Orbital maneuver
[2] A rocket applies acceleration to itself (a thrust) by expelling part of its mass at high speed.
With a good approximation of the delta-v budget designers can estimate the propellant required for planned maneuvers.
The off-set of the velocity vector after the end of real burn from the velocity vector at the same time resulting from the theoretical impulsive maneuver is only caused by the difference in gravitational force along the two paths (red and black in figure 1) which in general is small.
'Non-impulsive' refers to the momentum changing slowly over a long time, as in electrically powered spacecraft propulsion, rather than by a short impulse.
It is named after Hermann Oberth, the Austro-Hungarian-born, German physicist and a founder of modern rocketry, who apparently first described the effect.
[5] The Oberth effect is used in a powered flyby or Oberth maneuver where the application of an impulse, typically from the use of a rocket engine, close to a gravitational body (where the gravity potential is low, and the speed is high) can give much more change in kinetic energy and final speed (i.e. higher specific energy) than the same impulse applied further from the body for the same initial orbit.
The "assist" is provided by the motion (orbital angular momentum) of the gravitating body as it pulls on the spacecraft.
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).
[7] Hohmann was influenced in part by the German science fiction author Kurd Laßwitz and his 1897 book Two Planets.
Low energy transfers follow special pathways in space, sometimes referred to as the Interplanetary Transport Network.
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.
Maximum efficiency of inclination change is achieved at apoapsis, (or apogee), where orbital velocity
[12] Constant-thrust and constant-acceleration trajectories involve the spacecraft firing its engine in a prolonged constant burn.
It has been suggested that some forms of nuclear (fission or fusion based) or antimatter powered rockets would be capable of this trajectory.
These types of engines have very high specific impulse (fuel efficiency) but currently are only available with fairly low absolute thrust.
Rendezvous is commonly followed by docking or berthing, procedures which bring the spacecraft into physical contact and create a link between them.
