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Spacecraft propulsion is any method used to change the velocity of spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by exhausting a gas from the back/rear of the vehicle at very high speed through a rocket engine nozzle. This sort of engine is called a Spacecraft propulsion#Rocket engines.

All current spacecraft use chemical rockets (bipropellant rocket or solid rocket) for launch, though some (such as the Pegasus rocket and SpaceShipOne) have used air-breathing engines on their Multistage rocket. Most satellites have simple reliable chemical rockets (often monopropellant rockets) or resistojet rockets to keep their station, although some use momentum wheels for attitude control. Newer geo-orbiting spacecraft are starting to use electric propulsion for north-south stationkeeping. Interplanetary vehicles mostly use chemical rockets as well, although a few have experimentally used ion thrusters (a form of electric propulsion) with some success.

The necessity for propulsion system Artificial satellites must be Rocket launch into orbit, and once there they must be placed in their nominal orbit. Once in the desired orbit, they often need some form of attitude control so that they are correctly pointed with respect to the Earth, the Sun, and possibly some astronomy object of interest.{{cite news| author=Hess, M.; Martin, K. K.; Rachul, L. J. | title=Thrusters Precisely Guide EO-1 Satellite in Space First | publisher=NASA | date=February 7, 2002 | url=http://www.gsfc.nasa.gov/news-release/releases/2002/02-020.htm Hi Watcha Doin| accessdate=2007-07-30 --> They are also subject to [Atmospheric drag from the thin [Earth's atmosphere, so that to stay in orbit for a long period of time some form of propulsion is occasionally necessary to make small corrections ([orbital stationkeeping).{{cite web | last=Phillips | first=Tony | date=May 30, 2000 | url=http://science.nasa.gov/headlines/y2000/ast30may_1m.htm | title=Solar S'Mores | publisher=NASA | accessdate=2007-07-30 --> Many satellites need to be moved from one orbit to another from time to time, and this also requires propulsion.{{cite web | last=Olsen | first=Carrie | date=September 21, 1995 | url=http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/hohmann.html | title=Hohmann Transfer & Plane Changes | publisher=NASA | accessdate=2007-07-30 --> When a satellite has exhausted its ability to adjust its orbit, its useful life is over.

Spacecraft designed to travel further also need propulsion methods. They need to be launched out of the Earth's atmosphere just as satellites do. Once there, they need to leave orbit and move around.

For interplanetary travel, a spacecraft must use its engines to leave Earth orbit. Once it has done so, it must somehow make its way to its destination. Current interplanetary spacecraft do this with a series of short-term trajectory adjustments.{{cite web| author=Staff | date=April 24, 2007 | url=http://mars.jpl.nasa.gov/odyssey/mission/cruise.html | title=Interplanetary Cruise | publisher=NASA | work=2001 Mars Odyssey | accessdate=2007-07-30 --> In between these adjustments, the spacecraft simply [freefall along its orbit. The simplest fuel-efficient means to move from one circular orbit to another is with a [Hohmann transfer orbit: the spacecraft begins in a roughly circular orbit around the Sun. A short period of [thrust in the direction of motion accelerates or decelerates the spacecraft into an elliptical orbit around the Sun which is tangential to its previous orbit and also to the orbit of its destination. The spacecraft falls freely along this elliptical orbit until it reaches its destination, where another short period of thrust accelerates or decelerates it to match the orbit of its destination.{{cite news | first=Dave | last=Doody | title=Chapter 4. Interplanetary Trajectories | work=Basics of Space Flight | publisher=NASA JPL | date=February 7, 2002 | url=http://www2.jpl.nasa.gov/basics/bsf4-1.html | accessdate=2007-07-30 --> Special methods such as [aerobraking are sometimes used for this final orbital adjustment.{{cite conference | first=S. | last=Hoffman | title=A comparison of aerobraking and aerocapture vehicles for interplanetary missions | booktitle =AIAA and AAS, Astrodynamics Conference | pages=25 p. | publisher=American Institute of Aeronautics and Astronautics | date=August 20-22, 1984 | location=Seattle, Washington | url=http://www.aiaa.org/content.cfm?pageid=406&gTable=mtgpaper&gID=44030 | accessdate = 2007-07-31 -->

