Mass driver
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A mass driver or electromagnetic catapult is a method of non-rocket spacelaunch 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 package consisting of a magnetisable holder containing a payload. Once the payload has been accelerated, the two separate, and the holder is slowed and recycled for another payload.
Mass drivers can be used to propel spacecraft in two different ways: A large, ground-based mass driver could be used to launch spacecraft away from the Earth or another planet. 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)
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[edit] 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. Heavier objects go proportionally more slowly; and lighter 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 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 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.
In the prototypes, the payload would be held in a bucket and then released, so that the bucket can be decelerated and reused. A disposable bucket, on the other hand, would avail acceleration along the whole track.
[edit] On Earth
In contrast to a space gun, a mass driver can have a length of hundreds of kilometers and therefore achieve acceleration without too high g forces to the passengers. It can be constructed as a very long and mainly horizontally aligned launch track for spacelaunch, targeted upwards at the end, partly by bending of the track upwards and partly by Earth's curvature in the other direction.
Natural elevations, such as mountains, may facilitate for the construction of the distal, upwards targetted part. The higher up the track terminates, the less resistance from the atmosphere will the launched object receive.[1]
By being mainly located slightly above, on or beneath the ground, a mass driver may be easier to maintain compared with many other structures of non-rocket spacelaunch. If not underground then it still needs to be housed in a pipe that is constantly vacuum pumped in order to reduce drag.
In order to be able to launch humans and delicate instruments, it would need to be several hundreds of kilometres long. For rugged objects, with magnetic assistance, a significantly smaller, circular, track may suffice.[2]
A possibility for building a mass driver on Earth 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.
[edit] 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[citation needed], an engine using one would have a specific impulse of about 30 km/s or 3,000 s.[citation needed] 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. While 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. (See propulsive efficiency for more details).
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. Alternately, liquid oxygen could be used as reaction mass, which upon release would boil down to its molecular state. 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.
[edit] 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.
[edit] Mass drivers as weapons
High-acceleration linear motors are currently undergoing active research by the military for use as (ground-based) armor-piercing weapons. (See railgun.) 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 used to attack 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.
[edit] Practical attempts
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.
[edit] Mass drivers in science fiction
- The first mass driver known in print was actually called the "electric gun" and described in detail as a way to launch vehicles into outer space from the earth's surface in the 1897 science fiction novel A Trip to Venus by John Munro and published in 1897 by Jarrold & Sons, London. In the book Munro describes in great detail multiple coils fired in sequence by solenoids at proper timing to achieve acceleration without too high g forces to the passengers. The gun would be angled on a hillside if desired. Amazingly this book also describes in detail combinations of electric gun launch for a passenger capsule with onboard rockets, compressed gas jets and even retrofired bullets as a means to increase velocity and change direction and the use of planetary atmosphere aerobraking and parachutes for landing on a planet.
- One of the first depictions of a mass driver for space exploration was in the 1936 movie Things to Come.
- In the computer game Descent 3, the mass driver is a weapon that can be attached to your ship's hull which fires a lethal projectile at its target.
- In the computer game Alien Legacy the mass driver is a colony installation that is used to transport ore from one colony to another, regardless of the type of colony (planetside, space station, or the CALYPSO).
- In the novel The Moon Is a Harsh Mistress, mass drivers were used to send grain grown on the Moon to Earth, then after the revolution, they are used by Lunar rebels to launch artificial meteorites in an orbital bombardment.
- In the game Ace Combat 5: The Unsung War, a mass driver was used to send supplies to the Arkbird. One is also seen in Ace Combat 2 but is never used.
- In the game "Tremulous", a mass driver is used as a weapon by the human forces as a long-range rifle.
- In the Babylon 5 episode "The Long, Twilight Struggle", the Centauri Republic used mass drivers in violation of their treaties in their final attack on the Narn homeworld to fire asteroids onto the planet.
- In the Japanese anime metaseries Gundam mass drivers are commonly seen throughout different timelines. In the 1981 published Gundam Century a thorough disccusion and report of mass driver technology at the time was included, quoting the then recent science journals as references.
- In Tekkaman Blade II and Teknoman, they are used to launch the blue earth into a high orbit, where the blue earth's engines can take them either into space or around the planet.
- In the game Star Wars: Empire at War: Forces of Corruption, the Zann Consortium uses mass drivers as a weapon on many of their starships.
- In the game Homeworld, mass drivers form the main multi-purpose weapon of almost every ship, ranging from the small multi barreled fighter based version to massive turret-based capital ship cannons.
- In the game Xenogears, the main characters use a mass driver to expel a cure for a disease into the atmosphere, saving the planet.
- In the novel The Two Faces of Tomorrow by James P. Hogan, a Maskelyne mass driver based in the Sea of Tranquility is used by the TITAN supercomputer to easily destroy a lunar ridge that needs to be cleared for the construction of a second mass driver site.
- In the game Stars!, space stations can use orbital mass driver drivers: i) as a fuel-free method of transporting mineral between planets and ii) as an effective long-range weapon.
- In the games Master of Orion and Master of Orion II, the mass driver is a weapon that can be fitted on ships. Its damage does not decrease with range.
- In the game Deus Ex, an orbital mass driver used to deliver asteroid-mined materials to earth mistakenly destroys part of a Nigerian city. This happens off-screen but is referenced by several in-game media sources.
- In the video game series Wing Commander, the mass driver is a cannon installed on a number of fighters and some capital ships through many different eras. It has medium range, and can deliver medium damage to a target, compared to its contemporaries.
- In the game Einhander, a mass driver is used in a cut scene to launch a ship into space, after which the player has to destroy that ship in the next stage.
- In the fictional future depicted in the role play game Cyberpunk 2020, a mass driver had been installed on the Moon in the so called Tycho colony.
- In the novel Pushing Ice by Alastair Reynolds, the crew of the Rockhopper use mass drivers to send comets back to earth for processing into water and other resources.
- In the video game Halo, most UNSC ships are armed with a MAC (Magnetic Acceleration Cannon), which is actually a very powerful mass driver.
- In the novel Halo: Contact Harvest by Joseph Staten, the UNSC forces in Harvest used a mass driver to damage an incoming Covenant ship.
- In the game Command & Conquer: Renegade, a personal Mass Driver is used as a weapon by the Nod Character General Gideon Raveshaw, in the multiplayer mode.
- In the game series X( X: Beyond the Frontier , X²: The Threat and X³: Reunion ), the Mass Driver is the only non-energy-based projectile weapon available. It's also the only weapon known that will pass through a ships shield and bring heavy damage to the hull alone, rendering the shields protection useless against this type of weapon.
[edit] See also
[edit] People
- Eric Laithwaite and the Maglifter project
- Gerard K. O'Neill
- Henry Kolm
[edit] External links
- Electromagnetic Guns - A page describing research into linear motors at MIT.
- Electromagnetic Launch of Lunar Material
- New Scientist: Huge 'launch ring' to fling satellites into orbit - October 2006 article about research into a circular mass driver.
