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Flying the Shuttle - Entry: Difference between revisions
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If there is insufficient RCS fuel in one or both of the rear pods, pre-entry preparations are a good time to set up RCS to RCS or OMS to RCS crossfeeding. | If there is insufficient RCS fuel in one or both of the rear pods, pre-entry preparations are a good time to set up RCS to RCS or OMS to RCS crossfeeding. | ||
The next thing is to activate the guidance computer. If you adjusted your starting position / deorbit burn such that the groundtrack intersects with a landing site, you can select the site and activate the computer - guidance information will then be displayed in the lower right part of the HUD. At the same time, you can monitor the predicted trajectory on the map. If the computed ballistic impact point is close to the landing site, the entry is flyable (note that due to the cross range capability of the Shuttle, there's a significant margin for deviation, so the ballistic end point does not need to be perfectly aimed). | The next thing is to activate the guidance computer. The details of what is supported depend on the version of the Shuttle: | ||
=== De-orbit and entry planning in FG 3.6 === | |||
If you adjusted your starting position / deorbit burn such that the groundtrack intersects with a landing site, you can select the site and activate the computer - guidance information will then be displayed in the lower right part of the HUD. At the same time, you can monitor the predicted trajectory on the map. If the computed ballistic impact point is close to the landing site, the entry is flyable (note that due to the cross range capability of the Shuttle, there's a significant margin for deviation, so the ballistic end point does not need to be perfectly aimed). | |||
This is how a guidance solution for an entry to Vandenberg looks like: | This is how a guidance solution for an entry to Vandenberg looks like: | ||
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Important HUD readings to monitor are pitch and roll angle, nose temperature, vertical speed, Mach number and acceleration and pf course the guidance information in the lower left. The guidance information includes range to target, current deceleration, deceleration needed to reach the landing site, azimuthal deviation to target and deviation from ideal AoA. | Important HUD readings to monitor are pitch and roll angle, nose temperature, vertical speed, Mach number and acceleration and pf course the guidance information in the lower left. The guidance information includes range to target, current deceleration, deceleration needed to reach the landing site, azimuthal deviation to target and deviation from ideal AoA. | ||
=== De-orbit and entry planning in the Shuttle devel version === | |||
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. | |||
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. | |||
The entry interface location is likewise shown as soon as the orbit intersects the lower atmosphere. Thus, in order to manage the de-orbit, make sure the projected groundtrack is close to the site, then start the de-orbit burn some 10.000 to 12.000 miles ahead. As soon as the EI is shown, burn as long as it moves across the range line and you'll have an entry trajectory with a specified length. | |||
However, what is crucially important is also the perigee. If it is too high (> 70 km) atmospheric capture will be slow or might not happen at all - this will lead to a very long trajectory. If it is too low (< 20 km), sink into the lower atmosphere will be rapid, reducing range quite a bit and making the trajectory difficult to fly (the Shuttle will however take a lot of punishment, which is required for the abort trajectories specifically). | |||
Here's an example of a good de-orbit burn stared from a 320 km orbit 10.000 miles to range to a perigee of 40 km. | |||
[[File:Shuttle coming home01.jpg|600pxb|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. | 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. | ||
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. | 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: | 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: | ||