Advances in aviation are nearly always combined with advances in propulsion technology. Better, more efficient engines allow greater speeds or ranges while consuming less fuel. This continual quest for greater efficiencies extends to spaceflight as well. Space launch vehicles have historically carried all their propellants with them, relying on the staging principle to achieve orbit. Berating the atmosphere during launch, regarding it as some sort of fiendish barrier to be crossed as quickly as possible, they nevertheless embrace it on the return journey, thankful for its free drag, which slows the spacecraft down prior to landing. Without this blanket of air around our planet, manned spaceflight would be far more difficult, as all returning vehicles would have to carry as much propel-lant to slow down as they now carry to accelerate spaceward in the first place.
It is clear that competent spaceflight begins with efficiently traversing the atmospheric curtain that envelops our planet. With this in mind, let us do a survey of some advanced propulsion concepts that may power spaceplanes of the future.
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