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Spacecraft propulsion is any method used to accelerate
spacecraft
A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
and artificial
satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...
s. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with
space launch
Space launch is the earliest part of a flight that reaches space. Space launch involves liftoff, when a rocket or other space launch vehicle leaves the ground, floating ship or midair aircraft at the start of a flight. Liftoff is of two main t ...
or
atmospheric entry
Atmospheric entry is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: ''uncontrolled entry'', such as the ent ...
.
Several methods of pragmatic spacecraft propulsion have been developed each having its own drawbacks and advantages. Most satellites have simple reliable chemical thrusters (often
monopropellant rocket
A monopropellant rocket (or "monochemical rocket") is a rocket that uses a single chemical as its propellant.
Chemical-reaction monopropellant rockets
For monopropellant rockets that depend on a chemical reaction, the power for the propulsive re ...
s) or
resistojet rocket
A resistojet is a method of spacecraft propulsion (electric propulsion) that provides thrust by heating a typically non-reactive fluid. Heating is usually achieved by sending electricity through a resistor consisting of a hot incandescent filament, ...
s for
orbital station-keeping
In astrodynamics, orbital station-keeping is keeping a spacecraft at a fixed distance from another spacecraft or celestial body. It requires a series of orbital maneuvers made with thruster burns to keep the active craft in the same orbit as its t ...
and some use
momentum wheel
A reaction wheel (RW) is used primarily by spacecraft for three-axis attitude control, and does not require rockets or external applicators of torque. They provide a high pointing accuracy, and are particularly useful when the spacecraft must be ...
s for
attitude control
Attitude control is the process of controlling the orientation of an aerospace vehicle with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, etc.
Controlling vehicle ...
. Soviet bloc satellites have used
electric propulsion
Spacecraft electric propulsion (or just electric propulsion) is a type of spacecraft propulsion technique that uses electrostatic or electromagnetic fields to accelerate mass to high speed and thus generate thrust to modify the velocity of a sp ...
for decades, and newer Western geo-orbiting spacecraft are starting to use them for north–south station-keeping and orbit raising. Interplanetary vehicles mostly use chemical rockets as well, although a few have used
ion thruster
An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity.
An ion thruster ionizes a neutral gas by extracting some electrons out of a ...
s and
Hall-effect thruster
In spacecraft propulsion, a Hall-effect thruster (HET) is a type of ion thruster in which the propellant is accelerated by an electric field. Hall-effect thrusters (based on the discovery by Edwin Hall) are sometimes referred to as Hall thruster ...
s (two different types of
electric propulsion
Spacecraft electric propulsion (or just electric propulsion) is a type of spacecraft propulsion technique that uses electrostatic or electromagnetic fields to accelerate mass to high speed and thus generate thrust to modify the velocity of a sp ...
) to great success.
Hypothetical in-space propulsion technologies describe the propulsion technologies that could meet future
space science and exploration needs. These propulsion technologies are intended to provide effective
exploration
Exploration refers to the historical practice of discovering remote lands. It is studied by geographers and historians.
Two major eras of exploration occurred in human history: one of convergence, and one of divergence. The first, covering most ...
of our
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
and will permit mission designers to plan missions to "fly anytime, anywhere, and complete a host of science objectives at the destinations" and with greater reliability and safety. With a wide range of possible missions and candidate propulsion technologies, the question of which technologies are "best" for future missions is a difficult one. A portfolio of propulsion technologies should be developed to provide optimum solutions for a diverse set of missions and destinations.
[
][
Mason, Lee S.]
A practical approach to starting fission surface power development.
proceedings of International Congress on Advances in Nuclear Power Plants (ICAPP’06), American Nuclear Society, La Grange Park, IL, 2006b, paper. Vol. 6297. 2006.
Purpose and function
In-space propulsion begins where the
upper stage
A multistage rocket or step rocket is a launch vehicle that uses two or more rocket ''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 ...
of the
launch vehicle
A launch vehicle or carrier rocket is a rocket designed to carry a payload (spacecraft or satellites) from the Earth's surface to outer space. Most launch vehicles operate from a launch pad, launch pads, supported by a missile launch contro ...
leaves off; performing the functions of
primary propulsion,
reaction control,
station keeping,
precision pointing, and
orbital maneuver
In spaceflight, an orbital maneuver (otherwise known as a burn) 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 ' ...
ing. The
main engines used in
space
Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consider ...
provide the primary propulsive force for
orbit transfer,
planetary trajectories and extra
planetary landing and
ascent
Ascent or The Ascent may refer to:
Publications
* ''Ascent'' (magazine), an independent, not-for-profit magazine
* ''Ascent'' (journal), a literary journal based at Concordia College
* ''Ascent'' (novel), by Jed Mercurio
* '' Times Ascent'', a ...
. The reaction control and orbital maneuvering systems provide the propulsive force for orbit maintenance, position control, station keeping, and spacecraft attitude control.
[
When in space, the purpose of a ]propulsion system
Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived from ...
is to change the velocity, or ''v'', of a spacecraft. Because this is more difficult for more massive spacecraft, designers generally discuss spacecraft performance in ''amount of change in momentum per unit of propellant consumed'' also called specific impulse
Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse i ...
. The higher the specific impulse, the better the efficiency. Ion propulsion engines have high specific impulse (~3000 s) and low thrust whereas chemical rockets like monopropellant
Monopropellants are propellants consisting of chemicals that release energy through exothermic chemical decomposition. The molecular bond energy of the monopropellant is released usually through use of a catalyst. This can be contrasted with biprop ...
or bipropellant
The highest specific impulse chemical rockets use liquid propellants (liquid-propellant rockets). They can consist of a single chemical (a monopropellant) or a mix of two chemicals, called bipropellants. Bipropellants can further be divided into ...
rocket engines have a low specific impulse (~300 s) but high thrust.
