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Flying the Shuttle - Entry: Difference between revisions
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For the next minutes, one important thing to monitor is whether we're above or below the line - if we're above, we are too fast for the remaining range and need to slow down more, if we're below the line we're too slow and need to brake less. | For the next minutes, one important thing to monitor is whether we're above or below the line - if we're above, we are too fast for the remaining range and need to slow down more, if we're below the line we're too slow and need to brake less. | ||
The second important quantity is ΔAz - the azimuthal offset to target. If it is zero, we're headed right towards the landing site, if it's not zero we're aiming past it. For a perfect guidance solution, we want the Shuttle to move right down the line and keep ΔAz between -10 and 10. | The second important quantity is ΔAz - the azimuthal offset to target. If it is zero, we're headed right towards the landing site, if it's not zero we're aiming past it. For a perfect guidance solution, we want the Shuttle to move right down the line and keep ΔAz between -10 and 10. You can also find the same quantity displayed on the FG native HUD layer on the lower right. | ||
There is nothing simple about either range or azimuth - for instance, if you aim past the target, the azimuthal difference will grow as range decreases (that's simple geometry, at the closest approach to target we miss ΔAz will always be 90 degrees). For the current approach trajectory, we cross a decent amount of latitude, and thus there will be a Coriolis force pushing the Shuttle left. The actual range we fly will thus almost never be the direct distance shown. But if we fly the line, it'll all be taken care of. | |||
* You'll also find qbar displayed on the ENTRY TRAJ screen (and on the FG HUD). Once it reaches 10 psf and 40 psf respectively, you'll see callouts that roll and pitch control is transferred from thrusters to aero surfaces. You're likely to not notice, because the Aerojet DAP is designed to give you a crisp response in both regimes, but you can verify it via the thruster firing indicators (as during entry the nose RCS thrusters are not used, you'll not hear any thruster firings). | |||
* Now, short recap: The pitch channel is under automatic control, determines AoA and can't be changed due to thermal considerations. The yaw channel is unstable and is also under automatic control to enforce zero sideslip (or we burn). That leaves the roll channel to control azimuth and range. | |||
How? | |||
Azimuth is obvious - we just roll into the direction we want to go. If we're going right of the landing site, we roll left to decrease ΔAz and vice versa. At hypersonic speeds, the turn rate will be very small even for large bank angles, as Mach number decreases we'll gradually get more nimble - so it's a good idea to never let ΔAz grow too large or it might be beyond the Shuttle's maneuverability to recover. | |||
Controlling range is more tricky. The chain of control goes like this: Range is controlled by velocity. We have control over velocity by controlling the deceleration. Deceleration is controlled by atmosphere density, the denser the atmosphere, the more drag, the stronger deceleration. Atmosphere density is controlled by altitude - the higher up, the less density, the less drag. Altitude is controlled by vertical speed. Vertical speed is controlled by aerodynamical lift - and aerodynamical lift is controlled by bank angle, the more we bank to the side, the less lift we'll have. | |||
Thus, if we get below the nominal trajectory, we want to fly small bank angle, if we get above we want to fly large bank angle. At the same time, we want to bank into the direction to keep ΔAz small, thus if we're right of the target, we bank left till we reach a -10 deg ΔAz, then we reverse attitude (that's called a 'roll reversal') and bank right to let ΔAz grow to +10 degrees, then we do another roll reversal. | |||
So, as the Shuttle symbol enters the display, watch vertical speed on the native HUD layer upper left. It'll move slowly upward from -170 m/s as lift builds - when it reaches about -100 m/s, bank towards your target. Do this carefully, don't roll much faster than 6-8 deg/s, control is tenuous at this altitude, the Aerojet DAP does a good job in hiding this from you, but don't overdo it or you'll lose control. | |||
If you're used to how this feels in an airplane, it'll feel exceedingly weird because the guidance holds 40 deg AoA all the time - in essence you'll do this: | |||
