From CSTART Wiki
This page contains a glossary of commonly used terms related to spaceflight, which anyone can feel free to help us in expanding. It's primary purpose is to help make the material published by CSTART accessible to interested non-specialists from the general public. With this in mind, the explanations of terms should be kept reasonably short and simple.
This term has the same meaning as apihelion but is reserved for cases where the body involved is orbiting the Earth.
The point in an orbit when the orbiting body is furthest from the central body (e.g. "the satellite was last at apihelion 2 hours ago"), or the orbiting body's altitude above the central body when at this position (e.g "the satellite's apehelion was 100 km"). This is a generic term which can be used no matter what the central body is.
This term has the same meaning as apihelion but is reserved for cases where the body involved is orbiting the moon.
 Commercial Off the Shelf (COTS)
 Direct Ascent
A class of flight plan for landing humans on the surface of the moon and then returning them to Earth. It involves launching a single spacecraft from the Earth's surface to the lunar surface on a single trajectory, without entering either Earth orbit or lunar orbit and without any kind of rendezvous. Direct ascent flight plans are extremely straightforward, but are considered impractical because they require extremely heavy amounts of propellant.
NASA originally planned for the Apollo project to use a direct ascent flight plan, but this approach was eventually discarded in favour of a Lunar Orbit Rendezvous (LOR) flight plan to reduce launch mass and hence require a smaller launch vehicle.
 Earth Orbit Rendezvous (EOR)
 Extra Vehicular Activity (EVA)
Activity conducted by a human in space but outside of a spacecraft, e.g. a spacewalk or walking on the surface of the moon.
 Guidance Navigation and Control (GNC)
 Hybrid rocket engine
A rocket engine in which one of the propellants (almost always the fuel) takes the form of a solid, while the other (almost always the oxidiser) takes the form of a liquid. This is in contrast to a solid rocket engine (in which fuel and oxidiser are combined into a single solid propellant), and to a liquid rocket engine (in which both fuel and propellant are stored as separate liquids and mixed together just prior to combustion).
Hybrid rocket engines are very popular with small rocketry organisations due to their excellent trade off between factors such as cost, safety, simplicity and performance. Hybrids are safer (and hence cheaper) than solid or liquid rockets, simpler than liquid rockets (but not simpler than solids), and offer better performance than solid rockets (but worse performance than liquid rockets).
 In-Situ Resource Utilization (ISRU)
Utilising the natural resources of a celestial body other then Earth (e.g the moon, nearby asteroids, or Mars) while on that body. ISRU allows these resources to be utilised more cheaply than sending them back to Earth, and also lowers the amount of material which must be carried from Earth to another body in order to support long term habitation on that body. A common example is using water ice to produce drinking water, breathing oxygen, and liquid oxygen and liquid hydrogen for use as rocket fuel. ISRU has never actually been done before, but many consider it one of the most important things to aim for as part of a spacefaring future.
 Kármán Line
An invisible line (actually, technically a sphere) 100 km above the surface of the Earth which serves as the nearest thing there is to an official "boundary of space".
 Low Earth Orbit (LEO)
Any orbit around the Earth with an altitude between 200 km and 2000 km.
 Lunar Orbit Insertion (LOI)
A propulsive maneuver performed nearby the moon intended to result in the spacecraft ending up in a lunar orbit. This most commonly involves slowing down a fast-moving spacecraft so that it does not simply pass the moon by (or loop around it), but could in principle involve speeding up a slow-moving spacecraft so that it does not simply crash into the moon.
For fast-moving spacecraft coming from Earth (such as the Apollo spacecraft), LOI is a slow down maneuver, with a delta v of around 1000 m/s, making it the "medium" sized maneuver of a flight: much smaller than Trans Lunar Injection (TLI), but somewhat larger than Trans Earth Injection (TEI).
 Lunar Orbit Rendezvous (LOR)
A class of flight plan for landing humans on the surface of the moon and then returning them to Earth. It involves a main spacecraft composed of two separable parts: a lunar orbiter and a lunar lander. The orbiter and lander are sent into lunar orbit as a single, attached unit. Once in lunar orbit, the lander detaches from the orbiter and descends to the surface by itself. After surface activities are complete, the lander returns to lunar orbit and undergoes a rendezvous with the lunar orbiter. At this stage, either the combined spacecraft may return to Earth, or once all astronauts have returned to the orbiter, the lander is jettisoned and either left in lunar orbit or allowed to crash into the moon. LOR flight plans were used by the Apollo landings and the moon landing which the Soviet union planned and tested hardware for but never completed. The reason LOR is so popular is that it results in a low total propellant requirement (and hence low launch mass), because heavy equipment necessary to perform large orbital maneuvers or to support the crew during the trip to the moon do not need to be gently carried down to the moon's surface and then back up: only a lander, which can be made comparatively light, needs to undergo the costly descent and ascent maneuvers.
LOR flight plans are in contrast to Direct Ascent flight plans, and may either be considered separate from or combined with Earth Orbit Rendezvous (EOR) flight plans.
See also: Wikipedia
 Main Engine Cut Off (MECO)
This term has the same meaning as perihelion but is reserved for cases where the body involved is orbiting the Earth.
The point in an orbit when the orbiting body is closest to the central body (e.g. "the satellite was last at perihelion 2 hours ago"), or the orbiting body's altitude above the central body when at this position (e.g "the satellite's perihelion was 100 km"). This is a generic term which can be used no matter what the central body is.
This term has the same meaning as perihelion but is reserved for cases where the body involved is orbiting the moon.
 Reaction Control System (RCS)
Hardware system on a spacecraft designed to facilitate small grained control of position and orientation, as opposed to the system used to make large propulsive maneuvers like changes in orbit. Typical forms of RCS are small gas or liquid fueled rocket thrusters, and flywheels.
An adjective applied to trajectories, rockets, spacecraft, etc. which involve traveling into space (i.e. passing the Kármán line) but do not involve achieving orbit. Suborbital spaceflight, while by no means simple, is much easier than orbital spaceflight.
 Trans Earth Injection (TEI)
An orbital maneuver performed while in lunar orbit which puts a spacecraft on a trajectory bound for Earth.
TEI is a fairly "small" maneuver compared to others made in a return trip to the moon: the required delta v is around 700 m/s.
 Trans Lunar Injection (TLI)
An orbital maneuver performed while in Earth orbit (typically LEO) which puts a spacecraft on a trajectory bound for the moon. This does not necessarily involve reaching Earth escape velocity, as it is sufficient to remain in Earth orbit as long as that orbit has an apogee which matches the moon's orbital radius.
TLI is by far the "largest" maneuver involved in a trip to the moon: the required delta v is around 3200 m/s.
See also: Wikipedia.
 Thermal Protection System (TPS)
Hardware on a spacecraft which protects it from extremely high temperatures during atmospheric reentry. Failure of the TPS can result in the complete destruction of a spacecraft (e.g. Columbia disaster).