When launching a spacecraft from Earth, a propulsion method must overcome a higher gravitational
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stron ...
pull to provide a positive net acceleration.
In orbit, any additional impulse, even very tiny, will result in a change in the orbit path.
1) Prograde/Retrogade (i.e. acceleration in the tangential/opposite in tangential direction) – Increases/Decreases altitude of orbit
2) Perpendicular to orbital plane – Changes orbital inclination
Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object.
For a satellite orbiting the Earth ...
The rate of change of velocity
Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity is a ...
is called acceleration
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...
, and the rate of change of momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
is called force
In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a p ...
. 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 manoeuvring 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.
Earth's surface is situated fairly deep in a gravity well
The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sp ...
. The escape velocity
In celestial mechanics, escape velocity or escape speed is the minimum speed needed for a free, non- propelled object to escape from the gravitational influence of a primary body, thus reaching an infinite distance from it. It is typically ...
required to get out of it is 11.2 kilometers/second. As human beings evolved in a gravitational field of 1 g (9.8 m/s²), an ideal propulsion system for human spaceflight would be one that provides a continuous acceleration of 1 g (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
In Newtonian physics, free fall is any motion of a body where gravity is the only force acting upon it. In the context of general relativity, where gravitation is reduced to a space-time curvature, a body in free fall has no force acting on i ...
, such as nausea, muscular weakness, reduced sense of taste, or leaching
Leaching is the loss or extraction of certain materials from a carrier into a liquid (usually, but not always a solvent). and may refer to:
* Leaching (agriculture), the loss of water-soluble plant nutrients from the soil; or applying a small amou ...
of calcium from their bones.
The law of conservation of momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
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
Working mass, also referred to as reaction mass, is a mass against which a system operates in order to produce acceleration.
In the case of a chemical rocket, for example, the reaction mass is the Product (chemistry), product of the burned fuel sh ...
.
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 ''mv''²/2, which must come from somewhere. In a conventional solid
Solid is one of the State of matter#Four fundamental states, four fundamental states of matter (the others being liquid, gas, and Plasma (physics), plasma). The molecules in a solid are closely packed together and contain the least amount o ...
, liquid
A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...
, or hybrid rocket
A hybrid-propellant rocket is a rocket with a rocket motor that uses rocket propellants in two different phases: one solid and the other either gas or liquid. The hybrid rocket concept can be traced back to the early 1930s.
Hybrid rockets avo ...
, 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
An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity.
An ion thruster ionizes a neutral gas by extracting some electrons out of a ...
, electricity is used to accelerate ions out the back. Here some other source must provide the electrical energy (perhaps a solar panel
A solar cell panel, solar electric panel, photo-voltaic (PV) module, PV panel or solar panel is an assembly of photovoltaic solar cells mounted in a (usually rectangular) frame, and a neatly organised collection of PV panels is called a photo ...
or a nuclear reactor
A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nu ...
), whereas 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 (usually abbreviated ) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse i ...
, the impulse per unit weight-on-Earth (typically designated by ). The unit for this value is seconds. Because 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, 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.
In-space propulsion represents technologies that can significantly improve a number of critical aspects of the mission. Space exploration
Space exploration is the use of astronomy and space technology to explore outer space. While the exploration of space is carried out mainly by astronomers with telescopes, its physical exploration though is conducted both by robotic spacec ...
is about getting somewhere safely (mission enabling), getting there quickly (reduced transit times), getting a lot of mass there (increased payload
Payload is the object or the entity which is being carried by an aircraft or launch vehicle. Sometimes payload also refers to the carrying capacity of an aircraft or launch vehicle, usually measured in terms of weight. Depending on the nature of ...
mass), and getting there cheaply (lower cost). The simple act of "getting" there requires the employment of an in-space propulsion system, and the other metrics are modifiers to this fundamental action.[
Development of technologies will result in technical solutions that improve thrust levels, Isp, power, specific mass, (or ]specific power
Power-to-weight ratio (PWR, also called specific power, or power-to-mass ratio) is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measuremen ...
), volume, system mass, system complexity, operational complexity, commonality with other spacecraft systems, manufacturability, durability, and cost. These types of improvements will yield decreased transit times, increased payload mass, safer spacecraft, and decreased costs. In some instances, development of technologies within this technology area (TA) will result in mission-enabling breakthroughs that will revolutionize space exploration. There is no single propulsion technology that will benefit all missions or mission types. The requirements for in-space propulsion vary widely due according to their intended application. The described technologies should support everything from small satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...
s and robotic deep space exploration to space stations
A space station is a spacecraft capable of supporting a human crew in orbit for an extended period of time, and is therefore a type of space habitat. It lacks major propulsion or landing systems. An orbital station or an orbital space station is ...
and human missions to Mars applications.[
]
Defining technologies
Furthermore, the term "mission pull" defines a technology or a performance characteristic necessary to meet a planned NASA mission requirement. Any other relationship between a technology and a mission (an alternate propulsion system, for example) is categorized as "technology push." Also, a space demonstration refers to the spaceflight of a scaled version of a particular technology or of a critical technology subsystem. On the other hand, a space validation would serve as a qualification flight for future mission implementation. A successful validation flight would not require any additional space testing of a particular technology before it can be adopted for a science or exploration mission.
Operating domains
Spacecraft operate in many areas of space. These include orbital maneuvering, interplanetary travel and interstellar travel.
Orbital
Artificial satellites are first launched into the desired altitude by conventional liquid/solid propelled rockets after which the satellite may use onboard propulsion systems for orbital stationkeeping. Once in the desired orbit, they often need some form of attitude control
Attitude control is the process of controlling the orientation of an aerospace vehicle with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, etc.
