Mass Driver - Fixed Mass Drivers

Fixed Mass Drivers

Mass drivers can have no physical contact between moving parts due to the projectile being guided by dynamic magnetic levitation, allowing extreme reusability in the case of solid-state power switching, a 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 1976–77 with a $2000 budget, a short test model firing a projectile at 40 m/s and 33 g, his next model was an order of magnitude greater acceleration after a comparable increase in funding, and, a few years later, 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 more slowly. Light 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 (having often a niche still 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 large capital investment.

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 much reduce the required velocity to reach lunar orbit.

Most serious mass driver designs use superconducting coils to achieve reasonable energetic efficiency (often 50% to 90+%, depending on design). Methods 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 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, a 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 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.