Early Flexible Path to the Moon destinations don't need heavy lift, and if we can lower launch costs through something closer to mass production of launchers, and at the same time we can demonstrate technologies like autonomous rendezvous and docking, propellant depots, in-space assembly, ISRU, reusable space-only vehicles, and/or reusable lunar landers, we won't need heavy lift for the lunar surface, either. We shouldn't expect success with all of these technologies, but success with a few should be good enough. Leveraging commercial and international participation should also help us develop affordable and sustainable lunar surface access and development without having to develop an unaffordable heavy lift vehicle like Ares V or its close relatives.
It may make sense to advance some sort of heavy lift capability to add to our bag of tricks, but only if it can be done in a way that is both useful and affordable. We don't need heavy lift, but we can surely benefit from useful and affordable heavy lift. Fortunately, the 2011 NASA budget already takes a useful and affordable approach. It develops an RD-180 class engine made in the U.S. that can be used on a future heavy lift vehicle. Such an engine doesn't require a heavy lift vehicle for its justification; it can be used in other rockets (the Atlas V uses the Russian RD-180). Thus, the budget's heavy lift development approach is inherently useful beyond the HLV. The budget's approach also appears to be affordable, leveraging the existing EELV infrastructure and expertise that will exist and need to be paid for whether or not NASA explores or develops an HLV.
The Flexible Path to the Moon would benefit from propulsion capabilities like the following:
- reusable propulsion for astronaut vehicles to go back and forth between LEO and beyond-LEO cislunar space destinations
- highly efficient (and possibly reusable) propulsion to get cargo from LEO to beyond-LEO cislunar space destinations
- propulsion for (possibly reusable) vehicles to get cargo and/or astronauts to and from the lunar surface
If the Flexible Path to the Moon is taken, propulsion research and demonstrations should concentrate on these crucial areas as a higher priority over propulsion to shorten long deep space missions or propulsion for heavy lift.