Controlling vehicle ...
so that they are correctly pointed with respect to the Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, the Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, and possibly some astronomical
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galaxies ...
object of interest. They are also subject to drag from the thin atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
, so that to stay in orbit for a long period of time some form of propulsion is occasionally necessary to make small corrections (orbital station-keeping
In astrodynamics, orbital station-keeping is keeping a spacecraft at a fixed distance from another spacecraft or celestial body. It requires a series of orbital maneuvers made with thruster burns to keep the active craft in the same orbit as its t ...
). 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 usually over once it has exhausted its ability to adjust its orbit.
Interplanetary
For interplanetary travel
Interplanetary spaceflight or interplanetary travel is the crewed or uncrewed travel between stars and planets, usually within a single planetary system. In practice, spaceflights of this type are confined to travel between the planets of the ...
, a spacecraft can use its engines to leave Earth's orbit. It is not explicitly necessary as the initial boost given by the rocket, gravity slingshot, monopropellant/bipropellent attitude control propulsion system are enough for the exploration of the solar system (see New Horizons
''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research ...
). 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 moves along its trajectory without accelerating. The most fuel-efficient means to move from one circular orbit to another is with a Hohmann transfer orbit
In astronautics, the Hohmann transfer orbit () is an orbital maneuver used to transfer a spacecraft between two orbits of different altitudes around a central body. Examples would be used for travel between low Earth orbit and the Moon, or ano ...
: the spacecraft begins in a roughly circular orbit around the Sun. A short period of thrust
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
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 is a spaceflight maneuver that reduces the high point of an elliptical orbit (apoapsis) by flying the vehicle through the atmosphere at the low point of the orbit (periapsis). The resulting drag slows the spacecraft. Aerobraking is ...
or aerocapture are sometimes used for this final orbital adjustment.
Some spacecraft propulsion methods such as solar sail
Solar sails (also known as light sails and photon sails) are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large mirrors. A number of spaceflight missions to test solar propulsion and navigation have been p ...
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 solar sail spacecraft.
Interstellar
No spacecraft capable of short duration (compared to human lifetime) interstellar travel
Interstellar travel is the hypothetical travel of spacecraft from one star system, solitary star, or planetary system to another. Interstellar travel is expected to prove much more difficult than interplanetary spaceflight due to the vast dif ...
has yet been built, but many hypothetical designs have been discussed. Because 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 remains a formidable challenge for spacecraft designers.
Propulsion technology
The technology areas are divided into four basic groups: (1) Chemical propulsion, (2) Electric propulsion, (3) Advanced propulsion technologies, and (4) Supporting technologies; based on the physics of the propulsion system and how it derives thrust as well as its technical maturity. Additionally, there may be credible meritorious in-space propulsion concepts not foreseen or reviewed at the time of publication, and which may be shown to be beneficial to future mission applications.
Chemical propulsion
A large fraction of the rocket engine
A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordanc ...
s in use today are chemical rockets; that is, they obtain the energy needed to generate thrust by chemical reactions
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...
to create a hot gas that is expanded to produce thrust
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
. Many different propellant combinations are used to obtain these chemical reactions, including hydrazine
Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, hydrazine ...
, liquid oxygen
Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries—is the liquid form of molecular oxygen. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an applica ...
, liquid hydrogen
Liquid hydrogen (LH2 or LH2) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.
To exist as a liquid, H2 must be cooled below its critical point of 33 K. However, for it to be in a fully li ...
, nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has a ...
, and hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
for example. They can be used as a monopropellant
Monopropellants are propellants consisting of chemicals that release energy through exothermic chemical decomposition. The molecular bond energy of the monopropellant is released usually through use of a catalyst. This can be contrasted with biprop ...
or in a bi-propellant configuration.
Green chemical propulsion
The dominant form of chemical propulsion for satellites has historically been hydrazine
Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, hydrazine ...
, however this fuel is highly toxic and at risk of being banned across Europe. Non-toxic 'green' alternatives are now being developed to replace hydrazine. Nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has a ...
-based alternatives are garnering a lot of traction and government support, with development being led by commercial companies Dawn Aerospace, Impulse Space, and Launcher. The first nitrous oxide-based system ever flown in space was by D-Orbit onboard their ION Satellite Carrier (space tug
''Space Tug'' is a young adult science fiction novel by author Murray Leinster. It was published in 1953 by Shasta Publishers in an edition of 5,000 copies. It is the second novel in the author's Joe Kenmore series. Groff Conklin gave it a mixed ...
) in 2021, using six Dawn Aerospace
Dawn Aerospace is a space company based in the United States of America, the Netherlands and New Zealand. The company currently manufactures satellite propulsion systems with lower greenhouse potential and nontoxic materials, as well as an unmanned ...
B20 thrusters, launched upon a Falcon 9
Falcon 9 is a partially reusable medium lift launch vehicle that can carry cargo and crew into Earth orbit, produced by American aerospace company SpaceX.
The rocket has two stages. The first (booster) stage carries the second stage and payl ...
rocket.
Reaction engines
Reaction engines produce thrust
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
by expelling reaction mass
Working mass, also referred to as reaction mass, is a mass against which a system operates in order to produce acceleration.
In the case of a chemical rocket, for example, the reaction mass is the Product (chemistry), product of the burned fuel sh ...
, in accordance with Newton's third law of motion
Newton's laws of motion are three basic Scientific law, laws of classical mechanics that describe the relationship between the motion of an object and the forces acting on it. These laws can be paraphrased as follows:
# A body remains at re ...
. This law of motion is most commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force."
Examples include jet engine
A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, Pump-jet, water jet, and hybrid propulsion, the term ...
s, rocket engine
A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordanc ...
s, pump-jet
A pump-jet, hydrojet, or water jet is a marine system that produces a jet of water for propulsion. The mechanical arrangement may be a ducted propeller (axial-flow pump), a centrifugal pump, or a mixed flow pump which is a combination of both ...
