Airplanes are economical for multiple reasons. First, their designs have continually been improved and perfected over the last century. Second, they are produced in large numbers, increasing their affordability for the private and commercial sectors. Third, they use the atmosphere in three different ways, as we will shortly see. Until recently, the only organizations that have launched and operated spaceships are national governments. Not surprisingly, spaceships have so far been neither economical nor affordable. The imminent Private Space Race is about to change all this, however. It is a very significant point, not to be overlooked, that the first reusable spaceships - the Space Shuttle and SpaceShipOne - are both spaceplanes.
The single most important factor that makes the air transport industry possible is people - lots of them. So it will be with space travel. A large number of space tourists will eventually drive down the cost of space access, improve the quality of space vehicles, and create affordable economies of scale for those that follow.
Spaceplanes will use the atmosphere to their advantage, instead of regarding it as a barrier to be crossed. Like all aircraft, and unlike ballistically launched rockets, spaceplanes will use the atmosphere's inherent lifting properties, its oxygen content, and its propulsive potential. Ballistic missiles and rockets do none of these. Hybrid engines incorporating elements of the turbofan and the aerospike can be designed into the successful spaceplane. Such "spinning aerospikes" will therefore be able to use bypass air in the atmosphere as propellant, and employ automatic altitude compensation, ensuring peak efficiency all the way into orbit. High-altitude oxidizer transfer will be another technique spaceplanes will use to reach low Earth orbit. Spaceplanes need not take off with heavy loads of both fuel and oxidizer onboard. Because liquid hydrogen - the most energetic rocket fuel available - has a very low specific weight, spaceplanes may take off fully fueled with LH2 but with empty oxidizer tanks. At some 50,000-ft altitude, the heavy oxidizer - liquid air, hydrogen peroxide, or liquid oxygen - will be transferred from an aerial tanker to the spaceplane, which will then immediately accelerate into orbit from this altitude.
Once in orbit, spaceplanes will replenish both their fuel and oxidizer tanks from an orbiting propellant depot supplied by regular spaceplane sorties. With fully replenished tanks, and with a required delta-V of only two additional miles per second on top of the 5 mps already attained for low Earth orbit, the spaceplane will have the capability of leaving Earth orbit. The spaceplane may now become a Lunar shuttle, landing on the Moon with relatively low-thrust ventral rockets. Returning to Earth after a quick Lunar turnaround, the Moonplane will make a direct reentry into Earth's atmosphere at 25,000 mph and land at any spaceport, to be refueled and reflown in a few hours.
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