rather than sending the entire package all the way from Earth and back on each trip to Mars.  This would save a lot of fuel as the masses would be minimized.  The mass penalties are the big killers and that's why you have propellant depots and reusable ships.

The SLS could install the big pieces into place on the Gateway, Deimos, Mars' orbit, and the surface of Mars itself.  Once in place, all of these pieces can be serviced as a part of a regular schedule that should only require one launch of the big SLS rocket.  Or perhaps, not even that would be necessary.  Only in cases where something big has to be replaced on a leg of the journey would the big rocket be needed.  Smaller, cheaper rockets can take up the rest of the slack.

The first mission for the SLS is scheduled for 2017.  Rather than do a lunar flyby, you'd set up the EML-2 Gateway.  This would be an unmanned mission and it could still test the Orion module.  The second mission is scheduled to be manned, but instead of doing that, they should set up the Deimo's architecture.  In support of this, there would be missions ahead of time in order to determine if there is water on Deimos.  If not, that show is off, but you could still try Phobos.  If possible, you'd install the Orion at EML-2 on this mission, and  then leave it there.

Orion could take you home from the Gateway, or perhaps not even used at all.  Perhaps a better way to get home can be devised than that.  Perhaps it could be used in order to aerobrake and slow down to orbital velocity only, and then it will dock with a lander that is specialized to go from Earth orbit back to the surface.  A tug could take the Orion back to the Gateway for future missions.

Now after just two trips, you have the Gateway and Deimos set up.  Now, you go for the Mars orbit leg.  Install a lander module in Mars' orbit with SLS-3.  You could also put in a shuttle craft between Deimos/Phobos and Mars' orbit.  After this third trip, you've got the capability to get to the surface, but not back up.  With the final mission, SLS-4, you can go all the way to the surface and install the refueling base.

The refueling base on Mars is a permanent Mars base in which operations on the ground can commence.  You spread out on Mars from there.  Craft will have to be devised to give transport on the surface.

The craft used for the Gateway to Deimos/Phobos could be the Nautilus craft that would supply the crew with artificial gravity.  It's mission would be to go back and forth between the Gateway and Mars' moons.  It should not take an SLS mission to launch that.

With 4 SLS launches and some support launches of lesser rockets, you could install the infrastructure that will allow missions to Mars every two years.  That compares with 7 to 10 big SLS rockets to emplace a one off mission to the surface of Mars and back.  The value of this approach should be obvious.