Fuel depots are a solution to the following problems:
- providing an on-ramp for commercial space participation (a key VSE goal)
- providing an on-ramp for international participation (a key VSE goal)
- providing an incentive to develop low-cost space access (an important secondary VSE goal)
- through low-cost space access, helping to achieve security, science, and economic benefits (the 3 central VSE goals) by making launch of security, science, and commercial space missions affordable
- providing a market for lunar ISRU of propellants (an important secondary VSE goal)
- demonstrating a capability (refueling) that would be useful for military, commercial, and science satellites (again addressing the 3 central VSE goals)
- allowing the development of a heavy-lift vehicle to be avoided, or a much smaller heavy lift vehicle to be sufficient, making the VSE development more affordable and sustainable (criteria mentioned numerous times in the VSE)
- allowing in-space vehicles to be refueled, and thus reused
In fact, heavy lift appears to be a solution in search of a problem. Who needs heavy lift? Apparently not NASA science, the communications satellite industry, DOD, intelligence agencies, NOAA, etc. It seems that the main reason NASA would develop heavy lift is to avoid addressing the real goals of the VSE (science, security, and economic benefits in the context of commercial and international participation).
It is difficult to understand how such an approach can offer an economically favorable alternative. The Ares-5 offers the lowest cost-per-pound for payload to orbit of any presently known heavy-lift launch vehicle design. The mass-specific cost of payload to orbit nearly always improves with increasing launch vehicle scale.
Griffin is saying Ares-5 is the cheapest because it's the biggest. That's an absurd law - why not build a rocket 1,000 times bigger at 10,000 times the cost then? The per-kg cost will be miniscule! I think Griffin's law of scale is easily violated when you consider the possibility of smaller, mass-produced rockets. Exploration, with its serious payload mass requirements, could provide the market for such mass-produced rockets.
Griffin's scale rule of thumb also ignores development costs. After all, it will be a long time before those tens of billions of dollars of Ares-5 (and related Ares-1) development efforts are amortized, at a maximum flight rate of 2 per year. We already have the EELVs and are already building Falcon 9 and Taurus 2 anyway, so their development cost for a job like fuel launch for exploration is $0. When you consider Ares-5 costs, you also have to consider the possibility that the development effort will fail, and all development costs will be wasted ... or the development effort will succeed, but the operations will be so expensive that they are canceled as happened with Apollo, and again the development costs will be wasted.
The recommendation in favor of an architectural approach based upon the use of many smaller vehicles to resupply a fuel depot ignores this fact, as well as the fact that a fuel depot requires a presently non-existent technology - the ability to provide closed-cycle refrigeration to maintain cryogenic fuels in the necessary thermodynamic state in space.
That's why we fund NASA research and development, technology demonstration missions like the New Millennium series, and innovation prizes. Well, we used to fund these things before Dr. Griffin diverted those funds to Ares.
This technology is a holy grail of deep-space exploration, because it is necessary for both chemical- and nuclear-powered upper stages.
Ok, let's get to work and develop it then!
To establish an architecture based upon a non-existent technology at the very beginning of beyond-LEO operations is unwise.
Ares-5 is also non-existent technology. No Ares-5 has ever been launched, and none will be for decades at best.
At any rate, why would be need a fully-developed depot technology now to include it in our long-range plans? There is so much for astronauts to do in LEO, GEO, Earth-Moon Lagrange points, and lunar orbit - as well as with robotic precursors at the more difficult destinations for HSF - before we even need to consider using either depots or HLV. We don't need to define *any* specific architecture for jobs like Mars landings at this point. Let's just take some affordable, achievable, and useful steps in the right direction for now. One of those steps is developing and demonstrating refueling technologies so that when we are at the point where we need to decide what the details of the next exploration phase will be, the important and enabling refueling capability will be available.