Elevons and rudders in space

[bluej774]

Why is it that elevons and rudders like those seen on NASA's various orbiters succeed in adjusting the pitch, roll, and yaw of crafts in the vacuum of space? I was under the impression that such devices relied upon the resistance of air (or water) to function. Why do they work in a vacuum?

 

[jtbell]

Where did you read that they're used in orbit? Surely they're only used during re-entry and landing.

 

[D H]

A brief primer on Shuttle operations: OPS 1 (Operation Sequence 1) is the ascent phase of the flight. The Shuttle switches to OPS 2 (on-orbit) after orbit insertion, and finally to OPS 3 (entry) to end the flight.

The Shuttle's aerosurfaces primary use is during entry. The elevons are used during ascent, but only to control vehicle loads. While on-orbit, the aerosurfaces are not used at all. The aerosurfaces aren't even powered during OPS 2. The Shuttle instead uses its Reaction Control System jets while on-orbit to change the vehicle's attitude.

During entry, the vehicle transitions from RCS to aerosurfaces to control vehicle attitude in a staged manner. The APUs that power the aerosurfaces are powered up about an hour before entry. The aerosurfaces themselves remain disabled for a while. The deorbit burn places the Shuttle in an orbit that intersects the atmosphere.

The forward RCS thrusters are disabled at entry interface (essentially the point at which the atmosphere starts to be noticeable). Attitude control during the early phases of entry comes from the rear RCS thrusters only. The ailerons are enabled and the roll jets are disabled at 10 pounds per square foot dynamic pressure. The elevators are enabled and the pitch jets are disabled at 20 pounds per square foot dynamic pressure. Finally, the rudder is activated and the yaw jets are disabled when the vehicle's speed drops below Mach 3.5.

 

[bluej774]

I was afraid of that. But, thank you, that was exactly the information I was looking for.

Here's a follow-up question. So for everything besides forward thrust which is provided by the main engines and the maneuvering engines they use the forward control thrusters and aft control thrusters in order to control pitch, roll, and yaw while in space?

 

[Anthea Lawn]

The wikipedia page has a pretty good basic description of steering the shuttle in space.

http://en.wikipedia.org/wiki/Space_Shuttle_orbiter

The aerodynamic look of the shuttle is basically only to help it through the Earth's atmosphere; up in space it could be shaped like the statue of liberty and it wouldn't make any difference.

Another common fallacy is the idea that a thruster needs air to "push" against in order to be effective; Newton's law of conservation of momentum means that they work perfectly well in space.