Flying the Shuttle - Entry: Difference between revisions

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Flying the Shuttle - Entry (view source)

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{{Space Shuttle navigation}}
{{Space Shuttle navigation}}


This mission phase can directly be started using <b>--aircraft=SpaceShuttle-entry</b> on the command line.
This mission phase can directly be started using '''--aircraft=SpaceShuttle-entry''' on the command line.


== What are we trying to do? ==
== What are we trying to do? ==
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The total energy in the friction heat is sufficient to destroy the orbiter (as the Columbia disaster has demonstrated). It is therefore mandatory that most of the energy never reaches the orbiter. This is accomplished by a blunt-body entry - for a streamlined hypersonic body, the shockwave of compressed air is attached to the surface, but for a blunt body it detatches, containing most of the energy, and only a small fraction of a few percent are radiated to the actual spacecraft structure. This remaining heat flow still leads to temperatures close to 3000 F at the nose and leading wing edges, but can be managed by the thermal protection system.
The total energy in the friction heat is sufficient to destroy the orbiter (as the Columbia disaster has demonstrated). It is therefore mandatory that most of the energy never reaches the orbiter. This is accomplished by a blunt-body entry - for a streamlined hypersonic body, the shockwave of compressed air is attached to the surface, but for a blunt body it detatches, containing most of the energy, and only a small fraction of a few percent are radiated to the actual spacecraft structure. This remaining heat flow still leads to temperatures close to 3000 F at the nose and leading wing edges, but can be managed by the thermal protection system.


<b>It is hence mandatory that the airstream during an entry never reaches the weakly protected upper fuselage and that the shockwave remains always detatched from the orbiter.</b>
'''It is hence mandatory that the airstream during an entry never reaches the weakly protected upper fuselage and that the shockwave remains always detatched from the orbiter.'''


This is accomplished by maintaining a high AoA, i.e. during the hot entry phase, the Shuttle is flown in what is technically close to a stall condition - pitched up at 40 degrees.
This is accomplished by maintaining a high AoA, i.e. during the hot entry phase, the Shuttle is flown in what is technically close to a stall condition - pitched up at 40 degrees.
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[[File:Visual roll reversal.jpg|1000px]]
[[File:Visual roll reversal.jpg|1000px]]


The combination of 40 degree upward pitch and 70 degree roll is something not usually experienced by pilots. In such a confguration, the lift now acts <i>sideways</i>, i.e. the orbiter changes course. This may be desirable to steer the trajectory towards a landing site, or it may not. In the second case, the roll needs to be reversed periodically ('roll reversal') to steer the ground track into an S-shape around the desired trajectory.
The combination of 40 degree upward pitch and 70 degree roll is something not usually experienced by pilots. In such a confguration, the lift now acts ''sideways'', i.e. the orbiter changes course. This may be desirable to steer the trajectory towards a landing site, or it may not. In the second case, the roll needs to be reversed periodically ('roll reversal') to steer the ground track into an S-shape around the desired trajectory.


Using a combination of high bank angle, low bank angle and roll reversals, direction and deceleration rate are managed to steer the orbiter to the landing site.
Using a combination of high bank angle, low bank angle and roll reversals, direction and deceleration rate are managed to steer the orbiter to the landing site.
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Note that the heat load is proportional to the dynamical pressure qbar times the velocity relative to the air, whereas the structural load is proportional to qbar. Thermal management is thus most important at the initial high-velocity phase in which the deceleration force is modest, and only when thermal management is over, trajectory deceleration control becomes important. For the same reason, the pitch angle can be gradually reduced with Mach number, ending in a just 14 degree pitch at Mach 2.5 where the TAEM interface is reached.
Note that the heat load is proportional to the dynamical pressure qbar times the velocity relative to the air, whereas the structural load is proportional to qbar. Thermal management is thus most important at the initial high-velocity phase in which the deceleration force is modest, and only when thermal management is over, trajectory deceleration control becomes important. For the same reason, the pitch angle can be gradually reduced with Mach number, ending in a just 14 degree pitch at Mach 2.5 where the TAEM interface is reached.


<b>Since aerodynamical forces push the shuttle into a low AoA configuration, a high pitch angle, once lost, is not easily recoverable. It is best established outside the atmosphere where qbar is low and kept during entry, only to be relaxed in the final phase.</b>
'''Since aerodynamical forces push the shuttle into a low AoA configuration, a high pitch angle, once lost, is not easily recoverable. It is best established outside the atmosphere where qbar is low and kept during entry, only to be relaxed in the final phase.'''


== De-orbit preparations in FG ==
== De-orbit preparations in FG ==
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<i>(you don't need to do this if you start with the entry scenario directly - but if you want to fly down from orbit rather than from entry interface, the Shuttle needs to be prepared)</i>
''(you don't need to do this if you start with the entry scenario directly - but if you want to fly down from orbit rather than from entry interface, the Shuttle needs to be prepared)''


Entry preparations begin in orbit - it is important to work through the entry preparation checklists, in particular payload bay door and ET umbilical doors need to be closed or the Shuttle will have incomplete thermal protection and burn up. APUs need to be running to provide hydraulic power for the aerodynamical control surfaces.  
Entry preparations begin in orbit - it is important to work through the entry preparation checklists, in particular payload bay door and ET umbilical doors need to be closed or the Shuttle will have incomplete thermal protection and burn up. APUs need to be running to provide hydraulic power for the aerodynamical control surfaces.  
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{{Note| You can find in complement of this tutorial a more advanced tutorial for entry once you feel confident [http://wiki.flightgear.org/Flying_the_Shuttle_-_Entry_TAEM_and_Landing_Advanced Entry Advanced]}}<br />
{{Note| You can find in complement of this tutorial a more advanced tutorial for entry once you feel confident [[Flying_the_Shuttle_-_Entry_TAEM_and_Landing_Advanced|Entry Advanced]]}}<br />




