Orbit insertion maneuvers leave a spacecraft in a destination orbit. In contrast, orbit injection maneuvers occur when a spacecraft enters a transfer orbit, e.g.
trans-lunar injection (TLI),
trans-Mars injection (TMI) and
trans-Earth injection (TEI). These are generally larger than small trajectory correction maneuvers. Insertion, injection and sometimes initiation are used to describe entry into a
descent orbit, e.g. the
Powered Descent Initiation maneuver used for Apollo lunar landings.
Hohmann transfer In
orbital mechanics, the
Hohmann transfer orbit is an elliptical orbit used to transfer between two
circular orbits of different altitudes, in the same
plane. The orbital maneuver to perform the Hohmann transfer uses two engine impulses which move a
spacecraft onto and off the transfer orbit. This maneuver was named after
Walter Hohmann, the
German scientist who published a description of it in his 1925 book
Die Erreichbarkeit der Himmelskörper (
The Accessibility of Celestial Bodies). Hohmann was influenced in part by the German science fiction author
Kurd Laßwitz and his 1897 book
Two Planets.
Bi-elliptic transfer In
astronautics and
aerospace engineering, the
bi-elliptic transfer is an orbital maneuver that moves a
spacecraft from one
orbit to another and may, in certain situations, require less
delta-v than a
Hohmann transfer maneuver. The bi-elliptic transfer consists of two half
elliptic orbits. From the initial orbit, a delta-v is applied boosting the spacecraft into the first transfer orbit with an
apoapsis at some point r_b away from the
central body. At this point, a second delta-v is applied sending the spacecraft into the second elliptical orbit with
periapsis at the radius of the final desired orbit, where a third delta-v is performed, injecting the spacecraft into the desired orbit. While they require one more engine burn than a Hohmann transfer and generally requires a greater travel time, some bi-elliptic transfers require a lower amount of total delta-v than a Hohmann transfer when the ratio of final to initial
semi-major axis is 11.94 or greater, depending on the intermediate semi-major axis chosen. The idea of the bi-elliptical transfer trajectory was first published by
Ary Sternfeld in 1934.
Low energy transfer A
low energy transfer, or low energy
trajectory, is a route in space which allows spacecraft to change
orbits using very little fuel. These routes work in the
Earth-
Moon system and also in other systems, such as traveling between the
satellites of Jupiter. The drawback of such trajectories is that they take much longer to complete than higher energy (more fuel) transfers such as
Hohmann transfer orbits. Low energy transfer are also known as
weak stability boundary trajectories, or ballistic capture trajectories. Low energy transfers follow special pathways in space, sometimes referred to as the
Interplanetary Transport Network. Following these pathways allows for long distances to be traversed for little expenditure of
delta-v.
Orbital inclination change Orbital inclination change is an orbital maneuver aimed at changing the
inclination of an orbiting body's
orbit. This maneuver is also known as an orbital plane change as the plane of the orbit is tipped. This maneuver requires a change in the orbital velocity vector (
delta v) at the
orbital nodes (i.e. the point where the initial and desired orbits intersect, the line of orbital nodes is defined by the intersection of the two orbital planes). In general, inclination changes can require a great deal of delta-v to perform, and most mission planners try to avoid them whenever possible to conserve fuel. This is typically achieved by launching a spacecraft directly into the desired inclination, or as close to it as possible so as to minimize any inclination change required over the duration of the spacecraft life. Maximum efficiency of inclination change is achieved at
apoapsis, (or
apogee), where orbital velocity v\, is the lowest. In some cases, it may require less total delta v to raise the spacecraft into a higher orbit, change the orbit plane at the higher apogee, and then lower the spacecraft to its original altitude.
Constant-thrust trajectory Constant-thrust and
constant-acceleration trajectories involve the spacecraft firing its engine in a prolonged constant burn. In the limiting case where the vehicle acceleration is high compared to the local gravitational acceleration, the spacecraft points straight toward the target (accounting for target motion), and remains accelerating constantly under high thrust until it reaches its target. In this high-thrust case, the trajectory approaches a straight line. If it is required that the spacecraft rendezvous with the target, rather than performing a flyby, then the spacecraft must flip its orientation halfway through the journey, and decelerate the rest of the way. In the constant-thrust trajectory, the vehicle's acceleration increases during thrusting period, since the fuel use means the vehicle mass decreases. If, instead of constant thrust, the vehicle has constant acceleration, the engine thrust must decrease during the trajectory. This trajectory requires that the spacecraft maintain a high acceleration for long durations. For interplanetary transfers, days, weeks or months of constant thrusting may be required. As a result, there are no currently available spacecraft propulsion systems capable of using this trajectory. It has been suggested that some forms of nuclear (fission or fusion based) or antimatter powered rockets would be capable of this trajectory. More practically, this type of maneuver is used in low thrust maneuvers, for example with
ion engines,
Hall-effect thrusters, and others. These types of engines have very high specific impulse (fuel efficiency) but currently are only available with fairly low absolute thrust, meaning that the thrust must be applied over a prolonged period to achieve the necessary delta-v. The variation of this thrust throughout the trajectory needs to be described by a comparatively large number of variables, meaning that computing such low thrust maneuvers is usually substantially more time-consuming, and less conceptually straightforward, than most high thrust impulsive maneuvers. ==Rendezvous and docking==