Section 2.5 - Bulk Matter Engines
This group of propulsion methods expel solid particles as reaction mass, or accelerate cargo directly.
45 Rotary Engine
Description: A one or two stage rotary mechanism mechanically accelerates a small amount of reaction mass, then releases it. In the two stage version, top speeds of ~6 km/s are possible. The advantage is to be able to use any available material as reaction mass. The disadvantage is the relatively low ejection velocity. There is an opportunity to mechanically augment another type of thruster by having it moving backwards relative to the vehicle center when running, thus adding up to 6 km/s to whatever the native exhaust velocity is. The duty cycle of the augmented thruster would be limited to about 25%, because the rest of the time it would be moving in the wrong direction to add velocity. This raises the issue of many on-off cycles of the thruster.
Status: Centrifuges and flywheels have long been in use. Versions optimized for ejecting mass at high velocity are still in the concept stage.
46 Coilgun Engine/Launcher
Alternate Names: Mass Driver Reaction Engine
Description: This is a type of electromagnetic accelerator. A carrier, or bucket, is accelerated by interaction of magnetic fields from 'driver' coils. The carrier holds a reaction mass/cargo, which is released. The bucket is slowed down and reused. There is no theoretical limit to velocity of this type of device - particle accelerators have reached near lightspeed - but practical limits are likely to be on the order of 10 km/s. As a vehicle engine it is not dramatically different than other electric thrusters in performance, aside from the benefit of using any bulk mass as propellant. As a fixed launcher for bulk materials, a large power supply and rapid recycling of the buckets allows very high mass delivery rates to orbit velocity. Unassisted bulk material will have an orbit that intersects the launch point, so a device in orbit will have to catch the cargo lots and add some velocity to them.
Status: This method was extensively studied as part of the "Space Colony" studies of the ~1970's, and prototype parts were built and tested.
47 Railgun Engine/Launcher
Description: A railgun consists of two current-carrying rails, and a projectile that creates a short circuit between the rails. The rails create a vertical magnetic field between them, as the current goes in opposite directions in each rail. The interaction of the field with the current in the short circuit creates a Lorentz force forward that accelerates a projectile. In a large version this would be used to launch vehicles in their entirety. In a smaller version it would eject a reaction mass and function as a propulsion engine.
Status: Railguns have been under development as replacements for conventional artillery, but are not yet operational as such. For launch vehicle acceleration, a much larger power supply would be needed, and this has so far been merely a concept