Several of the abortive man-rated vehicle concepts were overly ambitious given the state of the art. I was in love with the National Aerospace Plane (X-30 NASP) but the technology just didn't exist to make it happen. The X-33 was to be a technology demonstrator for a bigger VentureStar, but it suffered from too many problems to ever fly.

We need to look at it from a broader perspective.

1. We need to ability to launch heavy payloads into Earth orbit. You don't need to send along people for this, so a man-rated vehicle isn't necessary. You could launch small modules (up to 51,000 pounds) on the Delta IV Heavy or Atlas V Heavy vehicle that are about to be tested. You could launch much heavier payloads using Shuttle derived technology and have that capability in place in a couple years.

2. We need the ability to launch people into orbit. There are some critical questions that we don't know the answer to yet, such as how many people, into what kind of orbit, for what mission profiles, and for what duration. You may be able to launch about 4-6 people in a capsule using a Delta IV Heavy or Atlas V Heavy EELV. This vehicle wouldn't have to be any more advanced than a Gemini capsule, just a bit bigger with better systems. Designed to the purpose, such a vehicle could be flying in a few years. If you need reusability, then the time and cost to built it will increase, but perhaps the increased costs can be recouped during operations.

Eventually, we'll need to get the Lunar and later Mars vehicles into orbit. They may have to be launched in pieces like we did with the ISS and assembled in orbit. It might take multiple launches to carry the propellants and other expendables up to the vehicle, although there's a glimmer of hope at being able to use some lunar resources to help in that regard, at least in the long term.

My dream is for the vehicle used to fly between the Earth and the moon (and later Mars) to be reusable, with post flight refueling and servicing taking place in Earth orbit. Creating reusable landers for the moon is difficult but should be possible. Reusable landers for Mars could be a lot more difficult due to heat shield issues.

As I've written before, I'd like for the transit vehicle to be able to leave Earth orbit, fly to the moon, and enter lunar orbit with a sizeable payload, say 50,000 pounds. For the first mission, that payload would be the lander and critical surface equipment such as a Transhab-type module. The lander would carry the payload to the lunar surface and leave it there, then return to lunar orbit when finished. It would be left in lunar orbit while the transfer vehicle returns to the Earth orbit to prepare for the next trip. Subsequent missions would carry additional materials for the lunar surface and propellant for the lander.