   #copyright

Spacecraft propulsion

2007 Schools Wikipedia Selection. Related subjects: General Physics; Space
transport

   A remote camera captures a close-up view of a Space Shuttle Main Engine
   during a test firing at the John C. Stennis Space Center in Hancock
   County, Mississippi
   Enlarge
   A remote camera captures a close-up view of a Space Shuttle Main Engine
   during a test firing at the John C. Stennis Space Centre in Hancock
   County, Mississippi

   Propulsion means to add speed or acceleration to an object, by an
   engine or other similar device. The word 'propulsion' can be used with
   many other words (such as jet, rocket, spacecraft) to become-'jet
   propulsion', 'rocket propulsion', or 'space craft propulsion' etc.

   Spacecraft propulsion is used to change the velocity of spacecraft and
   artificial satellites. There are many different methods. Each method
   has drawbacks and advantages, and spacecraft propulsion is an active
   area of research. Most spacecraft today are propelled by heating the
   reaction mass and allowing it to EJECT out from the back/rear of the
   vehicle. This sort of engine is called a rocket engine.

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

The necessity for propulsion systems

   Artificial satellites must be launched into orbit, and once there they
   must be placed in their nominal orbit. Once in the desired orbit, they
   often need some form of attitude control so that they are correctly
   pointed with respect to the Earth, the Sun, and possibly some
   astronomical object of interest. They are also subject to drag from the
   thin atmosphere, so that to stay in orbit for a long period of time
   some form of propulsion is occasionally necessary to make small
   corrections ( orbital stationkeeping). Many satellites need to be moved
   from one orbit to another from time to time, and this also requires
   propulsion. When a satellite has exhausted its ability to adjust its
   orbit, its useful life is over.

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

   For interplanetary travel, a spacecraft must use its engines to leave
   Earth orbit. Once it has done so, it must somehow make its way to its
   destination. Current interplanetary spacecraft do this with a series of
   short-term orbital adjustments. In between these adjustments, the
   spacecraft simply falls freely along its orbit. The simplest
   fuel-efficient means to move from one circular orbit to another is with
   a Hohmann transfer orbit: the spacecraft begins in a roughly circular
   orbit around the Sun. A short period of thrust in the direction of
   motion accelerates or decelerates the spacecraft into an elliptical
   orbit around the Sun which is tangential to its previous orbit and also
   to the orbit of its destination. The spacecraft falls freely along this
   elliptical orbit until it reaches its destination, where another short
   period of thrust accelerates or decelerates it to match the orbit of
   its destination. Special methods such as aerobraking are sometimes used
   for this final orbital adjustment.
   Artist's conception of a solar sail
   Enlarge
   Artist's conception of a solar sail

   Some spacecraft propulsion methods such as solar sails 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.

   Spacecraft for interstellar travel also need propulsion methods. No
   such spacecraft has yet been built, but many designs have been
   discussed. Since interstellar distances are very great, a tremendous
   velocity is needed to get a spacecraft to its destination in a
   reasonable amount of time. Acquiring such a velocity on launch and
   getting rid of it on arrival will be a formidable challenge for
   spacecraft designers.

Effectiveness of propulsion systems

   The earth is situated fairly deep in the gravity well and it takes a
   velocity of 11.2 metres/second ( escape velocity) or more to jump out
   of it! As we are in the habit of living in the gravitational field of
   1g (9.8 m/second square), an ideal propulsion system would be one that
   provides a continuous acceleration of 1g (though human bodies can
   tolerate accelerations up to 15g in few cases).

   The rocket or spaceship having such a propulsion system will quickly
   (as compared with time involved in journeys made by existing rockets)
   accelerate to near the speed of light plus the occupants will be free
   from all the ill effects of free fall, such as nausea, muscular
   weakness, reduced sense of taste, or leaching of calcium from their
   bones.

   When in space, the purpose of a propulsion system is to change the
   velocity v of a spacecraft. Since this is more difficult for more
   massive spacecraft, designers generally discuss momentum, mv. The
   amount of change in momentum is called impulse. 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 pull to provide a net positive
   acceleration. In orbit, the spacecraft tangential velocity provides a
   centrifugal force that counterweighs the gravity pull at a given path
   (which is actually the orbit path) so that any additional impulse, even
   very tiny, will result in a change in the orbit path.

