When it comes to getting back to the moon and then off to Mars. I think we are going in the wrong direction. A couple of years ago, if asked, most of us could agree that the shuttle's 50,000 lb payload was a medium to heavy lift.

What NASA needs is an extreme heavy lift shuttle (500,000 lb payload) leaving the small payloads to private industry. The competition that would result would drive the price down for smaller payloads.

The EHLS would allow for the ISS to be built quickly and the next generation station planned/built. For instance, if the EHLS had a 60 W x 30 H x 240 L payload it would be able to store 10-15x the number of ISS modules that the current shuttle can.

Another great benefit of such a large platform is that it could easily be the platform that transports the future lunar landers to lunar orbit. Its capacity and payload alone allow it to function a make shift space station, which is ideal for lunar and future mars mission.

As for the engineering challenges, just think of the Antonov AN225 and the Airbus A380. If we can make these beasts then we can certainly make the EHLS.

NASA has also shown that they can not handle the complexities of a rapid launch schedule (i.e Columbia and Challenger).