Difference between revisions of "Orbit"

Revision as of 19:47, 6 September 2013

A spacecraft in orbit around Moho.

An orbit is an elliptical path around a celestial body. The point on an orbit which is closest to the orbited body is called the periapsis and the furthest point is the apoapsis. These points are indicated on the map view as "Pe" and "Ap", respectively.

An orbit is considered “stable” if all points in the orbit are above the terrain and atmosphere of the orbited body, which applies if the periapsis is above the terrain and atmosphere as this is the lowest point of the orbit. A spacecraft in such an orbit will not lose velocity due to atmospheric drag and won't collide with terrain.

To achieve an orbit, a spacecraft must reach a sufficient altitude and orbital velocity. During ascent, a gravity turn helps to achieve both of these goals in a fuel-efficient way. With basic maneuvers it is possible to change the orbital shape.

Types of orbits

Low Kerbin orbit (LKO)

In analogy to the real world low Earth orbit (LEO) an LKO describes a stable low orbit around Kerbin that can be achieved with relatively low cost of Delta-V. The lowest point of an LKO must not be lower than 70 km in order to stay clear of atmospheric drag. The altitude of an LKO typically does not exceed about 200 km.

Tons of payload delivered to LKO is often used to compare performance and size of launch vehicles.

In optimal circumstances LKO can be achieved with 4450-4700 m/s Delta-V.^[1]

Due to the Oberth effect a low orbit is a suitable starting point for transferring to other celestial bodies. ^{[citation needed]}

Stationary orbit and synchronous orbit

→ Main article: Stationary orbit

An orbit with the same orbital period as the rotational period of the orbited body is called a synchronous orbit. If the inclination is also 0° and there is no eccentricity it is called a stationary orbit. A satellite in this orbit doesn't appear to move when viewed from the body's surface.

Kerbisynchronous Equatorial Orbit (KEO)

→ Main article: KEO

The stationary orbit around Kerbin, where the orbiting craft appears to stand still at a point above Kerbin's equator. The name was chosen to abbreviate it similar to GEO the abbreviation for geostationary orbit, which is the real world equivalent on the Earth.

Other

Various other orbits can be defined. The list of orbits on Wikipedia contain many common ones in the real world. They can be recreated by modifying the core features of the orbit to match the smaller universe. Some kind of orbits, like a sun-synchronous orbit around Earth, are not possible, because they require Kerbin's gravity to be not perfect. Also the Lagrange points do not exist so orbits near/around a Lagrange point are impossible. But it is possible to imitate L₄ and L₅, although every orbit with the same semi-major axis has the same orbital period, so L₄ and L₅ aren't special positions then.

Orbits in the save file

The save files (and scenarios) in KSP are plain text and human readable. Inside you will find information on the orbit of every craft currently in the game. It looks something like this:

 ORBIT
 {
   SMA = 76875.4600066045
   ECC = 0.136808532664149
   INC = 32.6082297441138
   LPE = 91.4665699628126
   LAN = 305.802690796769
   MNA = 0.556028537338098
   EPH = 19189976.1161395
   REF = 3
   OBJ = 0
 }

Each of these terms has a meaning, and changing them (and then re-loading your save game) will change the orbit of the vessel in question. (definitions shamelessly lifted from Wikipedia)

• SMA : Semimajor axis – the average of the periapsis and apoapsis distances from the orbiting body center
• ECC : Eccentricity – shape of the ellipse, describing how much it elongated compared to a circle.
• INC : Inclination – vertical tilt of the ellipse with respect to the reference plane, measured at the ascending node.
• LPE : Longitude of periapsis – horizontally orients the periapsis of the ellipse.
• LAN : Longitude of the ascending node – horizontally orients the ascending node of the ellipse.
• MNA : Mean anomaly at epoch – defines the position of the orbiting body along the ellipse at a specific time.
• EPH : epoch – the reference time for the orbit.
• REF : reference body – the ID of the body around which the orbit occurs. 0 is the sun, 1 is Kerbin, 2 is Mun, 3 is Minmus.
• OBJ : Some sort of object reference (I wouldn't change this number). It appears that 0 corresponds to probes, and 1 corresponds to debris.

By altering these values in the save file, one can easily "slew" vehicles into any position desired. This is very useful for setting up scenarios.

Reference code

This is a table contain the reference codes for all bodies of the Kerbol System:

Notes

1. ↑ Kerbin delta-V chart

External links

• Orbit on Wikipedia
• Apsis on Wikipedia
• Low Earth orbit on Wikipedia