Parking orbit
Encyclopedia
A parking orbit is a temporary orbit used during the launch of a satellite
or other space probe
. A launch vehicle boosts into the parking orbit, then coasts for a while, then fires again to enter the final desired trajectory. The alternative to a parking orbit is direct injection, where the rocket fires continuously (except during staging) until its fuel is exhausted, ending with the payload on the final trajectory.
The figure shows the first two reasons. For this lunar mission, the desired location for the final firing is originally over southern Africa. As the day progresses, this point stays essentially fixed while the Earth moves underneath, and this is compensated for by changing the launch angle.
conditions. Furthermore the length of two of the burns (the initial injection burn, and the final burn) typically depend on where in the launch window the launch occurs. To do this without wasting fuel, a rocket stage that can fire, then stop, then start again is needed. This implies a liquid fuel engine since solid fuel rockets cannot be stopped or restarted - once ignited they burn to completion. But even in a liquid fueled engine, this multiple restart capability is non-trivial for a number of reasons:
The Centaur
and Agena families of upper stages were designed for such restarts and have often been used in this manner. (The last Agena flew in 1987 but Centaur is still in production.)
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....
or other space probe
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...
. A launch vehicle boosts into the parking orbit, then coasts for a while, then fires again to enter the final desired trajectory. The alternative to a parking orbit is direct injection, where the rocket fires continuously (except during staging) until its fuel is exhausted, ending with the payload on the final trajectory.
Rationale for parking orbits
There are several reasons why a parking orbit may be used:- It can increase the launch windowLaunch windowLaunch window is a term used in spaceflight to describe a time period in which a particular launch vehicle must be launched. If the rocket does not launch within the "window", it has to wait for the next window....
. For earth-escape missions, these are often quite short (seconds to minutes) if no parking orbit is used. With a parking orbit, these can often be increased up to several hours. - For non-LEOLow Earth orbitA low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
missions, the desired location for the final burn may not be in a convenient spot. In particular, for earth-escape missions that want good northern coverage of the trajectory, the correct place for the final burn is often in the southern hemisphere. - For geostationary orbitGeostationary orbitA geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
missions, the correct spot for the final (or next to final) firing is normally on the equator. In such a case, the rocket is launched, coasts in a parking orbit until it is over the equator, then fires again into a geostationary transfer orbitGeostationary transfer orbitA geosynchronous transfer orbit or geostationary transfer orbit is a Hohmann transfer orbit used to reach geosynchronous or geostationary orbit....
. - For manned lunar missions, a parking orbit allowed some checkout while still close to home, before committing to the lunar trip.
- It is needed if the desired orbit has a high perigeePerigeePerigee is the point at which an object makes its closest approach to the Earth.. Often the term is used in a broader sense to define the point in an orbit where the orbiting body is closest to the body it orbits. The opposite is the apogee, the farthest or highest point.The Greek prefix "peri"...
. In this case the booster launches into an elliptical parking orbit, then coasts until a higher point in the orbit, then fires again to raise the perigee. See Hohmann transfer orbitHohmann transfer orbitIn orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits, typically both in the same plane....
. In this case the use of a parking orbit can also reduce the fuel usage of an inclination change, since these take less delta-V at high elevations.
The figure shows the first two reasons. For this lunar mission, the desired location for the final firing is originally over southern Africa. As the day progresses, this point stays essentially fixed while the Earth moves underneath, and this is compensated for by changing the launch angle.
Disadvantages of parking orbits
The most notable disadvantage is that a rocket needs to coast for a while, then restart while under zero gWeightlessness
Weightlessness is the condition that exists for an object or person when they experience little or no acceleration except the acceleration that defines their inertial trajectory, or the trajectory of pure free-fall...
conditions. Furthermore the length of two of the burns (the initial injection burn, and the final burn) typically depend on where in the launch window the launch occurs. To do this without wasting fuel, a rocket stage that can fire, then stop, then start again is needed. This implies a liquid fuel engine since solid fuel rockets cannot be stopped or restarted - once ignited they burn to completion. But even in a liquid fueled engine, this multiple restart capability is non-trivial for a number of reasons:
- During the coast, the propellants will drift away from the bottom of the tank and the pump inlets. This must be dealt with in some way. Either tanks with diaphragms, or ullage rocketsUllage motorUllage motors are relatively small, independently-fueled rocket engines that may be fired to accelerate the rocket prior to main engine ignition, when the vehicle is in a zero-g situation....
for settling the propellant back to the bottom of the tank, are needed. - Longer life batteries and other consumables are needed.
- Some engines use special chemicals for ignition; multiple sets are needed for re-starts
- Better insulation is needed, particularly on cryogenicCryogenicsIn physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit,...
tanks, to prevent too much propellant boiloff during coast. - A better inertial guidance system is needed, to keep track of the state during the coast.
- A reaction control systemReaction control systemA reaction control system is a subsystem of a spacecraft whose purpose is attitude control and steering by the use of thrusters. An RCS system is capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow...
is needed, to orient the stage properly for the final burn, and perhaps to establish a suitable thermal orientation during coast.
The Centaur
Centaur (rocket stage)
Centaur is a rocket stage designed for use as the upper stage of space launch vehicles. Centaur boosts its satellite payload to geosynchronous orbit or, in the case of an interplanetary space probe, to or near to escape velocity...
and Agena families of upper stages were designed for such restarts and have often been used in this manner. (The last Agena flew in 1987 but Centaur is still in production.)
Examples
- The Apollo program used parking orbits, for all the reasons mentioned above except those that pertain to geostationary orbits.
- The Space ShuttleSpace ShuttleThe Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
missions to the International Space StationInternational Space StationThe International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...
did not use parking orbits, for several reasons. The station is in a high inclination Low Earth orbitLow Earth orbitA low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
, where parking orbits are not much help; the shuttle did not have multiple restart capability; and the short launch windows were not a critical problem (another one would occur a day later, due to the orbital geometry).
- On the other hand, when the shuttle launched interplanetary probes such as Galileo, it used a parking orbit to deliver the probe to the right injection spot.
- The Ariane 5Ariane 5Ariane 5 is, as a part of Ariane rocket family, an expendable launch system used to deliver payloads into geostationary transfer orbit or low Earth orbit . Ariane 5 rockets are manufactured under the authority of the European Space Agency and the Centre National d'Etudes Spatiales...
does not use parking orbits. This simplifies the launcher since multiple restart is not needed, and the penalty is small for their typical GTOGeostationary transfer orbitA geosynchronous transfer orbit or geostationary transfer orbit is a Hohmann transfer orbit used to reach geosynchronous or geostationary orbit....
mission, as their launch site is close to the equator. An upgrade to the second stage (ESC-B) will have multiple re-start capability, so future missions may use parking orbits.
- In a literal example of a parking orbit, the Automated Transfer VehicleAutomated Transfer VehicleThe Automated Transfer Vehicle or ATV is an expendable, unmanned resupply spacecraft developed by the European Space Agency . ATVs are designed to supply the International Space Station with propellant, water, air, payload and experiments...
(ATV) can park for several months in orbit while waiting to rendezvous with the International Space StationInternational Space StationThe International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...
. For safety reasons, the ATV cannot approach the station while a Space ShuttleSpace ShuttleThe Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
is docked or when a Soyuz or ProgressProgress spacecraftThe Progress is a Russian expendable freighter spacecraft. The spacecraft is an unmanned resupply spacecraft during its flight but upon docking with a space station, it allows astronauts inside, hence it is classified manned by the manufacturer. It was derived from the Soyuz spacecraft, and is...
are maneuvering to dock or depart.