Some spacecraft propulsion methods such as solar sails provide very low but inexhaustible thrust;{{cite web| author=Anonymous | year=2007 | url=http://www.planetary.org/programs/projects/innovative_technologies/solar_sailing/facts.html | title=Basic Facts on Cosmos 1 and Solar Sailing | publisher=The Planetary Society | accessdate=2007-07-26 --> an interplanetary vehicle using one of these methods would follow a rather different trajectory, either constantly thrusting against its direction of motion in order to decrease its distance from the Sun or constantly thrusting along its direction of motion to increase its distance from the Sun.

Spacecraft for interstellar travel also need propulsion methods. No such spacecraft has yet been built, but many designs have been discussed. Since interstellar distances are very great, a tremendous velocity is needed to get a spacecraft to its destination in a reasonable amount of time. Acquiring such a velocity on launch and getting rid of it on arrival will be a formidable challenge for spacecraft designers.{{cite web| last=Rahls | first=Chuck | date=December 07, 2005 | url=http://www.physorg.com/news8817.html | title=Interstellar Spaceflight: Is It Possible? | publisher=Physorg.com | accessdate=2007-07-31 -->

Effectiveness of propulsion systems When in space, the purpose of a propulsion system is to change the velocity, or v, of a spacecraft. Since this is more difficult for more massive spacecraft, designers generally discuss momentum, mv. The amount of change in momentum is called impulse.{{cite web| last=Zobel | first=Edward A. | year=2006 | url=http://id.mind.net/~zona/mstm/physics/mechanics/momentum/introductoryProblems/momentumSummary2.html | title=Summary of Introductory Momentum Equations | publisher=Zona Land | accessdate=2007-08-02 --> So the goal of a propulsion method in space is to create an impulse.

When launching a spacecraft from the Earth, a propulsion method must overcome a higher gravity drag pull to provide a net positive acceleration.{{cite web| last = Benson | first = Tom | url=http://exploration.grc.nasa.gov/education/rocket/guided.htm | title=Guided Tours: Beginner's Guide to Rockets | publisher=NASA | accessdate = 2007-08-02 --> In orbit, the spacecraft tangential velocity provides a centrifugal acceleration that counteracts the acceleration due to gravity at a given path (which is actually the orbit path) so that any additional impulse, even very tiny, will result in a change in the orbit path.

The rate of change of velocity is called acceleration, and the rate of change of momentum is called force. To reach a given velocity, one can apply a small acceleration over a long period of time, or one can apply a large acceleration over a short time. Similarly, one can achieve a given impulse with a large force over a short time or a small force over a long time. This means that for maneuvering in space, a propulsion method that produces tiny accelerations but runs for a long time can produce the same impulse as a propulsion method that produces large accelerations for a short time. When launching from a planet, tiny accelerations cannot overcome the planet's gravitational pull and so cannot be used.

The Earth's surface is situated fairly deep in a gravity well and it takes a velocity of 11.2 kilometers/second (escape velocity) or more to escape from it. As human beings evolved in a gravitational field of 1g (9.8 m/s²), an ideal propulsion system would be one that provides a continuous acceleration of 1g (though human bodies can tolerate much larger accelerations over short periods). The occupants of a rocket or spaceship having such a propulsion system would be free from all the ill effects of free fall, such as nausea, muscular weakness, reduced sense of taste, or leaching of calcium from their bones.

The law of conservation of momentum means that in order for a propulsion method to change the momentum of a space craft it must change the momentum of something else as well. A few designs take advantage of things like magnetic fields or light pressure in order to change the spacecraft's momentum, but in free space the rocket must bring along some mass to accelerate away in order to push itself forward. Such mass is called reaction mass.

In order for a rocket to work, it needs two things: reaction mass and energy. The impulse provided by launching a particle of reaction mass having mass m at velocity v is mv. But this particle has kinetic energy mv²/2, which must come from somewhere. In a conventional solid rocket, liquid rocket, or hybrid rocket, the fuel is burned, providing the energy, and the reaction products are allowed to flow out the back, providing the reaction mass. In an ion thruster, electricity is used to accelerate ions out the back. Here some other source must provide the electrical energy (perhaps a Photovoltaic module or a nuclear reactor), while the ions provide the reaction mass.

