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[[Category:Reentry tutorials]][[Category:Tutorials]]
 
[[Category:Reentry tutorials]][[Category:Tutorials]]
Intuitive Atmospheric Entry is a common complaint on the Orbiter message boards that the aerodynamics on the Space Shuttle model are screwed up. This is usually reported after a person deorbits the vehicle, puts it in the right angle of attack... and then bounces off the atmosphere several times and overshoots the Cape. I know, I've done it myself. Here's how to enter the correct way.
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It is a common complaint on the Orbiter message boards that the aerodynamics on the Space Shuttle model are screwed up. This is usually reported after a person deorbits the vehicle, puts it in the right angle of attack... and then bounces off the atmosphere several times and overshoots the Cape. I know, I've done it myself. Here's how to enter the correct way.
  
 
Note: The aerodynamics of the stock Shuttle Atlantis which comes with Orbiter ''really are'' screwed up. It works great during launch, orbit, and the final gliding approach (Mach < 5.0), but in the hypersonic region it just doesn't work. I don't believe it is possible to do a good entry with it. I personally fly the [[Shuttle Fleet]] 3.9.2 myself, but I have heard many reports in the forum that the DeltaGlider III works also.
 
Note: The aerodynamics of the stock Shuttle Atlantis which comes with Orbiter ''really are'' screwed up. It works great during launch, orbit, and the final gliding approach (Mach < 5.0), but in the hypersonic region it just doesn't work. I don't believe it is possible to do a good entry with it. I personally fly the [[Shuttle Fleet]] 3.9.2 myself, but I have heard many reports in the forum that the DeltaGlider III works also.
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Also, it is an interesting fact that within a large range, the higher the drag on an entering vehicle, the smaller the heat load the spacecraft has to deal with is. This is partly because the craft does a larger part of its braking high in the atmosphere where there is less air. So, we use a very high angle of attack all the way through the heat pulse, to increase drag, increase lift, and not melt the windshield.
 
Also, it is an interesting fact that within a large range, the higher the drag on an entering vehicle, the smaller the heat load the spacecraft has to deal with is. This is partly because the craft does a larger part of its braking high in the atmosphere where there is less air. So, we use a very high angle of attack all the way through the heat pulse, to increase drag, increase lift, and not melt the windshield.
  
The entry autopilot's job is to make sure that the spacecraft always has an appropriate angle of attack for your present speed. The shuttle DAP does a decent job of this, even though it always keeps the nose a little below the [[w:NASA|NASA]] standard. The [[Deltaglider III]] autopilot will set the angle of attack to whatever the pilot commands, but it does not automatically track the mach number.
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The entry autopilot's job is to make sure that the spacecraft always has an appropriate angle of attack for your present speed. The shuttle DAP does a decent job of this, even though it always keeps the nose a little below the [[NASA]] standard. The [[Deltaglider III]] autopilot will set the angle of attack to whatever the pilot commands, but it does not automatically track the mach number.
  
 
== Flying ==
 
== Flying ==
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The secret is that you roll, in order to only oppose gravity with a small component of lift, rather than all of it. For a conventional aircraft or winged entry vehicle, like the Shuttle, the lift vector is always perpendicular to the wings. Since you can control the wings, you can control the lift vector. So, roll the vehicle so that the vertical component becomes almost equal to the gravity vector. ''Now you are flying.'' You may have to roll close to 90&deg; to get only the lift you need. Go ahead and do it, there is nothing to worry about as long as your AoA is correct and you have no sideslip, and the DAP and tail fin will take care of those things. Don't worry about the predicted impact point on the Map MFD. It doesn't know about flying. But, continue to pay attention to the Map MFD.  
 
The secret is that you roll, in order to only oppose gravity with a small component of lift, rather than all of it. For a conventional aircraft or winged entry vehicle, like the Shuttle, the lift vector is always perpendicular to the wings. Since you can control the wings, you can control the lift vector. So, roll the vehicle so that the vertical component becomes almost equal to the gravity vector. ''Now you are flying.'' You may have to roll close to 90&deg; to get only the lift you need. Go ahead and do it, there is nothing to worry about as long as your AoA is correct and you have no sideslip, and the DAP and tail fin will take care of those things. Don't worry about the predicted impact point on the Map MFD. It doesn't know about flying. But, continue to pay attention to the Map MFD.  
  
