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Flying the Shuttle - Launch: Difference between revisions
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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. | 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. | 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. This is the reason we need control over the launch course. | ||
Now, on an airless world, we could reach orbit a few hundered meters above the highest surface feature and get the same balance of centrifugal and gravitational force - there is no bonus for altitude. However, on Earth we can not reach Mach 27 at sea level - there is no material capable of withstanding the stresses and no thruster powerful enough to overcome the friction. Hence the acceleration to orbital speed needs to be above the atmosphere. | |||
For this reason, a launch trajectory always has two parts. The first part is near-vertical and lifts the spacecraft out of the atmosphere, the second part is near horizontal and accelerates to orbital speed. | |||
From an energetic perspective, the vertical part is wasteful and should be over as soon as possible. Imagine you have one g of thrust - it just allows you to overcome gravity and hover, but once your propellant is spent, you will fall down. This is the reason the SRBs are part of the launch stack - they generate a lot of thrust to get the shuttle out of the atmosphere as quickly as possible. It follows that it is wasteful if you want to reach an orbit of 600 km altitude to fly upwards to 600 km altitude and accelerate to orbital speed there - it costs far less energy to fly to 150 km altitude, accelerate to orbital speed there and later raise the orbit so that no energy is wasted fighting against gravity. For this reason, good launch tracks go out of the atmosphere but then stay low above it (of course, the spacecraft should not drop <i>back</i> into the atmosphere...). | |||
As the acceleration depends both on thrust and the remaining mass of the stack (once the propellant is spent, the acceleration grows), the shuttle is initially highly maneuverable as long as the SRBs are burning, then thrust reduces dramatically and slowly grows towards the end as the ET depletes. Vertical launch trajectory management hence utilizes the enormous upward push of the SRBs to bridge the time till the main engine acceleration is strong enough to stabilize the trajectory vertically. In other words, after SRB separation vertical speed is initially high, then reduces due to gravity to even below zero (i.e. the tranjectory falls slightly) till the SSME and the centrifugal force push it up again, at which point it should be managed to near zero once injecting into orbit. | |||
Finally, somewhat counter-intuitively, the spacecraft gains most of its kinetic energy in the last stages of the ascent. This can be understood in many different ways. One is to realize that initially propellant only has chemical energy, | |||
== How it feels in FG == | == How it feels in FG == | ||