[[File:Entry tutorial03a.jpg|600px|Entry tutorial 4]] | [[File:Entry tutorial03a.jpg|600px|Entry tutorial 4]] | ||
Watch both vertical speed and change bank angle to stabilize it at -70 m/s (you'll have to bank towards some 60 degree for this trajectory). That's called 'hdot capture'. | |||
[[File:Entry tutorial04.jpg|600px|Entry tutorial 5]] | [[File:Entry tutorial04.jpg|600px|Entry tutorial 5]] | ||
Observe how with small changes in bank angle you can make vertical speed increase and decrease - there's really control here. Utilize it to maneuver the Shuttle symbol back to the trajectory (you'll inevitably go a bit below initially for the chosen trajectory). | |||
* Watch azimuth as well and do a roll reversal (again, keep roll rates below 6-8 deg/s) when it goes beyond either -10 deg or 10 deg. During each roll reversal, your descent will slow down because you're rolling through zero bank angle and create a spike of lift - you need to capture and adjust hdot every time anew. Practice will make you perfect. | |||
* The Aerojet DAP will leave the Shuttle in its current attitude if you do nothing, so once you have a good attitude, you can let go of the controls for half a minute (in fact, you can just nudge the Shuttle into attitude using keyboard input and center controls when you're there, this is probably easier than using a real stick). | |||
That gives us time to check on a few other things - APU status and SPI (surface position indicator). You can take in APU status at a glance - if all is green you're fine, if anything shows red, you might have to shut down the APU. Airfoils will still work with only one APU running (just in case) but the response of the Shuttle will be less crisp and immediate. | |||
SPI will show the need for trim. Usually the elevon indicators should be close to zero - if they're not, you're gradually losing roll control. Below Mach 10, you can use the body flap and the speedbrake to try to ease the trim load of the elevons. For the loadout of this entry scenario, you can trim body flap up if you like. | |||
[[File:Entry tutorial05.jpg|600px|Entry tutorial 6]] | [[File:Entry tutorial05.jpg|600px|Entry tutorial 6]] | ||
* As you slow down, the screen will automatically change from ENTRY TRAJ 1 to ENTRY TRAJ 5. If you have good control over your bank angle and sink rate, you should see somthing like this: | |||
[[File:Entry tutorial06.jpg|600px|Entry tutorial 7]] | [[File:Entry tutorial06.jpg|600px|Entry tutorial 7]] | ||
Once the craft slows down further, the computer will gradually reduce AoA (no need to be alarmed) and the Shuttle will become more responsive - which unfortunately means you'll hit the ΔAz limits rather sooner than later, so roll reversals become more frequent. | |||
* Close to TAEM interface, you can start to see the color of the sky change to a deep blue. If you've stayed with the nominal trajectory and kept the azimuth under control, you should be some 85.000 to 100.000 ft altitude with about Mach 3 with some 60 miles to site on the indicator. | |||
[[File:Entry tutorial07.jpg|600px|Entry tutorial 8]] | [[File:Entry tutorial07.jpg|600px|Entry tutorial 8]] | ||
That's the point where you do an OPS transition and change from entry guidance to TAEM guidance (which works rather differently). While entry trajectory control is really different from the way an airplane is flown, during the TAEM pattern the Shuttle resembles a supersonic airplane. But that's for a different tutorial. You can still switch off AUTO control of the pitch channel and try to fly to the runway on your own. | |||
* If you get much below the nominal trajectory, you need to aim at the target at the first reasonable opportunity (perhaps around Mach 14 when you have plenty of speed left and the Shuttle actually can already change direction) and otherwise fly zero bank angle for optimum glidepath. If you aim too early, it costs a lot of lift without changing direction, if you aim too late the additional distance you have to fly due to the azimuth offset is too large. | |||
If you get much above the nominal trajectory, you can let ΔAz grow to more than 10 degrees - this will not only make roll reversals (which give additional boosts of lift) less frequent but also lengthen the actual trajectory since you'll be flying an S-curve. You can decelerate quite rapidly by selecting a high vertical velocity, but never stress the Shuttle above structural limits. | |||
* 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! | |||
* 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. | |||
== Further reading == | == Further reading == | ||