Free return trajectory
Encyclopedia
A free return trajectory is one of a very small sub-class of trajectories in which the trajectory of a satellite
traveling away from a primary body (for example, the Earth
) is modified by the presence of a secondary body (for example, the Moon
) causing the satellite to return to the primary body. This method has been used by several spacecraft
, most notably the Apollo 8
, Apollo 10
, and Apollo 11
lunar missions. Those spacecraft were launched into a free-return trajectory to allow their safe return in the event of a systems failure after launch. They all successfully inserted into orbit upon arriving at the Moon, and so did not take advantage of the free return. Due to the landing site restrictions that resulted from constraining the launch to a free return that flew by the Moon, subsequent Apollo missions, starting with Apollo 12
and including the ill-fated Apollo 13
, used a hybrid trajectory that launched to a highly elliptical Earth orbit that fell short of the Moon with effectively a free return to the atmospheric entry corridor. They then performed a mid-course maneuver to change to a trans-Lunar trajectory that was not a free return. This retained the safety characteristics of being on a free return upon launch, and only departed from free return once the systems were checked out and the lunar module was docked with the command module, providing back-up maneuver capabilities. In fact, within hours of the accident, Apollo 13 used the lunar module to maneuver from its planned lunar orbit insertion trajectory to a free return trajectory. Apollo 13 was the only Apollo mission to actually complete a free return trajectory.
A true free return trajectory is completed without the assistance of any mid-course corrections or maneuvers. The free return trajectory allows a great distance to be covered relatively directly without the use of additional fuel
to return the satellite to the primary body, thus the term "free". The satellite or spacecraft will not actually be captured by the secondary body without the use of its rocket engine
to slow the craft.
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
traveling away from a primary body (for example, the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
) is modified by the presence of a secondary body (for example, the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
) causing the satellite to return to the primary body. This method has been used by several spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
, most notably the Apollo 8
Apollo 8
Apollo 8, the second manned mission in the American Apollo space program, was the first human spaceflight to leave Earth orbit; the first to be captured by and escape from the gravitational field of another celestial body; and the first crewed voyage to return to Earth from another celestial...
, Apollo 10
Apollo 10
Apollo 10 was the fourth manned mission in the American Apollo space program. It was an F type mission—its purpose was to be a "dry run" for the Apollo 11 mission, testing all of the procedures and components of a Moon landing without actually landing on the Moon itself. The mission included the...
, and Apollo 11
Apollo 11
In early 1969, Bill Anders accepted a job with the National Space Council effective in August 1969 and announced his retirement as an astronaut. At that point Ken Mattingly was moved from the support crew into parallel training with Anders as backup Command Module Pilot in case Apollo 11 was...
lunar missions. Those spacecraft were launched into a free-return trajectory to allow their safe return in the event of a systems failure after launch. They all successfully inserted into orbit upon arriving at the Moon, and so did not take advantage of the free return. Due to the landing site restrictions that resulted from constraining the launch to a free return that flew by the Moon, subsequent Apollo missions, starting with Apollo 12
Apollo 12
Apollo 12 was the sixth manned flight in the American Apollo program and the second to land on the Moon . It was launched on November 14, 1969 from the Kennedy Space Center, Florida, four months after Apollo 11. Mission commander Charles "Pete" Conrad and Lunar Module Pilot Alan L...
and including the ill-fated Apollo 13
Apollo 13
Apollo 13 was the seventh manned mission in the American Apollo space program and the third intended to land on the Moon. The craft was launched on April 11, 1970, at 13:13 CST. The landing was aborted after an oxygen tank exploded two days later, crippling the service module upon which the Command...
, used a hybrid trajectory that launched to a highly elliptical Earth orbit that fell short of the Moon with effectively a free return to the atmospheric entry corridor. They then performed a mid-course maneuver to change to a trans-Lunar trajectory that was not a free return. This retained the safety characteristics of being on a free return upon launch, and only departed from free return once the systems were checked out and the lunar module was docked with the command module, providing back-up maneuver capabilities. In fact, within hours of the accident, Apollo 13 used the lunar module to maneuver from its planned lunar orbit insertion trajectory to a free return trajectory. Apollo 13 was the only Apollo mission to actually complete a free return trajectory.
A true free return trajectory is completed without the assistance of any mid-course corrections or maneuvers. The free return trajectory allows a great distance to be covered relatively directly without the use of additional fuel
Rocket propellant
Rocket propellant is mass that is stored in some form of propellant tank, prior to being used as the propulsive mass that is ejected from a rocket engine in the form of a fluid jet to produce thrust. A fuel propellant is often burned with an oxidizer propellant to produce large volumes of very hot...
to return the satellite to the primary body, thus the term "free". The satellite or spacecraft will not actually be captured by the secondary body without the use of its rocket engine
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
to slow the craft.