# Difference between revisions of "Terminology"

In KSP, there are many terms pertaining to orbiting and physics that often can be confusing to non-technicians. In addition, various other scientific terms and abbreviations are used to describe common terms.

This sheet is designed as a concise lookup table of necessary terms to help you get started down the road to being a full fledged astronaut!

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## Ship Orientation

The ship orientation is always relative to a specific object. The terms are usually defined relative to the cockpit.

Zenith
Top side of the ship which is usually oriented away from the orbited body. Opposite of nadir.
Bottom side of the ship which usually oriented towards the orbited body. Opposite of zenith.
Port(side)
Left side of the ship. Opposite of starboard.
Starboard
Right side of the ship. Opposite of portside.
Front
Front side/end of the ship which is usually towards the nose or prograde vector. Opposite of aft.
Aft
Back side/end of the ship which is usually housing the primary rockets and facing in retrograde. Opposite of front.

## Space Maneuvers

Atmospheric braking
→ Main article: Aerobraking
Lowering the periapsis so it is inside a planetary atmosphere. This will lead to the vessel being slowed by atmospheric drag. Can lead to atmospheric entry, but also is used to reduce the necessary burn time for significant orbit alterations.
Lithobraking
An attempted aerobrake using the ground instead of air. Usage of this term implies jokingly that the resulting crash-landing was not in the original plan. An adaptation of the term aerobraking where aero was replaced by the Greek word lithos, meaning “rock” or “stone”.
Atmospheric entry
→ Main article: Atmospheric entry
Entering atmosphere and using drag to decelerate a vessel to a groundwards trajectory. In real-world science this causes intense heat stress on the object as the vessel requires sufficient speed to not "bounce" back from the atmosphere into space. Currently (0.25[outdated]) atmospheric entry is only partially implemented with effects but heat and bounce are not yet implemented, there are mods however which allow parts to overheat. This is usually called re-entry/reentry but in theory only correct in Kerbin's atmosphere; atmospheric entry being the more general term.
Burn
firing of the engines, usually to alter trajectory in some way.
Circularizing
Reducing an orbit's eccentricity to 0 or close to it. This is usually achieved by a burn close to an apsis.
Maneuver node
→ Main article: Maneuver node
A planned maneuver in the map view, so as to predict the effect of a burn in advance of performing it.
A burn performed directly towards the center of a celestial body. It rotates the orbit counter-clockwise until the periapsis passes the center of mass of a celestial body. Opposite of radial-out burn.
A burn performed directly away from the center of a celestial body. it rotates the orbit clockwise until the periapsis passes the center of mass of a celestial body. Opposite of radial-in burn.
Re-entry
Retroburn
A burn performed "backwards", e.g. with the engines facing towards prograde and nose towards retrogade (hence the name). This is a common maneuver to used to lower the periapsis or apoapsis.

