Although to be fair the Falcon 9 will use 9 Merlin engines and will not have enough thrust to lift the Orion CM/SM combo. It would require the Falcon 9 Heavy which would use 27 Merlin engines. That is a lot of points of potential failures.

NASA also had a nearly unlimited budget, post-1963, to get the job done. Modeling still can only go so far, you still have to go ahead and build it.

The problem lies in the size of the Altair lander that NASA wants to use not just for simple landing but long duration stays. It is significantly larger then the Apollo lunar lander. It would take a colossal rocket to launch the mission in one rocket. We are also much further along orbital rendezvous then they were in the '60s, the Progress spacecraft can automatically dock with ISS.

SRBs are more efficient, the RD-171 burns kerosene not liquid hydrogen so it takes more propellant. Plus it would take four of them on the first stage. Now you are talking about a stage the size of the first stage of the Saturn V. The first stage of the Saturn V weighed just over 5,000,000 lbs versus the the 2,600,000 lbs for the SRBs combined.

The SRBs have been redesigned since Challenger which is why there hasn't been another accident related to the Solid Rockets Boosters. If you remove the SRBs then you will have to design a whole new engine, in the class of the Apollo era F-1s since each SRB puts out the equivalent thrust of almost TWO F-1 rocket engines each.

http://www.directlauncher.com/

http://en.wikipedia.org/wiki/Direct_Launch_Vehicle

The DIRECT system is a better option:
1) Most of the hardware is man-rated; unlike Ares

2) NASA does not have to retool manufacturing; unlike with Ares

3)Can be ready sooner with heavy lifting as an option

Why NASA is completely dug in on Ares is mind boggling. Orion, the capsule, is a go no matter what.

Also, the contractors won't really be affected: ATK would still make the SRBs, Lockmart would still manufacture the capsule, and Boeing would get it's money from being part of United Space Allaince.