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Flying the Shuttle - Launch: Difference between revisions
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While Earth's gravitational force decreases with distance, the Shuttle doesn't reach even remotely high enough to experience weightlessness due to this effect. Rather, for low orbit spaceflight, weightlessness can be understood as gravitational force being balanced by centrifugal pseudo-force. The upshot is that in order to reach a stable orbit, the Shuttle needs to be moving fast - about 27.000 ft/sec, roughly 8 km/sec or Mach 27. | While Earth's gravitational force decreases with distance, the Shuttle doesn't reach even remotely high enough to experience weightlessness due to this effect. Rather, for low orbit spaceflight, weightlessness can be understood as gravitational force being balanced by centrifugal pseudo-force. The upshot is that in order to reach a stable orbit, the Shuttle needs to be moving fast - about 27.000 ft/sec, roughly 8 km/sec or Mach 27. | ||
This velocity is something almost never referred to in aerodynamical flight - an inertial velocity with respect to the center of Earth. Since the planet rotates, this is not the same as a groundspeed. In fact, the planet rotates eastward fairly fast. The precise speed depends on latitude and is highest at the equator where it reaches a value of over 450 m/sec or 1450 ft/sec. The groundtrack of an orbit (i.e. the projection of the path of a spacecraft onto a globe) does hence not close even if the orbit itself is closed and is distorted by this effect. | |||
The important implication is that launching close to the equator due east, we gain 1450 ft/s of inertial speed for free, whereas launching due west from the same location, we have to accelerate an extra 1450 ft/s. For this reason, launch courses always point eastward. | |||
The next thing to consider is the [http://en.wikipedia.org/wiki/Orbital_inclination inclination] (i.e. under which angle will the desired orbit intersect with the equator). Trying to wrap a circle (the orbit) around a sphere (Earth), it is fairly easy to convince oneself that the only way to reach a zero inclination orbit is to launch from the equator and that no launch site at higher or lower latitude can lead to an inclination lower than that latitude in a straight launch. | |||
If we aim to rendezvous with an object in orbit, we need to launch into the same orbital plane, and (short of long computations, see the linked website) that usually means launching into the same inclination when the target passes over the launch site. | |||
== How it feels in FG == | == How it feels in FG == | ||