Flying the Shuttle - Entry: Difference between revisions

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Flying the Shuttle - Entry (view source)

Revision as of 07:16, 22 March 2016

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→‎How it feels in FG
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<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>
<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>


== How it feels in FG ==
== De-orbit preparations in FG ==
 
<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>


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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For a real Shuttle mission, mission control would take care of trajectory planning, and the Shuttle crew would just execute the plan. In FG, this option is not available and you have to do the planning yourself. As of September 2015, several planning and guidance instruments have been added to give the Shuttle the required capability.
For a real Shuttle mission, mission control would take care of trajectory planning, and the Shuttle crew would just execute the plan. In FG, this option is not available and you have to do the planning yourself. As of September 2015, several planning and guidance instruments have been added to give the Shuttle the required capability.


A viable entry trajectory based on maintaining 70 m/s sink throughout the entry is part of the vertical trajectory guidance. This trajectory has a length of 4100 miles and it is possible to deviate both into the direction of a shorter and of a longer trajectory by changing sink.
A viable entry trajectory based on maintaining 70 m/s sink throughout the entry is part of the vertical trajectory guidance. This trajectory has a length of 4100 miles and it is possible to deviate both into the direction of a shorter and of a longer trajectory by changing descent rate.


This means that ideally the entry interface (EI) needs to be 4100 miles from landing site. This can be accomplished by using the range lines of the entry guidance computer.
This means that ideally the entry interface (EI) needs to be 4100 miles from landing site. This can be accomplished by using the range lines of the entry guidance computer.
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[[File:Shuttle coming home01.jpg|600px|De-orbit planning for the Space Shuttle]]
[[File:Shuttle coming home01.jpg|600px|De-orbit planning for the Space Shuttle]]


Pitch up and wait for the atmosphere to grab the shuttle. This will be felt initially by a very slow drift of the attitude, trying to reduce pitch. Apply thrusters to keep the nose up. The thrust level needed to hold the 40 degrees will increase with increasing qbar, and eventually the controls will revert to aerodynamical surfaces for roll (qbar = 10 psf) and pitch (qbar = 40 psf). Using the <b>RCS ROT ENTRY DAP</b> steering characteristics changes quite drastically - initially it is probably easier to make minute thruster adjustments with the keyboard, in the later phase a stick or mouse is a much better option. With the <b>Aerojet</b> DAP, the change will hardly be felt as the rate controller logic adapts automatically to all changes in qbar and Mach number.
The trajectory from orbit to entry interface is then pure spaceflight - all you need to do is make sure that the Shuttle has a good attitude at entry interface, then do an OPS transition and activate the Aerojet DAP. The following tutorial describes how to fly the actual entry.
 
Once the Shuttle is under aerodynamical control, watch descent rate slow and reverse. Once the rate comes back up to about -50 m/s, you can initiate the first careful high-bank roll. Do it gently in order not to lose the AoA if you're flying by hand! If you're maneuvering with automatic pitch axis control, aim for about 6 deg/s roll rate. Watch the response of nose cone temperature and acceleration and the slow drift in course. The descent rate will fall again, don't let it fall too fast, or you'll get too hot. Allow for some lag, get a feeling for how the trajectory responds to what you're doing. This is actually piloting, and you can influence a lot of what is happening here. With the <b>Aerojet</b> DAP flight characteristics are very stable and the vehicle holds attitude and AoA automatically, which means you can use roll to control sinkrate very precisely - flying by hand, that may not work so well. Once deviation to target azimuth exceeds 10 degrees, do a roll reversal.
 
In the devel version, you can open the vertical trajectory planner (lower left). This is a plot of range (x-axis) vs. speed (y-axis) and gives you an instant feedback how you are doing with regard to the reference trajectory. If you're above the line, you need to slow down, i.e. aim for a higher sinkrate, if you're below the trajectory, you need to reduce drag, i.e. reduce sinkrate. If you stay on the trajectory, you should be delivered to TAEM interface 60 miles to site.
 
[[File:Shuttle TAL03.jpg|600px|De-orbit planning for the Space Shuttle]]
 
Monitor Mach number and altitude decrease, reduce pitch angle later in the flight as commanded by the guidance computer. Around Mach 3.5, you should finally get the rudder back with RCS jets switched completely off, at which point the Shuttle definitely feels like an aircraft. It can now actually change course and turn, although still sluggishly. Steer the course towards the landing site if you're close. Aim for TAEM interface of 85.000 ft, Mach 2.5, around 60 miles before the runway. Don't try to brake too fast, as the manual has it:
 
<i>It is better to arrive at TAEM interface with too much energy than without wings.</i>
 
(NASA has a sense of humor...)


== Going home - a tutorial ==
== Going home - a tutorial ==
Thorsten
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