, and more uncommon variations such as Hall–effect thrusters, ion drive
An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity.
An ion thruster ionizes a neutral gas by extracting some electrons out of ...
s, mass drivers, and nuclear pulse propulsion
Nuclear pulse propulsion or external pulsed plasma propulsion is a hypothetical method of spacecraft propulsion that uses nuclear explosions for thrust. It originated as Project ''Orion'' with support from DARPA, after a suggestion by Stanislaw ...
.
Rocket engines
Most rocket engines are internal combustion
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combus ...
heat engines
In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state ...
(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
A nozzle is a device designed to control the direction or characteristics of a fluid flow (specially to increase velocity) as it exits (or enters) an enclosed chamber or pipe.
A nozzle is often a pipe or tube of varying cross sectional area, a ...
. 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
A magnetic mirror, known as a magnetic trap (магнитный захват) in Russia and briefly as a pyrotron in the US, is a type of magnetic confinement device used in fusion power to trap high temperature plasma using magnetic fields. The ...
and release it via a magnetic nozzle A magnetic nozzle is a convergent-divergent magnetic field that guides, expands and accelerates a plasma jet into vacuum for the purpose of space propulsion. The magnetic field in a magnetic nozzle plays a similar role to the convergent-divergent ...
, 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 is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manifest ...
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
A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordanc ...
for a listing of various kinds of rocket engines using different heating methods, including chemical, electrical, solar, and nuclear.
Electric propulsion
Electric propulsion
Spacecraft electric propulsion (or just electric propulsion) is a type of spacecraft propulsion technique that uses electrostatic or electromagnetic fields to accelerate mass to high speed and thus generate thrust to modify the velocity of a sp ...
is commonly used for station keeping on commercial communications satellites
A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. C ...
and for prime propulsion on some scientific space missions because of their high specific impulse. However, they generally have very small values of thrust and therefore must be operated for long durations to provide the total impulse required by a mission.[Tomsik, Thomas M.]
Recent advances and applications in cryogenic propellant densification technology
." NASA TM 209941 (2000).[Oleson, S., and J. Sankovic.]
Advanced Hall electric propulsion for future in-space transportation
" Spacecraft Propulsion. Vol. 465. 2000.
Rather than relying on high temperature and fluid dynamics
In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) an ...
to accelerate the reaction mass to high speeds, there are a variety of methods that use electrostatic or electromagnetic
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...
forces to accelerate the reaction mass directly. Usually the reaction mass is a stream of ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
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
Robert Hutchings Goddard (October 5, 1882 – August 10, 1945) was an American engineer, professor, physicist, and inventor who is credited with creating and building the world's first liquid-fueled rocket. Goddard successfully laun ...
considered the possibility in his personal notebook.
Konstantin Tsiolkovsky
Konstantin Eduardovich Tsiolkovsky (russian: Константи́н Эдуа́рдович Циолко́вский , , p=kənstɐnʲˈtʲin ɪdʊˈardəvʲɪtɕ tsɨɐlˈkofskʲɪj , a=Ru-Konstantin Tsiolkovsky.oga; – 19 September 1935) ...
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.
The Glenn Research Center
NASA John H. Glenn Research Center at Lewis Field is a NASA center within the cities of Brook Park and Cleveland between Cleveland Hopkins International Airport and the Rocky River Reservation of Cleveland Metroparks, with a subsidiary facilit ...
aims to develop primary propulsion technologies which could benefit near and mid-term science missions by reducing cost, mass, and/or travel times. Propulsion architectures of particular interest to the GRC are electric propulsion
Spacecraft electric propulsion (or just electric propulsion) is a type of spacecraft propulsion technique that uses electrostatic or electromagnetic fields to accelerate mass to high speed and thus generate thrust to modify the velocity of a sp ...
systems, such as Ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
and Hall
In architecture, a hall is a relatively large space enclosed by a roof and walls. In the Iron Age and early Middle Ages in northern Europe, a mead hall was where a lord and his retainers ate and also slept. Later in the Middle Ages, the gr ...
thrusters. One system combines solar sail
Solar sails (also known as light sails and photon sails) are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large mirrors. A number of spaceflight missions to test solar propulsion and navigation have been p ...
s, a form of propellantless propulsion which relies on naturally-occurring starlight for propulsion energy, and Hall thrusters. Other propulsion technologies being developed include advanced chemical propulsion and aerocapture.[Solar Electric Propulsion (SEP)](_blank)
Glenn Research Center. NASA. 2019[Ion propulsion system research](_blank)
. Glenn Research Center. NASA. 2013
For some missions, particularly reasonably close to the Sun, solar energy
Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy (including solar water heating), and solar architecture. It is an essenti ...
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
A nuclear electric rocket (more properly nuclear electric propulsion) is a type of spacecraft propulsion system where thermal energy from a nuclear reactor is converted to electrical energy, which is used to drive an ion thruster or other electrica ...
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.
Some electromagnetic methods:
* Ion thrusters
An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity.
An ion thruster ionizes a neutral gas by extracting some electrons out o ...
(accelerate ions first and later neutralize the ion beam with an electron stream emitted from a cathode called a neutralizer)
**Electrostatic ion thruster
The gridded ion thruster is a common design for ion thrusters, a highly efficient low-thrust spacecraft propulsion method running on electrical power by using high-voltage grid electrodes to accelerate ions with electrostatic forces.
History
The ...
**Gridded ion thruster
The gridded ion thruster is a common design for ion thrusters, a highly efficient low-thrust spacecraft propulsion method running on electrical power by using high-voltage grid electrodes to accelerate ions with electrostatic forces.
History
The ...
