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There are two types of lift:
 
There are two types of lift:
* Positive lift, means the lift vector points away from the surface.
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* Positive lift means, the lift vector points away from the surface.
* Negative lift, means the lift vector points to the surface.
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* Negative lift means, the lift vector points to the surface.
  
The kinetic energy the spacecraft loses during reentry gets conserved by heating the air and the outside of the spacecraft.
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The kinetic energy, the spacecraft looses during reentry gets conserved by heating the air and the outside of the spacecraft.
  
One very important value in atmospheric flight is the [[dynamic pressure]], which is defined as the product of density (<math>\rho</math>) and velocity (v) squared, multiplied by <math>\frac{1}{2}</math>:
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One very important value in atmospheric flight is the [[dynamic pressure]], which is defined as the product of density(<math>\rho</math>) and velocity ( v ) squared, multiplied by <math>\frac{1}{2}</math>:
  
 
<math>p_d = \frac{1}{2} \cdot \rho \cdot v^2</math>
 
<math>p_d = \frac{1}{2} \cdot \rho \cdot v^2</math>
  
The product of dynamic pressure and the velocity is called the [[aerodynamic heatflux]] - it's the energy the spacecraft puts into the air for heating it and its hull.  
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The product of dynamic pressure and the velocity is called the [[aerodynamic heatflux]] - its the energy, the spacecraft puts into the air for heating it and its hull.  
  
 
<math>Q = p_d \cdot v = \frac{1}{2} \cdot \rho \cdot v^3</math>
 
<math>Q = p_d \cdot v = \frac{1}{2} \cdot \rho \cdot v^3</math>
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===Ballistic coefficient===
 
===Ballistic coefficient===
  
The ballistic coefficient is a value to tell how much a object is affected by drag and lift.
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The ballistic coefficient is a value to tell, how much a object is affected by drag and lift.
  
  
===Relation between descent rate and dynamic pressure===
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===Relation between descend rate and dynamic pressure===
  
 
The most important task during reentry is to control heating and aerodynamic loads on the spacecraft. Both values are linked to the dynamic pressure and the velocity of the spacecraft.  
 
The most important task during reentry is to control heating and aerodynamic loads on the spacecraft. Both values are linked to the dynamic pressure and the velocity of the spacecraft.  
  
The dynamic pressure is a function of speed and air density (which depends on altitude). As such, it stays constant, if the speed decreases faster, as the air density increases:
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The dynamic pressure is a function of speed and air density (which depends on altitude). As such, it stays stays constant, if the speed decreases faster, as the air density increases:
  
 
<math>v = \sqrt{\frac {2 \cdot p_d}{\rho}}</math>
 
<math>v = \sqrt{\frac {2 \cdot p_d}{\rho}}</math>
  
If the density increases by 4, the velocity has to be reduced by 50% to keep the same dynamic pressure, and thus, the same deceleration (as deceleration is proportional to dynamic pressure).  
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If the density increases by 4, the velocity has to be reduced by 50% for keeping the same dynamic pressure, and thus, the same decelleration (as decelleration is proportional to dynamic pressure).  
  
That also means: If the spacecraft descends faster than a special descent rate, the deceleration and heating increases, if the spacecraft descends slower than this rate, the deceleration drops.
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That also means: If the spacecraft descends faster than a special descend rate, the deceleration and heating increases, if the spacecraft descends slower than this rate, the decelleration drops.
  
 
==Ballistic reentry==
 
==Ballistic reentry==
  
The ballistic reentry is the simplest strategy. The spacecraft just drops into the atmosphere and uses only drag for slowing down. For this strategy it is important to neutralize any lift, as negative lift would be very bad for the spacecraft. This is usually done by rotating the capsule slowly. Temperatures range from 3,500 °F (1,926 °C) up to as high as 6,000 °F (3,315 °C)
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The ballistic reentry is the simplest strategy. The spacecraft just drops into the atmosphere and uses only drag for slowing down. For this strategy it is important, to neutralize any lift, as negative lift would be very bad for the spacecraft. This is usually done by rotating the capsule slowly.  
  
 
During a ballistic reentry, if the reentry angle is big enough, the trajectory forms a straight line, because inertia and drag are much higher than the gravity of the planet.
 
During a ballistic reentry, if the reentry angle is big enough, the trajectory forms a straight line, because inertia and drag are much higher than the gravity of the planet.
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==Gliding reentry==
 
==Gliding reentry==
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A gliding reentry makes use of lift to control the trajectory through the atmosphere. The craft does not need much lift to do such a reentry, but it needs to control its lift vector. For controlling the lift vector, a vessel has two possible ways: By changing the AOA and by banking the craft.  
 
A gliding reentry makes use of lift to control the trajectory through the atmosphere. The craft does not need much lift to do such a reentry, but it needs to control its lift vector. For controlling the lift vector, a vessel has two possible ways: By changing the AOA and by banking the craft.  
  
With the angle of attack, the vessel only changes the amount of lift available (including to negative lift), while the full range of directions are possible by banking the craft. That's why the AOA is kept constant for most vessels at the ideal value for a given speed and altitude, while the trajectory gets controlled by banking the craft. This leads to the typical S-turn trajectory of such vessels.  
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With the angle of attack, the vessel only changes the amount of lift available (including to negative lift), while the full range of directions are possible by banking the craft. Thats why the AOA is kept constant for most crafts at the ideal value for this speed and altitude, while the trajectory gets controlled with banking the craft. This leads to the typical S-turn trajectory of such vessels.  
  
The craft stays in the atmosphere all the time, unlike the skipping reentry. The gliding reentry is the most effective reentry strategy in terms of complexity and effect.
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The craft stays in the atmosphere all the time, other than the skipping reentry. The gliding reentry is the most effective reentry strategy in terms of complexity and effect.  
  
 
==Skipping reentry==
 
==Skipping reentry==
  
The skipping reentry gets used if a long reentry ground track is possible and a lot of velocity has to be lost. The spacecraft enters the atmosphere, slows down, but leaves it again on a suborbital trajectory.
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The skipping reentry gets used, if a long reentry ground track is possible and a lot of velocity has to be decellerated. The spacecraft enters the atmosphere slows down, but leaves it again on a suborbital trajectory.
 
 
[[Category: Articles]]
 
[[Category: Glossary]]
 

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