CHEAP ACCESS TO SPACE
As of this writing, it costs about $10,000 a pound to put cargo, human or otherwise, into low earth orbit. Unless the costs drop by 2 to 4 orders of magnitude, space colonization just ain't gonna happen, especially for you and I. The cost of space travel won't drop if it primarily remains a domain of government sponsored spacecraft and program expenditure. To that end, I believe the government needs to do what it can to encourage private enterprise to get involved in space exploration.
I don't know how long microprocessors (originally developed for ICBMs
and the space program) were around before someone figured out that they
could be used for personal computers (circa 1976). Ten years after
that, everything changed. Then the Internet became available to the
masses, and so here we are with a completely new economy staring us in
the face. One thing that made the microcomputer possible was that
the infrastructure for microcomputers showed up at about the same time
as the microprocessor technology. This is probably true for all technological
breakthroughs and I suspect cheap access to space is the same way.
FAA+NORAD=SPACE TRAFFIC CONTROL
Something like the FAA will eventually be necessary to coordinate launch
windows, orbits, airspace over launch sites, etc. Right now, there's
a lot of junk orbiting the earth that spacecraft need to avoid. When
space travel goes private, somebody is going to have to provide information
on where it's safe to orbit. At 18,000 mph, a spacecraft has little
or no time to avoid a collision with a screw, bolt, satellite, or other
spacecraft traveling on an intercept course. I don't think automated
collision avoidance hardware will make much sense in an earth to orbit
vehicle when such speeds are involved. On the other hand, NORAD tracks
most of it now. So why not marry NORAD and the FAA together?
ACCESS TO PROVEN TECHNOLOGY
Since taxpayers pay for the space program on a yearly basis, I think
the technology know-how used by the space program should be made available
to any citizen who wants it. Plans for the entire apollo space craft,
source code for every successful space probe, design guides for every space
craft flown, etc. Obviously, there are national security issues,
but not many. Any way, most of NASA's hardware is several years behind
the cutting edge of commercial hardware due to radiation hardening requirements.
ROBUST FLIGHT TECHNOLOGY
Robust rocket engine designs need to be worked out so that engines do
not need to be overhauled or scrapped after each launch. There will
have to be some basic engine workhorses. Ones that use a standard
fuel/oxidizer mix, ones that are restartable, ones that are easily maintainable,
and don't cost millions of dollars each. About standard fuel, jet
airplanes use one standard aviation fuel. This means airports don't need
to keep a warehouse of 32 flavors of fuel standing by to keep planes running.
In the same way, I suspect rockets will need to limit themselves to one
or two standard rocket fuels/oxidizers to keep rocket travel economical.
Also, rocket fuels need to be relatively safe. Some fuels in use
today are quite toxic and in my opinion should be avoided. Also,
I would prefer a renewable source of fuel, such as hydrogen (not my first
choice) or some ethanol or methanol derivative, and either liquid oxygen
or something like hydrogen peroxide as an oxidizer.
THE RELATIVE NEED FOR SAFETY
Spacecraft will eventually have to be safe for the public. It is doubtful that the absolute safety of the first private rocket passengers can be guaranteed, but that shouldn't be used as an obstacle to developing private spacecraft. When the airlines first came out, the planes weren't always safe, but some people still traveled by airplane anyway. Certainly companies and individuals used airplanes for their businesses long before the planes were proven safe. Sadly, many of the safety precautions we take today were derived from tragedies in the past.
Obviously, rocket manufacturers and engineers must be dilligent in avoiding
bad design or taking shortcuts with safety, but one should expect that
the road to safe, regular, inexpensive rocket travel will also take its
toll in human lives. We must come to terms with that reality and
neither be surprised nor discouraged when accidents happen.
SINGLE STAGE TO ORBIT
Rocket boosters will need to be recyclable, or they can't be used.
Keep It Simple Stupid will need to be the mantra of spacecraft in the future.
