Perhaps the reason you are reading this book is because you want to go into space. And you want to enter space in style. You do not want to be lobbed, like a human cannonball. You do not want to ride piggyback on some ballistic behemoth. You do not want to be plastered into your seat like a piece of silly putty. In short, you do not want to be thrown into space. You want to be flown into space, in first-class comfort. OK, then, read on and find out how your wish may just come true.
Before a child can run, a toddler must walk. Before a toddler can walk, a baby must crawl. And before crawling, the infant must be carried everywhere. During this long period of growth and learning, every child eventually gets to ride piggyback, perching on the shoulders of a parent or clinging to the back of an older sibling. Farm children actually have more fun, riding on real pigs, for example. My brother and I did this. Contrary to popular opinion, pigs are amazingly clean animals when they escape their muddy sties; and they do not mind being ridden by skinny farm kids. They will root up the neighbor's lawn, however, if they should leave the property. We also threw chickens off the barn roof, to test their flight capabilities. These experiments were reasonably successful; the hens made more or less controlled landings despite their less-than-ideal lift-to-weight ratios and their utter lack of prior flight experience. Domestic ducks do better, but you still have to throw them. Ah, but I digress.
The X-15 rocketplane (1959-1968) and the Space Shuttle (1981-2010) are the two most successful spaceplanes to date, with a combined total of more than 300 flights into suborbital space and low Earth orbit. Neither of these vehicles could attain orbit by itself. The X-15 had to be carried aloft (Fig. 9.1) and then crawled into suborbit, while the Space Shuttle has always ridden piggyback to reach orbit.
What are the inherent limitations of these two test programs? In the case of the X-15, the little rocketplane simply did not have enough propellant to reach orbital velocity. Its top speed was 4,520 mph, only a quarter of the required orbital speed of 17,500 mph. Although it exceeded 50 miles altitude on 13 flights and reached 100 km on two of these, the baby spaceplane did so only at the expense of all its forward momentum. The space velocity of the X-15 at apogee was zero every time, having traded speed for altitude. Gravity always brought the little rocketplane back to Earth, converting its potential energy of altitude back into the kinetic energy of high speed. Compared with the mature spaceplanes of the future, the X-15 research aircraft was barely able to crawl into suborbit.
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In the case of the Space Shuttle, it has to cling to the back of another vehicle to have any hope of getting to where it wants to go. Instead of carrying its own rocket propellants, it relies on a huge throwaway external tank and two huge Roman candles - the powerful solid rocket boosters. The X-15 could not carry enough fuel, while the Shuttle carries cargo instead of fuel, and rides piggyback on its own pro-pellant tanks to get around the problem (Fig. 9.2) . The Space Shuttle could be described as a strapping juvenile - too big for its britches - that sometimes gets itself into more trouble than it can handle.
Using the X-15 and the Space Shuttle as benchmarks, is it possible to design a viable single-stage-to-orbit spaceplane that would take off under its own power from a runway and accelerate into space? This problem is not trivial, presenting enormous challenges to the aerospace engineer. In fact, the problem of reaching low Earth orbit from the ground with a fully reusable one-piece spaceship is likely the most difficult problem in astronautics. To fully appreciate the problem, it is useful to look at the history of conventional aviation development.
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