Mass driver
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Image:Lunar-quenchgun.jpg
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. Sequential firing of a row of electromagnets accelerates the payload along a path. After leaving the path, the payload continues to move through inertia.
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Overview
A mass driver is essentially a coil gun that magnetically accelerates a bucket device carrying a payload. Once the payload has been accelerated, the bucket is slowed and recycled and reused.
Prototype mass drivers have existed since 1975. Most were constructed by the US Space Studies Institute in order to prove their properties and practicality. 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.
Fixed mass drivers
Generally speaking, mass drivers are only practical for small objects at a few kilometers per second; for example 1kg at 2.5km/s. Larger objects usually go proportionally more slowly; and smaller objects may be projected at 20km/s or more. The limits are generally the cost of the silicon to switch the current and the cost of the power supply and temporary energy storage for it. Earth based Mass drivers for propelling one tonne vehicles to orbit are unlikely to be cost effective in the near future.
The Earth's strong gravity and thick atmosphere make such an installation difficult. So, many proposals have been put forward to install mass drivers on the Moon where the lower gravity and lack of atmosphere significantly reduce the costs.
Most serious mass driver designs use superconducting coils to achieve reasonable energetic efficiency (approximately 50%). 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 transport lunar surface material to space habitats so that it could be processed 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 the sun. One design ([1]) 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, 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. No theoretical limit is known for the size, acceleration or muzzle energy of linear motors. However, at higher muzzle velocities, energetic efficiency is inevitably very poor. Whilst linear motors can, with current technology, convert up to about 50% of the electrical energy into kinetic energy of the projectile, the energy of interest is the kinetic energy of the vehicle, and as the muzzle velocity increases, this is a smaller and smaller percentage of the generated power.
Since kinetic energy of the projectile is mv2/2, the energy requirements vary with the square of the specific impulse, so in a design one must choose a tradeoff between energy consumption and consumption of reaction mass. In addition, since momentum of a particle of mass m has momentum mv- proportional to velocity, but energy is a square law, so the average thrust for a given energy is inversely proportional to the velocity of the particles. In other words, heavier projectile masses give lower specific impulse but proportionately higher thrust.
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.
Now the fact remains that it shares the same fundamental limits that conventional rocket engines do, being that it has finite fuel sources (solid objects). Another hurdle would be the fact that most of space is empty, so scavaging for objects to use is not really feasible. also there are no liquids in space as they become gas in vacuum.
Another 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.
Mass drivers in fiction
Science fiction has dealt with mass drivers as weapons, tools of industry, or indeed both, in numerous works such as The Moon Is a Harsh Mistress and Babylon 5. 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 dramatically 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.
In Heavy Time (1991), a novel by C.J. Cherryh, mass drivers are used to propel ore and minerals mined in the asteroid belt to refineries orbiting the planet Jupiter. These mass drivers are actually massive spacecraft that move from one asteroid to another, processing them to separate the useful materials from the rock.
Mass drivers were frequently used to propel ships into space in the various Gundam series. They are the primary method of Earth-to-space transportation in the Cosmic Era and feature prominently in the Universal Century and After Colony series as well.
The Wing Commander Series used mass drivers as weapons on space combat fighters (although in the description of their operation, the "mass drivers" more closely resemble coilguns).
The Playstation 2 game Ace Combat 5: The Unsung War used one in Mission 6 (White Bird - Part I) to launch supplies to the Arkbird (a spacecraft in orbit).
In the cross format RPG Deus Ex (2000), a few of the readable newspapers spread across the game world talk about a historical launch from the Zhou Enlai mass driver by the People's Republic of China, to transport a payload of freshly mined ore from the Moon to Earth. Newspapers found later in the game indicate that the launch became a disaster when the payload crashed in Nigeria, killing thousands. The crash was heavily implied to be an act of sabotage. There was a planned playable moon mission which would almost certainly have involved the aforementioned event but it was cut from the final game to meet the ship date.
In the PlayStation game Xenogears, a mass driver launched nuclear missiles during the Zeboim era, which could have led to the extinction of the entire Zeboim civilization. It was also used to spread nanoassemblers to reverse the DNA mutation in animal life on the planet.
In Sierra's 1994 computer game Alien Legacy, mass drivers are used to transfer mined ore between mining colonies and space stations or other colonies when transport ships are not available.
In the Master Of Orion series, mass drivers are a type of weapon that can be installed on spaceships.
In Hideo Kojima's Policenauts, there is a mass driver located on the Moon, used for transporting mined minerals to space stations situated at some of the Lagrangian points.
In the turn-based strategy game, Stars!, mass drivers can be installed in starbases and are used to launch mineral packets from planet to planet. They can also be used as a weapon if they are fired toward an enemy planet without a mass driver of an equivalent or better level. Therefore, 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.
In the RPG Cyberpunk 2020 it's said Europa used mass drivers based on the Moon to wipe out Tampa and other USA cities.
In the online multi-player game Star Sonata a mass driver is a rapid firing physical-based weapon.
In various installments of Hajime Yatate & Yoshiyuki Tomino's Gundam metaseries. In the Universal Century timeline, the mass driver initially appears in ZZ Gundam when it is used to transport the crew of the Argama back into space. They are also seen in Char's Counterattack and V Gundam. In the Cosmic Era timeline, mass drivers are a critical commodity that frequently change hands between various nations, usually by force. There are 5 Mass drivers at the start of the Gundam SEED series, at the following locations: Victoria (Africa), Gibraltar (Europe), Kaohsiung (Taiwan), Porta Panama (Central America) and Kaguya (Onogoro Island, of the Orb Union). Much focus is placed on this final mass driver, later in the series.
See also
- Coilgun
- Linear motor
- Railgun
- Spacecraft propulsion
- Eric Laithwaite and the Maglifter project
External links
- Electromagnetic Guns - A page describing recent research into linear motors at MIT.
- Electromagnetic Launch of Lunar Materialja:マスドライバー