Lunar Vision

Once an advanced spaceplane has reached low Earth orbit (LEO), its orbital sojourn is normally limited only by the stores of food, water, oxygen, and maneuvering propellants onboard. Its main propellants have been exhausted, making it essentially an empty winged bulk tank, orbiting just outside the atmosphere. In this condition, the spaceplane's only option, at some point, is to reenter Earth's atmosphere and land.

But consider how dramatically the picture changes when the spaceplane is refueled in orbit instead of on the ground. With an existing kinetic advantage of 5 miles/s (not to mention a potential energy store of some 200 miles altitude), an orbitally refueled spaceplane will have an immediate capability of undertaking any space mission requiring a delta-V of five additional miles per second. Yet Lunar missions (and indeed, all Earth escape missions) require only a fraction of this amount - about 2 miles per second. This opens up huge potentialities: Breaking out

M.A. Bentley, Spaceplanes: From Airport to Spaceport, 149

doi:10.1007/978-0-387-76510-5_10, © Springer Science + Business Media, LLC 2009

of Earth orbit requires only two-fifths the delta-V that it took to get into orbit in the first place. As a result, an orbitally refueled spaceplane would have far more than enough propellant to fly all the way to the Moon and back. The extra propellants can be used to replenish Lunar landing craft, increase the space maneuvering capability of the spaceplane, and even reduce the atmospheric reentry speed on the return flight. The remaining reserves can be used for maneuvering inside the atmosphere, including making missed approaches or go-arounds during the landing phase. All of this adds up to increased safety.

Future spaceplanes will doubtless have the ability to land directly on the Moon, but initially some form of Lunar orbit rendezvous will likely be used. Lunar landers will shuttle back and forth between the Moon's surface and Lunar orbit. Spaceplanes, with their enormous propellant tanks, will refuel the landers in orbit, transfer Moon-bound passengers, and take on Earth-bound ones. Not only will they transport eager space tourists to and from Lunar orbit, but they will also serve as supply vessels for Lunar infrastructure. In this way, spaceline companies will optimize the economics of their operations.

A typical Lunar spaceplane mission will take about 1 week. Let us take a look at how such a mission might unfold. The 50-passenger spaceplane is pulled away from the gate and towed to the end of the runway by ground-based vehicles. This minimizes waste of precious propellant prior to takeoff. Once cleared for departure, the pilots start the air-breathing turborockets, advance the throttles, and release the brakes. The plane quickly accelerates and rapidly thunders skyward, using the atmosphere at this point for most of its propulsion. Within 10 min it has pulled up behind an aerial tanker carrying the heavy oxidizer propellant that the spaceplane will need to make orbit. Aerial propellant transfer of liquid oxygen (LOX) or hydrogen peroxide (H2O2) takes another 10 min, followed immediately by all-engine light-off. The vehicle pitches up to the optimum flight attitude, thereby minimizing gravity and drag losses while maximizing the benefit of aerodynamic lift. In moments, the vehicle has exited the sensible atmosphere and is operating as a pure space vessel. The aerospike engines provide altitude compensation from the upper atmosphere to orbital altitude, squeezing every ounce of efficiency out of the propellants. This, together with the earlier air-breathing portion of the ascent, provides for the critical increase in specific impulse and delta-V required to reach orbit. Within 30 min of brake release, the spaceplane is cruising at an altitude of 100 miles and a speed of 17,500 mph. It has reached LEO. Some 45 min later, upon reaching the 200-mile apogee, the captain circularizes the orbit with another short burn of the engines.

The next phase of the flight involves orbital propellant transfer (OPT), which means rendezvousing with an orbital fuel depot. Regular flights of spaceplane tankers keep the depot supplied on a continuous basis. Station-keeping with the orbital cache, a space robot plugs fuel and oxidizer lines into refueling ports in the spaceplane. A third line transfers pure water. Solar-charged batteries drive pumps to transfer the propellants, and soon the vehicle is ready for trans-Lunar insertion.