As you travel, you will decelerate constantly. At first, you are travelling at almost orbital speed, so there will be a strong "centrifugal force" component holding you up against gravity. (I know there's no such thing, but it is a convenient fiction at this point.) The combined gravity+centrifugal vector may be much less than the 9.8m/s^2 we are used to, more like less than 1m/s^2. So, you don't need very much vertical lift, and will roll over strongly. Once slower, the gravity vector will increase, and you will gradually need more and more lift to balance it. So roll less You can tell when you have rolled enough by looking at the Surface MFD. The vacc meter is precisely the difference between your lift vertical component  and the current gravity+centrifugal vector. If the vacc is much less than zero, you are dropping too fast, and need more lift, so roll less, and get the tail closer to vertical. If the vacc is much greater than zero, you are flying back up, and need less lift, so roll more.
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As you travel, you will decellerating constantly. At first, you are travelling at almost orbital speed, so there will be a strong "centrifugal force" component holding you up against gravity. (I know there's no such thing, but it is a convenient fiction at this point.) The combined gravity+centrifugal vector may be much less than the 9.8m/s^2 we are used to, more like less than 1m/s^2. So, you don't need very much vertical lift, and will roll over strongly. Once slower, the gravity vector will increase, and you will gradually need more and more lift to balance it. So roll less You can tell when you have rolled enough by looking at the Surface MFD. The vacc meter is precisely the difference between your lift vertical component  and the current gravity+centrifugal vector. If the vacc is much less than zero, you are dropping too fast, and need more lift, so roll less, and get the tail closer to vertical. If the vacc is much greater than zero, you are flying back up, and need less lift, so roll more.
  
 
== Sideslip ==
 
== Sideslip ==
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*NMi = Nautical mile, 1NMi=1.851km
 
*NMi = Nautical mile, 1NMi=1.851km
*kft = 1000 feet, 1000ft=304.8m
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*kft = 1000 feet, 1000ft=308.4m
*ft=foot, 1ft=0.3048m
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*ft=foot, 1ft=0.3084m
 
 
You can also use the following chart with speeds and distances converted to the metric system.
 
 
 
{| border=1
 
! Range, km !! Airspeed, m/s !! AoA deg !! Alt, km !! Vert spd, m/s !! Left Roll deg !! Right Roll deg
 
|-
 
| 8282 || 7620 || 40 || 122 || -- || -- || --
 
|-
 
| 4878 || 7315 || 40 || 75 || -14 ||  || R80
 
|-
 
| 3997 || 7010 || 40 || 72 || -63 ||  || 71
 
|-
 
| 3335 || 6706 || 40 || 70 || -62 ||  || 66
 
|-
 
| 2819 || 6401 || 40 || 69 || -61 || L63 || 
 
|-
 
| 2411 || 6096 || 40 || 66 || -60 || 60 || 
 
|-
 
| 2087 || 5791 || 40 || 64 || -59 || 61 || 
 
|-
 
| 1811 || 5486 || 40 || 62 || -58 || 62 || 
 
|-
 
| 1600 || 5182 || 40 || 60 || -57 || 63 || 
 
|-
 
| 1419 || 4877 || 40 || 58 || -56 || 64 || 
 
|-
 
| 1265 || 4572 || 40 || 56 || -55 || 64 || 
 
|-
 
| 1139 || 4267 || 40 || 55 || -54 || 62 || 
 
|-
 
| 1013 || 3962 || 40 || 54 || -53 || 59 || 
 
|-
 
| 891 || 3658 || 40 || 53 || -52 || 57 || 
 
|-
 
| 789 || 3353 || 39 || 51 || -51 ||  || R55
 
|-
 
| 696 || 3048 || 38 || 50 || -50 ||  || 46
 
|-
 
| 606 || 2743 || 36 || 48 || -49 ||  || 43
 
|-
 
| 515 || 2438 || 34 || 46 || -48 ||  || 40
 
|-
 
| 426 || 2134 || 30 || 43 || -47 ||  || 39
 
|-
 
| 341 || 1829 || 27 || 40 || -46 ||  || 40
 
|-
 
| 261 || 1524 || 23 || 36 || -45 ||  || 42
 
|-
 
| 196 || 1219 || 19 || 32 || -44 || L41 || 
 
|-
 
| 139 || 914 || 16 || 27 || -43 || 40 || 
 
|-
 
| 113 || 762 || 14 || 25 || -42 ||  || 
 
|-
 
| 91 || 610 || 12 || 23 || -41 ||  || 
 
|-
 
| 69 || 457 || 9 || 20 || -40 ||  || 
 
|-
 
| 52 || 305 || 8 || 16 || -39 ||  || 
 
|}
 
  
 
The most important thing is to have the correct altitude and airspeed for your distance from the target. Don't worry quite so much about vertical speed, just use it as a guideline to help you hit the altitude targets. Worry least of all about the roll. For one thing, this is for the real shuttle, and the Orbiter model doesn't have quite the same aerodynamic properties. Also, the roll reversals depend on exactly when and where you reenter at. Use whatever roll you need to get the sink rate you want, and roll reverse whenever you need to, to keep moving towards the target.
 
The most important thing is to have the correct altitude and airspeed for your distance from the target. Don't worry quite so much about vertical speed, just use it as a guideline to help you hit the altitude targets. Worry least of all about the roll. For one thing, this is for the real shuttle, and the Orbiter model doesn't have quite the same aerodynamic properties. Also, the roll reversals depend on exactly when and where you reenter at. Use whatever roll you need to get the sink rate you want, and roll reverse whenever you need to, to keep moving towards the target.
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{{HasPrecis}}
 
{{HasPrecis}}
 
[[Category: Articles]]
 

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