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 loses 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 - it's 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 coefficient

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

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:

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

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).

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.

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.

Gliding reentry

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.

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.

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.