There are a lot of implicit assumptions that heavy lifters of this or that throw weight must be used for future exploration beyond low Earth orbit.
These “needs” have never been logically derived from anything.
Yet space policy and exploration architectures must be based on rationality above all. There is no excuse whatsoever to do things on a whim. Hundreds of billions of dollars, and the future of humanity’s spacefaring are at stake.
There is no foreseeable need to launch over 25 tonne monolithic payloads to low Earth orbit in lunar exploration, and probably even that number could be seriously diminished with some more thorough planning. Orion, EDS, LSAM, all are below that weight, if they are refueled using a depot in space.
If the huge development and operational estimated costs for a heavy lifter rocket go away, then that money is freed for real exploration work. In-space hardware development, more launches, more missions and operations.
Flight rate is _the_ most important way of reducing launch costs, the single largest impediment for advancement of spacefaring, and the propellant depot enables a higher launch rate. Multi-launch scenarios with a propellant depot also enable competition, redundancy and flexibility, all very good things, ensuring safety, robustness and progress.
I repeat as a summary how
1) Solutions for space exploration, like any large endeavour, must be rationally justified. No baseless assumptions should remain.
2) The need of heavy lift is a baseless assumption. It can be one of the alternative ways of execution, but it can not be a starting point or an axiom.
3) The current architecture is heavily based on the implicit assumption of heavy lift. Hence a rational space exploration architecture would examine things from the ground up. It could end up with some radically different conclusions.
4) Propellant depots is one alternative way of executing space exploration beyond LEO, and it does not need heavy lift.
5) Propellant depots can, if executed correctly, increase launch rates many fold, and thus enable lower costs, progress, reliability, redundancy, robustness – all the things that the space shuttle promised but failed to do because it was a sole solution that could not sustain a high enough launch rate and was too costly.
6) NASA at the same time should keep on working with fundamental research, to enable continuous progress trends in space technology.
7) Space exploration should look as different from Apollo as possible – there should continuity and continuous improvement possibilities, robustness and progress. The architecture should be affordable as well.
8) New space technology, like cheaper launchers, should be demonstrated at a smaller, humble scale first. That way many things can be tried and progress is faster, for the same price and effort. One failure also will not be as critical.
9) There seem to be impediments for information flow inside NASA, and many professionally acknowledged things like propellant depots, EML2 rendezvous or space tethers are never even mentioned in NASA high level planning. This is not rational.