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

   The 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
   mv^2/2, which must come from somewhere. In a conventional solid,
   liquid, or hybrid rocket, the fuel is burned, providing the energy, and
   the reaction products are allowed to flow out the back, providing the
   reaction mass. In an ion thruster, electricity is used to accelerate
   ions out the back. Here some other source must provide the electrical
   energy (perhaps a solar panel or a nuclear reactor), while the ions
   provide the reaction mass.

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

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

Calculations

   Burning the entire usable propellant of a spacecraft through the
   engines in a straight line in free space would produce a net velocity
   change to the vehicle; this number is termed ' delta-v'.

   The total Δv of a vehicle can be calculated using the rocket equation,
   where M is the mass of fuel (or rather the mass of propellant), P is
   the mass of the payload (including the rocket structure), and v[e] is
   the velocity of the rocket exhaust. This is known as the Tsiolkovsky
   rocket equation:

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

   For historical reasons, as discussed above, v[e] is sometimes written
   as

          v[e] = I[sp]g[o]

   where I[sp] is the specific impulse of the rocket, measured in seconds,
   and g[o] is the gravitational acceleration at sea level.

   For a long voyage, the majority of the spacecraft's mass may be
   reaction mass. Since a rocket must carry all its reaction mass with it,
   most of the first reaction mass goes towards accelerating reaction mass
   rather than payload. If we have a payload of mass P, the spacecraft
   needs to change its velocity by Δv, 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 I[sp]

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

   For Δv much smaller than v[e], this equation is roughly linear, and
   little reaction mass is needed. If Δv 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.

   In order to achieve this, some amount of energy must go into
   accelerating the reaction mass. Every engine will waste some energy,
   but even assuming 100% efficiency, the engine will need energy
   amounting to

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

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

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

   Suppose we want to send a 10,000 kg space probe to Mars. The required
   Δv from LEO is approximately 3000 m/s, using a Hohmann transfer orbit.
   (A manned probe would need to take a faster route and use more fuel).
   For the sake of argument, let us say that the following thrusters may
   be used:
   Engine Effective Exhaust Velocity
   (m/s) Specific impulse
   (s) Fuel mass
   (kg) Energy required
   (GJ) Energy per kg
   of propellant minimum power
   per N of thrust
   Solid rocket
   1,000 100 190,000 95 500 kJ 0.5 kW
   Bipropellant rocket
   5,000 500 8,200 103 12.6 MJ 2.5 kW
   Ion thruster 50,000 5,000 620 775 1.25 GJ 25 kW

   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 much more than unity. To do this
   they would have to be supplied with Gigawatts of power — equivalent to
   a major metropolitan generating station. This would need to be carried
   on the vehicle, which is clearly impractical.

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

   Interestingly, for a mission delta-v, there is a fixed I[sp] that
   minimises the overall energy used by the rocket. This comes to an
   exhaust velocity of about ⅔ of the mission delta-v (see 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, and thus end up
   powersource limited and give very low thrust. Where the vehicle
   performance is power limited, e.g. if solar power or nuclear power is
   used, then in the case of a large v[e] the maximum acceleration is
   inversely proportional to it. Hence the time to reach a required
   delta-v is proportional to v[e]. Thus the latter should not be too
   large.

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

   Some drives (such as VASIMR) actually can significantly vary their
   exhaust velocity. This can help reduce propellant usage and improve
   acceleration at different stages of the flight. However the best
   energetic performance and acceleration is still obtained when the
   exhaust velocity is close to the vehicle speed. Proposed ion and plasma
   drives usually have exhaust velocities enormously higher than that
   ideal (in the case of VASIMR the lowest quoted speed is around 15000
   m/s compared to a mission delta-v from high Earth orbit to Mars of
   about 4000m/s).

Propulsion methods

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

Rocket engines

   A "cold" (un-ignited) rocket engine test at NASA
   Enlarge
   A "cold" (un-ignited) rocket engine test at NASA

   Most rocket engines are internal combustion 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. This bell-shaped nozzle is what gives a
   rocket engine its characteristic shape. The effect of the nozzle is to
   dramatically accelerate the mass, converting most of the thermal energy
   into kinetic energy. Exhaust speeds as high as 10 times the speed of
   sound at sea level are not uncommon.