{{Note|This tutorial is for an up to date version by the official [http://www.science-and-fiction.org/bookstore.html Flightgear Space Shuttle Flight Manual].}}
{{Note|This tutorial is for an up to date version by the official [http://www.science-and-fiction.org/bookstore.html Flightgear Space Shuttle Flight Manual].}}


<i>In this tutorial, you'll learn to use the Shuttle's avionics to fly the hypersonic trajectory from entry interface to TAEM interface. The tutorial assumes that you have (by doing the on-orbit tutorials) gained basic familiarity with the avionics and the systems such that you can configure the MFDs to your needs and enter commands.</i>
''In this tutorial, you'll learn to use the Shuttle's avionics to fly the hypersonic trajectory from entry interface to TAEM interface. The tutorial assumes that you have (by doing the on-orbit tutorials) gained basic familiarity with the avionics and the systems such that you can configure the MFDs to your needs and enter commands.''


* Start Flightgear with the commandline options <code>--aircraft=SpaceShuttle-entry --lat=0.0 --lon=-160.0 --heading=45 --timeofday=morning</code>
* Start Flightgear with the commandline options <code>--aircraft=SpaceShuttle-entry --lat=0.0 --lon=-160.0 --heading=45 --timeofday=morning</code>
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As soon as guidance is active, the FG native HUD should show a few new figures, among them the distance to destination.
As soon as guidance is active, the FG native HUD should show a few new figures, among them the distance to destination.


Now, let's take a minute to consider our situation. The red trajectory line is a ballistic trajectory, and it's endpoint is where the Shuttle would hit the surface <i>if there wasn't an atmosphere.</i> Since there is an atmosphere however, we may end up somewhere else - which is the whole point of entry trajectory control.  
Now, let's take a minute to consider our situation. The red trajectory line is a ballistic trajectory, and it's endpoint is where the Shuttle would hit the surface ''if there wasn't an atmosphere.'' Since there is an atmosphere however, we may end up somewhere else - which is the whole point of entry trajectory control.  


Since the Shuttle is an aerodynamical object, it can (within limits) control its lift and drag and thus change the trajectory (shorten, lengthen or extend laterally) around the ballistic impact point - by a good 1085 miles in fact (that's technically known as 'cross range capability'). So we will aim such that the actual point where we hit the lower atmosphere is some 60 miles away from Vandenberg (at which point TAEM guidance takes over and this tutorial ends).<br />
Since the Shuttle is an aerodynamical object, it can (within limits) control its lift and drag and thus change the trajectory (shorten, lengthen or extend laterally) around the ballistic impact point - by a good 1085 miles in fact (that's technically known as 'cross range capability'). So we will aim such that the actual point where we hit the lower atmosphere is some 60 miles away from Vandenberg (at which point TAEM guidance takes over and this tutorial ends).<br />
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The purpose of this is as follows: During entry, thermal protection requires that we turn the blunt end of the Shuttle into the plasma stream. This translates into a rather strict Mach-dependent AoA requirement - for instance at Mach 23 we <i>must</i> fly 40±3 deg AoA or we'll burn and break up. Setting the pitch channel to AUTO makes the flight computer control AoA very precisely - which gives us the room to worry about trajectory control.
The purpose of this is as follows: During entry, thermal protection requires that we turn the blunt end of the Shuttle into the plasma stream. This translates into a rather strict Mach-dependent AoA requirement - for instance at Mach 23 we ''must'' fly 40±3 deg AoA or we'll burn and break up. Setting the pitch channel to AUTO makes the flight computer control AoA very precisely - which gives us the room to worry about trajectory control.


Of course, that means that all we can control is the roll channel, so we need to use that.
Of course, that means that all we can control is the roll channel, so we need to use that.
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* If you're in for a challenge, try to control pitch yourself using the guidance information displayed in the FG native HUD. It's not as bad as it seems because of the 'attitude hold' characteristics of the DAP.
* If you're in for a challenge, try to control pitch yourself using the guidance information displayed in the FG native HUD. It's not as bad as it seems because of the 'attitude hold' characteristics of the DAP.


If you're in for a real challenge, use the {{Key press|m}} <i>outside the atmosphere</i> to change to the ENTRY DAP. This will allow you to directly control the airfoils with the stick like in a plane - you'll find control in the upper atmosphere extremely difficult and only gradually see a measure of stability return. You'll be hard-pressed to stay alive rather than to reach any precision on trajectory control!
If you're in for a real challenge, use the {{Key press|m}} ''outside the atmosphere'' to change to the ENTRY DAP. This will allow you to directly control the airfoils with the stick like in a plane - you'll find control in the upper atmosphere extremely difficult and only gradually see a measure of stability return. You'll be hard-pressed to stay alive rather than to reach any precision on trajectory control!


* You can also try varying the entry interface location (by changing lat and lon) when you re-do this scenario - explore what to do if you're coming in higher or lower on energy, what to do if azimuthal offset is different.
* You can also try varying the entry interface location (by changing lat and lon) when you re-do this scenario - explore what to do if you're coming in higher or lower on energy, what to do if azimuthal offset is different.
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