Heard this today on
NASA TV, and they didn't bother to expand the
acronym. So here goes.
TAEM (pronounced
tay-em) stands for
Terminal
Area
Energy
Management. It is a phase in the
Space Shuttle reentry and landing process. After the
orbiter has completed transitioning from
spaceflight to
aerodynamic lift, there is a good possibility (due to conservative planning) that the vehicle's
velocity will be higher than required to complete the remainder of the flight plan.
As the Orbiter descends through the atmosphere, it manages its energy (velocity is not the same as energy; energy takes into account altitude, velocity, and attitude) by altering its bank angle (for more information on this, see Space Shuttle Re-entry Phase). As it passes through an altitude of approximately 83,000 feet, it is (well, it should be) around 52 nautical miles from the runway, travelling at or near Mach 2.5, and its angle of attack should have reached 14 degrees. This point is referred to in Shuttle-speak as the TAEM Interface, and when it is reached, the TAEM program in the Orbiter computers takes control. On those missions which offer cockpit-eye TV views, this is when the cameras will typically be switched on; NASA TV offers the best views of this, usually maintaining the cockpit view as a screen quadrant with mission control data in the other 3/4 along with running commentary from Control as well as broadcasting voice traffic between the Orbiter and Ground.
From here, the Orbiter turns onto a tangent with the nearest Heading Alignment Cylinder (HAC). Imagine the runway as a line segment; now, using the approach end of the runway as an intersection point, draw two circles that touch right at that point. Each circle is approximately 36,000 feet in diameter. The cylinders of airspace defined by these circles, rising up from ground level, are the HACs. Their purpose is to serve as a course guide for the Orbiter such that when its course intersects the edge of either, it can begin a constant-rate turn which will terminate with the orbiter above the runway threshold, its heading aligned with the runway. The point at which the Orbiter intersects the HAC is referred to as Waypoint 1.
This phase of the flight can be flown on automatic or manually; however, to fly this phase manually takes a great deal of training and experience, since the demands of holding the Orbiter onto a constantly-changing profile, with only flight computer feedback, are great. The pilot must keep track of an enormous number of variables (airspeed, altitude, bank angle, sink rate, distance to runway, heading, etc.) without any direct sensory feedback whatsoever - it's purely instrument flight. The reason for this is that the demands of the shuttle flight profile are such that until end of the approach and landing, there are no external visible references which can help maintain this profile.
TAEM is called TAEM because this is the last chance (terminal) that the Orbiter has to adjust its energy profile for landing. If it has retained too much energy, the flight control computers will bleed energy the same way they have been doing since the aerodynamic control surfaces became active - they will increase the bank angle of the Orbiter, in order to increase drag and sink rate. As the energy profile approaches nominal, the orbiter will return to a level attitude. In order to maintain course, the orbiter will reverse bank periodically, which means it will be flying a series of S-turns until the computers are happy with its profile.
In addition to the S-turn, the shuttle speed brake is activated in this phase, as airspeed has dropped enough to deploy it. The speed brake (which can be seen in video at the back of the rudder; the rudder's trailing edge can 'split,' the two halves of the speed brake portion of the rudder can then extend variably into the slipstream) can be used in early TAEM to modify the Orbiter's drag, lift-to-drag ratio, and hence flightpath angle.
At around 22 nautical miles from the landing site, and at an altitude of approximately 49,000 feet, the Orbiter will drop subsonic as it approaches Waypoint 1. When it reaches Waypoint 1, the computers switch to TAEM heading alignment phase, and the Orbiter will follow the HAC until it is within 20 degrees heading of the runway. Note that it will fly the HAC high, so that it comes out of the heading alignment with enough altitude for adjustments on final. It will fly a higher sink rate on final to compensate.
Once it has reached a course 20 degrees from runway centerline, the Orbiter transitions to TAEM Pre-final mode. It leaves the HAC by banking to acquire the runway centerline, and decreases angle of attack to acquire the steeper glideslope of final approach. At approximately 10,000 feet of altitude, 6.9 nautical miles from the runway and travelling at approximately 290 knots, the Orbiter is finished with TAEM and enters the Orbiter Approach and Landing phase.
Sources:
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