Aerobraking
Aerobraking is a technique for reducing velocity relative to a celestial body through the controlled use of atmospheric drag. Aerocapture is special form of aerobraking which is used to go from escape velocity into a orbit instead of adjust an already stable orbit.
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, perform an aerocapture.
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 turn 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 stable orbit.
Disadvantages
Aerobraking is a dangerous maneuver, as it is extremely sensitive to initial conditions meaning that it is very hard to get an aerobraking manoeuvre to have the exactly desired results.
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 spent to lower the apoapsis, some fuel is then still saved.
As of version 1.0, atmospheric reentry heat has been implemented. It will overheat and destroy exposed parts with low temperature tolerance. Most parts must be protected behind heat shields or within cargo bays. Heat shields consume ablator, with which they come pre-filled, to protect components behind them. Furthermore, craft stability during reentry is vital in order for heat shields to remain in front of more vulnerable components.
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).
Lithobraking
Lithobraking is a semi-serious term used to refer to a spacecraft being slowed by contact with the ground. Lithobraking at too high a speed will cause a catastrophic failure of the mission, but it can be used as a low-delta-v landing technique. A lithobraking vehicle can non-destructively roll or skid and use friction over a considerable distance in order to dissipate excess kinetic energy relative to the body being landed on, or a portion of the spacecraft can be sacrificed with the energy of landing absorbed by components breaking.
The only sensible place to lithobrake as a serious method is the flats on Minmus. They are perfectly flat allowing a craft with landing gear to land and slow down under brakes. The aircraft landing gear are needed otherwise the wheels break due to excessive force. To execute this maneuver the player should start in a low Minmus orbit the lower the periapsis to just below the surface in the centre of one of the flats. Proof-of-concept lithobraking has been carried out successfully, without retro-rockets or parachutes, on both the Mun and Kerbin.
In real life, the Mars Climate Orbiter miscalculated its aerobraking trajectory and crashed into Mars in 1999. The Genesis mission failed to deploy a drogue on re-entry and crashed into North America in 2004.
See also
- KSP Aerobraking Calculator by Alterbaron
- Aerobraking on Wikipedia
- Aerocapture on Wikipedia
