Aerobraking

**Aerobraking** is a technique that uses a planet's atmosphere to slow down a rocket in its atmosphere or decrease the apoapsis of an orbiting rocket. Aerobraking works similarly to aerocapture since they both use the atmosphere of a planet to slow down a rocket. The only difference between them is that aerocapture is used to get to orbit.

## Entering orbit

*Main article: Aerocapture*

Aerobraking comes with huge advantages if you want to save fuel. If you send a rocket towards another planet, it will enter that planet's sphere of influence. Without changing the trajectory, the rocket will continue its trajectory until it escapes back into a heliocentric orbit.

## Decreasing apoapsis

Aerobraking also refers to decreasing the apoapsis of a rocket that is orbiting that body. This is a more fuel-efficient method than just using engines to lower the orbit. To do that, fire the engines retrograde, and the periapsis must intersect at the upper layers of the atmosphere. Then the atmospheric drag slightly decreases the apoapsis. After it exits the atmosphere and is at its apoapsis, the rocket must conduct a **periapsis raise maneuver** to raise the periapsis out of the atmosphere and into the designated orbit.

### Prograde aerobraking

This is done by having the rocket enter the atmosphere facing the direction of travel. It must be performed in the lower layers of the atmosphere to slow the rocket enough.

### Retrograde aerobraking

This is done by having the rocket enter the atmosphere facing away from the direction of travel. This cannot be an option for some rockets.

### Perpendicular aerobraking

This is done by having the rocket point straight up during aerobraking. This is difficult to perform on Venus, Jupiter, or a custom planet with a thick atmosphere, since the atmosphere is too thick.

## Aerobraking on celestial bodies

The celestial bodies that can be used to aerobrake are the Sun, Venus, Earth, Mars, and Jupiter. They are explained further down here:

### Sun

The sun has a dense corona that can be used for aerobraking or aerocapture to gain a lower heliocentric orbit. A rocket passing in or close to the corona will melt, so it is advised to use heat shields to survive this type of maneuver.

### Venus

Venus has a thick atmosphere that is five times as thick as Earth's, which is well-suited for aerobraking.

A rocket approaching Venus from Earth or Mercury can use aerocapture to gain orbit, saving fuel, but it's risky. To do this, try to set the flight path to intersect the atmosphere at 25–30 kilometers high. If it is outside of these altitudes, the rocket will either miss Venus or burn up in the atmosphere. Try using fuel to slow down on Venus to get to orbit, or use the No Heat Damage cheat.

Venus has a very thick atmosphere. As the rocket is slowing down, it will rotate uncontrollably due to the aerodynamic forces and will experience a great heating effect. The rocket will possibly burn up in the atmosphere before landing, failing the mission. To counteract this, **never go below 25 kilometers**, as the rotation will become strong below that height. If the rocket gets lower than the height, make sure it has a low center of mass to avoid unnecessary rotation. To do so, put a large fuel tank (fully loaded with fuel) below the rocket.

### Earth

The Earth has a dense atmosphere. Usually, Earth is the final destination for rockets returning from other planets. However, someone might want to reuse a rocket, refuel it, and send it somewhere else. To do so, a rocket must intersect Earth's atmosphere somewhere around 15 to 12 kilometers high. If the rocket is returning from the moon, the trajectory can be set anywhere (even on an impact trajectory), but the high velocity may heat up the rocket to temperatures up to 4000 °C. To avoid burning up in the atmosphere and the atmosphere turning the rocket to the wrong angle for reentry, set the trajectory above 8 kilometers. Below 10 kilometers, the atmosphere greatly slows down a rocket.

### Mars

Mars has a thin atmosphere. A rocket coming from Earth can be slowed down enough to gain orbit if it crosses the Martian atmosphere; in this case, it is called aerocapture. To do this, set the flight path at around 10 kilometers high. This will send the rocket into an elliptical orbit, with the apoapsis inside the Martian sphere of influence and the periapsis intersecting the atmosphere. The rocket must be encased in a special case called an "aeroshell" to protect it from the harsh environments. After passing the atmosphere once, increase the periapsis to 9–10 kilometers. When the rocket enters the atmosphere, face the heat shield toward the direction of travel. Once it gains an impact trajectory, wait for the rocket to drop to 250 m/s, then deploy the parachutes. For rovers (and possibly heavy rockets), the aerobraking height must be between 6.2 kilometers and 5.5 kilometers.

