The Spacecraft propulsion reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Spacecraft propulsion

There are many different methods of spacecraft propulsion used to change the velocity of spacecraft and artificial satellites. 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 flow out the back of the vehicle. This sort of engine is called a rocket engine.

Table of contents

1 The necessity for propulsion systems 2 Effectiveness of propulsion systems 3 Propulsion methods 3.1 Rockets 3.2 Electromagnetic acceleration of reaction mass 3.3 Systems without reaction mass 3.4 Launch mechanisms 3.5 Methods requiring new principles of physics 4 Table of methods and their efficiencies 5 Further information

The necessity for propulsion systems

Artificial satellites must be launched up to orbit, and once there they must accelerate to circularize their 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. 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 do satellites. 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 efficient way to do this 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.

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

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 first overcome the Earth's gravitational pull before it can begin accelerating.

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 mv²/2, which must come from somewhere. In a conventional solid fuel 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 the reaction mass. After all, energy can in principle be produced without much difficulty, but the reaction mass must be carried along with the rocket and irretrievably consumed when used. A way of measuring the amount of impulse that can be obtained from a fixed amount of reaction mass is the specific impulse. This is the impulse per unit mass; examining the formula for impulse, it turns out that this is in fact the exhaust velocity.

A rocket with a high exhaust velocity can achieve the same impulse with less reaction mass. However, the kinetic energy is proportional to the square of the exhaust velocity, so that more efficient engines require more energy to run.

A second problem is that if the engine is to provide a large amount of thrust, that is, a large amount of impulse per second, it must also provide 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.

Propulsion methods

The many different propulsion methods can be classified based on their means of accelerating the reaction mass.

Rockets

A rocket engine accelerates its reaction mass by heating it, giving hot high-pressure gas or plasma. The reaction mass is then allowed to escape from the rear of the vehicle by passing through a de Laval nozzle, which dramatically accelerates the reaction mass, converting thermal energy into kinetic energy. It is this nozzle which gives a rocket engine its characteristic shape.

Rockets emitting gases are limited by the fact that their exhaust temperature cannot be so high that the nozzle and reaction chamber are damaged; most large rockets have elaborate cooling systems to prevent damage to either component. 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.

Rocket engines that could be used in space (all emit gases unless otherwise noted):

• Solid rocket (chemical energy)
• Hybrid rocket (chemical energy)
• Monopropellant rocket (chemical energy)
• Bipropellant rocket (chemical energy)
• Tripropellant rocket (chemical energy)
• Dual mode propulsion rocket (chemical energy)
• Resistojet rocket (electric heating)
• Arcjet rocket (chemical burning aided by electrical discharge)
• Pulsed plasma thruster (electric arc heating; emits plasma)
• Variable specific impulse magnetoplasma rocket (electromagnetic heating; emits plasma)
• Solar thermal rocket (solar energy)
• Nuclear thermal rocket (nuclear fission energy)
• Nuclear pulse propulsion (exploding fission/fusion bombs)
• Antimatter catalyzed nuclear pulse propulsion (antimatter catalyzed fission and fusion)
• Gaseous fission reactor (nuclear fission energy)
• Nuclear salt-water rocket (nuclear fission energy)
• Fusion rocket (nuclear fusion energy)
• Antimatter rocket (annihilation energy)
• Redshift rocket (annihilation energy; hypothetical modification to allow high exhaust temperature without causing damage)

When launching a vehicle from the Earth's surface, the atmosphere poses certain problems. For example, the precise shape of the most efficient de Laval nozzle for a rocket depends sharply on the ambient pressure. For this reason, various exotic nozzle designs such as the plug nozzle, the expanding nozzle and the aerospike have been proposed, each having some way to adapt to changing ambient air pressure.

On the other had, certain rocket engines have been proposed that take advantage of the air in some way (as do jet engines):

• the Air-augmented rocket, and
• the Liquid air cycle engine.

Electromagnetic acceleration of reaction mass

Rather than relying on high temperature and fluid dynamics to accelerate the reaction mass to high speeds, there are a variety of methods that use electrostatic or 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 efficiency engines require large amounts of power to run. For some missions, solar energy may be sufficient, 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, the maximum amount of power that can be generated limits the maximum amount of thrust that can be produced.

Some electromagnetic methods:

• Hall effect thruster
• Ion thruster
• Field Emission Electric Propulsion
• Magnetoplasmadynamic thruster
• Pulsed inductive thruster
• Mass drivers (for propulsion)

The Biefeld-Brown effect is a somewhat exotic electrical effect. In air, a voltage applied across a particular kind of capacitor produces a thrust. There have been claims that this also happens in vacuum due to some sort of coupling between the electromagnetic field and gravity, but recent experiments show no sign of this.

