For a decade or two now the proponents of Bob Zubrin's Mars Direct have been bemoaning the lack of super heavy lift (any vehicle that can lift more than 50 ton to LEO). While few advocates claim a heavy lift launch vehicle is the only component of the Mars Direct plan which is missing, most consider it a necessary prerequisite.
Before I tell you why I think that's simply wrong, let's just make a quick list of all the other things we still need: decent life support, space suits for Mars, rovers that run on methane/LOX, the ISRU propellant production system, bigger entry/descent/landing systems than have ever been flown, portable field equipment for the science mission, artificial gravity generation, and the habitat itself. Oh yeah, and a space rated nuclear reactor.
The beautiful innovation of the Mars Direct plan was the use of the Mars atmosphere to produce the return propellant so it doesn't need to be carried all the way from Earth. Without this simple idea the size of the launch vehicle would have to be exponentially larger. The architecture is littered with examples like this. The use of direct aerocapture over aerobreaking or just using propellant to enter Mars orbit. The advance staging of the Earth Return Vehicle, etc. It's such a shame that this willingness to trade mission complexity for reduced launch capability has been be lost. So we sit on the ground, waiting for a big enough rocket.
Why can't we make do with the rockets we've got? Why can't we start now? While many of the technologies in the list above could be tested with the smaller launch vehicles we have now, it's hard to imagine how to develop the capability of putting 30 ton payloads onto the surface of Mars without first having the capability of throwing 30 ton payloads to Mars transfer orbit.
An LH2/LOX third stage, these days often called an Earth Departure Stage, with an initial mass of 100 tons in low Earth orbit, will do the job (dry mass 6328 kg). To get any smaller we need to start using multiple launches. The traditional approach is to send up the EDS and then send subsequent flights to fill it up. The primary difficulty with this approach is that the LH2 starts boiling off as soon as the sun hits it, so you have to get the propellant up there as quickly as possible.
I'd like to propose an alternative. Using a storable propellant would mean we could take as much time as we like to build the EDS but it would be more like 177 tons in low Earth orbit (dry mass 13359kg). While I'm going backwards, why stop? All that rendezvous and docking and propellant transfer is heavy and eats into the inert mass ratio, so amassing all this propellant is likely to be a slow process. How can we reduce it?
Most EDS designs call for more than just the one engine. This is because the entire burn has to occur in less than a few minutes to make the orbital calculations workable and avoid prolonged exposure to the radiation belts when sending the crew. If we're going to use multiple engines anyway, we could just use multiple stages firing in parallel. Of course, they'd have to be docked together pretty well.
Ok, enough stalling, how many flights do we need?
In just a few years, SpaceX will be fielding the Falcon 9 Heavy. It will be able to throw 32 tons to low Earth orbit. A storable propellant upper stage (dry mass 2226kg) can throw 5 tons to Mars transfer orbit and still give us 2.5 tons of propellant to perform rendezvous and docking with the other stages. A final flight is required to deliver the payload to the cluster of stages. As said earlier, these 7 flights can be stretched out over as long a period as is required. The total cost of each 30t Mars throw is $665M.
Compared to the reported $300M price for a launch of the proposed SpaceX super heavy lift vehicle, this seems like a pretty bad deal. The argument can no doubt be made that paying SpaceX $2.5B now and waiting for super heavy lift is a better idea. Remembering that SpaceX will be developing the Falcon 9 Heavy on their own dime, from a simple cost perspective, more than six flights of the super heavy lift vehicle have to be flown to justify the development costs - but more important than this, sitting on the ground and waiting is not the right choice.
Update April 12, 2011: Obviously SpaceX's announcement of the Falcon Heavy, which will take 53 tons to LEO and yes, they plan to develop on their own dime, completely changes the plausibility of this architecture. It is now conceivable to do just a few launches to build a big enough EDS. I still think storable propellant is worth the extra mass in LEO.. it's just more mature right now, and NOFBX in particular makes it even more viable.
I'm not questioning the virtues of LH2, but we need both development to reduce cryogenic boiloff and an increased flight rate before it will be a plausible technology. We should be doing this development, and supporting the launch industry to increase flight rate, but there is no reason to wait for either to be successful before we start lofting heavy payloads to Mars to demonstrate the techniques required on the business end of Mars colonization.