**Field-emission electric propulsion
Field-emission electric propulsion (FEEP) is an advanced electrostatic space propulsion concept, a form of ion thruster, that uses a liquid metal as a propellant – usually either caesium, indium, or mercury.
A FEEP device consists of an emitter ...
** MagBeam
**Hall-effect thruster
In spacecraft propulsion, a Hall-effect thruster (HET) is a type of ion thruster in which the propellant is accelerated by an electric field. Hall-effect thrusters (based on the discovery by Edwin Hall) are sometimes referred to as Hall thruster ...
**Colloid thruster
A colloid thruster (or "electrospray thruster") is a type of low thrust electric propulsion rocket engine that uses electrostatic acceleration of charged liquid droplets for propulsion. In a colloid thruster, charged liquid droplets are produced b ...
* Electrothermal thrusters (electromagnetic fields are used to generate a plasma to increase the heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...
of the bulk propellant, the thermal energy imparted to the propellant gas is then converted into kinetic energy by a nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow (specially to increase velocity) as it exits (or enters) an enclosed chamber or pipe.
A nozzle is often a pipe or tube of varying cross sectional area, a ...
of either physical material construction or by magnetic means)
** DC arcjet
** Microwave arcjet
**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.
The ...
* Electromagnetic thrusters (ions are accelerated either by the Lorentz Force
In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...
or by the effect of electromagnetic fields where the electric field is not in the direction of the acceleration)
**Plasma propulsion engine
A plasma propulsion engine is a type of electric propulsion that generates thrust from a quasi-neutral plasma. This is in contrast with ion thruster engines, which generate thrust through extracting an ion current from the plasma source, which ...
**Magnetoplasmadynamic thruster
A magnetoplasmadynamic (MPD) thruster (MPDT) is a form of electrically powered spacecraft propulsion which uses the Lorentz force (the force on a charged particle by an electromagnetic field) to generate thrust. It is sometimes referred to as L ...
** Electrodeless plasma thruster
**Pulsed inductive thruster
A pulsed inductive thruster (PIT) is a form of ion thruster, used in spacecraft propulsion. It is a plasma propulsion engine using perpendicular electric and magnetic fields to accelerate a propellant with no electrode.
Operation
A nozzle r ...
**Pulsed plasma thruster
A pulsed plasma thruster (PPT), also known as a plasma jet engine, is a form of electric spacecraft propulsion. PPTs are generally considered the simplest form of electric spacecraft propulsion and were the first form of electric propulsion to be ...
** Variable specific impulse magnetoplasma rocket (VASIMR)
**Vacuum arc thruster A vacuum arc thruster (VAT) is a form of Electrically powered spacecraft propulsion, electric spacecraft propulsion. It uses a vacuum arc discharge, across an insulator, between two electrodes to produce thrust. A metal plasma is produced from micro ...
*Mass driver
A mass driver or electromagnetic catapult is a proposed method of non-rocket spacelaunch which would use a linear motor to Acceleration, accelerate and catapult Payload (air and space craft), payloads up to high speeds. Existing and contemplated ...
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
In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves over time. Exact conservation laws include conservation of energy, conservation of linear momentum, co ...
of momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
is usually taken to imply 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
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
s, electromagnetic waves
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) lig ...
, solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
and solar radiation. Electromagnetic waves in particular are known to contain momentum, despite being massless; specifically the momentum flux density P of an EM wave is quantitatively 1/c2 times the Poynting vector
In physics, the Poynting vector (or Umov–Poynting vector) represents the directional energy flux (the energy transfer per unit area per unit time) or '' power flow'' of an electromagnetic field. The SI unit of the Poynting vector is the watt ...
S, i.e. P = S/c2, where c is the velocity of light. 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, such as gravitational and magnetic fields f ...
methods which do not rely on reaction mass thus must try to take advantage of this fact by coupling to a momentum-bearing field such as an EM wave that exists in the vicinity of the craft. However, because many of 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
Space tethers are long cables which can be used for propulsion, momentum exchange, stabilization and attitude control, or maintaining the relative positions of the components of a large dispersed satellite/spacecraft sensor system. Depending on t ...
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 sails (also known as light sails and photon sails) are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large mirrors. A number of spaceflight missions to test solar propulsion and navigation have been p ...
s rely on radiation pressure
Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength that is a ...
from electromagnetic energy, but they require a large collection surface to function effectively.
A magnetic sail
A magnetic sail is a proposed method of spacecraft propulsion that uses a static magnetic field to deflect a plasma wind of charged particles radiated by the Sun or a Star thereby transferring momentum to accelerate or decelerate a spacecraft. ...
deflects charged particles from the solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
with a magnetic field, thereby imparting momentum to the spacecraft. The Magsail is a large superconducting loop proposed for acceleration/deceleration in the solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
and deceleration in the Interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
. A variant is the mini-magnetospheric plasma propulsion
A magnetic sail is a proposed method of spacecraft propulsion that uses a static magnetic field to deflect a plasma wind of charged particles radiated by the Sun or a Star thereby transferring momentum to accelerate or decelerate a spacecraft. ...
system and its successor, the Magnetoplasma sail inject plasma at a low rate to enhance the magnetic field to more effectively deflect charged particles in a plasma wind.
An E-sail would use very thin and lightweight wires holding an electric charge to deflect these particles, and may have more controllable directionality.
As a proof of concept, 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 , with a mass of . The satellite was lost shortly after launch ...
became the first nanosatellite to orbit Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
. As of August 2017, NASA confirmed the Sunjammer solar sail project was concluded in 2014 with lessons learned for future space sail projects.
Cubesail will be the first mission to demonstrate solar sailing in low Earth orbit, and the first mission to demonstrate full three-axis attitude control of a solar sail.
Japan also launched its own solar sail powered spacecraft IKAROS in May 2010. IKAROS successfully demonstrated propulsion and guidance and is still flying today.
