Glide Range and Energy
After MECO, the shuttle has a certain amount of energy that determines how far and how long it can glide. This amount of energy is a function of the speed and altitude of the shuttle. The initial energy state, determined by the speed and altitude at the time the SSMEs are shut down, decreases continually during GRTLS, reaching zero at orbiter wheels stop.
The horizontal distance or range that the shuttle can glide with a given amount of energy is determined by the vehicle's lift and drag characteristics. These lift and drag characteristics are usually summarized as a ratio of lift to drag (L/D). The L/D of the shuttle can be varied by changing the shuttle basic flight parameters, such as angle of attack (alpha), or by moving aerodynamic control surfaces which affect drag and/or lift.
For example, L/D varies with angle of attack as well as sideslip angle (beta) and bank angle. L/D also varies with the position of the elevons, body flap, speedbrake, and rudder. Some of these controls, such as the rudder, have very little effect on the shuttle L/D, while others, such as the speedbrake, exist primarily to alter L/D. The speedbrake simply alters drag without affecting lift, thereby decreasing the L7D ratio.
The flight condition of the orbiter can be adjusted to maximize the L/D ratio. This state, referred to as MAX L OVER D, determines the maximum glide range of the orbiter for a given amount of energy.
In practice, it is not a good idea to plan on flying at maximum L/D. An extra headwind or unplanned drag results in insufficient range to make the runway. To avoid this, the MECO energy state is planned to be comfortably more than the minimum necessary. The nominal amount of excess energy can be easily dissipated during GRTLS. Should the shuttle arrive at MECO with less than the planned amount, it is be said to be low on energy. This usually means that there is enough energy to make the runway, but less than planned. If the shuttle is extremely low on energy, such that the planned runway cannot be reached, then the shuttle has to downmode.
Downmoding is doing something other than what was planned because it takes less energy to do so. An example of downmoding is changing the path to the runway from an overhead turn approach to a straight-in approach. It is also sometimes possible to change the targeted runway to one that requires a shorter approach.
It is also possible that the shuttle might have more energy at MECO than was planned on. This might not appear to be a problem since energy can always be dissipated but never increased. The problem is that if the shuttle is too high on energy it could conceivably go by the runway and not be able to land. The shuttle has a very limited turning capability and cannot just orbit above the field waiting to slow down to landing speed. This problem is avoided, if possible, by dissipating excess energy relatively early in the GRTLS profile so that the energy state is as normal as possible when the shuttle approaches the runway.
The role of GRTLS guidance is to manage the shuttle energy state to maximize the chances of a safe landing. The techniques that guidance uses to manage energy are given at the end of section 3.
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