Systems without reaction mass

The law of conservation of momentum states that any engine which truly uses no reaction mass cannot move the center of mass of a spaceship (changing orientation, on the other hand, is possible). But space is not empty, especially space inside the Solar Systems; there are countless photons sleeting through it, there is a magnetic field, and there is the solar wind. Various propulsion methods try to take advantage of this; since all these things are very diffuse, propulsion structures need to be large.

Space drives that need no (or little) reaction mass:

• Momentum wheel
• Tether propulsion
• Solar sails
• Magnetic sails
• Mini-magnetospheric plasma propulsion

Launch mechanisms

The launch of a spacecraft from the surface of a planet into space places special requirements on the methods of propulsion used. Generally speaking high thrust is of vital importance for launch, and many of the propulsion methods above do not provide sufficient thrust to be used in this capacity. Exhaust toxicity or other side effects can also have detrimental effects on the environment the spacecraft is launching from, ruling out other propulsion methods. Currently, only chemical rockets are used for the launch of spacecraft from Earth's surface.

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
• Space fountain
• Hypersonic skyhook
• Electromagnetic catapult (rail gun, coil gun)
• Ballistic acceleration (Project HARP, ram accelerator)
• Laser propulsion (Lightcraft)

Methods requiring new principles of physics

In addition, a variety of hypothetical propulsion techniques have been considered that would require entirely new principles of physics to realize. As such, they are currently highly speculative:

• Alcubierre drive (Warp drive)
• Wormholes
• Differential sail
• Disjunction drive
• Diametric drive
• Pitch drive
• Bias drive
• Time machines
• RS Model Warp Drives

Table of methods and their efficiencies

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 specific impulse: the amount of thrust that can be produced using a unit of fuel. This is the most important characteristic of the propulsion method as it determines the top speed available for the propulsion method.

The second and third are the typical amounts of thrust and the typical burn times of the method. One interesting and somewhat counterinituitive physics result is that outside a gravity well, the total energy provided by a propulsion mechanism is equal to the thrust times the time the thrust is applied. Hence, 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 influenced by gravity.

Propulsion methods

Method	Specific Impulse (seconds)	Thrust (Newtons)	Duration
Propulsion methods in current use
Solid rocket	100-400	103- 107	minutes
Hybrid rocket	150-420		minutes
Monopropellant rocket	100-300	0.1-100	milliseconds - minutes
Momentum wheel (attitude control only)	n/a	0.001-100	indefinite
Bipropellant rocket	100-470	0.1-107	minutes
Tripropellant rocket	250-450		minutes
Resistojet rocket	200-600	10-2-10	minutes
Arcjet rocket	400-1200	10-2-10	minutes
Hall effect thruster (HET)	800-5000	10-3-10	months
Ion thruster	1500-8000	10-3-10	months
Field Emission Electric Propulsion (FEEP)	10000-13000	10-6-10-3	weeks
Magnetoplasmadynamic thruster (MPD)	2000-10000	100	weeks
Pulsed plasma thruster (PPT)
Pulsed inductive thruster (PIT)	5000	20	months
Nuclear electric rocket	As electric propulsion method used
Tether propulsion	N/A	1-1012	minutes
Currently feasible propulsion methods
Dual mode propulsion rocket
Air-augmented rocket	500-600		seconds-minutes
Liquid air cycle engine	450		seconds-minutes
SABRE	3000/450		minutes
Variable specific impulse magnetoplasma rocket (VASIMR)	1000-30000	40-1200	days - months
Solar thermal rocket	700-1200	1-100	weeks
Nuclear thermal rocket	900	105	minutes
Solar sails	N/A	9 per km2 (at 1 AU)	Indefinite
Mass drivers (for propulsion)	3000-?	104-108	months
Technologies requiring further research
Magnetic sails	N/A	Indefinite	Indefinite
Mini-magnetospheric plasma propulsion	20,000	~1N/kW	months
Gaseous fission reactor	1000-2000	103-106
Nuclear pulse propulsion (Orion drive)	2000-100,000	109-1012	half hour
Antimatter catalyzed nuclear pulse propulsion	2000-40,000		days-weeks
Nuclear salt-water rocket	10,000	103-107	half hour
Beam-powered propulsion	As propulsion method powered by beam
Nuclear photonic rocket	3×107	10-5-1	years-decades
Biefeld-Brown effect (see also Lifter)	N/A	0.01-1 (currently)	weeks, probably months
Significantly beyond current engineering
Fusion rocket
Bussard ramjet
Antimatter rocket
Redshift rocket

Further information

See also: Rocket, satellite, interplanetary travel, interstellar travel

External links:

• NASA Beginner's Guide to Propulsion
• Advanced Propulsion Concepts at islandone.org
• NASA Breakthrough Propulsion Physics project

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