Aerobraking
Aerobraking (sometimes aerocapture) is a technique for reducing velocity relative to a celestial body through the controlled use of atmospheric drag.
Usually, when a craft enters the sphere of influence of a celestial body, its relative speed will be much too high to be captured by that body and without intervention it will simply fly out the other side of the target's SOI and return to orbiting whichever body it was previously in the SOI of. To reduce velocity and be captured into an orbit, the craft can either fire its engines retrograde, perform a gravity assist around a moon (when present), or, if the target body has an atmosphere, aerobrake.
Contents
Execution
To perform an aerobraking maneuver, the trajectory of a craft is altered to pass through the body's upper atmosphere. The atmospheric drag will slow the craft down enough to convert its escape trajectory into an eccentric orbit. After the apoapsis is lowered to the desired height, it is necessary to raise the periapsis back outside the atmosphere to stop losing velocity and enter a sustainable orbit.
Disadvantages
Aerobraking is a dangerous maneuver. If a craft dips too deep and stays too long in the atmosphere, it could lose so much speed to drag that its apoapsis will no longer be above the atmosphere and it becomes captured inside the atmosphere. If this happens and the craft doesn't have enough thrust to return above the atmosphere, that could mean an inevitable meeting with the surface. If the craft's mission wasn't to land on the body it's aerobraking with, then its mission might be over. At worst, the craft could have a TWR < 1 on this body with no parachutes, leading to a collision.
The converse is a little less dangerous. If the craft doesn't dip deep enough, it might not lose enough speed to be captured by the body. Usually it is safer to aerobrake too high than too low, because even if the orbit's apoapsis is just barely within the SOI of the planet, the periapsis will remain in the atmosphere and allow additional aerobraking maneuvers to further lower the apoapsis. And if the apoapsis is outside the SOI a manual burn afterwards will lower it and the thrust is not to important then. Even though fuel has to be spend to lower the apoapsis, some fuel is then still saved.
As of the current version (0.23[outdated]) traveling through an atmosphere at high velocity will cause dangerous-looking red atmospheric entry flames to appear around the craft, but this is almost completely harmless. The only parts which can currently be damaged by drag are deployed solar panels. Retracting solar panels prior to entering the atmosphere is enough to protect them. However, Squad has announced that in the future, atmospheric entry heat will be implemented which will be able to cause damage to a vessel. This factor could make aerobraking even more dangerous.
For the ideal height for an aerobraking maneuver, consult the article about that body. The following planets and moons have atmospheres that allow for aerobraking:
Real world
In real world space operations, an aerocapture is a maneuver intended to enter an orbit around a target body, while aerobraking refers to lowering the apoapsis of an existing orbit. However, the terms are generally used interchangeably in Kerbal Space Program. Additionally, in real life aerobraking maneuvers are planned to be extremely conservative and a craft will typically perform many passes at a height at which the atmosphere is very thin, losing small amounts of velocity each time while avoiding the effects of turbulence and frictional heating (neither of which are present in KSP).
See also
- KSP Aerobraking Calculator by Alterbaron
- Aerobraking on Wikipedia
- Aerocapture on Wikipedia
