Maneuvers During Re-Entry

Why does the Orbiter perform strange maneuvers and banks to slow it down before landing?

Other than its rocket-propulsion system, which the Shuttle uses to get into its final Earth-approaching orbit, the vehicle is essentially an unpowered falling glider during re-entry.  Because it's a falling object, the Orbiter's re-entry speed is determined by the Earth's gravitational pull.

Applying the physics of re-entry, the Shuttle's speed at that time is usually around 25,405 feet per second (or ... 17,322 statute miles per hour).

The Shuttle needs to slow down that speed (by increasing its drag) in order to make a safe landing. 

It has small rockets which the crew use to position the Orbiter (to give it maximum help during the slow-down process). 

Once the Orbiter has slowed-down enough, the mission's pilot selects the best trajectory and target (which NASA calls the "deorbit maneuver").  That procedure slows-down the Shuttle's speed by another 300 (or so) feet-per-second (or, 205 miles per hour slower). 

Then the crew selects the best angle of approach (to keep the Shuttle from burning up when it returns to the Earth's atmosphere and to further slow it down). 

This maneuver is called the hypersonic split-S maneuver.  It's really a zigzag pattern of flight which produces even more drag (just like downhill skiers execute zigzags to decrease their speed as they descend a mountain).

But ... because the Shuttle is going so fast on re-entry (around 17,500 miles per hour), there is insufficient time to execute enough of those zigzags before the Orbiter reaches the Earth's atmosphere.  That is why it goes through various types of maneuvers, banks and repositioning - so it can land at a safe speed.

Media Credits

Information and quoted passage, NASA's Glenn Research Center.


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