Architecture has propellant depots, “depot tugs” between LEO and EML1, and landers from EML1 and the moon. Breaking up propulsion steps makes system more efficient. Can be launched and supported in 25-ton chunks (no HLV needed). Can also get tonnage back to LEO via aerocapture, to allow delivery of lunar water there.
Consists of personnel modules (zero-gee and g-oriented), propellant carrier, two modular depots, reusable transfer vehicles, aerobraked reusable vehicle, lander, all Lox/hydrogen.
Top ten techs:
10. Variable mixture ratio lox/hydrogen engine.
9. Low-g and zero-g oxygen/hydrogen liquefaction
8. Low-g water electrolysis
7. Deep-space autonomous rendezvous and docking (AR&D)
6. Aerocapture (need to fly aerocapture experiment from eighties that never flew)
5. Long-life reusable lox/hydrogen engine
4. Aero-assisted entry, descent and landing
3. Long-term zero-g cryo storage
2. Zero-g cryo transfer
1. Zero-g cryo fluid management (storage). Can be done with cryo coolers.
NASA flight technology demos (FTDs) support some but not all, but schedule far too long. Really important stuff out in 2025 time frame.
10, 9, 8, 7 and 5 (half of them) not covered by FTDs.
Needed now, cryo management, storage, transfer.
Next, AEDL, then aerocapture.
First three technologies enable depots, AEDL enhances ETO propellant tankers, long-life engines help cost, deep-space enables depot assembly and lander/stage mating.
Overall, enable reusability, enhance efficiency, promote reduced propellant delivery cost to LEO.
[Update a while later]
Dallas went too fast for me to capture everything, but in answer to a comment here, the reason for variable mixture ratio is that due to other uses (e.g. oxygen for life support), differential boil-off rates in storage, etc., you can’t count on any particular mixture ratio. Electrolysis gives you stoichiometric output, but while that’s the most efficient ratio in terms of energy production, it doesn’t maximize specific impulse (6:1 is the best for that). But the point is that you don’t want to waste any propellant when it cost so much, so you don’t care about Isp per se, as long as the engine can turn whatever ratio into useful thrust. The trades for this problem are very different than the ones for launch systems, when propellant, in whatever ratio desired, is a trivial part of the launch cost.