A satellite or other space vehicle is subject to the law of conservation of angular momentum, which constrains a body from a net change in angular velocity
In physics, angular velocity or rotational velocity ( or ), also known as angular frequency vector,(UP1) is a pseudovector representation of how fast the angular position or orientation of an object changes with time (i.e. how quickly an objec ...
. 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
A reaction wheel (RW) is used primarily by spacecraft for three-axis attitude control, and does not require rockets or external applicators of torque. They provide a high pointing accuracy, and are particularly useful when the spacecraft must be ...
or control moment gyroscopes to control orientation in space.
A gravitational slingshot
In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement (e.g. orbit around the Sun) and gravity of a planet or other astronomical object to alter the ...
can carry a space probe
A space probe is an artificial satellite that travels through space to collect scientific data. A space probe may orbit Earth; approach the Moon; travel through interplanetary space; flyby, orbit, or land or fly on other planetary bodies; or ent ...
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.
Beam-powered propulsion
Beam-powered propulsion, also known as directed energy propulsion, is a class of aircraft or spacecraft propulsion that uses energy beamed to the spacecraft from a remote power plant to provide energy. The beam is typically either a microwave or a ...
is another method of propulsion without reaction mass. Beamed propulsion includes sails pushed by laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
, microwave, or particle beams.
Advanced propulsion technology
Advanced, and in some cases theoretical, propulsion technologies may use chemical or nonchemical physics to produce thrust, but are generally considered to be of lower technical maturity with challenges that have not been overcome. For both human and robotic exploration, traversing the solar system is a struggle against time and distance. The most distant planets are 4.5–6 billion kilometers from the Sun and to reach them in any reasonable time requires much more capable propulsion systems than conventional chemical rockets. Rapid inner solar system missions with flexible launch dates are difficult, requiring propulsion systems that are beyond today's current state of the art. The logistics, and therefore the total system mass required to support sustained human exploration beyond Earth to destinations such as the Moon, Mars or Near Earth Objects,
are daunting unless more efficient in-space propulsion technologies are developed and fielded.[
]
A variety of hypothetical propulsion techniques have been considered that require a deeper understanding of the properties of
space, particularly inertial frames
In classical physics and special relativity, an inertial frame of reference (also called inertial reference frame, inertial frame, inertial space, or Galilean reference frame) is a frame of reference that is not undergoing any acceleration ...
and the vacuum state
In quantum field theory, the quantum vacuum state (also called the quantum vacuum or vacuum state) is the quantum state with the lowest possible energy. Generally, it contains no physical particles. The word zero-point field is sometimes used as ...
. To date, such methods are highly speculative and include:
*Black hole starship
In astronautics, a black hole starship is the theoretical concept of a starship capable of interstellar travel using a black hole as an energy source for spacecraft propulsion. The concept was first discussed in science fiction, notably in the book ...
* Differential sail
*Gravitational shielding
The term gravitational shielding refers to a hypothetical process of shielding an object from the influence of a gravitational field. Such processes, if they existed, would have the effect of reducing the weight of an object. The shape of the shie ...
*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, such as gravitational and magnetic fields f ...
**Diametric drive
**Disjunction drive
**Pitch drive
**Bias drive
*Photon rocket
A photon rocket is a rocket that uses thrust from the momentum of emitted photons ( radiation pressure by emission) for its propulsion. Photon rockets have been discussed as a propulsion system that could make interstellar flight possible, which r ...
*Quantum vacuum thruster
A reactionless drive is a hypothetical device producing motion without the exhaust of a propellant. A propellantless drive is not necessarily reactionless when it constitutes an open system interacting with external fields; but a reactionless ...
* Nano electrokinetic thruster
*Reactionless drive
A reactionless drive is a hypothetical device producing motion without the exhaust of a propellant. A propellantless drive is not necessarily reactionless when it constitutes an open system interacting with external fields; but a reactionless ...
** Abraham—Minkowski drive
**Alcubierre drive
The Alcubierre drive () is a speculative warp drive idea according to which a spacecraft could achieve apparent faster-than-light travel by contracting space in front of it and expanding space behind it, under the assumption that a configurable ...
** Dean drive
** EmDrive
**Heim theory
Heim theory, first proposed by German physicist Burkhard Heim publicly in 1957, is an attempt to develop a theory of everything in theoretical physics. The theory claims to bridge some of the disagreements between quantum mechanics and general rel ...
**Woodward effect James F. Woodward (born 1941) is a professor emeritus of history and an adjunct professor of physics at California State University, Fullerton. He is best known for a physics hypothesis that he proposed in 1990, later expanded, that predicts several ...
** Thornson Inertial Engine (TIE)
** Gyroscopic Inertial Thruster (GIT)
A NASA assessment of its Breakthrough Propulsion Physics Program The Breakthrough Propulsion Physics Project (BPP) was a research project funded by NASA from 1996-2002 to study various proposals for revolutionary methods of spacecraft propulsion that would require breakthroughs in physics before they could be re ...
divides such proposals into those that are non-viable for propulsion purposes, those that are of uncertain potential, and those that are not impossible according to current theories.
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 (usually abbreviated ) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse is ...
: 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.
Table Notes
Testing
Spacecraft propulsion systems are often first statically tested on 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
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. Ground testing is very inexpensive i ...
well away from habitation and other buildings for safety reasons. Ion drive
An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity.
An ion thruster ionizes a neutral gas by extracting some electrons out of ...
s 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.
