Spacecraft propulsion is any method used to change the velocity of spacecraft or artificial satellites. In-space propulsion is used to maneuver the spacecraft after the launch vehicle has lifted it into outer space.
Several methods of pragmatic spacecraft propulsion have been developed, each having its own advantages and drawbacks. Most satellites use simple reliable chemical thrusters such as monopropellant rockets or resistojet rockets to maintain orbital station-keeping, while a few use momentum wheels for attitude control. Russian and antecedent Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north–south station-keeping and orbit raising. Most interplanetary spacecraft use chemical rocket propulsion as well; a few have used electric propulsion methods such as ion thrusters and Hall-effect thrusters.
Various propulsion technologies are actively being developed to pioneer space missions of wide vairety, especially to create faster and more efficient propulsion technologies. Hypothetical propulsion technologies are intended to provide effective exploration of the Solar System and permit humanity to reach farther distances more with greater reliability, safety, and efficiency. Some examples of future propulsion technology are nuclear propulsion and solar sails.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, Illinois, 2006b, paper. Vol. 6297. 2006.
Purpose and function A spacecraft propulsion system provides propulsive force for orbital transfer, interplanetary trajectories, and potentially for landing and ascent. In addition, propulsion is needed for orienting and positioning the spacecraft in route as well as orbit maintenance (reaction control, station keeping, precision pointing, and orbital maneuvering).
While a spacecraft is in orbit, any additional impulse, even tiny, will result in a change in the orbit path, in three ways, corresponding to the three dimensions of space:
Prograde/retrograde (i.e. acceleration in the tangential/opposite in tangential direction), which increases/decreases altitude of orbit. Perpendicular to orbital plane, which changes orbital inclination. Radially, i.e. towards or away from the body being orbited, which alters the eccentricity of the orbit.
Earth's surface is situated fairly deep in a gravity well; the escape velocity required to leave its orbit is 11.2 kilometers per second. Thus for destinations beyond, propulsion systems need enough propellant and to be of high enough efficiency. The same is true for other planets and moons, albeit some have lower gravity wells.
As human beings evolved in a gravitational field of "one g" (), it would be safer for acceleration rates of the spacecraft to be about or not too far from 1G, as human bodies can only tolerate up to 9Gs, which is approximately . The occupants of a rocket or spaceship with the propulsion system constantly accelerating at 1G would experience artificial gravity, however this is not a practical method as it requires constant acceleration (which requires large amounts fuel to be effective).
Theory Nearly all types of modern spacecraft propulsion are reaction engines, which produce thrust by expelling reaction mass, in accordance with Newton's third law of motion. The Tsiolkovsky rocket equation shows, using the law of conservation of momentum, that for a rocket engine propulsion method to change the momentum of a spacecraft, it must change the momentum of something else in the opposite direction. In other words, the rocket must exhaust mass opposite the spacecraft's acceleration direction, with such exhausted mass called propellant or reaction mass. For this to happen, both reaction mass and energy are needed. The impulse provided by launching a particle of reaction mass with mass at velocity is . But (following Newton's second law of motion) this particle has kinetic energy , which must come from somewhere. In a conventional solid, liquid, or hybrid rocket, fuel is burned, providing the energy, and the reaction products are allowed to flow out of the engine nozzle, providing the reaction mass. In an ion thruster, electricity is used to accelerate ions behind the spacecraft. Here other sources must provide the electrical energy (e.g. a solar panel or a nuclear reactor), whereas the ions provide the reaction mass.
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 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 for a long time can often produce the same impulse as another which produces large accelerations for a short time. However, when launching from a planet, tiny accelerations cannot overcome the planet's gravitational pull and so cannot be used.
Some designs however, operate without internal reaction mass by taking advantage of magnetic fields or light pressure to change the spacecraft's momentum.
Efficiency When discussing the efficiency of a propulsion system, designers often focus on the effective use of the reaction mass, which must be carried along with the rocket and is irretrievably consumed when used. Spacecraft performance can be quantified in amount of change in momentum per unit of propellant consumed, also called specific impulse. This is a measure of the amount of impulse that can be obtained from a fixed amount of reaction mass. 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 or bipropellant rocket engines have a low specific impulse (~300 s) but high thrust.
The impulse per unit weight-on-Earth (typically designated by ) has units of 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, with the same units as velocity (e.g., meters per second). This measure is equivalent to the effective exhaust velocity of the engine, and is typically designated . Either the change in momentum per unit of propellant used by a spacecraft, or the velocity of the propellant exiting the spacecraft, can be used to measure its specific impulse. The two values differ by a factor of the standard acceleration due to gravity, gn, 9.80665 m/s² ().
In contrast to chemical rockets, electrodynamic rockets use electric or magnetic fields to accelerate a charged propellant. The benefit of this method is that it can achieve exhaust velocities, and therefore , more than 10 times greater than those of a chemical engine, producing steady thrust with far less fuel. With a conventional chemical propulsion system, 2% of a rocket's total mass might make it to the destination, with the other 98% having been consumed as fuel. With an electric propulsion system, 70% of what is aboard in low Earth orbit can make it to a deep-space destination.
However, there is a trade-off. Chemical rockets transform propellants into most of the energy needed to propel them, but their electromagnetic equivalents must carry or produce the power required to create and accelerate propellants. Because there are currently practical limits on the amount of power available on a spacecraft, these engines are not suitable for launch vehicles or when a spacecraft needs a quick, large impulse, such as when it brakes to enter a capture orbit. Even so, because electrodynamic rockets offer very high , mission planners are increasingly willing to sacrifice power and thrust (and the extra time it will take to get a spacecraft where it needs to go) in order to save large amounts of propellant mass.
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 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 station-keeping). 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, 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). Once it has done so, it must 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 typically 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: 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 or aerocapture 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. The concept has been successfully tested by the Japanese IKAROS solar sail spacecraft.
Interstellar
Because interstellar distances are 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. No spacecraft capable of short duration (compared to human lifetime) interstellar travel has yet been built, but many hypothetical designs have been discussed.
Propulsion technology The four conventional and widely used types of spacecraft propulsion are cold gas, monopropellant, bipropellant, and solid motor systems.