Alan Boyle reports on the work of the Space Development Steering Committee in laying out a lower cost approach to human Mars missions than the current vague NASA plans based on the SLS rocket: Spaceflight experts work on alternate vision for Mars trips – NBC News.com

As Alan mentions, alternatives to the giant rocket approach to space exploration and development have been proposed for years. However, they have not gotten nearly enough exposure and discussion. Congresspersons who lead NASA policy have avoided as best they can to have such ideas presented in public hearings.

United Launch Alliance (ULA) several years ago laid out an architecture  for lunar exploration based on the use of their Atlas V and Delta IV launchers. See the Exploration section of the  ULA papers collection, which includes, for example, Affordable Exploration Architecture 2009  (pdf). The abstract lays out the problems with the monster rocket approach and the advantages of medium launchers coupled with propellant depots:

The present ESAS architecture [i.e. the Constellation program canceled in 2010] for lunar exploration is dependent on a large launcher. It has been assumed that either the ARES V or something similar [i.e. the SLS], such as the proposed Jupiter “Direct” lifters are mandatory for serious lunar exploration. These launch vehicles require extensive development with costs ranging into the tens of billions of dollars and with first flight likely most of a decade away. In the end they will mimic the Saturn V programmatically: a single-purpose lifter with a single user who must bear all costs. This programmatic structure has not been shown to be effective in the long term. It is characterized by low demonstrated reliability, ballooning costs and a glacial pace of improvements.

The use of smaller, commercial launchers coupled with orbital depots eliminates the need for a large launch vehicle. Much is made of the need for more launches- this is perceived as a detriment. However since 75% of all the mass lifted to low earth orbit is merely propellant with no intrinsic value it represents the optimal cargo for low-cost, strictly commercial launch operations. These commercial launch vehicles, lifting a simple payload to a repeatable location, can be operated on regular, predictable schedules. Relieved of the burden of hauling propellants, the mass of the Altair and Orion vehicles for a lunar mission is very small and can also be easily carried on existing launch vehicles. This strategy leads to high infrastructure utilization, economic production rates, high demonstrated reliability and the lowest possible costs.

(My emphasis.) Competition among commercial launch firms to fill orbital propellant depots will offer redundancy and also encourage lower costs and innovation, especially with respect to reusability. The economics of reusable launch vehicles benefit greatly from the high launch rates that depot filling would generate.

An internal NASA group produced a study in the 2010-2011 time frame that found similar benefits from an exploration architecture based on medium scale launchers and propellant depots but it has never been released.

A group led by Dr. Alan W. Wilhite, Langley Professor of Engineering at Georgia Tech, has published a series of papers on such architectures:

• A Sustainable Evolved Human Space Exploration Architecture Using Commercial Launch and Propellant Depots, 
  FISO Telecon – Feb.13.13 (pdf, Slide presentation) 
• Reusable Lunar Transportation Architecture Utilizing  Orbital Propellant Depots, Erik Axdahl et al (pdf) 

From Wilhite’s FISO presentation:

Rick Boozer‘s book The Plundering of NASA: an Exposé, which I’ve mentioned here several times, discusses these alternative exploration architectures in detail. It’s available at Amazon.