Rockets are typically composed of two or more stages, or segments, designed to fall off as they use up their propellant. This greatly reduces the weight of the remaining vehicle to be boosted into orbit, and is a highly effective means of achieving orbital speed. The rocket equation is applied as many times as there are stages on the vehicle, with the final mass, m, being equal each time to the weight of the stack just before the expended stage drops off. As each successive stage ignites, the new initial mass, M, is equal to the sum of the remaining fully fueled stages and the spacecraft atop the rocket. The great advantage in using multiple stages is an increase in effective mass ratio. This results in a greater payload weight being delivered to an intended orbit, either around Earth or for transfer to an objective beyond. The increased payload capability can be traded for speed, using a given launch vehicle, by placing a much lighter payload on top of the booster stages. This was done with the Pluto-bound New Horizons spacecraft, which had to be made as light as possible in order to reach the environs of Pluto within a decade. New Horizons took only 10 h to pass the orbit of the Moon, compared to 3 days for the Apollo missions.
Rockets have good reasons to use multiple stages to accomplish their typically one-time missions. These reasons, however, do not apply to the successful spaceplanes of the future, because the respective missions of ballistic rockets and spaceplanes will be fundamentally dissimilar. The most important difference between spaceplanes and rockets is that spaceplanes will be reusable.
Now that we have taken a quick look at airplanes and rockets, let us examine the vehicles of the future, the spaceplanes that will open up space to the traveling public.
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