When discussing the efficiency of a propulsion system, designers often focus on effectively using the reaction mass. Reaction mass must be carried along with the rocket and is irretrievably consumed when used. One way of measuring the amount of impulse that can be obtained from a fixed amount of reaction mass is the specific impulse, the impulse per unit weight-on-Earth (typically designated by I_{sp}). The unit for this value is seconds. Since the weight on Earth of the reaction mass is often unimportant when discussing vehicles in space, specific impulse can also be discussed in terms of impulse per unit mass. This alternate form of specific impulse uses the same units as velocity (e.g. m/s), and in fact it is equal to the effective exhaust velocity of the engine (typically designated v_{e}). Confusingly, both values are sometimes called specific impulse. The two values differ by a factor of standard gravity, the acceleration due to gravity on the Earth's surface (I_{sp} g = v_{e}).

A rocket with a high exhaust velocity can achieve the same impulse with less reaction mass. However, the energy required for that impulse is proportional to the square of the exhaust velocity, so that more mass-efficient engines require much more energy, and are typically less energy efficient. This is a problem if the engine is to provide a large amount of thrust. To generate a large amount of impulse per second, it must use a large amount of energy per second. So highly (mass) efficient engines require enormous amounts of energy per second to produce high thrusts. As a result, most high-efficiency engine designs also provide very low thrust.

Delta-v and propellant use s versus final velocity, as calculated from the rocket equation.Burning the entire usable propellant of a spacecraft through the engines in a straight line in free space would produce a net velocity change to the vehicle; this number is termed 'delta-v' (\Delta v).

If the exhaust velocity is constant then the total \Delta v of a vehicle can be calculated using the rocket equation, where M is the mass of fuel (or rather the mass of propellant), P is the mass of the payload (including the rocket structure), and v_e is the velocity of the rocket exhaust. This is known as the Tsiolkovsky rocket equation:

\Delta v = -v_e \ln \left(\frac{M+P}{P}\right)

For historical reasons, as discussed above, v_e is sometimes written as

v_e = I_{sp} g_{o}

where I_{sp} is the specific impulse of the rocket, measured in seconds, and g_{o} is the gravitational acceleration at sea level.

For a high delta-v mission, the majority of the spacecraft's mass needs to be reaction mass. Since a rocket must carry all of its reaction mass, most of the initially-expended reaction mass goes towards accelerating reaction mass rather than payload. If the rocket has a payload of mass P, the spacecraft needs to change its velocity by\Delta v, and the rocket engine has exhaust velocity ve, then the mass M of reaction mass which is needed can be calculated using the rocket equation and the formula for I_{sp}:

M = P \left(e^{\Delta v/v_e}-1\right)

For \Delta v much smaller than ve, this equation is roughly linear, and little reaction mass is needed. If \Delta v is comparable to ve, then there needs to be about twice as much fuel as combined payload and structure (which includes engines, fuel tanks, and so on). Beyond this, the growth is exponential; speeds much higher than the exhaust velocity require very high ratios of fuel mass to payload and structural mass.

Energy use Some energy must go into accelerating the reaction mass. Every engine will waste some energy, but even assuming 100% efficiency, the engine will need energy amounting to

\begin{matrix} \frac{1}{2} \end{matrix} Mv_e^2

Comparing the rocket equation (which shows how much energy ends up in the final vehicle) and the above equation (which shows the total energy required) shows that even with 100% engine efficiency, certainly not all energy supplied ends up in the vehicle - some of it, indeed usually most of it, ends up as kinetic energy of the exhaust.

Interestingly, if the I_{sp} is fixed, for a mission delta-v, there is a particular I_{sp} that minimises the overall energy used by the rocket. This comes to an exhaust velocity of about ⅔ of the mission delta-v (see Tsiolkovsky rocket equation#Energy). Drives with a specific impulse that is both high and fixed such as Ion thrusters have exhaust velocities that can be enormously higher than this ideal, and thus end up powersource limited and give very low thrust. Where the vehicle performance is power limited, e.g. if solar energy or nuclear power is used, then in the case of a large v_{e} the maximum acceleration is inversely proportional to it. Hence the time to reach a required delta-v is proportional to v_{e}. Thus the latter should not be too large.

On the other hand if the exhaust velocity can be made to vary so that at each instant it is equal and opposite to the vehicle velocity then the absolute minimum energy usage is achieved. When this is achieved, the exhaust stops in space and has no kinetic energy; and all the energy ends up in the vehicle (in principle such a drive would be 100% efficient, in practice there would be thermal losses from within the drive system and residual heat in the exhaust). However in most cases this uses an impractical quantity of propellant, but is a useful theoretical consideration.

Some drives (such as Variable specific impulse magnetoplasma rocket or Electrodeless plasma thruster ) actually can significantly vary their exhaust velocity. This can help reduce propellant usage and improve acceleration at different stages of the flight. However the best energetic performance and acceleration is still obtained when the exhaust velocity is close to the vehicle speed. Proposed ion and plasma drives usually have exhaust velocities enormously higher than that ideal (in the case of VASIMR the lowest quoted speed is around 15000 m/s compared to a mission delta-v from high Earth orbit to Mars of about Delta-v#Delta-vs around the Solar System).

For a mission, for example, when launching from or landing on a planet, the effects of gravitational attraction and any atmospheric drag must be overcome by using fuel. It is typical to combine the effects of these and other effects into an effective mission delta-v. For example a launch mission to low Earth orbit requires about 9.3-10 km/s delta-v. These mission delta-vs are typically numerically integrated on a computer.

Example Suppose we want to send a 10,000 kg space probe to Mars. The required \Delta v from Low Earth orbit is approximately 3000 m/s, using a Hohmann transfer orbit. (A manned probe would need to take a faster route and use more fuel). For the sake of argument, let us say that the following thrusters may be used:

{]
(m/s)!Specific impulse
(s)!Fuel mass
(kg)!Energy required
(GJ)!Energy per kg
of propellant!minimum power
per N of thrust|-|Solid rocket
]
|5,000|500|8,200|103|12.6 MJ|2.5 kW|-|Ion thruster. This would need to be carried on the vehicle, which is clearly impractical.

Instead, a much smaller, less powerful generator may be included which will take much longer to generate the total energy needed. This lower power is only sufficient to accelerate a tiny amount of fuel per second, but over long periods the velocity will be finally achieved. For example. it took the [Smart 1 more than a year to reach the Moon, while with a chemical rocket it takes a few days. Because the ion drive needs much less fuel, the total launched mass is usually lower, which typically results in a lower overall cost.

Mission planning frequently involves adjusting and choosing the propulsion system according to the mission delta-v needs, so as to minimise the total cost of the project, including trading off greater or lesser use of fuel and launch costs of the complete vehicle.

Propulsion methods Propulsion methods can be classified based on their means of accelerating the reaction mass. There are also some special methods for launches, planetary arrivals, and landings.

Rocket engines Most rocket engines are internal combustion engine heat engines (although non combusting forms exist). Rocket engines generally produce a high temperature reaction mass, as a hot gas. This is achieved by combusting a solid, liquid or gaseous fuel with an oxidiser within a combustion chamber. The extremely hot gas is then allowed to escape through a high-expansion ratio de Laval nozzle. This bell-shaped nozzle is what gives a rocket engine its characteristic shape. The effect of the nozzle is to dramatically accelerate the mass, converting most of the thermal energy into kinetic energy. Exhaust speeds as high as 10 times the speed of sound at sea level are common.

Rockets emitting plasma can potentially carry out reactions inside a magnetic bottle and release the plasma via a magnetic nozzle, so that no solid matter need come in contact with the plasma. Of course, the machinery to do this is complex, but research into nuclear fusion has developed methods, some of which have been used in speculative propulsion systems.

See rocket engine for a listing of various kinds of rocket engines using different heating methods, including chemical, electrical, solar, and nuclear.

Electromagnetic acceleration of reaction mass Rather than relying on high temperature and fluid dynamics to accelerate the reaction mass to high speeds, there are a variety of methods that use electrostatic or electromagnetism forces to accelerate the reaction mass directly. Usually the reaction mass is a stream of ions. Such an engine very typically uses electric power, first to ionise atoms, and then uses a voltage gradient to accelerate the ions to high exhaust velocities.

For these drives, at the highest exhaust speeds energetic efficiency and thrust are all inversely proportional to exhaust velocity. Their very high exhaust velocity means they require huge amounts of energy and thus with practical power sources provide low thrust, but use hardly any fuel.

For some missions, particularly reasonably close to the Sun, solar energy may be sufficient, and has very often been used, but for others further out or at higher power, nuclear energy is necessary; engines drawing their power from a nuclear source are called nuclear electric rockets.

With any current source of electrical power, chemical, nuclear or solar, the maximum amount of power that can be generated limits the amount of thrust that can be produced to a small value. Power generation adds significant mass to the spacecraft, and ultimately the weight of the power source limits the performance of the vehicle.

Current nuclear power generators are approximately half the weight of solar panels per watt of energy supplied, at terrestrial distances from the Sun. Chemical power generators are not used due to the far lower total available energy. Beamed power to the spacecraft shows some potential. However, the dissipation of waste heat from any power plant may make any propulsion system requiring a separate power source infeasible for interstellar travel.

Some electromagnetic methods:

Systems without reaction mass carried within the spacecraft The conservation law of momentum states that any engine which uses no reaction mass cannot move the center of mass of a spaceship (changing orientation, on the other hand, is possible). But space is not empty, especially space inside the Solar System; there are gravitation fields, magnetic fields, solar wind and solar radiation. Various propulsion methods try to take advantage of these. However, since these phenomena are diffuse in nature, corresponding propulsion structures need to be proportionately large.

There are several different space drives that need little or no reaction mass to function. A tether propulsion system employs a long cable with a high tensile strength to change a spacecraft's orbit, such as by interaction with a planet's magnetic field or through momentum exchange with another object.{{cite news| first=Dave | last=Drachlis | title=NASA calls on industry, academia for in-space propulsion innovations | publisher=NASA | date=October 24, 2002 | url=http://www.msfc.nasa.gov/news/news/releases/2002/02-269.html | accessdate=2007-07-26 --> [Solar sails rely on [radiation pressure from electromagnetic energy, but they require a large collection surface to function effectively. The [magnetic sail deflects charged particles from the [solar wind with a magnetic field, thereby imparting momentum to the spacecraft. A variant is the [mini-magnetospheric plasma propulsion system, which uses a small cloud of plasma held in a magnetic field to deflect the Sun's charged particles.

For changing the orientation of a satellite or other space vehicle, conservation of angular momentum does not pose a similar constraint. Thus many satellites use momentum wheels to control their orientations. These cannot be the only system for controlling satellite orientation, as the angular momentum built up due to torques from external forces such as solar, magnetic or tidal forces eventually needs to be "bled off" using a secondary system.

Gravitational slingshots can also be used to carry a probe onward to other destinations.

Launch mechanisms High thrust is of vital importance for Earth launch, thrust has to be greater than weight (see also gravity drag). Many of the propulsion methods above give a thrust/weight ratio of much less than 1, and so cannot be used for launch.

All current spacecraft use chemical rocket engines (bipropellant rocket or solid rocket) for launch. Other power sources such as nuclear have been proposed, and tested, but safety, environmental and political considerations have so far curtailed their use.

One advantage that spacecraft have in launch is the availability of infrastructure on the ground to assist them. Proposed ground-assisted launch mechanisms include:



Airbreathing engines for launch Studies generally show that conventional air-breathing engines, such as ramjets or turbojets are basically too heavy (have too low a thrust/weight ratio) to give any significant performance improvement when installed on a launch vehicle itself. However, launch vehicles can be air launched from separate lift vehicles (e.g. B-29 Superfortress, Pegasus rocket and Scaled Composites White Knight) which do use such propulsion systems.

On the other hand, very lightweight or very high speed engines have been proposed that take advantage of the air during ascent: | author=Anonymous | year=2006 | url=http://www.reactionengines.co.uk/sabre.html | title=The Sabre Engine | publisher=Reaction Engines Ltd. | accessdate=2007-07-26 -->
 

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