Do spaceplanes have a future farther out than the Moon? We have seen how spaceplanes will serve both as tour vessels and as tankers, thereby fueling the spacelanes between Earth and Moon. Can the same logic justify a spaceplane infrastructure on the much longer, interplanetary routes? Similar to the shorter Terra-Luna runs, the interplanetary spaceways will be traveled by good spaceships that need to refuel periodically as well. The places they do this will obviously be their points of departure and arrival. We will limit the discussion to the Earth-Mars run for now, but the same logic would apply to any destination in the Solar System.
To fully understand the dynamics of interplanetary infrastructure, it is important to understand how good spaceships will arrive. Both Earth and Mars have atmospheres, which good spaceships will use for deceleration upon arrival. The atmosphere on Mars is very tenuous - only about 1% of Earth's - so it is less effective for aerobraking than Earth's own thick veil. This is not as much of a problem as it might seem, because spaceships arriving at Mars from Earth will have a relatively reduced velocity due to their uphill climb out of the Sun's gravitational well. They will be moving slower when they reach Mars than Earthbound spaceships from Mars will be moving when they reach Earth. Also, the Martian gravity field is substantially weaker than Earth's, and so Mars gravity will accelerate an arriving spaceship less than is the case with Earth. Third, spacecraft arriving at Mars encounter the atmosphere at a greater altitude, because it extends farther into space than Earth's. All of these factors tend to offset the fact that Mars's thin atmosphere seems incapable of decelerating a spaceship. The upshot of this discussion so far is that both the Martian and the Terran atmospheres are useful for slowing down arriving spaceships. This means that they must be shaped aerodynamically (Fig. 10.12).
Similar to the Lunar spaceplane, a good interplanetary spaceship will be reusable, and therefore it must be able to land anywhere and refuel. Landings on Earth require wings to generate lift before arrival at the spaceport. And landings on Mars require an aerodynamic shape for deceleration, in addition to ventral thrusters to make a VLHA landing. The alternatives dictate that the spaceship becomes some sort of partially reusable or nonreusable module, much like Orion or Apollo. This discussion concerns only good spaceships, however, which rules out modules completely.
Mars is replete with resources, both in its atmosphere and underground. Rocket propellants can be manufactured on Mars from either of these sources. When the good spaceship arrives on Mars, or in Martian orbit, the first thing it will do is fill its propellant tanks. Martian gravity is twice the Lunar, but still only a little more than one third of Earth's.
Fig. 10.12 Properly designed advanced spaceplanes will be able to aero-brake in the atmospheres of Earth, Mars, Venus, and eventually atmosphere-enshrouded moons such as Saturn's Titan (courtesy Reaction Engines Limited)
Departing from Mars, the spaceplane can use the atmosphere for lift and working mass, just as it does on Earth, even though the Martian air contains no oxygen. Two of the three components in flight and fuel management, therefore, still apply. To generate the required lift in the tenuous atmosphere of Mars requires faster speeds, but again, the gravity field is weaker, and so only a fraction of the lift required on Earth is needed on Mars. Some arrangement of air-augmented or rotating turborocket engine will also allow the Martian atmosphere to serve as working mass, just as turbofan engines regularly use bypass air for a large percentage of thrust on Earth. These considerations show that spaceplanes are beneficial in leaving the surfaces of planets such as Mars, with very thin atmospheres.
Arriving on Earth, the interplanetary spaceplane will have an atmosphere entry velocity about the same as the Lunar spaceplanes encounter, some 25,000 mph. At this point, all of the arguments in favor of the Lunar spaceplane apply also to the interplanetary spaceplane. Aerodynamic design slows the ship down, wings or lifting bodies allow the ship to navigate safely to a spaceport, and full reusability allows it to park on the ramp and refuel. It is also possible for the returning Mars spaceplane to aerobrake through Earth's upper atmosphere, reenter space on an ellipse that intersects the Moon's orbit, and land at a Lunar colony. Again, the spaceplane shows its amazing versatility and practical potential.
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