Mass driver
|
A mass driver for lunar launch (artist's conception) |
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 due to
inertia.
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
1976 (
Mass Driver 1). 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 (see
coil gun,
railgun, or
helical railgun)
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.
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 mv
2/2, 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. Propelling the reaction mass to solar escape velocity is another way to ensure that it will not remain a hazard.
As with all propulsion systems most of space is almost completely empty, so propellant sources are only to be found at asteroids, comets, moons and planets.
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.
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.
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, long range over-the-horizon indirect fire, or from a nearby planetary body, such as a
moon.
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
Real-time strategy games,
Homeworld and
Homeworld 2, massdrivers are the most common weapon, ranging from small units used aboard strike craft (fighters and bombers) to the massive units used on the
Heavy Cruiser class ships.
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
Interplay's 1998 computer game
Hardwar, there is a broken down mass driver which you at the end of the game use to get off
Titan.
In the
Master Of Orion series, mass drivers are a type of relatively low technology 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. Mass drivers seem to be somewhat widespread in the UC timeline, with notable examples located at Hong Kong on Earth, and
Anaheim Electronics' factory on Luna. 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. Specifically, this mass driver was used by the crew of the
Archangel, and the crew of the
Izumo class battleship Kusanagi to escape into space. During this operation, the ships and the mass driver were under attack by the
Blue Cosmos Advanced unit consisting of 3 Gundams: Calamity, Forbidden and Raider. However, the attack was not successful as the ships and mass driver were being protected by
Athrun Zala piloting the
ZGMF-X09A Justice Gundam, and
Kira Yamato piloting the
ZGMF-X10A Freedom Gundam. The mass driver, along with its facilities, was then self-destructed by
Uzumi Nara Athha, head of the ORB Union, such that the
Blue Cosmos forces could not use it. A sixth Mass Driver was actually built by the
Earth Alliance for civilian use, with assistance from the
Junk Guild and
Serpent Tail: The enormous
Giga-Float, an aritificial island which, after its construction, remainedconstantly on the move, making it very difficult to locate - and therefore preserving it from attack.
Rondo Ghina Sahaku of Orb tried to destroy the Giga-Float before it could be completed, but he was run off by the
Blue Frame, and the Giga-Float was repaired and completed.In addition to this, linear launch catapults are standard in CE military spacecraft, and are used - as theorized - much like aircraft carrier launch catapults for both mobile armor and mobile suit units. The Mass Drivers in
Gundam SEED are far larger than the currently existing prototypes, at about fifty kilometers long.It is also worth noting that rail guns and other forms of linear weaponry have made numerous appearances throughout Gundam.
In
Star Wars Episode III: Revenge of the Sith, massdrivers are used by the Invisible Hand against a
Venator class
Star Destroyer during a broadside attack that eventually results in the Invisible Hand plummeting toward Coruscant's surface.
In the video game Halo, MAC guns (Magnetic Accelerator Cannons) are used as planetary defences against large enemy vessels.
In television series
Battlestar Galactica the
Viper space fighters could be viewed as being launched off the mothership by a coil mass driver. The launch bay and exit passage is shaped closely enough for being a mass driver, and the take-off acceleration is obviously special for the small craft. It is unlikely that the TV-series launch bay was designed with this in mind however.
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Coilgun*
Linear motor*
Railgun*
Helical railgun*
Spacecraft propulsion*
Eric Laithwaite and the Maglifter project
*
Gerard K. O'Neill*
Henry Kolm*
Electromagnetic Guns - A page describing recent research into linear motors at MIT.
*
Electromagnetic Launch of Lunar Material
