A mass driver or electromagnetic catapult is a proposed method of non-rocket spacelaunch which would use a linear motor to accelerate and catapult payloads up to high speeds. Existing and proposed mass drivers use coils of wire energized by electricity to make electromagnets, though a rotary mass driver has also been proposed. Sequential firing of a row of electromagnets accelerates the payload along a path. Although any device used to propel a ballistic payload is technically a mass driver, in this context a mass driver is essentially a coilgun that magnetically accelerates a package consisting of a magnetizable holder containing a payload. Once the payload has been accelerated, the two separate, and the holder is slowed and recycled for another payload. Alternatively, a mass driver may be based on linear induction motors used as the Electromagnetic Aircraft Launch System (EMALS) on the aircraft carrier USS Gerald R. Ford. Mass drivers can be used to propel spacecraft in three different ways: A large, ground-based mass driver could launch spacecraft away from Earth, the Moon, or another body. A small mass driver could act as a rocket engine on board a spacecraft, flinging pieces of material into space to propel itself. Another variation would have a massive facility on a moon or asteroid send projectiles to assist a distant craft. Miniaturized mass drivers can also be used as weapons in a similar manner as classic firearms or cannon using chemical combustion. Hybrids between coilguns and railguns such as helical railguns are also possible.
Fixed mass drivers Mass drivers need no physical contact between moving parts because they guide their projectiles by dynamic magnetic levitation, allowing extreme reusability in the case of solid-state power switching, and a functional life of – theoretically – up to millions of launches. While marginal costs tend to be accordingly low, initial development and construction costs are highly dependent on performance, especially the intended mass, acceleration, and velocity of projectiles. For instance, while Gerard O'Neill built his first mass driver in 1976–1977 with a $2000 budget, a short test model firing a projectile at 40 m/s and 33 g, his next model had an order-of-magnitude greater acceleration after a comparable increase in funding, and, a few years later, researchers at the University of Texas estimated that a mass driver firing a 10 kilogram projectile at 6000 m/s would cost $47 million. For a given amount of energy involved, heavier objects go proportionally slower. Lightweight objects may be projected at 20 km/s or more. The limits are generally the expense of energy storage able to be discharged quickly enough and the cost of power switching, which may be by semiconductors or by gas-phase switches (which still often have a niche in extreme pulse power applications). However, energy can be stored inductively in superconducting coils. A 1 km long mass driver made of superconducting coils can accelerate a 20 kg vehicle to 10.5 km/s at a conversion efficiency of 80%, and average acceleration of 5,600 g. Earth-based mass drivers for propelling vehicles to orbit, such as the StarTram concept, would require considerable capital investment. The Earth's relatively strong gravity and relatively thick atmosphere make the implementation of a practical solution difficult. Also, most if not all plausible launch sites would propel spacecraft through heavily-traversed air routes. Due to the massive turbulence such launches would cause, significant air traffic control measures would be needed to ensure the safety of other aircraft operating in the area. Most serious mass-driver designs use superconducting coils to achieve reasonable energetic efficiency (often 50% to 90+%, depending on design). Equipment may include a superconducting bucket or aluminum coil as the payload. The coils of a mass driver can induce eddy currents in a payload's aluminum 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 synchronizes 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. Based on this mode, a major proposal for the use of mass drivers involved transporting lunar-surface material to space habitats for processing using solar energy. The Space Studies Institute showed that this application was reasonably practical. In some designs, 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. Alternatively, if a track were constructed along the entire circumference of the Moon (or any other celestial body without a significant atmosphere) then a reusable bucket's acceleration would not be limited by the length of the track – however, such a system would need to be engineered to withstand substantial centrifugal forces if it were intended to accelerate passengers and/or cargo to very high velocities.
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