If the trajectory is set to intersect with the Martian atmosphere at 10 kilometers or more on the initial approach, the rocket will not slow down enough, and it will be required to burn retrograde to slow down enough.

### Jupiter

Jupiter has a dense atmosphere that is similarly thick as Venus's atmosphere, so it is largely used for aerobraking. The rocket needs to be in the upper layers of the atmosphere to do this, or the atmosphere will destroy any rocket passing through its atmosphere, with or without a heat shield. For orbiter and atmospheric probe missions, this technique is used, but with a protective aeroshell and a very strong heat shield to survive this objective.

To do this maneuver, you must be in the upper layers of the atmosphere (ideally 80–70 kilometers above the surface), as said earlier. A strong heat shield is needed, or use the "No Heat Damage" cheat.

### Other celestial bodies

It is known that Mercury, the Moon and the 4 Galilean moons (Io, Europa, Ganymede, and Callisto) have no atmosphere, so they cannot be used for aerobraking in any way in-game. In real life, the planets have very little atmospheres, so their atmospheres are too thin for aerobraking.

## Landing

Aerobraking is largely used for landing: it reduces the speed of a rocket down to the point where parachutes can operate safely, sometimes even without retrorockets.

### Sun

The sun has a dense corona and can slow down any rocket. If a player tries to *land* on the sun, the rocket will melt instantaneously when it touches the atmosphere. If, by chance, the rocket survives the heat of reentry on the sun, it will slow down to 500 m/s, reach *ground zero*, and then continue to sink into the sun to the hot core, forever, until it glitches at the core of the sun.

### Venus

Venus has a dense atmosphere that is enough to slow down any rocket to the point where no parachutes are needed. Its atmosphere is thick, so it can destroy any rocket that's passing above 2 kilometers per second.

### Earth

Earth's atmosphere can sufficiently slow down any rocket that is approaching in its atmosphere. If a rocket is moving very fast (for example, a return mission from Mercury, Jupiter, or a custom distant planet), the atmosphere will destroy any rocket that's passing through the atmosphere, with or without a heat shield.

### Mars

Mars, with its thin atmosphere, can slow down a rocket that is not entering direct orbit, but sometimes not enough. This is why aerobraking or using aerocapture can be very useful. The heating effect on Mars is not as intense as that on Earth or Venus.

### Jupiter

Jupiter has a very dense atmosphere like Venus and can destroy any rocket, with or without a heat shield. If the rocket manages to pass through its atmosphere without burning up, it will then slow down to the point that parachutes aren't needed anymore. It will then reach *ground zero* and slowly sink to the core. If it reaches the core, the rocket will glitch.

## Ascending from the surface

The atmosphere slows down any rocket passing through. It depends on velocity, engine type, and atmospheric pressure. This is why it is very important to have a rocket with an aerodynamic shape, like a stretched "A" shape rather than a "V", "H", or "I" shape.

### Venus

When getting to orbit on Venus, you need atmospheric engines (the Hawk or Titan engine), just like on the first stage of a rocket. The rocket will be designed in such a way that it resembles a rocket from Earth. After a couple hundred meters, the rocket will stop accelerating because of the thick atmosphere. After around 5–8 kilometers, the rocket will start accelerating slowly because of the low atmospheric pressure at that altitude. After passing 8 kilometers, it is a good time to turn the rocket slowly into orbit.

### Earth

The Earth's atmosphere can slow down a rocket ascending its atmosphere, but not significantly. It is recommended for rockets to fly vertically upward for about 500 meters before slowly turning to get into orbit.

### Mars

The effect is weak. A rocket can ascend through the Martian atmosphere very easily and quickly. When reaching the cloud tops, turn the rocket slowly.

## Bugs

* Aerobraking does not occur when time warp is in progress because of locked orbits.

## See also

* Aerocapture
* Deep Space Maneuvers
* Gravity Assists
* Oberth effect
* Trajectory Correction Maneuvers