I think the only way to do this is to make single stage to orbit rockets
work. From the looks of it the Roton,
by RotaryRocket, looks like a very strong contender for the first SSTO
craft. This spacecraft goes up as a rocket and comes down as a helicopter.
This approach answers a whole set of problems related to vertical takeoff
and landing. The technical explanation on the RotaryRocket website
is thorough and makes sense. The lightcraft
is another unusual approach to SSTO. It looks very promising too,
especially since it uses very little on-board fuel to reach orbit due to
it's novel beamed energy propulsion system. The lightcraft currently
exists as a small mirror surfaced probe that has been propelled into the
air using a pulsed, high power laser. The mirrored surface tightly
focuses each laser beam pulse into a ring shaped zone in the air behind
the probe, which literally causes the air to explode. In theory (and
in practice, using a small probe) it works pretty well. However,
it has been proposed that a full sized craft would be accelerated into
the air at such high acceleration, that passengers would be required to
be suspended in a fluid bath just to survive takeoff. I'm not sure
the general public will be too enthusiastic about such an approach.
SPACE ELEVATORS AND TETHERS
Assuming cheap access to space is working, there are many mechanisms that make travelling through the rest of space easier. One such mechanism is the space tether. A space tether is just a very long fiber cable, perhaps hundreds or thousands of kilometers long. A space elevator is a type of space tether that hangs perpendicular to its orbit around a planet due to gravity gradient effects. Although the center of mass of the elevator orbits in the conventional manner, the end of the tether closest to the planet travels at far less than the orbital speed for it's altitude, the far end of the tether travels faster than the orbital speed for it's altitude. Since spacecraft can dock with the bottom of the elevator at velocities far below orbital speed, the spacecraft can save substantial amounts of propellant. The space elevator is then used as a vertical railroad, transporting cargo from a disadvantageous orbital speed/altitude to a highly advantageous speed/altitude. Meanwhile, masses at either end of the elevator keep the line tight and stable while electric hoists raise and lower the cargo. As long as the movement of mass up the elevator is about the same as the amount of mass that is moved down the elevator (traffic in both directions is about the same) the system requires no additional amounts of fuel to stay in orbit. Since the hoists on the elevator use solar electric power, no fuel is needed to move spacecraft and cargo from one orbit to another. This means of transportation has been suggested for an orbiting earth elevator, but there are substantial technical hurdles to overcome to make this work in earth orbit. A space elevator could be also be useful (and much simpler to construct) in a lunar orbit, due to the weaker gravitational field of the moon.
Another technique is to use a rotating space tether, as has been suggested by Dr. Robert
Forward of Tethers Unlimited Inc. The tether rotates in
the same direction as it's orbit around the planetary body. This way, when the end of the tether
is closest to the planet, it is travelling far below orbital speed, and when the end of the tether
is farthest from the planet, it is travelling far above orbital speed. By rotating, this kind of
tether can be much shorter and therefore much less expensive than the space elevator. In the case of
a rotating lunar tether, it is possible that at it's closest approach to the moon, the end of a rotating tether
could momentarily be stationary above the lunar surface, cargo released or picked up at this point would
require drastically reduced amounts of fuel, as compared to conventional orbitting techniques.
USING MASS DRIVERS IN LUNAR LAUNCHES AND LANDINGS
This is an idea that evolved from thinking about how useful a mass drive would be in launching cargo from the moon. It became apparent that the same mass driver that delivered cargo to lunar orbit could, with a little modification, be used to launch spacecraft into lunar orbit using only electric power. With no air on the moon, a maglev train car could be constructed with a large flat platform onto which a spacecraft could be attached. At the appropriate speed (about 8000 kph/5000 mph), the spacecraft would be released and (because of the curvature of the lunar surface) would coast into orbit using very little fuel (only a little fuel would be used to circularize the orbit as necessary). Meanwhile, the mass driver/maglev train would rapidly decelerate to a stop. Assuming the mass driver accelerates and decelerates at a constant 1g, the train would need about 565 km of "track." One important point to note, because the train would be operating at or above lunar orbital speed or even escape velocity, it would be necessary to have parts of the train essentially magnetically clamped to the rails to prevent the train itself from attaining lunar orbit!
