Spacecraft propulsion

A rocket engine test at NASA

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.

All current spacecraft use chemical rocket engines for launch. Most satellites have simple reliable chemical rockets or resistojet rockets to keep their station, although some use momentum wheels for attitude control. A few use some sort of electrical engine for stationkeeping. Interplanetary vehicles mostly use chemical rockets as well, although a few have experimentally used ion thrusters with some success.

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 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.

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.

File:Xenon-ion-engine.jpg

An ion engine test

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.

Calculations

In orbit, a spacecraft will continue to move along its trajectory unless it uses its engines. Getting from one "place" to another in this context means changing from one orbit to another. The actual location of the spacecraft will change "by itself" due to the spacecraft's orbital motion. So most mission plans (such as a Hohmann transfer orbit) have the spacecraft change orbits several times, letting orbital motion take care of actually moving the spacecraft. So one way to measure the "range" of a spacecraft in orbit is to give its $\Delta v$ , that is, the maximum amount by which it can change its velocity. This determines how much it can change its orbits, and it takes into account that the mass of the spacecraft changes as it uses up its fuel. When launching from or landing on a planet, $\Delta v$ does not tell the whole story, since the planet's gravitational attraction must be overcome by using fuel.

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 $\Delta v$ , and the rocket engine has exhaust velocity v_e, then the mass M of reaction mass which is needed is given by

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

For $\Delta v$ much smaller than v_e, this equation is roughly linear, and little reaction mass is needed. If $\Delta v$ is comparable to v_e, then there needs to be about twice as much fuel as payload (which includes engines, fuel tanks, and so on). Beyond this, the growth is exponential; speeds much higer than the exhaust velocity require heroic measures.

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

{\frac {1}{2}}Mv_{e}^{2}.

To visualize these numbers, suppose we want to send a 10000 kg space probe to Mars. The required $\Delta v$ is approximately 3000 m/s, using a Hohmann transfer orbit. (A manned probe would need to take a faster route and use more fuel).

Kind of engine	Exhaust velocity (m/s)	Fuel mass (kg)	Energy required (MWh)
Solid rocket (inefficient)	1000	190000	27
Bipropellant rocket (efficient)	5000	8200	29
Ion thruster	50000	620	210
VASIMR	300000	100	1300

Observe that the more efficient engines can use far less fuel; the amount is almost negligible, for the best engines. However, note also the amount of energy required. At one gravity, the total acceleration takes about 300 s, or about five minutes. So for one of the high-efficiency engines to generate a gravity of thrust, they must be supplied with 2.5 or 15 GW of power - a major metropolitan generating station, which would have to be included in the 10000 kg of payload weight. So either of these methods, used in this mode, must produce tiny thrusts over long periods. Since transit times for this Hohmann transfer orbit are on the order of years in any case, this need not slow down the mission any; in fact, the best orbit will probably no longer be a Hohmann transfer orbit at all, but instead some constant-acceleration orbit which might be faster or slower. So these numbers are only very approximate.

Propulsion methods

The many different 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.

Rockets

File:NASA-SSME-test-firing.jpg

A test firing of the Space Shuttle Main Engine

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, but research into nuclear fusion has developed methods for doing this.

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 hand, 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

File:Ion-engine-NASA.jpg

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 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:

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:

Launch mechanisms

File:Lunar-quenchgun.jpg

An electromagnetic catapult on the Moon

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:

Planetary arrival and landing

File:NASA-pathfinder-airbag-test.jpg

A test version of the MARS Pathfinder airbag system

When a vehicle wishes to enter orbit around its destination planet, or when it wishes 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.

aerobraking brings a probe into orbit
parachutes can land a probe on a planet with an atmosphere
airbags can soften the final landing

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

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:

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 velocity at which exorbitant amounts of fuel begin to be necessary (see the section on calculations, above).

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 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

External links:

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