Possible mission architectures
Several studies have been conducted regarding manned missions to Mars and for the above reasons none of them consider a single stage giant rocket. Other alternatives have been considered, tending to reduce the mass ratio.
Aerocapture and aerobraking
Instead of braking the spacecraft to get into Martian orbit by using thrusters and in the same way land with propulsive braking, these two operations could take advantage of the presence of the feeble yet useful Martian atmosphere.
Aerocapture is the process by which the atmosphere slows down the incoming spacecraft to attain orbit. A heat shield protects the spacecraft against the high temperatures generated during the process and is afterwards jettisoned. Mars Global Surveyor utilizes this technique to get into Martian orbit.
Although the atmospheric drag is not enough by any means to land a spacecraft, it can assist any propulsive maneuver to slow down the vehicle for landing, thus reducing the delta V required of the rocket engine. A notorious example of aerobraking application is the Mars Pathfinder probe.
Following the Pathfinder example, the use of parachutes can also be contemplated to slow down the landing, although some use of propulsive braking will still have to be needed. Airbags, however, which were also used successfully in the Pathfinder mission will obviously not become acceptable options for a manned landing.
Staging the mission
The other viable option is to stage the mission, that is, to use more than one spacecraft for the different stages in the mission.
There are various options for doing this in order to economize fuel. An Apollo type mission architecture would call for a command module vehicle attached to a smaller lander that would descend to Mars. After completing the mission, the lander would ascend to rendezvous with the command module in Martian orbit. The lander would be discarded before starting the return trip, thus minimizing the weight returned to Earth orbit. This mission is often referred to as Mars Orbit rendezvous (MOR).
Another similar option would involve launching an unmanned and fully equipped (although without fuel for the return trip) return vehicle from Earth orbit to land on the surface of Mars. Using ISMU, the return vehicle will be loaded with enough propellant to launch from the Martian surface and make the trip back Only when there is confirmation that the vehicle has been stacked up with propellant, at the next launch opportunity the crew will be launched on the manned vehicle that will only be used for the ongoing trip, as the return vehicle is awaiting them at a predetermined landing site on the Martian surface. This type of mission is often designated as Mars Surface Rendezvous (MSR)