Planetary and atmospheric propulsion
Launch-assist mechanisms
There have been many ideas proposed for launch-assist mechanisms that have the potential of drastically reducing the cost of getting into orbit. Proposed non-rocket spacelaunch
Non-rocket spacelaunch refers to theoretical concepts for launch into space where much of the speed and altitude needed to achieve orbit is provided by a propulsion technique that is not subject to the limits of the rocket equation. Although al ...
launch-assist mechanisms include:
* Skyhook (requires reusable suborbital launch vehicle, not feasible using presently available materials)
*Space elevator
A space elevator, also referred to as a space bridge, star ladder, and orbital lift, is a proposed type of planet-to-space transportation system, often depicted in science fiction. The main component would be a cable (also called a tether) anc ...
(tether from Earth's surface to geostationary orbit, cannot be built with existing materials)
*Launch loop
A launch loop, or Lofstrom loop, is a proposed system for launching objects into orbit using a moving cable-like system situated inside a sheath attached to the Earth at two ends and suspended above the atmosphere in the middle. The design conce ...
(a very fast enclosed rotating loop about 80 km tall)
*Space fountain
A space fountain is a proposed form of an extremely tall tower extending into space. As known materials cannot support a static tower with this height, a space fountain has to be an active structure: A stream of pellets is accelerated upwards f ...
(a very tall building held up by a stream of masses fired from its base)
*Orbital ring
An orbital ring is a concept of an artificial ring placed around a body and set rotating at such a rate that the apparent centrifugal force is large enough to counteract the force of gravity. For the Earth, the required speed is on the order of 1 ...
(a ring around Earth with spokes hanging down off bearings)
*Electromagnetic catapult
An electromagnetic catapult, also called EMALS ("electromagnetic aircraft launch system") after the specific US system, is a type of aircraft launching system. Currently, only the United States and China have successfully developed it, and it i ...
(railgun
A railgun or rail gun is a linear motor device, typically designed as a weapon, that uses Electromagnet, electromagnetic force to launch high velocity projectiles. The projectile normally does not contain explosives, instead relying on the proj ...
, coilgun
A coilgun, also known as a Gauss rifle, is a type of mass driver consisting of one or more coils used as electromagnets in the configuration of a linear motor that accelerate a ferromagnetic or conducting projectile to high velocity. In almost ...
) (an electric gun)
*Rocket sled launch
A rocket sled launch, also known as ground-based launch assist, catapult launch assist, and sky-ramp launch, is a proposed method for launching space vehicles. With this concept the launch vehicle is supported by an eastward pointing rail or ma ...
*Space gun Space Gun may refer to:
* Space gun, a method of launching an object into space
* ''Space Gun'' (album), a 2018 album by Guided by Voices
* ''Space Gun'' (video game), a 1990 arcade game
* Ljutic Space Gun, a 12 gauge single-shot shotgun
See also
* ...
(Project HARP
Project HARP, short for High Altitude Research Project, was a joint venture 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 and collecting ...
, ram accelerator
A ram accelerator is a device for accelerating projectiles or just a single projectile to extremely high speeds using jet-engine-like propulsion cycles based on ramjet or scramjet combustion processes. It is thought to be possible to achieve no ...
) (a chemically powered gun)
*Beam-powered propulsion
Beam-powered propulsion, also known as directed energy propulsion, is a class of aircraft or spacecraft propulsion that uses energy beamed to the spacecraft from a remote power plant to provide energy. The beam is typically either a microwave or a ...
rockets and jets powered from the ground via a beam
*High-altitude platform
Atmospheric satellite (United States usage, abbreviated atmosat) or pseudo-satellite (British usage) is a marketing term for an aircraft that operates in the atmosphere at high altitudes for extended periods of time, in order to provide servic ...
s to assist initial stage
Air-breathing engines
Studies generally show that conventional air-breathing engines, such as ramjets
A ramjet, or athodyd (aero thermodynamic duct), is a form of airbreathing jet engine that uses the forward motion of the engine to produce thrust. Since it produces no thrust when stationary (no ram air) ramjet-powered vehicles require an ass ...
or turbojets
The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and ...
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 launching is the practice of releasing a rocket, missile, parasite aircraft or other aircraft payload from a mother ship or launch aircraft. The payload craft or missile is often tucked under the wing of the larger mother ship and then "drop ...
ed from separate lift vehicles (e.g. B-29
The Boeing B-29 Superfortress is an American four-engined propeller-driven heavy bomber, designed by Boeing and flown primarily by the United States during World War II and the Korean War. Named in allusion to its predecessor, the B-17 Fly ...
, Pegasus Rocket
Pegasus is an Air launch to orbit, air-launched launch vehicle developed by Orbital Sciences Corporation (OSC) and now built and launched by Northrop Grumman. Capable of carrying small payloads of up to into low Earth orbit, Pegasus first fl ...
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 sabre ( French: sabʁ or saber in American English) is a type of backsword with a curved blade associated with the light cavalry of the early modern and Napoleonic periods. Originally associated with Central European cavalry such as th ...
– a lightweight hydrogen fuelled turbojet with precooler
* ATREX
The ATREX engine (Air Turbo Ramjet Engine with eXpander cycle) 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.
ATREX uses liquid hydrogen fuel in a fairly exotic sin ...
– a lightweight hydrogen fuelled turbojet with precooler
* Liquid air cycle engine
A liquid air cycle engine (LACE) is a type of spacecraft propulsion engine that attempts to increase its efficiency by gathering part of its oxidizer from the atmosphere. A liquid air cycle engine uses liquid hydrogen (LH2) fuel to liquefy the a ...
– a hydrogen fuelled jet engine that liquifies the air before burning it in a rocket engine
* Scramjet
A scramjet (supersonic combustion ramjet) is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to compress the incoming air forcefully ...
– jet engines that use supersonic combustion
* Shcramjet – similar to a scramjet engine, however it takes advantage of shockwaves produced from the aircraft in the combustion chamber to assist in increasing overall efficiency.