## Physics

Acceleration
Rate of change to the velocity. Acceleration is a vector, measured in "m/s2".
Ballistic trajectory
A falling object's trajectory is ballistic. In rocketry it usually indicates that the object in question is only influenced by gravity and does not exert any force (i.e. thrust) of its own.
Delta-v (Δv)
The change in velocity that has or can be exerted by the spacecraft. This is measured in meters per second (m/s). More mass can reduce the delta-v, while more propulsion can increase it. This makes it a useful value to calculate the effectiveness of launch vehicles. For example, a launch vehicle requires about 4,500 m/s of delta-v to escape Kerbin's atmosphere and achieve a stable orbit.
as a definition: ${\displaystyle \Delta {v}=\int _{t_{0}}^{t_{1}}{\frac {|T|}{m}}\,dt}$
where T is the thrust, m is the actual mass
If there is no external force and direction changing, the result is changing of the speed:
${\displaystyle \Delta {v}=\int _{t_{0}}^{t_{1}}{\frac {|T|}{m}}\ dt=\int _{t_{0}}^{t_{1}}{|a|}\ dt=|{v}_{1}-{v}_{0}|\,}$
In case of counting with the specific impulse:
${\displaystyle \Delta {v}=|{v}_{1}-{v}_{0}|_{max}\ =-\int _{t_{0}}^{t_{1}}{I_{sp}\cdot g\ \cdot {\frac {\dot {m}}{m}}}\ dt={I_{sp}\cdot g\cdot ln({\frac {m_{0}}{m_{1}}})}\,}$
Energy
→ See also: Specific orbital energy on Wikipedia
The energy of an object in an orbit is the sum of its potential and kinetic energy. The potential energy is ${\displaystyle E_{p}=-{\frac {GMm}{R}}}$ and kinetic energy ${\displaystyle E_{k}={\frac {1}{2}}mv^{2}}$ where G is the gravitational constant, M is the mass of the body, m is the mass of the craft, R is the distance from the center of the body and v is the velocity. This results in ${\displaystyle E=E_{k}+E_{p}={\frac {1}{2}}mv^{2}-{\frac {GMm}{R}}}$. This sum stays the same when not thrusting: When approaching periapsis potential energy is transferred into kinetic energy. After passing the periapsis the kinetic energy is converted back into potential energy. When the energy or specific orbital energy is greater than zero the vehicle is on an escape trajectory.
This is the basic idea behind Kepler's laws of planetary motion, which is what gives rise to KSP's patched conics approximation. An ellipse is the set of all points on a plane such that the sum of the distances to two points - the foci - is some constant. One focus of a Kepler orbit is the centre of mass of the object being orbited; as an object approaches it, it exchanges potential energy for kinetic energy. As the object moves away from this focus - equivalently, if the orbit is elliptical, as the object approaches the other focus - it exchanges kinetic energy for potential energy. If the craft going directly towards or away from the object, the foci coincide with the apsides, where the kinetic (apoapsis) or potential (periapsis) energy is zero. If it's perfectly circular (e.g. the Mun's orbit around Kerbin), the two foci coincide and the locations of the apsides are undefined, since every point of the orbit is an apsis.
There is also the specific orbital energy (${\displaystyle \epsilon }$) which doesn't require the mass of the craft: ${\displaystyle E_{p}=\epsilon _{p}m}$, ${\displaystyle E_{k}=\epsilon _{k}m}$, ${\displaystyle E=\epsilon \,m=(\epsilon _{k}+\epsilon _{p})m=-{\frac {GM}{2a}}}$. All orbits with the same semi-major axis (a) have the same specific orbital energy.
Escape Velocity
The velocity needed to escape a given planet's gravity well, as given by ${\displaystyle v_{e}={\sqrt {\frac {2GM}{r}}},}$ where G is the gravitational constant, M is the mass of the planet, and r is the radius of the planet.
g-force (G)
A measurement of acceleration as expressed in the sea-level force of Earth's gravity with 1 G being about 9.81 m/s². An object at Earth's surface is accelerated at 1 G. The object weighs twice as much when at 2 G acceleration and is weightless when accelerated with 0 G. In free fall, like in orbit, and without an engine running or an atmosphere applying drag all objects experience no acceleration which can be expressed as 0 G.
Gravity
The force exerted by all objects with mass. Very weak. Usually only objects with very high mass - i.e. planets, moons - have any noticeable effect. Diminishes with the square of distance from the center of mass. So for an object twice as far, experiences only 1/22 = 1/4 of the gravity.
Gravity Well
The area around a planet affected by gravity. Actually extends to infinity, but as gravity decreases quadratically with distance (after twice the distance the gravity is only a quarter), it is only significant within the body's sphere of influence. In fact, in KSP, gravity isn't simulated at all beyond a body's sphere of influence due to its use of the "patched conic approximation".
Orbit
→ Main article: Orbit
When an object has sufficient tangential velocity (and is outside the atmosphere, so drag won't slow it down) so that it will keep falling "next" to the planet (never touching ground) its trajectory is called an orbit. Stable orbits are elliptical (a circle is an ellipse with zero eccentricity). If the objects tangential speed exceeds escape velocity it's orbit will be either para- or hyperbolic.
Specific Impulse (Isp)
→ Main article: Specific impulse
${\displaystyle I_{sp}={\frac {T}{\dot {m}}},}$${\displaystyle [I_{sp}]={\frac {m}{s}}}$
The Isp defines how effective a propulsion system is. The higher the Isp the more powerful is the thrust applied to the rocket with the same fuel mass. The Isp is usually given in seconds but actually the physically correct unit is distance per time which is usually given in meters per second or feet per second. To avoid confusion which unit of speed is used, the physical correct Isp (in distance/time) is divided by the surface gravity of Earth (9.81 m/s²). This results in a value given in seconds. To use this Isp in formulas it must to be converted back into distance per time which requires multiplying with the surface gravity of Earth again. As this value is only used to convert between those two units, the specific impulse doesn't change when the gravity changes. It appears that KSP use a value like 9.82 m/s² and thus using a little less fuel.
As the specific impulse is the ratio of thrust and fuel flow ${\displaystyle {\frac {Ns}{kg}}}$ is sometimes given as the unit. This is mathematically another form of ${\displaystyle {\frac {m}{s}}=kg\cdot {\frac {m}{s^{2}}}\cdot {\frac {s}{kg}}}$ because force is the multiplication of mass and acceleration defining ${\displaystyle N=kg\cdot {\frac {m}{s^{2}}}}$. So ${\displaystyle 1{\frac {Ns}{kg}}=1{\frac {m}{s}}}$ with the latter being simply only in SI base units.
Sphere of influence
→ Main article: Sphere of influence
The radius around a celestial body within which its gravity well is non-negligible. Commonly known as SoI/SOI.
Tangential velocity
The component of the velocity that is tangential to the trajectory. Instantaneous velocity - velocity when the time of measurement approaches zero - is always tangential to the trajectory.
Thrust-to-weight ratio
${\displaystyle {\text{TWR}}={\frac {T}{W}}={\frac {T}{m\cdot g}}}$The Ratio between the total mass of the vehicle and the available thrust of all propulsion devices of the vehicle/current stage. A TWR greater than 1 means the craft will have enough thrust to accelerate vertically and gain altitude. A TWR below 1 means that the craft won't be able to counteract gravity and drag at low altitudes, although in space it only means that maneuvers will take longer. Because the weight (W) depends on the current gravitational acceleration (g) the TWR depends on which body is currently influencing the craft. The acceleration on the Mun's surface is only 16.6 % of Kerbin's acceleration, so at the surface a TWRKerbin = 1 would be a TWRMun = 6.