"There is no question that this plan involves considerable risk, and a variety of missions, technology developments and testing programs in advance might reduce that risk. But if we try to do even a significant fraction before committing to the mission, we will never get to Mars. [..] If we want to reduce risk to human life, there are vastly more effective ways of doing so than by spending $10 billion per year for the next two or three decades on a human spaceflight program mired in low Earth orbit."
And at this point I had to start laughing, for I know what $10 billion per year with the shuttle program would deliver, using some pieces of work Zubrin himself even discussed in the past.
In the 1990's, General Dynamics proposed a program called "Early Lunar Access" using a pairing of Shuttle and ELV to deliver a lunar program for low cost and rapid development. Studying this, and applying todays capabilities, let us explore what can be done.
Let us assume that we push the Shuttle to maximum capability, as many launches as we can safely, considering we have only one launch pad and three orbiters. As a Shuttle takes 2 months to turn it around on average, that means each shuttle can be launched 6 times per year, maximum. I'll divide that by half, as these are old vehicles, for a total of 9 launches. Knowing the numbers for the Space Shuttle, this would cost a grand total of $5.1 billion per year. Now, we need to be able to afford the craft, so I will take an approach of adapting the Nautilus-X program and cost projections to this combined Shuttle/EELV approach. It's cost is $3.7 billion for R&D and construction, with testing of equipment on the ISS. Because we still have the Shuttle, the spacecraft does not require automated assembly systems, which means speeding up of development. Now, 8 assembly launches will not have a complete system, but it will have all of the components *but* the inflatable habs and the fuel units. $10 billion a year, subtracting the Shuttle and vehicle costs comes to $8.8 billion. The inflatable habs, fuel, and the lander would be lifted with traditional EELV's, requiring 3 heavy launches (two Delta and one Atlas 551) to accomplish the final assembly in the 2 weeks after the initial crew launch, utilizing the crew left by the last Shuttle launch, all three vehicles utilizing both Cape EELV launch sites and the Vandenburg launch site. This would cost $1.1 billion, making the whole cost of the Mars mission $9.9 billion. No new launch vehicles, and no use of technology we do not have within the near future.
Now, let us compare to Zubrin's plan here. Unlike the Shuttle, which has the tools and capability for on-orbit assembly, Falcon is just a BFR. You'd have more launches to reach the same capability, even if each launch could lift more, due to the need for on-orbit assembly. By a calculation, for the same level of craft as the Nautilus-X, it would take 20 Falcon 9 Heavy launches to reach the same level of capability. While yes, the launch strength of F9H is higher, it lacks any kind of on-orbit assembly system, requiring an automated deploy, tug, and assembly, which would increase the weight of each payload. Based on the automated Russian assembly of modules for both Mir and ISS, and comparing to the weight of those brought by the Shuttle with their capability for the ISS, each module would have to add approximately 40% weight to reach the same level of capability based on history with not one, but 4 space agencies. This of course means the craft would need a more powerful engine, which adds even more weight to the process, further increasing the number of flights. It would need 12 flights for the main structure, 3 more for the habitat, and 5 for the fuel, totalling 20 flights. At $150 million per launch, that comes to $3 billion. The turnaround for Falcons is not fast enough, nor do they have the number of pads needed to enable a staffed spaceship to handle the final stage of assembly. And adding to it, the development cost of the vehicle would itself be higher, due to it's need to self-assemble. You'd be looking, based on experience we've had with the ISS, at the craft itself having the cost at least doubled, bringing it to $7.4 billion. So, $3 billion + $7.4 billion == More expensive than the Shuttle + EELV program.
Welcome to space flight. You're talking a Mars mission, and claiming superiority of one system over another over cost, while ignoring the full capability of the other system. This is the exact kind of mission the Shuttle is ideal for. You're not going to Mars in a capsule, you need a full spaceship. And a full spaceship is not going to be all-up, but assembled on-orbit. For on-orbit Assembly, nothing beats the Space Shuttle.