The main issue is storability. Hydrogen-oxygen rocketry gives the best performance, but only water is storable over long periods of time. The hydrogen and oxygen would best be made from the water shortly before use, and can be used in fuel cells as well. A variety of alternative, storable fuel propellants can be made from asteroidal materials for various applications, including methyl alcohol and hydazine. Oxidizers other than oxygen can include hydrogen peroxide, nitrogen tetroxide and nitric acid.
Solid propellants based on atomized metals are also feasible.
Fuel propellants for long range interceptors Long range interceptors will be mostly fuel by weight. They must accelerate quickly and maneuver to intercept ballistic missiles and antisatellite weapons. It's possible that today's aircraft carriers and long range bombers could be replaced with space based interceptors which simply fire to zero out their orbital velocity and fall straight down, as gently and stealthily as desired. The closer a platform can get to the surface of the earth, the better it can get reconnaissance information, communicate with small receivers, and intercept tactical ballistic missiles. However, the lower a platform goes, the greater the drag of the earth's atmosphere and thus the need for considerable stationkeeping fuels.