Engineering a Spacefaring Society
In some ways, I think that Project Constellation ignores the technological needs of a spacefaring society in the name of budgetary and schedule expediency. Transportation to the moon and beyond will be achieved with brute-force, by launching large rockets from the earth. But if Constellation is truly a marathon (rather than a sprint,) it should focus on the long-term development of the essential technologies which will enable human exploration of the solar system.
My "technologies for a spacefaring society" list is nothing new. Many of the all-time great visionaries in the space business have said the same things that I'm saying here. Nonetheless, the admission of things that require development is an admission that humanity is not ready to truly leave the cradle for good.
The following is a list of eight technologies that I feel are essential to human exploration of our own solar system. Rather than a "moon-first" focus, I'm beginning to feel that Constellation should be redirected towards developing these technologies before we return to the moon.
1) Space Nuclear Power
Albeit a controversial technology, portable nuclear reactors have the capability to make bases on the moon and Mars sustainable without being hostage to the sun (or Martian weather.) While they would require periodic replenishment from earth, space surface reactors are the way to truly power long-term human bases on other bodies of our solar system.
2) Large-Scale Electric Propulsion
This is not a prerequisite for lunar travel, but they certainly make for a more fuel-efficient cruise to Mars, asteroids or beyond. Current electric thrusters have put out very tiny amounts of thrust. We need much larger thrusters for human interplanetary missions.
While solar power is a possible power source for electric propulsion units, the large array size needed to drive the electric thrusters would make a spacecraft more vulnerable to micrometeoroid impacts. For human missions, nuclear reactors are the preferred power source for driving electric rockets.
Electric propulsion isn't useful for human travel to the moon due to their low thrust. My preferred lunar transport architecture uses nuclear-thermal rockets. Without losing too much thrust over chemical engines, they offer almost twice the specific impulse of hydrogen-oxygen rockets.
3) Artificial Gravity
While we don't understand everything there is to know about long-term exposure to weightlessness, we've seen enough to realize that zero-G is largely detrimental to the human body. We still don't fully understand what rotation rates the human body can tolerate while subjected to an artificial-G system. We also need lightweight materials that can build strong structures for artificial-G spacecraft. An open question is whether it's better to subject a person to a slow, constant spin on a large-diameter wheel spaceship, or if an astronaut could spend short periods of time in a very fast centrifuge to counteract the effects of weightlessness.
4) On-Orbit Fueling
On-Orbit refueling has so much to offer for spacefaring societies. It allows us to launch massive spacecraft from earth into orbit, as long as those craft are launched with empty propellant tanks and refueled on-orbit. Propellant stocks make for cheap payloads and increase the demand for earth-to-orbit transportation.
Beyond their use in earth orbit, propellant depots on the moon and Mars will enable reusable transportation to ferry astronauts between space transportation hubs and the lunar/Martian surface. Whether it's a space station at EML1/2 or on the Moons of Mars, it will serve as a useful staging ground for landings and for inbound or earthbound astronauts.
5) In-Situ Resource Utilization
The ability to make propellant on the moon or Mars will save the expense of launching so much propellant mass into earth orbit. The Sabatier reaction and electrolysis on Mars can produce methane, oxygen, and water by simply using Martian atmosphere combined with hydrogen feedstock. Future missions can make use of water and other substances we find on the moon and Mars.
If astronauts are to survive with little or no resupply from earth, they will need to adopt a "live off the land" philosophy. Just as the pioneers of the American frontier learned how to be resourceful with the things they found in the lands where they settled, so too will the astronauts who settle the moon, Mars and beyond.
6) Closed-Loop Life Support Systems
Unless astronauts can carry massive amounts of consumables with them for long space voyages, they will need to close the life-support loop. Practially everything will need to be recycled. That even means having to find a way to recycle the astronauts' poop. It's a dirty job, but NASA or somebody will need to develop a "biodome" capable of sustaining life with a minimum of mass that will need to be replaced.
7) Aerobraking & aerocapture
The ability to use the atmosphere of Earth or Mars to brake large payloads will save much propellant mass in an earth-Mars transportation system. It's essentially like getting a free ride, as long as we can build heat shields and guidance systems that can make aerobraking effective.
8) Reliable, routine transport to earth orbit
This is a major sticking point for a lot of the space pundits. Many people can't get past the idea that expendible rockets are so wasteful. But it's also true that reusable launchers are more expensive to develop and more expensive to operate. In the current paradigm, the best way to provide human transportation for earth to orbit is with simple, throwaway rockets and simple capsules.
As we build simpler rockets and capsules that can reliably increase the demand and ability to put humans in orbit, we'll get closer to the launch rates that will make reusable rockets cost-effective. It may take 50 or more launches per year to make economic sense of a reusable spacecraft. That day may not come in my lifetime, but it eventually will come.
My current thinking on reusable launchers is that a scramjet-powered first stage would be required for a manned spaceship. It would need an assist from the upper stage's rocket engines for takeoff, but it would then accelerate to Mach 12 (approximately) before releasing its "spaceliner" upper stage. The scramjet-powered mothership would be capable of airliner-like operations, as opposed to the relatively maintenance-intensive upper stage. In the system I envision, there would actually be fewer motherships than spaceliners in order to meet the demand for spaceflights.
Epilogue--Growing Up in the Cradle
As Tsiolkovsky wrote, "Earth is the cradle of mankind, but man cannot remain in the cradle forever." While his words still ring true today, it's clear to me that we are still mere babies in this wide universe. We're trying to stick our hands out of the cradle, but we just don't have the strength to pull ourselves up and out. We will only grow up when we invest time, money, brainpower, willpower, and patience (most importantly) in the tools that will make us strong enough to rise from the cradle.