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 Earth's surface until leaving the atmosphere then perform a rocket burn to bridge the final delta-v
Delta-''v'' (more known as "change in velocity"), symbolized as ∆''v'' and pronounced ''delta-vee'', as used in spacecraft flight dynamics, is a measure of the impulse per unit of spacecraft mass that is needed to perform a maneuver such as ...
to orbital velocity.
For spacecraft already in very low-orbit, air-breathing electric propulsion would use residual gases in the upper atmosphere as propellant. Air-breathing electric propulsion could make a new class of long-lived, low-orbiting missions feasible on Earth, Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
or Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
.Conceptual design of an air-breathing electric propulsion system
. (PDF). 30th International Symposium on Space Technology and Science. 34th International Electric Propulsion Conference and 6th Nano-satellite Symposium. Hyogo-Kobe, Japan July 4, 2015.
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 is a spaceflight maneuver that reduces the high point of an elliptical orbit (apoapsis) by flying the vehicle through the atmosphere at the low point of the orbit (periapsis). The resulting drag slows the spacecraft. Aerobraking is ...
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 because it takes much less delta-V to enter an elliptical orbit compared to a low circular orbit. Because 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'' (MGS) was an American robotic space probe developed by NASA's Jet Propulsion Laboratory and launched November 1996. MGS was a global mapping mission that examined the entire planet, from the ionosphere down through the a ...
'', ''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 spectr ...
'', and ''Mars Reconnaissance Orbiter
''Mars Reconnaissance Orbiter'' (MRO) is a spacecraft designed to study the geology and climate of Mars, provide reconnaissance of future landing sites, and relay data from surface missions back to Earth. It was launched on August 12, 2005, an ...
'', and at least one Venus mission, ''Magellan
Ferdinand Magellan ( or ; pt, Fernão de Magalhães, ; es, link=no, Fernando de Magallanes, ; 4 February 1480 – 27 April 1521) was a Portuguese explorer. He is best known for having planned and led the 1519 Spanish expedition to the East ...
''.
* Aerocapture
Aerocapture is an orbital transfer maneuver in which a spacecraft uses
aerodynamic drag force from a single pass through a planetary
atmosphere to decelerate and achieve orbit insertion.
Aerocapture uses a planet's or moon's atmosphere to accom ...
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, because 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 by Zond 6
Zond 6 was a formal member of the Soviet Zond program, and an unmanned version of the Soyuz 7K-L1 manned Moon-flyby spacecraft. It was launched on a lunar flyby mission from a parent satellite (68-101B) in Earth parking orbit. The spacecraft ...
and Zond 7
Zond 7, a formal member of the Soviet Zond program and unmanned version of Soyuz 7K-L1 manned Moon-flyby spacecraft, the first truly successful test of L1, was launched towards the Moon
The Moon is Earth's only natural satellite. I ...
upon lunar return were aerocapture maneuvers, because they turned a hyperbolic orbit into an elliptical orbit. On these missions, because 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
The ballute (a portmanteau of ''balloon'' and ''parachute'') is a parachute-like braking device optimized for use at high altitudes and supersonic velocities.
The original ballute configuration was invented in 1948 by the Goodyear company. The ...
is an inflatable drag device.
* Parachute
A parachute is a device used to slow the motion of an object through an atmosphere by creating drag or, in a ram-air parachute, aerodynamic lift. A major application is to support people, for recreation or as a safety device for aviators, who ...
s can land a probe on a planet or moon with an atmosphere, usually after the atmosphere has scrubbed off most of the velocity, using a heat shield
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...
.
* Airbag
An airbag is a vehicle occupant-restraint system using a bag designed to inflate extremely quickly, then quickly deflate during a collision. It consists of the airbag cushion, a flexible fabric bag, an inflation module, and an impact sensor. Th ...
s can soften the final landing.
* Lithobraking, or stopping by impacting the surface, is usually done by accident. However, it may be done deliberately with the probe expected to survive (see, for example, Deep Impact (spacecraft)
''Deep Impact'' was a NASA space probe launched from Cape Canaveral Air Force Station on January 12, 2005. It was designed to study the interior composition of the comet Tempel 1 (9P/Tempel), by releasing an impactor into the comet. At 05:52 U ...
), in which case very sturdy probes are required.
In fiction
In science fiction, space ships use various means to travel, some of them scientifically plausible (like solar sails or ramjets), others, mostly or entirely fictitious (like anti-gravity
Anti-gravity (also known as non-gravitational field) is a hypothetical phenomenon of creating a place or object that is free from the force of gravity. It does not refer to the lack of weight under gravity experienced in free fall or orbit, or t ...
, warp drive
A warp drive or a drive enabling space warp is a fictional superluminal spacecraft propulsion system in many science fiction works, most notably '' Star Trek'', and a subject of ongoing physics research. The general concept of "warp drive" wa ...
, spindizzy
''Cities in Flight'' is a four-volume series of science fiction novels and short stories by American writer James Blish, originally published between 1950 and 1962, which were first known collectively as the "Okie" novels. The series features ent ...
or hyperspace travel).
See also
References
External links
NASA Breakthrough Propulsion Physics project
Earth-to-Orbit Transportation Bibliography
– a detailed survey by Greg Goebel, in the public domain
Johns Hopkins University, Chemical Propulsion Information Analysis Center
Tool for Liquid Rocket Engine Thermodynamic Analysis
Smithsonian National Air and Space Museum's How Things Fly website
* Fullerton, Richard K.
Advanced EVA Roadmaps and Requirements
" Proceedings of the 31st International Conference on Environmental Systems. 2001.
''Atomic Rocket'' – Engines
A site listing and detailing real, theoretical and fantasy space engines.
{{DEFAULTSORT:Spacecraft Propulsion
Spacecraft propulsion
Spacecraft components
Spaceflight technology
NASA programs
Glenn Research Center
Discovery and exploration of the Solar System