I am wondering if there is any work being done on a manned vehicle able to execute a 6.5 kilometer-per-second velocity change. This is the requirement for a vehicle to get from LEO to the lunar surface. It seems at times we derail transportation work by imposing Apollo-era constraints. For example, a three-stage rocket with the third stage performing both a LEO insertion burn AND the TLI burn - on a single tank load of propellant. This has to be dreadfully large and the launching vehicle much larger still.
A simpler solution may be to design a two-stage vehicle where the first stage provides only some 4 km/sec velocity change. The manned second stage then takes over and performs the remaining 6 km/sec velocity increment to achieve orbit. Both stages are reuseable.
On orbit, the now empty manned stage docks with a propellant depot and takes on a full load of propellant. This is the only 'proximity' operation in the entire flight. The vehicle then performs the TLI, lunar orbit insertion and landing maneuvers. It arrives on the lunar surface with little or no propellant in its tanks.
A good conception of what the vehicle might look like is the DC-X vehicle tested in the 1990's. Imagine that on the payload end of a Solid Rocket Booster and you get a pretty good idea of how the system would operate.
This is basically the Apollo era plan of Earth Orbit Rendezvous. The Saturn V could go to orbit on only two stages (Skylab I) but the third stage added considerable payload (think not dragging the empty 2nd stage to orbit) and used its very efficient engines for the TLI ( like two hours later). The later parts of the mission were all performed with much less efficient, but storable, propellants.
When you look at the details of the Apollo program, you see that they were really bright people!
Also, I might make the distinction between the Apollo program and the Saturn program. Kinda different people. (Von Braun had Saturn) The Apollo was going to use the Nova Booster but Von Braun built the Saturn about twice as big as he was supposed to and Lunar orbit rendezvous saved a lot of mass.
With launch vehicles the more stages (generally) the more efficient they are and the more complex they are. look at a modern launcher like a delta. 4 solids fire, then two more, then just the first stage, then the upper stage, maybe then a payload assist module, then the satellite may get in the act. Thisis all trying to get rid of dead mass as fast as possible.
The re-usability question just goes to development and maintenance costs. - I like it, but I won't pay for it.