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

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

Airbreathing engines for launch

   Studies generally show that conventional air-breathing engines, such as
   ramjets or turbojets are basically too heavy (have too low a
   thrust/weight ratio) to give any significant performance improvement
   when installed on a launch vehicle. However, they can be air launched
   from a separate lift vehicle (e.g. X-1, Pegasus and SS1). 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 - a lightweight hydrogen fuelled turbojet with precooler
     * Liquid air cycle engine - a hydrogen fuelled jet engine that
       liquifies the air before burning it in a rocket engine
     * Scramjet - jet engines that use supersonic combustion

Electromagnetic acceleration of reaction mass

   This test engine accelerates ions using electrostatic forces
   Enlarge
   This test engine accelerates ions using electrostatic forces

   Rather than relying on high temperature and fluid dynamics to
   accelerate the reaction mass to high speeds, there are a variety of
   methods that use electrostatic or electromagnetic forces to accelerate
   the reaction mass directly. Usually the reaction mass is a stream of
   ions. Such an engine requires electric power to run, and high exhaust
   velocities require large amounts of energy.

   For these drives it turns out that to a reasonable approximation fuel
   use, 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 powersources provide low
   thrust, but use hardly any fuel.

   For some missions, solar energy may be sufficient, and has very often
   been used, but for others nuclear energy will be necessary; engines
   drawing their power from a nuclear source are called nuclear electric
   rockets.

   With any current source of power, chemical, nuclear or solar, the
   maximum amount of power that can be generated greatly limits the
   maximum amount of thrust that can be produced to a small value. Power
   generation also 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 potential.

   The dissipation of waste heat from the powerplant may make any
   propulsion system requiring a separate power source infeasible for
   interstellar travel.

   Some electromagnetic methods:
     * Ion thruster
          + Electrostatic ion thruster
          + Field Emission Electric Propulsion
          + Hall effect thruster
          + Helicon Double Layer Thruster
          + Electrodeless plasma thruster (acceleration by electromagnetic
            forces; emits plasma)
          + Pulsed inductive thruster
     * Magnetoplasmadynamic thruster
     * Variable specific impulse magnetoplasma rocket
     * Mass drivers (for propulsion)

Systems without reaction mass carried within the spacecraft

   NASA study of a solar sail. The sail would be half a kilometer wide.
   Enlarge
   NASA study of a solar sail. The sail would be half a kilometer wide.

   The law of conservation of momentum states that any engine which uses
   no reaction mass cannot move the centre of mass of a spaceship
   (changing orientation, on the other hand, is possible). But space is
   not empty, especially space inside the Solar System; there are
   gravitation fields, magnetic fields, solar wind and solar radiation.
   Various propulsion methods try to take advantage of these. However,
   since these phenomena are diffuse in nature, corresponding propulsion
   structures need to be proportionately large.

   Space drives that need no (or little) reaction mass:
     * Tether propulsion
     * Solar sails
     * Magnetic sails
     * Mini-magnetospheric plasma propulsion

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

Launch mechanisms

   An artist's conception of an electromagnetic catapult on the Moon
   Enlarge
   An artist's conception of an electromagnetic catapult on the Moon

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

   Exhaust toxicity or other side effects can also have detrimental
   effects on the environment the spacecraft is launching from, ruling out
   other propulsion methods, such as most nuclear engines, at least for
   use from the Earths surface.

   Therefore, all current spacecraft use chemical rocket engines (
   bipropellant or solid-fuel) for launch.

   One advantage that spacecraft have in launch is the availability of
   infrastructure on the ground to assist them. Proposed ground-assisted
   launch mechanisms include:
     * Space elevator
     * Orbital airship
     * Space fountain
     * Hypersonic skyhook
     * Electromagnetic catapult ( railgun, coilgun)
     * Space gun ( Project HARP, ram accelerator)
     * Laser propulsion ( Lightcraft)

Planetary arrival and landing

   A test version of the MARS Pathfinder airbag system
   Enlarge
   A test version of the MARS Pathfinder airbag system

   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 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 Odyssey and Mars
       Reconnaissance Orbiter, and at least one Venus mission, Magellan.
     * Aerocapture 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 by Zond 6 and Zond 7 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.
     * Parachutes 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.
     * Airbags can soften the final landing.
     * Lithobraking, 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).
       Very sturdy probes and low approach velocities are required.

