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Space Shuttle: Difference between revisions
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→Lateral stability
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As mentioned above, during most of the entry phase, the Space Shuttle has no rudder action and the yawing moment as a function of sideslip angle beta is negative, indicating instability. This means that the FCS has to manage yaw stability by commanding yaw thrusters to maintain near zero beta, which is increasingly more challenging as the Shuttle penetrates deeper into the atmosphere and aerodynamical forces grow while thrust is reduced as compared to nominal vacuum values. This implies that a sizable amount of RCS propellant (about 1/3 of the capacity to be on the safe side) needs to be available before atmospheric entry. | As mentioned above, during most of the entry phase, the Space Shuttle has no rudder action and the yawing moment as a function of sideslip angle beta is negative, indicating instability. This means that the FCS has to manage yaw stability by commanding yaw thrusters to maintain near zero beta, which is increasingly more challenging as the Shuttle penetrates deeper into the atmosphere and aerodynamical forces grow while thrust is reduced as compared to nominal vacuum values. This implies that a sizable amount of RCS propellant (about 1/3 of the capacity to be on the safe side) needs to be available before atmospheric entry. | ||
Below approximately Mach 6, the rudder starts to contribute to yaw stability and from Mach 3.5 down to Mach 2 where the yawing moment finally becomes positive only the rudder is used. | Below approximately Mach 6, the rudder starts to contribute to yaw stability and from Mach 3.5 down to Mach 2 where the yawing moment finally becomes positive only the rudder is used. The roll behavior of the orbiter before any FCS is somewhat skittish as the roll moment as a function of roll rate is not a large damping term over most of the Mach range. The FCS of the Shuttle in FG therefore does not place yaw and roll axis directly under pilot control. The rudder is always commanded to minimize beta and no pilot input for the rudder should be needed or used unless sideslip is explicitly desired. The elevons are commanded to provide a simple roll damper to make control smoother. | ||
The real Shuttle has in addition a <b>NO Y JET</b> mode to stabilize the orbiter during entry in which the elevons are used to control yaw. This leads to significantly reduced roll control since roll then needs to be driven by adverse yaw till the rudder picks up sufficient airflow. Currently this mode is not yet available in FG. | The real Shuttle has in addition a <b>NO Y JET</b> mode to stabilize the orbiter during entry in which the elevons are used to control yaw. This leads to significantly reduced roll control since roll then needs to be driven by adverse yaw till the rudder picks up sufficient airflow. Currently this mode is not yet available in FG. | ||
=== A note on thruster efficiency in the atmosphere === | |||
Thrusters used in the hypersonic rarefied airflow of the upper atmosphere do not only cause the yaw, pitch and roll moment by the thrust acting at a certain distance to the CoG, but also are subject to plume impingement on the orbiter fuselage and interactions with the air flow field. | |||
While impingement generically degrades the effectivity, the interaction moment can somewhat counter-intuitively act both directions. In particular the yaw moment is increased by the airflow, helping to stabilize the Shuttle. | |||
As of May 2015, none of these effects is modeled in Flightgear. | |||
== Glossary of acronyms == | == Glossary of acronyms == | ||