Space transportation 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.

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 to 2-4 km/s, than a single system accelerating cargo to 8 km/s.

The minimum mass of a system of transportation is defined as the mass required for a 1-ton cargo.

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 acceleration of 9 G. They can withstand up to 14 G when immersed in water.

Earth-to-orbit transportation can be classified into four groups:

  1. Terrestrial transportation either rests on the ground, or floats on a balloon. Most guns are included in this group.
  2. All components of suborbital transportation accelerate from a standstill to orbital velocity. A typical example is rocket launcher.
  3. Orbital transportation orbits the Earth and transfers momentum to cargo carried from the Earth by other means. The simplest example is the electrotube.
  4. Relay transportation combines several elements of the above groups.

Space colonization is a symphony of many instruments. Instead of asking which instrument is the best, we need to ask which contraption is the most suitable for a particular stage of space colonization.

The ice gun is my favorite prelude. The projectile-rocket relay is suitable for intermezzo; it can carry large cargoes unsuitable for the ice gun. The Moon-Earth momentum exchange makes good fortissimo. It can provide both the momentum and 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 only cheap sources of the raw materials are Trojan asteroids, comets, and some small satellites. 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.

Robert Forward compiled a bibliography of low-thrust propulsion techniques intended for interplanetary transportation:

AIAA has published useful books about spacecraft design.

Bibliography of interstellar travel: