Spacecraft propulsion is any method used to accelerate 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....

 and artificial satellite

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....

s. 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 forcing a gas from the back/rear of the vehicle at very high speed through a supersonic de Laval nozzle. This sort of engine

Engine

An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...

 is called a 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...

.

All current spacecraft use chemical rockets (bipropellant or solid-fuel

Solid rocket

A solid rocket or a solid-fuel rocket is a rocket engine that uses solid propellants . The earliest rockets were solid-fuel rockets powered by gunpowder; they were used by the Chinese in warfare as early as the 13th century and later by the Mongols, Arabs, and Indians.All rockets used some form of...

) for launch, though some (such as the Pegasus rocket

Pegasus rocket

The Pegasus rocket is a winged space launch vehicle capable of carrying small, unmanned payloads into low Earth orbit. It is air-launched, as part of an expendable launch system developed by Orbital Sciences Corporation . Three main stages burning solid propellant provide the thrust...

 and SpaceShipOne) have used air-breathing engines on their first stage

Multistage rocket

A multistage rocket is a rocket that usestwo or more stages, each of which contains its own engines and propellant. A tandem or serial stage is mounted on top of another stage; a parallel stage is attached alongside another stage. The result is effectively two or more rockets stacked on top of or...

. Most satellites have simple reliable chemical thrusters (often monopropellant rocket

Monopropellant rocket

A monopropellant rocket is a rocket that uses a single chemical as its propellant.-Chemical-reaction monopropellant rockets:...

s) or resistojet rocket

Resistojet rocket

A resistojet is a method of Spacecraft propulsion that provides thrust by heating a fluid. Heating is usually achieved by sending electricity through a resistor consisting of a hot incandescent filament, with the expanded gas expelled through a conventional nozzle.Resistojets have been flown in...

s for orbital station-keeping and some use momentum wheel

Momentum wheel

A reaction wheel is a type of flywheel used primarily by spacecraft for attitude control without using fuel for rockets or other reaction devices....

s for attitude control. Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north-south stationkeeping. Interplanetary vehicles mostly use chemical rockets as well, although a few have used ion thruster

Ion thruster

An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and...

s and Hall effect thruster

Hall effect thruster

In spacecraft propulsion, a Hall thruster is a type of ion thruster in which the propellant is accelerated by an electric field. Hall thrusters trap electrons in a magnetic field and then use the electrons to ionize propellant, efficiently accelerate the ions to produce thrust, and neutralize the...

s (two different types of electric propulsion) to great success.

Requirements

Artificial satellites must be launched

Rocket launch

A rocket launch is the takeoff phase of the flight of a rocket. Launches for orbital spaceflights, or launches into interplanetary space, are usually from a fixed location on the ground, but may also be from a floating platform such as the San Marco platform, or the Sea Launch launch...

 into orbit

Orbit

In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

, 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

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...

, the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, and possibly some astronomical

Astronomy

Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 object of interest. They are also subject to drag from the thin atmosphere

Earth's atmosphere

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

, 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

Orbital stationkeeping

In astrodynamics orbital station-keeping is a term used to describe the orbital maneuvers made by thruster burns that are needed to keep a spacecraft in a particular assigned orbit.For many Earth satellites the effects of the non-Keplerian forces, i.e...

). Many satellites need to be moved from one orbit to another from time to time, and this also requires propulsion. A satellite's useful life is over once it has exhausted its ability to adjust its orbit.

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

Interplanetary travel

Interplanetary spaceflight or interplanetary travel is travel between planets within a single planetary system. In practice, spaceflights of this type are confined to travel between the planets of the Solar System....

, 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. In between these adjustments, the spacecraft simply falls freely along its trajectory. The most fuel-efficient means to move from one circular orbit to another is with a Hohmann transfer orbit

Hohmann transfer orbit

In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits, typically both in the same plane....

: the spacecraft begins in a roughly circular orbit around the Sun. A short period of thrust

Thrust

Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....

 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. Special methods such as aerobraking

Aerobraking

Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit by flying the vehicle through the atmosphere at the low point of the orbit . The resulting drag slows the spacecraft...

 are sometimes used for this final orbital adjustment.
Some spacecraft propulsion methods such as solar sail

Solar sail

Solar sails are a form of spacecraft propulsion using the radiation pressure of light from a star or laser to push enormous ultra-thin mirrors to high speeds....

s provide very low but inexhaustible thrust; 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. The concept has been successfully tested by the Japanese IKAROS

IKAROS

IKAROS is a Japan Aerospace Exploration Agency experimental spacecraft. The spacecraft was launched on 21 May, 2010, aboard an H-IIA rocket, together with the Akatsuki probe and four other small spacecraft...

 solar sail spacecraft.

Spacecraft for interstellar travel

Interstellar travel

Interstellar space travel is manned or unmanned travel between stars. The concept of interstellar travel in starships is a staple of science fiction. Interstellar travel is much more difficult than interplanetary travel. Intergalactic travel, or travel between different galaxies, is even more...

 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.

Effectiveness

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

Momentum

In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...

, mv. The amount of change in momentum is called impulse. 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 gravitational

Gravity drag

In astrodynamics and rocketry, gravity drag is a measure of the loss in the net performance of a rocket while it is thrusting in a gravitational field...

 pull to provide a positive net acceleration.
In orbit, any additional impulse, even very tiny, will result in a change in the orbit path.

The rate of change of velocity

Velocity

In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

 is called acceleration

Acceleration

In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...

, and the rate of change of momentum

Momentum

In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...

 is called force

Force

In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

. 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

Gravity well

A gravity well or gravitational well is a conceptual model of the gravitational field surrounding a body in space. The more massive the body the deeper and more extensive the gravity well associated with it. The Sun has a far-reaching and deep gravity well. Asteroids and small moons have much...

. The escape velocity

Escape velocity

In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...

 required to get out of it is 11.2 kilometers/second. 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

Free fall

Free fall is any motion of a body where gravity is the only force acting upon it, at least initially. These conditions produce an inertial trajectory so long as gravity remains the only force. Since this definition does not specify velocity, it also applies to objects initially moving upward...

, such as nausea, muscular weakness, reduced sense of taste, or leaching

Leaching

In general, leaching is the extraction of certain materials from a carrier into a liquid . Specifically, it may refer to:...

 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

Solid rocket

A solid rocket or a solid-fuel rocket is a rocket engine that uses solid propellants . The earliest rockets were solid-fuel rockets powered by gunpowder; they were used by the Chinese in warfare as early as the 13th century and later by the Mongols, Arabs, and Indians.All rockets used some form of...

, liquid

Liquid rocket

A liquid-propellant rocket or a liquid rocket is a rocket engine that uses propellants in liquid form. Liquids are desirable because their reasonably high density allows the volume of the propellant tanks to be relatively low, and it is possible to use lightweight pumps to pump the propellant from...

, or hybrid rocket

Hybrid rocket

A hybrid rocket is a rocket with a rocket motor which uses propellants in two different states of matter - one solid and the other either gas or liquid. The Hybrid rocket concept can be traced back at least 75 years....

, 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

Ion thruster

An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and...

, electricity is used to accelerate ions out the back. Here some other source must provide the electrical energy (perhaps a solar panel

Photovoltaic module

A solar panel is a packaged, connected assembly of solar cells, also known as photovoltaic cells...

 or a nuclear reactor

Nuclear reactor

A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

), 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

Specific impulse

Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

, the impulse per unit weight-on-Earth (typically designated by ). 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 ). Confusingly, both values are sometimes called specific impulse. The two values differ by a factor of g_n

Standard gravity

Standard gravity, or standard acceleration due to free fall, usually denoted by g0 or gn, is the nominal acceleration of an object in a vacuum near the surface of the Earth. It is defined as precisely , or about...

, the standard acceleration due to gravity 9.80665 m/s² ().

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 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 high-mass-efficient engines require enormous amounts of energy per second to produce high thrusts. As a result, most high-mass-efficient engine designs also provide lower thrust due to the unavailability of high amounts of energy.

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.

Reaction engines

A reaction engine is an engine which provides propulsion by expelling reaction mass, in accordance with Newton's third law of motion. This law of motion is most commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force".

Examples include both duct engines and 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...

s, and more uncommon variations such as Hall effect thruster

Hall effect thruster

In spacecraft propulsion, a Hall thruster is a type of ion thruster in which the propellant is accelerated by an electric field. Hall thrusters trap electrons in a magnetic field and then use the electrons to ionize propellant, efficiently accelerate the ions to produce thrust, and neutralize the...

s, ion drives and mass drivers. Duct engines are obviously not used for space propulsion due to the lack of air; however some proposed spacecraft have these kinds of engines to assist takeoff and landing.

Delta-v and propellant

Exhausting 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

In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....

' ().

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

Tsiolkovsky rocket equation

The Tsiolkovsky rocket equation, or ideal rocket equation is an equation that is useful for considering vehicles that follow the basic principle of a rocket: where a device that can apply acceleration to itself by expelling part of its mass with high speed and moving due to the conservation of...

:


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


where is the specific impulse

Specific impulse

Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

 of the rocket, measured in seconds, and is the gravitational acceleration

Gravitational acceleration

In physics, gravitational acceleration is the acceleration on an object caused by gravity. Neglecting friction such as air resistance, all small bodies accelerate in a gravitational field at the same rate relative to the center of mass....

 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
, and the rocket engine has exhaust velocity v_e, then the mass M of reaction mass which is needed can be calculated using the rocket equation and the formula for :


For much smaller than v_e, this equation is roughly linear

Linear

In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...

, and little reaction mass is needed. If is comparable to v_e, 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.

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

Delta-v

In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....

. 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.

Some effects such as Oberth effect

Oberth effect

In astronautics, the Oberth effect is where the use of a rocket engine when travelling at high speed generates much more useful energy than one at low speed...

 can only be significantly utilised by high thrust engines such as rockets, i.e. engines that can produce a high g-force

G-force

The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

 (thrust per unit mass, equal to delta-v per unit time).

Power use and propulsive efficiency

For all reaction engine

Reaction engine

A reaction engine is an engine or motor which provides propulsion by expelling reaction mass, in accordance with Newton's third law of motion...

s (such as rockets and ion drives) some energy must go into accelerating the reaction mass.
Every engine will waste some energy, but even assuming 100% efficiency, to accelerate an exhaust the engine will need energy amounting to


This energy is not necessarily lost- some of it usually ends up as kinetic energy of the vehicle, and the rest is wasted in residual motion of the exhaust.

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.

The exact amount depends on the design of the vehicle, and the mission. However there are some useful fixed points:

• if the is fixed, for a mission delta-v, there is a particular that minimises the overall energy used by the rocket. This comes to an exhaust velocity of about ⅔ of the mission delta-v (see the energy computed from the rocket equation). 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 for many missions.

• 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 the propulsive efficiency is 100%- 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. Anyway the vehicle has to move before the method can be applied.

Some drives (such as VASIMR

Variable specific impulse magnetoplasma rocket

The Variable Specific Impulse Magnetoplasma Rocket is an electro-magnetic thruster for spacecraft propulsion. It uses radio waves to ionize and heat a propellant and magnetic fields to accelerate the resulting plasma to generate thrust...

 or Electrodeless plasma thruster

Electrodeless plasma thruster

The electrodeless plasma thruster is a spacecraft propulsion engine. It was created by Mr. Gregory Emsellem based on technology developed by French Atomic Energy Commission scientist Dr Richard Geller and Dr...

) actually can significantly vary their exhaust velocity. This can help reduce propellant usage or 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

Plasma (physics)

In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 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 4000m/s).

It might be thought that adding power generation capacity is helpful, and while initially this can improve performance, this inevitably increases the weight of the power source, and eventually the mass of the power source and the associated engines and propellant dominates the weight of the vehicle, and then adding more power gives no significant improvement.

For, although solar power and nuclear power are virtually unlimited sources of energy, the maximum power they can supply is substantially proportional to the mass of the powerplant (i.e. specific power

Specific power

In physics and engineering, surface power density or sometimes simply specific power is power per unit area.-Applications:* The intensity of electromagnetic radiation can be expressed in W/m2...

 takes a largely constant value which is dependent on the particular powerplant technology). For any given specific power, with a large which is desirable to save propellant mass, it turns out that the maximum acceleration is inversely proportional to . Hence the time to reach a required delta-v is proportional to . Thus the latter should not be too large.

Power to thrust ratio

The power to thrust ratio is simply:


Thus for any vehicle power P, the thrust that may be provided is:

Example

Suppose we want to send a 10,000 kg space probe to Mars. The required from LEO

Low Earth orbit

A 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...

 is approximately 3000 m/s, using a Hohmann transfer orbit

Hohmann transfer orbit

In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits, typically both in the same plane....

. For the sake of argument, let us say that the following thrusters may be used:

Engine	Effective Exhaust Velocity (km/s)	Specific impulse Specific impulse Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,... (s)	Fuel mass (kg)	Energy required (GJ)	Energy per kg of propellant	minimum power/thrust	Power generator mass/thrust*
Solid rocket Solid rocket A solid rocket or a solid-fuel rocket is a rocket engine that uses solid propellants . The earliest rockets were solid-fuel rockets powered by gunpowder; they were used by the Chinese in warfare as early as the 13th century and later by the Mongols, Arabs, and Indians.All rockets used some form of...	1	100	190,000	95	500 kJ	0.5 kW/N	N/A
Bipropellant rocket	5	500	8,200	103	12.6 MJ	2.5 kW/N	N/A
Ion thruster Ion thruster An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and...	50	5,000	620	775	1.25 GJ	25 kW/N	25 kg/N
Advance electrically powered drive	1,000	100,000	30	15,000	500 GJ	500 kW/N	500 kg/N

* - assumes a specific power of 1kW/kg

Observe that the more fuel-efficient engines can use far less fuel; its mass is almost negligible (relative to the mass of the payload and the engine itself) for some of the engines. However, note also that these require a large total amount of energy. For Earth launch, engines require a thrust to weight ratio of more than one. To do this with the ion or more theoretical electrical drives, the engine would have to be supplied with one to several gigawatts of power — equivalent to a major metropolitan generating station

Electricity generation

Electricity generation is the process of generating electric energy from other forms of energy.The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday...

. From the table it can be seen that this is clearly impractical with current power sources.

Alternative approaches include some forms of laser propulsion

Laser propulsion

Laser propulsion is a form of beam-powered propulsion where the energy source is a remote laser system and separate from the reaction mass...

, where the reaction mass does not provide the energy required to accelerate it, with the energy instead being provided from an external laser or other beamed power system. Small models of some of these concepts have flown, although the engineering problems are complex and the ground based power systems are not a solved problem.

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, and would be insufficient for launching from the Earth. However, over long periods in orbit where there is no friction, the velocity will be finally achieved. For example. it took the SMART-1

SMART-1

SMART-1 was a Swedish-designed European Space Agency satellite that orbited around the Moon. It was launched on September 27, 2003 at 23:14 UTC from the Guiana Space Centre in Kourou, French Guiana. "SMART" stands for Small Missions for Advanced Research in Technology...

 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, but takes longer.

Mission planning therefore frequently involves adjusting and choosing the propulsion system so as to minimise the total cost of the project, and can involve trading off launch costs and mission duration against payload fraction.

Rocket engines

Most rocket engines are internal combustion

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the 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 nozzle

De Laval nozzle

A de Laval nozzle is a tube that is pinched in the middle, making a carefully balanced, asymmetric hourglass-shape...

. 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 speed reaching as high as 10 times the speed of sound at sea level are common.

Rocket engines provide essentially the highest specific powers and high specific thrusts of any engine used for spacecraft propulsion.

Ion propulsion rockets can heat a plasma or charged gas inside a magnetic bottle and release it 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

Nuclear fusion

Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

 has developed methods, some of which have been proposed to be used in propulsion systems, and some have been tested in a lab.

See 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...

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

Electromagnetic propulsion

Rather than relying on high temperature and fluid dynamics

Fluid dynamics

In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...

 to accelerate the reaction mass to high speeds, there are a variety of methods that use electrostatic or electromagnetic

Electromagnetism

Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...

 forces to accelerate the reaction mass directly. Usually the reaction mass is a stream of ion

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s. Such an engine typically uses electric power, first to ionize atoms, and then to create a voltage gradient to accelerate the ions to high exhaust velocities.

The idea of electric propulsion dates back to 1906, when Robert Goddard considered the possibility in his personal notebook.
Konstantin Tsiolkovsky

Konstantin Tsiolkovsky

Konstantin Eduardovich Tsiolkovsky was an Imperial Russian and Soviet rocket scientist and pioneer of the astronautic theory. Along with his followers the German Hermann Oberth and the American Robert H. Goddard, he is considered to be one of the founding fathers of rocketry and astronautics...

 published the idea in 1911.

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 rocket

Nuclear electric rocket

In a nuclear electric rocket, nuclear thermal energy is changed into electrical energy that is used to power one of the electrical propulsion technologies. Technically the powerplant is nuclear, not the propulsion system, but the terminology is standard. A number of heat-to-electricity schemes...

s.

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:

• Ion thrusters (accelerate ions first and later neutralize the ion beam with an electron stream emitted from a cathode called a neutralizer)
  • Electrostatic ion thruster
    Electrostatic ion thruster
    An electrostatic ion thruster is a design for ion thrusters . These designs use high voltage electrodes in order to accelerate ions with electrostatic forces.-History:...
    
    
  • Field Emission Electric Propulsion
    Field Emission Electric Propulsion
    Field-emission electric propulsion is an advanced electrostatic propulsion concept, a form of ion thruster, that uses liquid metal as a propellant. A FEEP device consists of an emitter and an accelerator electrode...
    
    
  • Hall effect thruster
    Hall effect thruster
    In spacecraft propulsion, a Hall thruster is a type of ion thruster in which the propellant is accelerated by an electric field. Hall thrusters trap electrons in a magnetic field and then use the electrons to ionize propellant, efficiently accelerate the ions to produce thrust, and neutralize the...
    
    
  • Colloid thruster
    Colloid thruster
    ]A colloid thruster is a type of thruster which uses electrostatic acceleration of charged liquid droplets for propulsion. It is closely related to electrospray ionization and other hydrodynamic spraying processes. In a colloid thruster charged liquid droplets are produced by an electrospray...
    
    
• Electrothermal thrusters (electromagnetic fields are used to generate a plasma to increase the heat
  Heat
  In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
  
   of the bulk propellant, the thermal energy imparted to the propellant gas is then converted into kinetic energy by a nozzle
  Nozzle
  A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....
  
   of either physical material construction or by magnetic means)
  • DC arcjet
    Arcjet rocket
    Arcjets are a form of electric propulsion for spacecraft, whereby an electrical discharge is created in a flow of propellant . This imparts additional energy to the propellant, so that one can extract more work out of each kilogram of propellant, at theexpense of increased power consumption and ...
    
    
  • microwave arcjet
    Arcjet rocket
    Arcjets are a form of electric propulsion for spacecraft, whereby an electrical discharge is created in a flow of propellant . This imparts additional energy to the propellant, so that one can extract more work out of each kilogram of propellant, at theexpense of increased power consumption and ...
    
    
  • Pulsed plasma thruster
    Pulsed plasma thruster
    Pulsed plasma thrusters are a method of spacecraft propulsion also known as Plasma Jet Engines in general. They use an arc of electric current adjacent to a solid propellant, to produce a quick and repeatable burst of impulse...
    
    
  • Helicon Double Layer Thruster
    Helicon Double Layer Thruster
    The Helicon Double Layer Thruster is a prototype spacecraft propulsion engine. It was created by Australian scientist Dr. Christine Charles, based on a technology invented by Professor Rod Boswell, both of the Australian National University....
    
    
• Electromagnetic thrusters (ions are accelerated either by the Lorentz Force
  Lorentz force
  In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...
  
   or by the effect of electromagnetic fields where the electric field is not in the direction of the acceleration)
  • Magnetoplasmadynamic thruster
    Magnetoplasmadynamic thruster
    The Magnetoplasmadynamic thruster is a form of electrically powered spacecraft propulsion which uses the Lorentz force to generate thrust...
    
    
  • Electrodeless plasma thruster
    Electrodeless plasma thruster
    The electrodeless plasma thruster is a spacecraft propulsion engine. It was created by Mr. Gregory Emsellem based on technology developed by French Atomic Energy Commission scientist Dr Richard Geller and Dr...
    
    
  • Pulsed inductive thruster
    Pulsed inductive thruster
    Pulsed inductive thrusters are a form of ion thruster, used in spacecraft propulsion. A PIT uses perpendicular electric and magnetic fields to accelerate a propellant. A nozzle releases a puff of gas which spreads across a flat induction coil of wire about 1 meter across...
    
    
  • Variable specific impulse magnetoplasma rocket (VASIMR)
    Variable specific impulse magnetoplasma rocket
    The Variable Specific Impulse Magnetoplasma Rocket is an electro-magnetic thruster for spacecraft propulsion. It uses radio waves to ionize and heat a propellant and magnetic fields to accelerate the resulting plasma to generate thrust...
    
    
• Mass driver
  Mass driver
  A mass driver or electromagnetic catapult is a proposed method of non-rocket spacelaunch which would use a linear motor to accelerate and catapult payloads up to high speeds. All existing and contemplated mass drivers use coils of wire energized by electricity to make electromagnets. Sequential...
  
  s (for propulsion)

In electrothermal and electromagnetic thrusters, both ions and electrons are accelerated simultaneously, no neutralizer is required.

Without internal reaction mass

The law of conservation

Conservation law

In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves....

 of momentum

Momentum

In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...

 states that any engine which uses no reaction mass cannot accelerate 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 field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

s, solar wind

Solar wind

The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

 and solar radiation. Field propulsion

Field propulsion

Field propulsion is the concept of spacecraft propulsion where no propellant is necessary but instead momentum of the spacecraft is changed by an interaction of the spacecraft with external force fields....

 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. Solar sail

Solar sail

Solar sails are a form of spacecraft propulsion using the radiation pressure of light from a star or laser to push enormous ultra-thin mirrors to high speeds....

s rely on radiation pressure

Radiation pressure

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...

 from electromagnetic energy, but they require a large collection surface to function effectively. The magnetic sail

Magnetic sail

A magnetic sail or magsail is a proposed method of spacecraft propulsion which would use a static magnetic field to deflect charged particles radiated by the Sun as a plasma wind, and thus impart momentum to accelerate the spacecraft...

 deflects charged particles from the solar wind

Solar wind

The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

 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. An E-sail would use very thin and lightweight wires holding an electric charge to deflect these particles, and may have more controllable directionality.

Proof of concept - NanoSail-D

NanoSail-D

NanoSail-D was a small satellite which was to have been used by NASA's Ames Research Center to study the deployment of a solar sail in space. It was a three-unit CubeSat measuring 30 by 10 by 10 centimetres , with a mass of...

 becomes first nanosatellite to orbit 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...

. Plans to add them to future Earth orbit satellites, enabling them to de-orbit and burn up once they are no longer needed. Cube sail aims to tackle Space Junk

A satellite or other space vehicle is subject to the law of conservation of angular momentum, which constrains a body from a net change

Net force

In physics, net force is the total force acting on an object. It is calculated by vector addition of all forces that are actually acting on that object. Net force has the same effect on the translational motion of the object as all actual forces taken together...

 in angular velocity

Angular velocity

In physics, the angular velocity is a vector quantity which specifies the angular speed of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second, although it may be measured in other units such as degrees per second, revolutions per...

. Thus, for a vehicle to change its relative orientation without expending reaction mass, another part of the vehicle may rotate in the opposite direction. Non-conservative external forces, primarily gravitational and atmospheric, can contribute up to several degrees per day to angular momentum, so secondary systems are designed to "bleed off" undesired rotational energies built up over time. Accordingly, many spacecraft utilize reaction wheels or control moment gyroscopes

Control moment gyroscope

A control momentum gyroscope is an attitude control device generally used in spacecraft attitude control systems. A CMG consists of a spinning rotor and one or more motorized gimbals that tilt the rotor’s angular momentum. As the rotor tilts, the changing angular momentum causes a gyroscopic...

 to control orientation in space.

A gravitational slingshot

Gravitational slingshot

In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement and gravity of a planet or other celestial body to alter the path and speed of a spacecraft, typically in order to save propellant, time, and expense...

 can carry a 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...

 onward to other destinations without the expense of reaction mass. By harnessing the gravitational energy of other celestial objects, the spacecraft can pick up kinetic energy. However, even more energy can be obtained from the gravity assist if rockets are used.

Launch mechanisms

High thrust is of vital importance for Earth launch. Thrust has to be greater than weight (see also gravity drag

Gravity drag

In astrodynamics and rocketry, gravity drag is a measure of the loss in the net performance of a rocket while it is thrusting in a gravitational field...

). 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 or solid-fuel

Solid rocket

A solid rocket or a solid-fuel rocket is a rocket engine that uses solid propellants . The earliest rockets were solid-fuel rockets powered by gunpowder; they were used by the Chinese in warfare as early as the 13th century and later by the Mongols, Arabs, and Indians.All rockets used some form of...

) 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 non-rocket spacelaunch

Non-rocket spacelaunch

Non-rocket space launch is a launch into space where some or all needed speed and altitude is provided by non-rocket means, rather than simply using conventional chemical rockets from the ground. A number of alternatives to rockets have been proposed...

 ground-assisted launch mechanisms include:

• Space elevator
  Space elevator
  A space elevator, also known as a geostationary orbital tether or a beanstalk, is a proposed non-rocket spacelaunch structure...
  
   (a geostationary tether to orbit)
• Launch loop
  Launch loop
  A launch loop or Lofstrom loop is a proposed system for launching objects into space orbit using a moving cable-like system attached to the earth at two ends and suspended above the atmosphere in the middle...
  
   (a very fast enclosed rotating loop about 80 km tall)
• Space fountain
  Space fountain
  A space fountain is a proposed form of space elevator that does not require the structure to be in geostationary orbit, and does not rely on tensile strength for support. In contrast to the original space elevator design , a space fountain is a tremendously tall tower extending up from the ground...
  
   (a very tall building held up by a stream of masses fired from base)
• Orbital ring
  Orbital ring
  An Orbital Ring is a concept for a space elevator that consists of a ring in low earth orbit that rotates at above orbital speed, that has fixed tethers hanging down to the ground.The structure is intended to be used for space launch....
  
   (a ring around the Earth with spokes hanging down off bearings)
• Hypersonic skyhook (a fast spinning orbital tether)
• Electromagnetic catapult
  Mass driver
  A mass driver or electromagnetic catapult is a proposed method of non-rocket spacelaunch which would use a linear motor to accelerate and catapult payloads up to high speeds. All existing and contemplated mass drivers use coils of wire energized by electricity to make electromagnets. Sequential...
  
   (railgun
  Railgun
  A railgun is an entirely electrical gun that accelerates a conductive projectile along a pair of metal rails using the same principles as the homopolar motor. Railguns use two sliding or rolling contacts that permit a large electric current to pass through the projectile. This current interacts...
  
  , coilgun
  Coilgun
  A coilgun is a type of projectile accelerator that consists of one or more coils used as electromagnets in the configuration of a synchronous linear motor which accelerate a magnetic projectile to high velocity...
  
  ) (an electric gun)
• Rocket sled launch
  Rocket sled launch
  A rocket sled launch is a method of launching space vehicles. A rail or maglev track and a rocket or jet booster is used to accelerate a sled holding a vehicle up an eastward facing mountain slope...
  
  
• Space gun
  Space gun
  A space gun is a method of launching an object into outer space using a large gun, or cannon. It provides a method of non-rocket spacelaunch‎.In the HARP Project a U.S...
  
   (Project HARP
  Project HARP
  Project HARP, short for High Altitude Research Project, was a joint project of the United States Department of Defense and Canada's Department of National Defence created with the goal of studying ballistics of re-entry vehicles at low cost; whereas most such projects used expensive rockets, HARP...
  
  , ram accelerator
  Ram accelerator
  A ram accelerator has the same function as a gun; i.e., it is a device for accelerating projectiles; however, it is entirely different in that jet-engine-like propulsion cycles utilizing ramjet and/or scramjet combustion processes are used to accelerate a projectile to extremely high speeds...
  
  ) (a chemically powered gun)
• Beam-powered propulsion
  Beam-powered propulsion
  Beam-powered propulsion is a class of aircraft or spacecraft propulsion mechanisms that use energy beamed to the spacecraft from a remote power plant to provide energy...
  
   rockets and jets powered from ground via a beam
• High-altitude platforms to assist initial stage
• Orbital airship
  Orbital airship
  The orbital airship, also called the space blimp, is a proposed space transportation system that carries payloads to and from low Earth orbit...
  
  

Airbreathing engines

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 launch

Air launch

Air launching is the practice of dropping a parasite aircraft, rocket, or missile from a mothership. The parasite aircraft or missile is usually tucked under the wing of the larger mothership and then "dropped" from underneath the wing while in flight...

ed from separate lift vehicles (e.g. B-29

B-29 Superfortress

The B-29 Superfortress is a four-engine propeller-driven heavy bomber designed by Boeing that was flown primarily by the United States Air Forces in late-World War II and through the Korean War. The B-29 was one of the largest aircraft to see service during World War II...

, Pegasus Rocket

Pegasus rocket

The Pegasus rocket is a winged space launch vehicle capable of carrying small, unmanned payloads into low Earth orbit. It is air-launched, as part of an expendable launch system developed by Orbital Sciences Corporation . Three main stages burning solid propellant provide the thrust...

 and White Knight) which do use such propulsion systems. Jet engines mounted on a launch rail could also be so used.

On the other hand, very lightweight or very high speed engines have been proposed that take advantage of the air during ascent:

• SABRE - a lightweight hydrogen fuelled turbojet with precooler
• ATREX
  ATREX
  The ATREX engine developed in Japan is an experimental precooled jet engine that works as a turbojet at low speeds and a ramjet up to mach 6.0....
  
   - a lightweight hydrogen fuelled turbojet with precooler
• Liquid air cycle engine
  Liquid air cycle engine
  A Liquid Air Cycle Engine is a type of spacecraft propulsion engine that attempts to increase its efficiency by gathering part of its oxidizer from the atmosphere...
  
   - a hydrogen fuelled jet engine that liquifies the air before burning it in a rocket engine
• Scramjet
  Scramjet
  A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow...
  
   - jet engines that use supersonic combustion

Normal rocket launch vehicles fly almost vertically before rolling over at an altitude of some tens of kilometers before burning sideways for orbit; this initial vertical climb wastes propellant but is optimal as it greatly reduces airdrag. Airbreathing engines burn propellant much more efficiently and this would permit a far flatter launch trajectory, the vehicles would typically fly approximately tangentially to the earth surface until leaving the atmosphere then perform a rocket burn to bridge the final delta-v

Delta-v

In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....

 to orbital velocity.

Planetary arrival and landing

When a vehicle is to enter orbit around its destination planet, or when it is to land, it must adjust its velocity. This can be done using all the methods listed above (provided they can generate a high enough thrust), but there are a few methods that can take advantage of planetary atmospheres and/or surfaces.

• Aerobraking
  Aerobraking
  Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit by flying the vehicle through the atmosphere at the low point of the orbit . The resulting drag slows the spacecraft...
  
   allows a spacecraft to reduce the high point of an elliptical orbit by repeated brushes with the atmosphere at the low point of the orbit. This can save a considerable amount of fuel since it takes much less delta-V to enter an elliptical orbit compared to a low circular orbit. Since the braking is done over the course of many orbits, heating is comparatively minor, and a heat shield is not required. This has been done on several Mars missions such as Mars Global Surveyor
  Mars Global Surveyor
  The Mars Global Surveyor was a US spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996. It began the United States's return to Mars after a 10-year absence. It completed its primary mission in January 2001 and was in its third extended mission phase when, on 2...
  
  , Mars Odyssey
  2001 Mars Odyssey
  2001 Mars Odyssey is a robotic spacecraft orbiting the planet Mars. The project was developed by NASA, and contracted out to Lockheed Martin, with an expected cost for the entire mission of US$297 million. Its mission is to use spectrometers and electronic imagers to hunt for evidence of past or...
  
   and Mars Reconnaissance Orbiter
  Mars Reconnaissance Orbiter
  Mars Reconnaissance Orbiter is a NASA multipurpose spacecraft designed to conduct reconnaissance and Exploration of Mars from orbit...
  
  , and at least one Venus mission, Magellan
  Magellan probe
  The Magellan spacecraft, also referred to as the Venus Radar Mapper, was a 1,035-kilogram robotic space probe launched by NASA on May 4, 1989, to map the surface of Venus using Synthetic Aperture Radar and measure the planetary gravity...
  
  .
• Aerocapture
  Aerocapture
  Aerocapture is a technique used to reduce velocity of a spacecraft, arriving at a celestial body with a hyperbolic trajectory, in order to bring it in an orbit with an eccentricity of less than 1. It uses the drag created by the atmosphere of the celestial body to decelerate. Only one pass in the...
  
   is a much more aggressive manoeuver, converting an incoming hyperbolic orbit to an elliptical orbit in one pass. This requires a heat shield and much trickier navigation, since it must be completed in one pass through the atmosphere, and unlike aerobraking no preview of the atmosphere is possible. If the intent is to remain in orbit, then at least one more propulsive maneuver is required after aerocapture—otherwise the low point of the resulting orbit will remain in the atmosphere, resulting in eventual re-entry. Aerocapture has not yet been tried on a planetary mission, but the re-entry skip
  Skip reentry
  Skip reentry is a reentry technique involving one or more successive "skips" off the atmosphere to achieve greater entry range or to slow the spacecraft before final entry, which helps to dissipate the huge amount of heat that is usually generated on faster descents...
  
   by Zond 6
  Zond 6
  Zond 6, a formal member of the Soviet Zond program and unmanned version of Soyuz 7K-L1 manned moon-flyby spacecraft, was launched on a lunar flyby mission from a parent satellite in Earth parking orbit...
  
   and Zond 7
  Zond 7
  This article was originally based on material from ...
  
   upon lunar return were aerocapture maneuvers, since they turned a hyperbolic orbit into an elliptical orbit. On these missions, since there was no attempt to raise the perigee after the aerocapture, the resulting orbit still intersected the atmosphere, and re-entry occurred at the next perigee.
• a Ballute
  Ballute
  A ballute was invented by Goodyear in 1958. It is a parachute braking device that is optimized for use at high altitudes and high supersonic velocities. The original ballute was a cone-shaped balloon with a toroidal burble fence fitted around its widest point...
  
   is an inflatable drag device
• Parachute
  Parachute
  A parachute is a device used to slow the motion of an object through an atmosphere by creating drag, or in the case of ram-air parachutes, aerodynamic lift. Parachutes are usually made out of light, strong cloth, originally silk, now most commonly nylon...
  
  s can land a probe on a planet with an atmosphere, usually after the atmosphere has scrubbed off most of the velocity, using a heat shield
  Atmospheric reentry
  Atmospheric entry is the movement of human-made or natural objects as they enter the atmosphere of a celestial body from outer space—in the case of Earth from an altitude above the Kármán Line,...
  
  .
• Airbag
  Airbag
  An Airbag is a vehicle safety device. It is an occupant restraint consisting of a flexible envelope designed to inflate rapidly during an automobile collision, to prevent occupants from striking interior objects such as the steering wheel or a window...
  
  s can soften the final landing.
• Lithobraking
  Lithobraking
  Lithobraking is a technique of descent by an unmanned space vehicle to the surface of a body by which the vehicle is slowed by impact with the body's surface....
  
  , or stopping by simply smashing into the target, is usually done by accident. However, it may be done deliberately with the probe expected to survive (see, for example, Deep Space 2
  Deep Space 2
  Deep Space 2 was a NASA probe which was part of the New Millennium Program. It included two highly advanced miniature space probes which were sent to Mars aboard the Mars Polar Lander in January 1999. The probes were named "Scott" and "Amundsen", in honor of Robert Falcon Scott and Roald Amundsen,...
  
  ), in which case very sturdy probes and low approach velocities are required.

Hypothetical methods

A variety of hypothetical propulsion techniques have been considered that would require entirely new principles of physics to realize and that may not actually be possible. To date, such methods are highly speculative and include:

• Diametric drive
• Pitch drive
• Bias drive
• Disjunction drive
• Alcubierre drive
  Alcubierre drive
  The Alcubierre drive, also known as the Alcubierre metric, is a speculative, but valid solution of the Einstein field equations. It is a mathematical model of a spacetime exhibiting features reminiscent of the fictional "warp drive" from Star Trek, which can travel "faster than light", although...
  
   (a form of Warp drive)
• Differential sail
• Wormhole
  Wormhole
  In physics, a wormhole is a hypothetical topological feature of spacetime that would be, fundamentally, a "shortcut" through spacetime. For a simple visual explanation of a wormhole, consider spacetime visualized as a two-dimensional surface. If this surface is folded along a third dimension, it...
  
  s – theoretically possible, but unachieveable in practice with current technology
• Reactionless drive
  Reactionless drive
  A reactionless drive is a fictional or theorized method of propulsion where thrust is generated without any need for any outside force or net momentum exchange to produce linear motion...
  
  s – breaks the law of conservation of momentum; theoretically impossible
• EmDrive
  EmDrive
  EmDrive is the name of a spacecraft propulsion system proposed, and reportedly developed, by Roger Shawyer. New Scientist ran a cover story on EmDrive in its 8 September 2006 issue. The device is a magnetron with a specially shaped, fully enclosed tapering resonator cavity whose area is greater...
  
   – tries to circumvent the law of conservation of momentum; may be theoretically impossible
• A "hyperspace" drive based upon Heim theory
  Heim theory
  Heim theory is a physics theory, initially proposed by a German physicist, the late Burkhard Heim, that attempts to develop a theory of everything. Heim theory's six dimensional model was later extended to eight and twelve dimensions, in collaboration with W. Dröscher...
  
  

A NASA assessment is found at Marc G Millis Assessing potential propulsion breakthroughs (2005)

Table of methods

Below is a summary of some of the more popular, proven technologies, followed by increasingly speculative methods.

Four numbers are shown. The first is the effective exhaust velocity

Specific impulse

Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

: the equivalent speed that the propellant leaves the vehicle. This is not necessarily the most important characteristic of the propulsion method; thrust and power consumption and other factors can be. However:

• if the delta-v is much more than the exhaust velocity, then exorbitant amounts of fuel are necessary (see the section on calculations, above)
• if it is much more than the delta-v, then, proportionally more energy is needed; if the power is limited, as with solar energy, this means that the journey takes a proportionally longer time

The second and third are the typical amounts of thrust and the typical burn times of the method. Outside a gravitational potential small amounts of thrust applied over a long period will give the same effect as large amounts of thrust over a short period. (This result does not apply when the object is significantly influenced by gravity.)

The fourth is the maximum delta-v this technique can give (without staging). For rocket-like propulsion systems this is a function of mass fraction and exhaust velocity. Mass fraction for rocket-like systems is usually limited by propulsion system weight and tankage weight. For a system to achieve this limit, typically the payload may need to be a negligible percentage of the vehicle, and so the practical limit on some systems can be much lower.

style="background-color: lightgray"|Propulsion methods

Method	Effective Exhaust Velocity Specific impulse Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,... (km/s)	Thrust Thrust Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system.... (N)	Firing Duration	Maximum Delta-v Delta-v In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver.... (km/s)	Technology readiness level
Solid-fuel rocket	1 – 4	100,000103 - 107	60minutes	7~ 7	9:Flight proven
Hybrid rocket Hybrid rocket A hybrid rocket is a rocket with a rocket motor which uses propellants in two different states of matter - one solid and the other either gas or liquid. The Hybrid rocket concept can be traced back at least 75 years....	1.5 – 4.2	1,000<0.1 - 107	60minutes	3> 3	9:Flight proven
Monopropellant rocket Monopropellant rocket A monopropellant rocket is a rocket that uses a single chemical as its propellant.-Chemical-reaction monopropellant rockets:...	21 – 3	0.1 – 100	1milliseconds-minutes	3~ 3	9:Flight proven
Liquid-fuel rocket	1 – 4.7	1,0000.1 – 107	60minutes	9~ 9	9:Flight proven
Electrostatic ion thruster Electrostatic ion thruster An electrostatic ion thruster is a design for ion thrusters . These designs use high voltage electrodes in order to accelerate ions with electrostatic forces.-History:...	15 – 210	10−3 – 10	months/years	100> 100	9:Flight proven
Hall effect thruster Hall effect thruster In spacecraft propulsion, a Hall thruster is a type of ion thruster in which the propellant is accelerated by an electric field. Hall thrusters trap electrons in a magnetic field and then use the electrons to ionize propellant, efficiently accelerate the ions to produce thrust, and neutralize the...  (HET)	298 - 50	10−3 – 10	months/years	100> 100	9:Flight proven
Resistojet rocket Resistojet rocket A resistojet is a method of Spacecraft propulsion that provides thrust by heating a fluid. Heating is usually achieved by sending electricity through a resistor consisting of a hot incandescent filament, with the expanded gas expelled through a conventional nozzle.Resistojets have been flown in...	42 - 6	10−2 - 10	60minutes		8:Flight qualified
Arcjet rocket Arcjet rocket Arcjets are a form of electric propulsion for spacecraft, whereby an electrical discharge is created in a flow of propellant . This imparts additional energy to the propellant, so that one can extract more work out of each kilogram of propellant, at theexpense of increased power consumption and ...	104 - 16	10−2 - 10	60minutes		8:Flight qualified
Field Emission Electric Propulsion Field Emission Electric Propulsion Field-emission electric propulsion is an advanced electrostatic propulsion concept, a form of ion thruster, that uses liquid metal as a propellant. A FEEP device consists of an emitter and an accelerator electrode...  (FEEP)	115100-130	10−6-10−3	months/years		8:Flight qualified
Pulsed plasma thruster Pulsed plasma thruster Pulsed plasma thrusters are a method of spacecraft propulsion also known as Plasma Jet Engines in general. They use an arc of electric current adjacent to a solid propellant, to produce a quick and repeatable burst of impulse...  (PPT)	20~ 20	~ 0.1	~2,000–10,000 hours		7:Prototype demoed in space
Dual mode propulsion rocket Dual mode propulsion rocket Dual mode propulsion systems combine the high efficiency of bipropellant rockets with the reliability and simplicity of monopropellant rockets. Dual mode systems are either hydrazine/N2O4, or MMH/hydrogen peroxide...	1 – 4.7	1,0000.1 – 107	1milliseconds-minutes	6~ 3 – 9	7:Prototype demoed in space
Solar sail Solar sail Solar sails are a form of spacecraft propulsion using the radiation pressure of light from a star or laser to push enormous ultra-thin mirrors to high speeds.... s	299790:Light Speed of light The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time... 145–750:Wind	9/km2 @ 1 AU Astronomical unit An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance.... 230/km2@0.2AU 10−10/km2@4 ly Light-year A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...	indefinite	40> 40	9:Light pressure attitude-control flight proven 6:Deploy-only demoed in space 5:Light-sail validated in lit vacuum
Tripropellant rocket Tripropellant rocket A tripropellant rocket is a rocket that uses three propellants, as opposed to the more common bipropellant rocket or monopropellant rocket designs, which use two or one fuels, respectively...	2.5 - 5.3	1,0000.1 - 107	60minutes	9~ 9	6:Prototype demoed on ground
Magnetoplasmadynamic thruster Magnetoplasmadynamic thruster The Magnetoplasmadynamic thruster is a form of electrically powered spacecraft propulsion which uses the Lorentz force to generate thrust...  (MPD)	6020 - 100	100	weeks		6:Model-1 kW demoed in space
Nuclear thermal rocket Nuclear thermal rocket In a nuclear thermal rocket a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor, and then expands through a rocket nozzle to create thrust. In this kind of thermal rocket, the nuclear reactor's energy replaces the chemical energy of the propellant's...	9	10,000,000107	60minutes	20> ~ 20	6:Prototype demoed on ground
Mass driver Mass driver A mass driver or electromagnetic catapult is a proposed method of non-rocket spacelaunch which would use a linear motor to accelerate and catapult payloads up to high speeds. All existing and contemplated mass drivers use coils of wire energized by electricity to make electromagnets. Sequential... s (for propulsion)	150 - ~30	1,000,000104 - 108	months		6:Model-32MJ demoed on ground
Tether propulsion	N/A	1,000,0001 - 1012	60minutes	7~ 7	6:Model-31.7 km demoed in space
Air-augmented rocket Air-augmented rocket Air-augmented rockets use the supersonic exhaust of some kind of rocket engine to further compress air collected by ram effect during flight to use as additional working mass, leading to greater effective thrust for any given amount of fuel than either the rocket or a ramjet...	5 - 6	1,0000.1 - 107	seconds-minutes	7> 7?	6:Prototype demoed on ground
Liquid air cycle engine Liquid air cycle engine A Liquid Air Cycle Engine is a type of spacecraft propulsion engine that attempts to increase its efficiency by gathering part of its oxidizer from the atmosphere...		100,000103 - 107	seconds-minutes		6:Prototype demoed on ground
Pulsed inductive thruster Pulsed inductive thruster Pulsed inductive thrusters are a form of ion thruster, used in spacecraft propulsion. A PIT uses perpendicular electric and magnetic fields to accelerate a propellant. A nozzle releases a puff of gas which spreads across a flat induction coil of wire about 1 meter across...  (PIT)	4510-80	20	months		5:Component validated in vacuum
Variable Specific Impulse Magnetoplasma Rocket Variable specific impulse magnetoplasma rocket The Variable Specific Impulse Magnetoplasma Rocket is an electro-magnetic thruster for spacecraft propulsion. It uses radio waves to ionize and heat a propellant and magnetic fields to accelerate the resulting plasma to generate thrust...  (VASIMR)	15510 - 300	62040 - 1,200	days - months	100> 100	5:Component-200 kW validated in vacuum
Magnetic field oscillating amplified thruster Magnetic field oscillating amplified thruster The Magnetic Field Oscillating Amplified Thruster is a versatile electrothermodynamic system, which is able to accelerate nearly every medium to extremely high velocities, thereby generating a high energetic plasma jet in the exhaust.To do so, MOA utilises a so-called Alfvén wave, a physical...	7010 - 130	0.1 - 1	days - months	100> 100	5:Component validated in vacuum
Solar thermal rocket Solar thermal rocket Solar thermal propulsion is a form of spacecraft propulsion that makes use of solar power to directly heat reaction mass, and therefore does not require an electrical generator as most other forms of solar-powered propulsion do. A solar thermal rocket only has to carry the means of capturing solar...	7 - 12	101 - 100	weeks	20> ~ 20	4:Component validated in lab
Radioisotope rocket Radioisotope rocket The radioisotope rocket is a type of rocket engine that uses the heat generated by the decay of radioactive elements to heat a working fluid, which is then exhausted through a rocket nozzle to produce thrust...	7 - 8	1.3 - 1.5	months		4:Component validated in lab
Nuclear electric rocket Nuclear electric rocket In a nuclear electric rocket, nuclear thermal energy is changed into electrical energy that is used to power one of the electrical propulsion technologies. Technically the powerplant is nuclear, not the propulsion system, but the terminology is standard. A number of heat-to-electricity schemes... (As electric prop. method used)	Variable	Variable	Variable		4:Component-400kW Safe Affordable Fission Engine Safe Affordable Fission Engine are NASA's small experimental nuclear fission reactors for electricity production in space. Most known is the SAFE-400 reactor producing 400 kW thermal power, giving 100 kW of electric energy using a Brayton cycle gas turbine. The fuel is uranium nitride in...  validated in lab
Orion Project Project Orion (nuclear propulsion) Project Orion was a study of a spacecraft intended to be directly propelled by a series of explosions of atomic bombs behind the craft...  (Near term nuclear pulse propulsion)	6020 - 100	109 - 1012	several days	45~30-60	3:Validated-900 kg proof-of-concept
Space elevator Space elevator A space elevator, also known as a geostationary orbital tether or a beanstalk, is a proposed non-rocket spacelaunch structure...	N/A	N/A	indefinite	12> 12	3:Validated proof-of-concept
Reaction Engines SABRE	30/4.5	1,0000.1 - 107	60minutes		3:Validated proof-of-concept
Magnetic sail Magnetic sail A magnetic sail or magsail is a proposed method of spacecraft propulsion which would use a static magnetic field to deflect charged particles radiated by the Sun as a plasma wind, and thus impart momentum to accelerate the spacecraft... s	145-750:Wind	70/40Mg Tonne The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...	indefinite		3:Validated proof-of-concept
Magnetic sail#Mini-magnetospheric plasma propulsion	200	400~1 N/kW	months		3:Validated proof-of-concept
Beam-powered Beam-powered propulsion Beam-powered propulsion is a class of aircraft or spacecraft propulsion mechanisms that use energy beamed to the spacecraft from a remote power plant to provide energy... /Laser Laser propulsion Laser propulsion is a form of beam-powered propulsion where the energy source is a remote laser system and separate from the reaction mass... (As prop. method powered by beam)	Variable	Variable	Variable		3:Validated-71m proof-of-concept
Launch loop Launch loop A launch loop or Lofstrom loop is a proposed system for launching objects into space orbit using a moving cable-like system attached to the earth at two ends and suspended above the atmosphere in the middle... /Orbital ring Orbital ring An Orbital Ring is a concept for a space elevator that consists of a ring in low earth orbit that rotates at above orbital speed, that has fixed tethers hanging down to the ground.The structure is intended to be used for space launch....	N/A	10,000~104	60minutes	>>11-30	2:Technology concept formulated
Nuclear pulse propulsion Nuclear pulse propulsion Nuclear pulse propulsion is a proposed method of spacecraft propulsion that uses nuclear explosions for thrust. It was first developed as Project Orion by DARPA, after a suggestion by Stanislaw Ulam in 1947...  (Project Daedalus Project Daedalus Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible unmanned interstellar spacecraft. Intended mainly as a scientific probe, the design criteria specified that the spacecraft had to use current or near-future technology and had to... ' drive)	51020 - 1,000	109 - 1012	years	15,000~15,000	2:Technology concept formulated
Gas core reactor rocket Gas core reactor rocket Gas core reactor rockets are a conceptual type of rocket that is propelled by the exhausted coolant of a gaseous fission reactor. The nuclear fission reactor core may be either a gas or plasma...	1510 - 20	103 - 106			2:Technology concept formulated
Nuclear salt-water rocket Nuclear salt-water rocket A nuclear salt-water rocket is a proposed type of nuclear thermal rocket designed by Robert Zubrin that would be fueled by water bearing dissolved salts of Plutonium or U235...	100	100,000103 - 107	1,800half hour		2:Technology concept formulated
Fission sail Fission sail The fission sail is a type of spacecraft propulsion proposed by Robert Forward that uses fission fragments to propel a large solar sail-like craft...					2:Technology concept formulated
Fission-fragment rocket Fission-fragment rocket The fission-fragment rocket is a rocket engine design that directly harnesses hot nuclear fission products for thrust, as opposed to using a separate fluid as working mass...	15,000				2:Technology concept formulated
Nuclear photonic rocket Nuclear photonic rocket In a nuclear photonic rocket, a nuclear reactor would generate such high temperatures that the blackbody radiation from the reactor would provide significant thrust. The disadvantage is that it takes a lot of power to generate a small amount of thrust this way, so acceleration is very slow. The...	299,790	10−5 - 1	years-decades		2:Technology concept formulated
Fusion rocket Fusion rocket A fusion rocket is a theoretical design for a rocket driven by fusion power which could provide efficient and long-term acceleration in space without the need to carry a large fuel supply. The design relies on the development of fusion power technology beyond current capabilities, and the...	550100 - 1,000				2:Technology concept formulated
Antimatter catalyzed nuclear pulse propulsion Antimatter catalyzed nuclear pulse propulsion Antimatter catalyzed nuclear pulse propulsion is a variation of nuclear pulse propulsion based upon the injection of antimatter into a mass of nuclear fuel which normally would not be useful in propulsion...	2,100200 - 4,000		days-weeks		2:Technology concept formulated
Antimatter rocket Antimatter rocket An antimatter rocket is a proposed class of rockets that use antimatter as their power source. There are several designs that attempt to accomplish this goal...	55,00010,000-100,000				2:Technology concept formulated
Bussard ramjet Bussard ramjet The Bussard ramjet is a theoretical method of spacecraft propulsion proposed in 1960 by the physicist Robert W. Bussard, popularized by Larry Niven in his Known Space series of books, and referred to by Carl Sagan in the television series and book Cosmos....	2.2 - 20,000		indefinite	30,000~30,000	2:Technology concept formulated
Gravitoelectromagnetic toroidal launchers	299,790:GEM			299,790<299790	1:Basic principles observed & reported
Method	Effective Exhaust Velocity Specific impulse Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,... (km/s)	Thrust Thrust Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system.... (N)	Firing Duration	Maximum Delta-v Delta-v In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver.... (km/s)	Technology readiness level

Testing

Spacecraft propulsion systems are often first statically tested on the Earth's surface, within the atmosphere but many systems require a vacuum chamber to test fully. Rockets are usually tested at a rocket engine test facility

Rocket engine test facility

A rocket engine test facility is a location where rocket engines may be tested on the ground, under controlled conditions. A ground test program is generally required before the engine is certified for flight...

 well away from habitation and other buildings for safety reasons. Ion drives are far less dangerous and require much less stringent safety, usually only a large-ish vacuum chamber is needed.

Famous static test locations can be found at Rocket Ground Test Facilities

Some systems cannot be adequately tested on the ground and test launches may be employed at a Rocket Launch Site.

See also

• Interplanetary travel
  Interplanetary travel
  Interplanetary spaceflight or interplanetary travel is travel between planets within a single planetary system. In practice, spaceflights of this type are confined to travel between the planets of the Solar System....
  
  
• Interstellar travel
  Interstellar travel
  Interstellar space travel is manned or unmanned travel between stars. The concept of interstellar travel in starships is a staple of science fiction. Interstellar travel is much more difficult than interplanetary travel. Intergalactic travel, or travel between different galaxies, is even more...
  
  
• List of aerospace engineering topics
• List of rockets
• Magnetic sail
  Magnetic sail
  A magnetic sail or magsail is a proposed method of spacecraft propulsion which would use a static magnetic field to deflect charged particles radiated by the Sun as a plasma wind, and thus impart momentum to accelerate the spacecraft...
  
  
• Orbital maneuver
  Orbital maneuver
  In spaceflight, an orbital maneuver is the use of propulsion systems to change the orbit of a spacecraft.For spacecraft far from Earth—for example those in orbits around the Sun—an orbital maneuver is called a deep-space maneuver .-delta-v:...
  
  
• Orbital mechanics
• Pulse detonation engine
  Pulse detonation engine
  A pulse detonation engine, or "PDE", is a type of propulsion system that uses detonation waves to combust the fuel and oxidizer mixture. The engine is pulsed because the mixture must be renewed in the combustion chamber between each detonation wave initiated by an ignition source. Theoretically, a...
  
  
• Rocket
  Rocket
  A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...
  
  
• Rocket engine nozzles
  Rocket engine nozzles
  A rocket engine nozzle is a propelling nozzle used in a rocket engine to expand and accelerate the combustion gases produced by burning propellants so that the exhaust gases exit the nozzle at hypersonic velocities.-History:...
  
  
• Rocket Propulsion Analysis
• Satellite
  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....
  
  
• Solar sail
  Solar sail
  Solar sails are a form of spacecraft propulsion using the radiation pressure of light from a star or laser to push enormous ultra-thin mirrors to high speeds....
  
  
• Specific impulse
  Specific impulse
  Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
  
  
• Tsiolkovsky rocket equation
  Tsiolkovsky rocket equation
  The Tsiolkovsky rocket equation, or ideal rocket equation is an equation that is useful for considering vehicles that follow the basic principle of a rocket: where a device that can apply acceleration to itself by expelling part of its mass with high speed and moving due to the conservation of...
  
  
• Stochastic electrodynamics
  Stochastic electrodynamics
  In theoretical physics, Stochastic Electrodynamics is a variant of Classical Electrodynamics which posits the existence of a classical Lorentz Invariant radiation field having statistical properties similar to that of the electromagnetic zero-point field of Quantum Electrodynamics...
  
  

External links

• NASA Beginner's Guide to Propulsion
• NASA Breakthrough Propulsion Physics project
• Rocket Propulsion
• Journal of Advanced Theoretical Propulsion
• Different Rockets
• Earth-to-Orbit Transportation Bibliography
• Spaceflight Propulsion - a detailed survey by Greg Goebel, in the public domain
• Rocket motors on howstuffworks.com
• Johns Hopkins University, Chemical Propulsion Information Analysis Center
• Tool for Liquid Rocket Engine Thermodynamic Analysis
• ALTA-SPACE
• NASA Jet Propulsion Laboratory