Orbital Mechanics: What is Orbital Nodes?

To understand Orbital Mechanics, the physics around Orbits, and the mathematics, we have to have some sort of understanding of what Orbital Nodes are. Several of these terms are also explained in Kerbal Space Program.

Basic terms

To begin with, lets do some basic terms. Δv is a term often used when we talk about adding or removing energy to an orbit. Δv, or delta V as it is pronounced referes to the change in speed. Positive Δv is acceleration, and negative is deceleration.

Directions are a vector compound. Speed is also part of this same vector. To change this vector you apply another vector with a vector direction compound, and Δv as its length. The result of the previous vector and the added vector is the new direction and speed vector.

Direction of Movement

From the surface we start by going UP. At this point we still don’t operate with Orbital, not even Suborbital terms. After giving a tilt, we start creating a suborbital flight path, and Orbital Mechanics terms starts to apply.

STABILITY ASISTED / MANEUVER

STABILITY ASISTED, or just STABILITY, is a SAS mode where you lock current heading. This can be just to avoid spinning, or it can be to prepare for a maneuver. MANEUVER is to lock the vessel to a pre-calculated vector, i.e. to achieve a pre-calculated orbit.

PROGRADE / RETROGRADE

Continue to acellerate along the suborbital flight path is called PROGRADE. This will add energy to the suborbital flightpath and it will eventually become an orbital flight path. An orbital flight path, or just ORBIT, is when no part of the flight path passes below surface of a celestial body, i.e. the Periapsis is above ground. A stable orbit, is when the entire flight path is above any athmosphere. If we want to come down from an orbit, we need to remove energy from the flight path, we do that by a RETROGRADE maneuver.

NORMAL / ANTI-NORMAL

The normal vectors are perpendicular to the orbital plane. Burning NORMAL or ANTI-NORMAL will change the orbital inclination.

RADIAL IN / RADIAL OUT

These vectors are paralell to the orbital plane, and perpendicular to the prograde vector. The radial, or RADIAL IN vector, points inside the orbit, towards the focus of the orbit. The anti-radial, or RADIAL OUT vector, points outside the orbit, away from the body. Performing a radial burn will rotate the orbit around the craft.

TARGET / ANTI-TARGET

These vectors are not really part of orbital mechanics, but I do include them as they are part of the game. TARGET and ANTI-TARGET are vectors pointing directly towards or away from a selected target. Since the target moves with a different speed than the craft, these vectors will be drifting. These are useful when performing a rendevouz or a docking sequence.

MANEUVER

This is the resulting vector of a planned maneuver. A MANEUVER can be any one of the above mentioned vectors, or a combination of two or more of them.

Points of Reference

To calculate the flight path of a particular orbit, we have several points of reference to orient us, either describing positions, directions, angles, speed, or shapes. I will discuss a little about how to manipulate these nodes.

Apoapsis

This term comes from the prefix apo- (ἀπό = away from), and the greek word apsis (ἁψίς), and refers to the orbits arcs highest altitude above the baricentre. This is the point where an orbit is the furtherest away from the object it is orbiting.

Periapsis

This is the oposite of Apoapsis, where the prefix peri- (περί = near), the point of the orbits arc where it is nearest the baricenter.

Inclunation i

The INCLINATION is the angle between the orbital plane, and the bodys’s plane of reference, normally measured in degrees. A prograde orbit (same direction as the rotation of the center body) has an inclination between 0° and 90°, if the inclination is between 90° and 180° it is considered a retrograde orbit. Inclinations of 0° and 180° are following the equatorial plane or reference plane, and an inclination of 90° is a polar orbit passing above both poles.

Eccentricity e

Orbital eccentricity is a number describing the shape of an orbit. An ECCENTRICITY of 0 means the orbit is a perfect circle, a number between 0 and 1 is an eliptic orbit, 1 is an parabolic trajectory, and any number higher than 1 is an hyperbolic trajectory. That means, if the eccentricity is 1 or higher, you will pass or depart the object instead of orbiting it.

Ascending Node ☊

Any orbit that is not perfectly in the plane of reference will pass the plane two times for each orbit. When passing from the southern hemisphere to the northern, we call that point the ASCENDING NODE, and it can be referenced with ☊. The opposit node, where we pass to southern hemisphere is called the DESCENDING NODE, which have the symbol ☋. The node referes to the angle from the orbital reference direction ♈︎, the symbol comes from the zodiac sign for Aries, which is the earth’s reference direction.

Argument of Periapsis ω

The Argyment of Periapsis is the orientation of the ellipse in the orbital plane, as an angle measured from the Ascending Node to the periapsis.

True Anomaly ν

True Anomaly defines the position of the orbiting body along the ellipse at a specific time. It is measured in the angle along the ellipse from Periapsis to the current location.

End Note

My descriptions might not be scientifically accurate, I have tried to explain the terms how I feel a physics teacher should have thought me. Though I am trying to be as accurate as possible, I might deviate from actually Orbital Physics to fit the terms of the game rather than real world accuracy.

You can read more about these terms on wikipedia, there are some good articles about Orbital Elements, Orbital Nodes, and Apsis, just to mention a few.