For most of the passengers, who are leaving Earth for the first time in their lives, the thrill of breaking out of Earth's gravitational grip and seeing the home planet floating in space is awe-inspiring. The sight of Earth as a beautiful oasis in space, as experienced by millions of future space tourists, will surely change forever humankind's perspective of themselves and their place in the universe. The benefits from these kinds of experiences, repeated many times over for people from around the globe, can only result in a better future for the entire human race.

Now the fun begins. Passengers look forward to 3 days of free-fall on the uphill coast to the Moon, during which they are free to float and cavort about the cabin, gaze through the many portholes at Earth, the Moon, or the stars, and generally accustom themselves to the weightless environment. Every spaceliner has an observation deck equipped with telescopes for the viewing pleasure of all passengers. Through these, everyone enjoys unobstructed views of the heavens.

As the ship nears Luna, cabin chatter is abuzz in anticipation of seeing the far side of the Moon, which is never visible from Earth. All eyes are glued to the portholes as the ship arcs around the Lunar limb and Earth disappears from view. For the first time in their lives, passengers cannot see their world. One of the most conspicuous features of the far side is the huge crater Tsiolkovskiy, named after the famous Russian rocket scientist (Fig. 10.1). Presently, the captain announces that all passengers and crew are to take their seats and buckle in securely for the deceleration maneuver into Lunar orbit. The rocket burn is accomplished by turning the spaceplane around so that it is flying backward, then firing the rearward-facing maneuvering engines. To the passengers, it seems as if they are accelerating forward rather than decelerating backward. The flight crew checks their instruments to ensure that

Fig. 10.1 Tsiolkovskiy Crater on the Lunar far side, as photographed by the crew of Apollo 8. This feature was discovered by the unmanned Russian space probe Luna III in 1959 and named in honor of Konstantin Eduardovich Tsiolkovskiy, father of Russian cosmonautics. The feature measures some 250 miles in diameter (courtesy NASA)

Fig. 10.1 Tsiolkovskiy Crater on the Lunar far side, as photographed by the crew of Apollo 8. This feature was discovered by the unmanned Russian space probe Luna III in 1959 and named in honor of Konstantin Eduardovich Tsiolkovskiy, father of Russian cosmonautics. The feature measures some 250 miles in diameter (courtesy NASA)

the proper orbit has been achieved, and passengers are once again released from the confines of their seats. After seeing Earth-set and the far side of the Moon, the next event is Earth-rise above the desolate Lunar hills below. Cameras click away as each space tourist vies for his or her own version of the famous Apollo 8 Earth-rise picture, taken by the first Earthlings to fly to the Moon.

After circling the Moon perhaps one more time, passengers are once again instructed to take their places and secure their safety belts. It is time for rendezvous and docking with the Lunar shuttle. As passengers crane their necks for a better view, some catch a glimpse of an ungainly vehicle completely lacking any semblance of airworthiness. This is the vessel that will transport them from the sleek spaceplane to the dusty surface of the Moon. As this vehicle draws nearer and nearer, its strange outlines become apparent. It looks more like a crab than anything else, with a short and squat body, six gangly legs, and various antennae sprouting from its arthropodlike midsection. Yet the design is eminently spaceworthy and is intended to provide the greatest level of safety, comfort, and efficiency for operators and passengers alike. Its overall dimensions - short and squat - ensure that it can never tip over, even if it should land on the slope of a crater. Like the spaceplane, it carries 50 passengers and a crew of five. Upon docking, two hatches are opened between the vessels, so that passengers can be transferred quickly from vessel to vessel with a minimum of confusion. A total of 100 passengers are efficiently "transfloated" in a well-practiced sort of space ballet. At the same time as this is taking place, pure water is transferred from the spaceplane to the thirsty Lunar crab. This will be refined into rocket propellants after landing, to be used by this and other Lunar shuttles.

This vision of the near future is a glimpse of a relatively immature Lunar infrastructure. Propellants are being brought from Earth to the Moon, rather than from the Moon to LEO, and Lunar spaceplanes are not yet sufficiently developed to land on the Moon themselves. Nevertheless, Lunar tourism is in full swing, with 50-passenger Lunar shuttles making regular flights to and from Lunar orbit.

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