Reentry

Reentry is in spaceflight the process of (re-)entering a atmosphere from space. Commonly this term means to reduce the very high speed by drag, requiring heavy heat shields. But its also possible to slow the speed down using propulsion systems, with very high fuel costs.

For aerodynamic reentry, there are three major strategies available:

• Ballistic reentry (eg Mercury)
• Gliding reentry (eg Gemini, Apollo, Soyuz and STS )
• Skip reentry (eg Zond)

Aerodynamic basics

Any object moving through the atmosphere usually creates two forces: Lift and drag. Even a sphere can create lift, if it rotates. The drag vector is always in the opposite direction of the airspeed vector, the lift vector always orthogonal to the drag vector (yes, that means the lift vector can point in many directions).

Drag slows the vessel down, lift changes its trajectory.

There are two types of lift:

• Positive lift means, the lift vector points away from the surface.
• Negative lift means, the lift vector points to the surface.

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($\rho$) and velocity ( v ) squared, multiplied by ${\frac {1}{2}}$:

$p_{d}={\frac {1}{2}}\cdot \rho \cdot v^{2}$

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.

$Q=p_{d}\cdot v={\frac {1}{2}}\cdot \rho \cdot v^{3}$

A typical satellite is designed for only withstanding 1800 W/m² - thats the same energy it can get from the sun during solar maximum.

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.

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.