Maglev Takeoff. Almost fuel-less access to low lunar orbit.
The inverse of maglev takeoff, maglev landing, might also be possible. A spacecraft could follow an elliptical orbit down to the surface of the moon, in line with the mass driver track. At the appropriate time, the mass driver platform would be accelerated to match speed and position with the spacecraft overhead. The spacecraft's orbit would be planned to just touch the surface of the maglev train. A landing hook on the rocket, or a net on the platform would be used to secure the spacecraft to the platform. Once secured, the platform would decelerate to a stop. This would be sort of like a navy aircraft landing on an aircraft carrier, with the difference being most aircraft and aircraft carriers aren't travelling at 8000 kph on their final approach. Without an atmosphere, this could be possible. Of course, there would be the need for many safety precautions. In an accident, it goes without saying that the results would most certainly be fatal. On the other hand, any passenger on the ill-fated spacecraft, wouldn't have time to notice.
Maglev Landing. Undoubtedly a barrel of fun for a naval aviator.
There are highlands on the back side of the moon that would seem to
be the ideal place for a maglev train like this. The Apollo flights
always circled around the backside of the moon prior to firing their engine
and achieving lunar orbit. Essentially the same path would be taken
by a ship arriving from earth. The difference in the maglev landing
scheme is that there would be no decelerating burn to put the spacecraft
into lunar orbit. Instead the train would catch them. Similarly,
takeoff from the moon using the maglev launch follows the same Apollo style
orbit. Thus a craft would only use fuel to leave earth orbit, perform
(hopefully) minor orbital corrections, and perhaps to slow down on it's
way back from the moon.
LUNAR-BASED SPACECRAFT PRODUCTION
With a reasonably gentle launch option available from the moon (maglev
launch), it would be reasonable to consider spacecraft assembly on the
moon. One advantage that a planetary body has for construction work
is that it can be alot easier for people to work in a gravity field than
in the zero gravity of orbit: traction is an option; tools, materials,
and people slowly drop to the ground, they don't drift off to become deadly
projectiles for some later flight; resources don't have to be shipped up
to the orbital construction site, instead they can be manufactured in situ;
workers can go home after the job is done for the day; etc. Even
delicately engineered rockets could be built on the moon, constructed directly on
a train launch platform, then gently heaved into orbit when they are complete.
Rockets with nuclear engines might become the 'hot' product for loonies.
Far from earth and her sensitive environment, nuclear engines could
again gain popularity, as they are an order of magnitude more efficient
(and probably much safer) than chemical rockets.
Space colonization will not happen until space travel becomes relatively affordable: afordable for you and I. For space travel to become affordable, private companies and entrepreneurs need to be involved in the space travel business. We need to make sure our government is assisting in this effort. NASA should offload much of their current low earth orbit launch activities to up and coming space transportation companies, and NASA needs to stay out of the commercial space launch business. Additionally, it would be helpful for NASA to share their technology with any citizen who thinks they can use it. Eventually, the earth's 'airspace' will need to be managed by some kind of traffic control system, and the release of 'space junk' will need to be prevented. Some kind of combination of the talents of the FAA and NORAD would be the likely model for such a system. Aids to orbital navigation will also be important as traffic increases.
Undoubtedly, space colonists will come up with their own novel solutions
to their space transportation needs. It can be regarded as certain that
our chrystal ball holds no monopoly over the future of space travel, in truth
we have no idea what the future will bring for space colonization and space travel.
Who knows, in the next ten years, someone could come up with impulse engines
and warp drives, and the whole discussion about the future of fuel guzzling
rockets, dangerous moon catapults, and spinning space tethers will be elegantly
finessed, but I for one am not betting on it.