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

Methods that may require breaking the laws of physics

   Artist's conception of a warp drive design
   Enlarge
   Artist's conception of a warp drive design

   In addition, a variety of hypothetical propulsion techniques have been
   considered that would require entirely new principles of physics to
   realize. To date, such methods are highly speculative and include:
     * Diametric drive
     * Pitch drive
     * Bias drive
     * Disjunction drive
     * Alcubierre drive (Warp drive)
     * Differential sail
     * Wormholes - theoretically possible, but impossible in practice with
       current technology
     * Antigravity - requires the concept of antigravity; theoretically
       impossible
     * Reactionless drives - breaks the law of conservation of momentum;
       theoretically impossible
     * EmDrive - again, breaks the law of conservation of momentum;
       theoretically impossible

Table of methods and their specific impulse

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

   Three numbers are shown. The first is the effective exhaust velocity:
   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.)

   CAPTION: Propulsion methods

   Method Effective Exhaust Velocity
   (m/s) Thrust
   (N) Duration
   Propulsion methods in current use
   Solid rocket 1,000 - 4,000 10^3 - 10^7 minutes
   Hybrid rocket 1,500 - 4,200 <0.1 - 10^7 minutes
   Monopropellant rocket 1,000 - 3,000 0.1 - 100 milliseconds - minutes
   Bipropellant rocket 1,000 - 4,700 0.1 - 10^7 minutes
   Tripropellant rocket 2,500 - 4,500 minutes
   Resistojet rocket 2,000 - 6,000 10^-2 - 10 minutes
   Arcjet rocket 4,000 - 12,000 10^-2 - 10 minutes
   Hall effect thruster (HET) 8,000 - 50,000 10^-3 - 10 months
   Electrostatic ion thruster 15,000 - 80,000 10^-3 - 10 months
   Field Emission Electric Propulsion (FEEP) 100,000 - 130,000 10^-6 -
   10^-3 weeks
   Magnetoplasmadynamic thruster (MPD) 20,000 - 100,000 100 weeks
   Pulsed plasma thruster (PPT)
   Pulsed inductive thruster (PIT) 50,000 20 months
   Nuclear electric rocket As electric propulsion method used
   Currently feasible propulsion methods
   Solar sails N/A 9 per km²
   (at 1 AU) Indefinite
   Tether propulsion N/A 1 - 10^12 minutes
   Mass drivers (for propulsion) 30,000 - ? 10^4 - 10^8 months
   Orion Project (Near term nuclear pulse propulsion) 20,000 - 100,000
   10^9 - 10^12 several days
   Variable specific impulse magnetoplasma rocket (VASIMR) 10,000 -
   300,000 40 - 1,200 days - months
   Nuclear thermal rocket 9,000 10^5 minutes
   Solar thermal rocket 7,000 - 12,000 1 - 100 weeks
   Radioisotope rocket 7,000-8,000 months
   Air-augmented rocket 5,000 - 6,000 0.1 - 10^7 seconds-minutes
   Liquid air cycle engine 4,500 1000 - 10^7 seconds-minutes
   SABRE 30,000/4,500 0.1 - 10^7 minutes
   Dual mode propulsion rocket
   Technologies requiring further research
   Magnetic sails N/A Indefinite Indefinite
   Mini-magnetospheric plasma propulsion 200,000 ~1 N/kW months
   Nuclear pulse propulsion ( Project Daedalus' drive) 20,000 - 1,000,000
   10^9 - 10^12 half hour
   Gas core reactor rocket 10,000 - 20,000 10^3 - 10^6
   Antimatter catalyzed nuclear pulse propulsion 20,000 - 400,000
   days-weeks
   Nuclear salt-water rocket 100,000 10^3 - 10^7 half hour
   Beam-powered propulsion As propulsion method powered by beam
   Fission sail
   Fission-fragment rocket 1,000,000
   Nuclear photonic rocket 300,000,000 10^-5 - 1 years-decades
   Space Elevator N/A N/A days
   Significantly beyond current engineering
   Fusion rocket 1,300,000-36,000,000
   Bussard ramjet
   Antimatter rocket 10,000,000-100,000,000
   Redshift rocket
   gravitoelectromagnetic toroidal launchers

   Retrieved from " http://en.wikipedia.org/wiki/Spacecraft_propulsion"
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   with only minor checks and changes (see www.wikipedia.org for details
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