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[[Category:Reentry tutorials]][[Category:Tutorials]]
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[[Category:Reentry 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.
 
  
 
== Deorbit Burn ==
 
== Deorbit Burn ==
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These are pretty easy constraints.  
 
These are pretty easy constraints.  
  
Basically this has to be done by trial and error, to find the correct deorbit profile for each spacecraft and orbit. For a Space Shuttle, get it down to a circular orbit with an altitude of around 225km. Then, wait until the orbit track passes "close enough" to the target base. It doesn't even have to be all that close, since the space shuttle has a large crossrange capability. Next, wait until the distance to the target is 18000km and dropping. At this point, do a retrograde burn until the perigee altitude is -20km. This will result in an entry interface at about 4400 nautical miles (8000km) from the target, at 400,000 feet (121.920km), with an appropriate sink rate.
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Basically this has to be done by trial and error, to find the correct deorbit profile for each spacecraft and orbit. For a Space Shuttle, get the it down to a circular orbit with an altitude of around 225km. Then, wait until the orbit track passes "close enough" to the target base. It doesn't even have to be all that close, since the space shuttle has a large crossrange capability. Next, wait until the distance to the target is 18000km and dropping. At this point, do a retrograde burn until the perigee altitude is -20km. This will result in an entry interface at about 4400 nautical miles (8000km) from the target, at 400,000 feet (121.920km), with an appropriate sink rate.
  
 
You will notice on the map MFD that the ground impact point is well short of the target, but don't worry. We will be flying there, not falling.
 
You will notice on the map MFD that the ground impact point is well short of the target, but don't worry. We will be flying there, not falling.
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== Angle of Attack (AoA) ==
 
== Angle of Attack (AoA) ==
[[Image:Shuttle Entry 1.jpg|frame|right|Figure 1a. Shuttle flying at AoA of 17deg, at about mach 3. Higher mach numbers require a higher AoA]]
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[[Image:Shuttle Entry 1.jpg|frame|right|Figure 1. Shuttle flying at AoA of 17deg, at about mach 3. Higher mach numbers require a higher AoA]]
[[Image:Shuttle Entry 3.jpg|240px|thumb|right|Figure 1b. Shuttle flying at AoA of 35deg, at mach 20]]
 
  
The [[Space Shuttle]] is a remarkably good aircraft, when travelling above Mach 5. Then again, you can fly a barn door if you get it going fast enough. Now, the bottom of the space shuttle is your heat shield, and it is covered with all those nice black tiles. Obviously we want that side towards the wind. The top is not shielded to the same extent. It is basically just a blanket to keep the swirling hot wind from below, off the vehicle. So, the shuttle was carefully designed to fly with a high angle of attack at high mach numbers.
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The Space shuttle is a remarkably good aircraft, when travelling above Mach 5. Then again, you can fly a barn door if you get it going fast enough. Now, the bottom of the space shuttle is your heat shield, and it is covered with all those nice black tiles. Obviously we want that side towards the wind. The top is not shielded to the same extent. It is basically just a blanket to keep the swirling hot wind from below, off the vehicle. So, the shuttle was carefully designed to fly with a high angle of attack at high mach numbers.
  
 
When the spacecraft is flying, rather than falling, it relies on lift to keep it up. The amount of lift varies depending on the angle of attack. In the hypersonic range, the drag of a vehicle increases until the angle of attack reaches 90&deg;. The lift also increases, but there is a certain maximum lift point, above which the lift will drop again. This maximum lift point depends on the shape of the vehicle and its speed, and is about 40&deg; for the space shuttle from orbital speed down to about mach 10.  
 
When the spacecraft is flying, rather than falling, it relies on lift to keep it up. The amount of lift varies depending on the angle of attack. In the hypersonic range, the drag of a vehicle increases until the angle of attack reaches 90&deg;. The lift also increases, but there is a certain maximum lift point, above which the lift will drop again. This maximum lift point depends on the shape of the vehicle and its speed, and is about 40&deg; for the space shuttle from orbital speed down to about mach 10.  
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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 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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While we want the pitch angle of attack to be large, and the bank to be whatever necessary to control lift, we want the yaw angle of attack, or ''sideslip'' angle, to be as small as possible. Basically we want the wind to hit the well-shielded nose dead on and slide along the side of the spacecraft, and not hit the relatively unprotected side of the ship dead on.
 
While we want the pitch angle of attack to be large, and the bank to be whatever necessary to control lift, we want the yaw angle of attack, or ''sideslip'' angle, to be as small as possible. Basically we want the wind to hit the well-shielded nose dead on and slide along the side of the spacecraft, and not hit the relatively unprotected side of the ship dead on.
  
So, what does this look like? Some spacecraft have a sideslip angle meter. If yours does, great. If not, you can still get what you need from the orbit or surface HUD. Basically, all you have to do is keep the nose pointing vector exactly above the velocity vector. At high AoA, it is easier to see the arrow pointing towards the velocity vector. Just make sure this is down. Once you start lowering the nose and can see the velocity vector on screen, switch to the surface HUD.
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So, what does this look like? Some spacecraft have a sideslip angle meter. If yors does, great. If not, you can still get what you need from the orbit or surface HUD. Basically, all you have to do is keep the nose pointing vector exactly above the velocity vector. At high AoA, it is easier to see the arrow pointing towards the velocity vector. Just make sure this is down. Once you start lowering the nose and can see the velocity vector on screen, switch to the surface HUD.
  
If you are using either the DeltaGlider autopilot or Shuttle DAP, the autopilot will automatically take care of this for you. One more thing off your mind. If not, yaw towards the velocity vector and get it right under the nose.
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If you are using either the DeltaGlider autopilot or Shuttle DAP, the autopilot will automatically take care of this for you. One more thing off your mind.
  
 
== Roll Reversal ==
 
== Roll Reversal ==
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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
 
*ft=foot, 1ft=0.3048m
 
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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#During entry, you will be spending most of your time looking at the Map MFD, Surface MFD, and cue card, checking that your sink rate and speed are appropriate for your distance from the target.
 
#During entry, you will be spending most of your time looking at the Map MFD, Surface MFD, and cue card, checking that your sink rate and speed are appropriate for your distance from the target.
 
#Don't forget to do roll reversals, to stay heading roughly towards the target.
 
#Don't forget to do roll reversals, to stay heading roughly towards the target.
#Start flying like a normal airplane and begin to line up on the runway somewhere between mach 2 and 1
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#Start flying like a normal airplane and begin to line up on the runway runway somewhere between mach 2 and 1
  
 
{{HasPrecis}}
 
{{HasPrecis}}
 
[[Category: Articles]]
 

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