How will future spaceplanes utilize their inherent strengths to enter space? Will they assault the problem with brute force, as ballistic rockets do? No! Instead, they will use a flanking maneuver, hitting the problem sideways in a clever, more competent fashion. At the core of every spaceplane, of course, will be its efficient and able engine.
The key to the advanced spaceplane will be advanced propulsion technology, new, lightweight engines that will propel us into the future. As we have already seen, these engines will be air-breathing hybrids - part turbojet and part rocket -able to function as efficiently in the atmosphere as they do in space. Yet these new engines are only part of the strategy in out-flanking the future. A proper ascent profile through the atmosphere is also required.
There are many ways to enter space, and just as many ways to return. There is the tried and true vertical launch with ballistic landing capsule (Fig. 9.15), demonstrated from 1961 through 1975 in the US manned space program, and soon to be rehabilitated. There is the vertical launch with horizontal landing (Fig. 9.16), as in the Space Shuttle. And there is the horizontal takeoff and horizontal landing, the airplane approach of the advanced spaceplane. In the first two methods, the launch vehicle punches through the atmosphere, then arcs over to build up enough speed to stay in orbit. But what ascent profile might the advanced spaceplane use?
It turns out that the advanced spaceplane's ascent trajectory will be inextricably linked to its onboard power plant - the air-breathing turborocket. Working more
like a turbojet at takeoff and at low altitudes, the advanced spaceplane will depart the runway much like any ordinary airliner. The flight profile in the first part of the ascent to orbit will therefore be very conventional. A small amount of onboard fuel will be burned with a large quantity of air, and this will serve as the working mass during this stage of the flight. The duration of this phase will depend on whether the spaceplane is designed to collect, liquefy, and store air before accelerating into space, whether it "refuels" in mid-air, or whether it has taken off with all the propel-lants it needs to reach orbit. Either way, at some point the spaceplane will make a relatively rapid acceleration, get above the atmosphere, and enter orbit. It will remain in the atmosphere as long as there is a benefit to doing so, but as soon as the air becomes too much of a "drag," it will be time to get out.
The lift generated by the advanced spaceplane will completely cancel gravity losses as long as it operates inside the atmosphere, and drag will be overcome by sleek design and by exiting the atmosphere before the speeds get too high. Thermodynamic - or heat - loads will be kept down the same way as aerodynamic loads, because both arise together. Let us now take an even closer look at the advanced spaceplane's bag of tricks.
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