Our dying planet can no longer support the exploding human population. Wars, plagues, and famines may bring temporary relief, but only a massive colonization of outer space will provide a long-term solution. Our technology is well-suited for space, except for Earth-to-orbit transportation. One does not have to be a rocket scientist to know that modern rocket launchers are too expensive. This publication describes cheap, reusable rocket launcher called rocket cluster, and very cheap alternatives to the rocket launchers.
I like my old electric razor. In addition to being a paragon of durability, it is cheaper than disposable blades and requires less maintenance. The razor works well in the benign setting of my bedroom, but it wouldn't last long in a rocket exhaust plume! Refractory tiles and shock-absorbing suspension could prolong its life but at a cost far exceeding the price of the razor. What is true of my razor is also true of any contraption, including space transportation: it must be reusable to be economical, and it cannot be reusable, unless it operates in a benign environment and has a fail-safe design. The most common space hazards are: ablation, vibration, corrosion, thermal fatigue, radiation, and atomic oxygen erosion.
Earth-to-orbit transportation can be classified into four groups:
Some contraptions subject cargo to such an extreme acceleration that they are unsuitable for people. Gravitational acceleration on the Earth's surface equals g = 9.8 ms-2. Fighter pilots pass out in the acceleration of 9 g. They can withstand up to 60 g when they are immersed in a liquid, and their lungs are filled with oxygenated saline solution.
A system of transportation which accelerates cargo to 8 km/s demands either the use of extremely hot propellant, or a long and massive system, or a system which subjects cargo to extreme acceleration. It is easier to design a relay of independent systems, each accelerating cargo by 1-4 km/s, than a single system accelerating cargo to 8 km/s. In the short term, the relay, sometimes called launch assist, makes much more sense than eliminating rockets in favor of an untested contraption.
Guidance systems used in most rocket launchers rely on inertial navigation systems which include gyroscopes and accelerometers. Some inertial navigation systems used in aircraft do not work well when exposed to the high acceleration of the rocket launcher. Global Positioning System (GPS) is difficult to use in private rocket launchers because all commercial GPS receivers must comply with Missile Technology Control Regime (MTCR) guidelines and therefore they must shut themselves down when they compute that they are traveling faster than 515 m/s or are at an altitude higher than 18 km.
Space colonization is a symphony of many instruments. Instead of asking which instrument is the best, we should ask which contraption is the most suitable for a particular stage of space colonization.
My favorite prelude is the helicopter-rocket relay. Engine cluster combined with terrestrial bolo is suitable for intermezzo. The Moon-Earth momentum exchange makes good fortissimo. It can provide raw materials for the first, cramped space colonies.
Pioneers may not mind existing in a "tin can ," but others would prefer living in a garden. To build the garden, i.e., orbital greenhouse, we have to mine and process huge quantities of raw materials. The Moon is devoid of volatiles, so we have to get them from Trojan asteroids, comets, and small objects in the outer solar system. They can be brought to the vicinity of the Earth with a combination of solar-thermal propulsion (solar boiler spewing steam) and gravity assist from Jupiter. Humans packed in the "tin cans" can then reach the diverted asteroids with the help of solar sails.
The largest collection of space links: http://www.hobbyspace.com/Links/index.html.
The most extensive publication about interplanetary and interstellar transportation: Advanced Propulsion Concepts (APC) Notebook Online, posted at http://sec353.jpl.nasa.gov/apc/. (If you can not use this publication, ask Grace Fisher-Adams for explanation. She is Associate Intellectual Property Counsel for Jet Propulsion Laboratory, 4800 Oak Grove Drive, M/S 180-305, Pasadena, CA 91109, phone: (818) 393-4612, fax: (818) 393-4621)
Dani Eder also compiled a bibliography of space transportation: Canonical List of Space Transportation and Engineering Methods
The best overview of electric propulsion for interplanetary transportation: M. Martinez-Sanchez and J. E. Pollard, "Spacecraft Electric Propulsion: An Overview," Journal of Propulsion and Power, Vol. 14, No. 5, September-October 1998, pp. 688-699.
Robert Forward compiled a bibliography of low-thrust propulsion techniques intended for interplanetary transportation:
Bibliography of interstellar transportation.
AIAA has published useful books about spacecraft design.