Mass driver
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A mass driver or electromagnetic catapult is a method of spacecraft propulsion that would use a linear motor to accelerate payloads up to high speeds. All existing and contemplated mass drivers use coils of wire energized by electricity to make electromagnets.
Mass drivers can be used to propel spacecraft in two different ways: A large, ground-based mass driver can be used to launch spacecraft away from the Earth or another planet, or a spacecraft could have a mass driver on board, flinging large pieces of material into space to propel itself. A hybrid design is also possible.
Prototype mass drivers have existed since 1975. Most were constructed by the Space Studies Institute in order to prove their properties and practicality.
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Ground-based mass drivers
A ground-based mass driver would be a large installation for launching vehicles. It would consist of some electrical power station and a long linear motor. Vehicles would be placed on the launcher and accelerated. They could be released at escape velocity for interplanetary voyages, or they could be launched at near orbital velocity and use a short rocket burn to circularize their orbit. The Earth's strong gravity and thick atmosphere make such an installation difficult, so many proposals have been put forward to place such an installation on the Moon.
The coils can be constructed of aluminum to save mass, and to permit them to be constructed from lunar materials. The best known performance occurs with an aluminum coil as the payload. The coils of the mass-driver induce eddy-currents (paramagnetism) in the payload's coil, and then act on the resulting magnetic field. There are two sections of a mass-driver. The maximum acceleration part spaces the coils at constant distances, and synchronize the coil currents to the bucket. In this section, the acceleration increases as the velocity increases, up to the maximum that the bucket can take. After that, the constant acceleration region begins. This region spaces the coils at increasing distances to give a fixed amount of velocity increase per unit of time.
In the prototypes, the payload would be held in a bucket and then released, so that the bucket can be decelerated and reused.
In this mode, the major proposal for use of mass-drivers was to throw lunar dirt at space habitats so that they could process it using solar energy. The Space Studies Institute showed that this application was reasonably practical.
A second possibility is to build a mass driver on Earth that can launch radioactive waste into space (or into the Sun, which is already extremely radioactive). One design ([1] (http://www.oz.net/~coilgun/theory/electroguns.htm)) for such a launcher that could be constructed using current technology requires the launcher to be about 2 km long and accelerate the cargo at 10,000 g (100 km/s²). Each launch would require about 300 GJ. If this were driven by a nuclear power plant, only about 10% of the power generated by the plant would be required to dispose of its waste safely and permanently.
A third possibility for building a mass driver is a compromise system: a mass driver accelerates a payload up to some high speed which is not high enough for launch. It then releases the payload, which completes the launch under its own power. This would drastically reduce the amount of thrust that would be required for a launch, while allowing the mass driver design to use well-tested maglev components.
Spacecraft-based mass drivers
A spacecraft could carry a mass driver as its primary engine. With a suitable source of electrical power (probably a nuclear reactor) the spaceship could then use the mass driver to accelerate pieces of matter of almost any sort, boosting itself in the opposite direction.
Since current linear motors can accelerate cargo to 30 km/s, an engine using one would have a specific impulse of about 30 km/s or 3,000 s. However, no theoretical limit is known for the size, acceleration or muzzle energy of linear motors, so this can probably be improved signficantly. Efficiency is also quite good; linear motors can, with current technology, convert up to about 50% of the electrical energy into kinetic energy. Since a particle of mass m has momentum mv and kinetic energy mv2/2, the energy requirements vary inversely with the specific impulse, so in a design one must choose a tradeoff between energy consumption and consumption of reaction mass.
Since a mass driver could use any type of mass for reaction mass to move the spacecraft, this, or some variation, seems ideal for deep-space vehicles that scavenge reaction mass from found resources.
One possible drawback of the mass driver is that it has the potential to send solid reaction mass travelling at dangerously high relative speeds into useful orbits and traffic lanes. To overcome this problem, most schemes plan to throw finely-divided dust, or liquids. Propelling the reaction mass to solar escape velocity is another way to ensure that it will not remain a hazard.
The other use for mass drivers on board spacecraft is as a means of launching smaller vessels from an internal hangar (akin to an aircraft catapult.) Such mass drivers would mainly be deployed on military vessels as a means of launching combat units into the battlefield rapidly, though it is possible that a commercial vessel might use a mass driver launcher for probes or exploratory vessels.
Hybrid mass drivers
Another variation is to have a mass-driver on a spacecraft, and use it to "reflect" masses from a stationary mass-driver. Each deceleration and acceleration of the mass contributes to the momentum of the spacecraft. The spacecraft need not carry reaction mass, and doesn't even need much electricity, beyond the amount needed to replace losses in the electronics. The system could also be used to deliver pellets of fuel to the spacecraft for use in powering some other propulsion system. This could be considered a form of beam-powered propulsion.
Another theoretical use for this concept of propulsion can be found in space fountains, a system in which a continuous stream of pellets in a circular track hold up a tall (and heavy) structure.
Mass drivers as weapons
High-acceleration linear motors are currently undergoing active research by the military for use as (ground-based) armor-piercing weapons. Since a mass driver is essentially a very large, very high-velocity linear motor, it could in principle be used as a very large weapon, either firing directly on a target in space, or as weapon of mass destruction, attacking a location on a planet's surface from a position in orbit, or from a nearby planetary body, such as a moon.
Science fiction has dealt with this subject in numerous works, such as The Moon is a Harsh Mistress and Babylon 5. This usage also appears in the much-liked game Stars!, although in Stars!, mass drivers are orbital devices installed on a planet's space station, and so the bombardment must be from another planet, and no closer. However, the lack of atmospheric resistance in space should make this a non-issue, as long as one's own ships stay out of the way.
Mass drivers in fiction
On the show Babylon 5 in the Episode The Long, Twilight Struggle the Centauri use mass drivers to bomb the Narn homeworld, despite the fact that mass drivers had been outlawed by "every civilized planet". In later episodes, they elaborated that the mass drivers had been loaded with small asteroids from the Narn system. Not only did the mass drivers utterly devastate every major city on Narn, it dramtically affected the environment of the planet as well.
In the novel The Moon Is a Harsh Mistress (1966) by Robert A. Heinlein, Colonists on the Moon use mass drivers, designed for shipping goods down to Earth, as weapons of mass destruction in a war of independence (although they do go to great lengths to avoid civilian casualties). The projectiles (when being used for exports or as weapons) are fitted with conventional thrusters to alter their course and slow their descent, and are controlled and monitored by a self-aware computer called Mike.
Mass drivers were also used almost exclusively to propel aircraft into space in Gundam SEED.
Further information
See also: spacecraft propulsion, linear motor, railgun, coilgun
External links
- Electromagnetic Guns (http://www.oz.net/~coilgun/theory/electroguns.htm) - A page describing recent research into linear motors at MIT.
- Electromagnetic Launch of Lunar Material (http://lifesci3.arc.nasa.gov/spacesettlement/spaceresvol2/electromag.html)ja:マスドライバー