There are three distinct uses for the Lunar spaceplane: (1) shipping propellants to the Moon, (2) shipping propellants from the Moon, and (3) transporting passengers and bulk cargo both to and from the Moon. The third use can be combined with either of the others.
The first use assumes that no Lunar ice deposits have been discovered. To keep Lunar shuttles operating between the Lunar surface and Lunar orbit, propellants will have to be transported from Earth. This will be expensive, but the alternative is to build a new Lunar spacecraft for every mission. Without a constant and reliable source of propellants to support Lunar vehicles, operations cannot be sustained. How, then, will we get propellants to the Moon? The first step is to use a series of spaceplane flights to fill a LEO depot with a propellant supply. A determination will have to be made about what types of propellant are best. The best solution is to ship ordinary water to the depot, and then to the Moon. Once there, it can be electrolyzed into its constituent rocket propellants: liquid hydrogen and LOX. The reason for this strategy has to do with the density of water compared to the density of rocket propellant. Water takes up far less volume. In addition, it is far safer to transport in its inert, nonexplosive form. Finally, transferring water from one spacecraft to another is easy and risk-free.
This raises the question, could a Lunar infrastructure be supported by simple rocket tankers instead of spaceplanes? Could such tankers be used to resupply the orbital depot, or even Lunar spacecraft directly? The answer is no, unless the rocket is also a reusable launch vehicle. Barring this, these would be one-time missions, and cost-benefit analysis exposes this idea as unworkable. The Lunar spacecraft could be reused by refueling it from a rocket tanker, while the wingless rocket tanker itself would be a worthless hulk as soon as it had delivered its propellants, and so the net gain would be zero. Rocket tankers with no wings could never reenter Earth's atmosphere and never be reused. When it comes to future space infrastructure, it is apparent that winged vehicles are just as important in space as they are on Earth.
The second use assumes that water ice has been discovered on the Moon, and in appreciable quantities. Well, this changes the picture completely. The Moon now becomes the key to space access itself, but again, only with an infrastructure based on winged space vessels. Lunar spaceplanes will ship Lunar ice, in the form of purified water, to LEO propellant supply depots. They will accomplish this by entering the Earth's atmosphere at an initial speed of 25,000 mph and aerobraking to 17,500 mph, relying entirely on their own aerodynamic designs and the atmosphere to achieve this delta-V. Supplies of Lunar water can be delivered to LEO "gas stations" in this manner, where they can be electrolyzed into useful propellants by solar energy. Spaceplanes arriving at these depots from the ground will refuel, and continue on to the Moon with their passengers, relief crews, and bulk cargo, returning in a few days with another tank of Lunar water. In this way, every spaceplane customer contributes to an efficient Lunar transport infrastructure.
But why ship water all the way from the Moon instead of just bringing it up from Earth? After all, the ground is )iterally a thousand times closer to LEO than the Lunar surface is, and there is a lot more water on Earth than on the barren Moon. The delta-V required to go from the Moon to LEO is negligible when compared with the DV needed to get from Earth's surface to an orbit 200 miles above. The most difficult aspect of spaceflight is that first 200 miles and, more important, that initial 17,500 mph. It is the difference between coasting down a long, steep hill and climbing a tall mountain. It is always easier going downhill, because gravity is doing all the work. The same is true in the case of Lunar spaceflight.
Modular Moon missions represent an extension of rocket staging, at extreme altitude. It takes three stages to get the space vehicle out of Earth orbit, and three more to reach the Moon. Modular Moon missions are therefore far more costly in terms of operational complexity than are spaceplanes. While the spaceplane simply fills up with liquid propellants in LEO and flies to the Moon, both Apollo and Orion depend on no less than six separate rocket stages, each with their own engines, tanks, interstages, propellant pressurization systems, etc. Moon missions are thereby accomplished only by throwing off each stage when it runs out of propel-lant and continuing on with the remaining fully fueled stages. After the three boost stages are expended and the spacecraft is on its way to the Moon, there remain three stages, or modules. These are the Service Module, the Lunar Descent stage, and the Lunar Ascent stage. In the case of Apollo, the Service Module had the task of getting the spacecraft both into and out of Lunar orbit, while the Lunar stages were responsible for getting two astronauts to the surface of the Moon and back to Lunar orbit. In the case of Orion, the large Lunar descent stage will get the spacecraft into Lunar orbit and land on the Moon. The Service Module's only job is to get the crew out of Lunar orbit for the ride home. In both Apollo and Orion, the ascent stage has one purpose only, and that is to transport astronauts and about 100 kg of rocks from the Moon to the orbiting crew module. As each stage is abandoned, the capabilities of the remaining spacecraft diminish accordingly. The system therefore has no more versatility than the capabilities of any one component. By the time the spacecraft has been reduced to conical crew-return capsule, its only capability is to protect the astronauts during the searing high-G reentry.
Spaceplanes, on the other hand, are good spaceships, because they are versatile and capable. There are no throwaway parts, and nothing is unnecessarily repeated. Because of their huge propellant tanks, they can burn their engines longer and therefore transport greater payloads, provided they can be refueled in space. And they can be, because they are part of the refueling infrastructure. The spaceplane is the space tanker for all good spaceships, including itself. It does not have to splash down under a canopy of parachutes, or rely on an armada of Navy vessels for recovery. It just lands at a spaceport and refuels, as any good plane.
Missiles and modules worked with Apollo, and the concept will work with Orion just as well, probably better. There is a small amount of reusability built into the system already. But without any resupply of propellants in space, the modular Moon mission is unsustainable. Reusable spaceships are the key, but reusability requires refueling, and refueling requires propellant transfer. Space tankers are therefore necessary for a long-term Lunar base to be feasible, practical, and safe.
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