Magnetoconstricted Metastable Metallic Hydrogen Engines
Magnetoconstricted Metastable Metallic Hydrogen Engines (MMMH engines) are a commonly used propulsion technology for on-orbit and other short-range applications.
Principle
Metastable Metallic Hydrogen releases extreme amounts of energy when decomposing into molecular hydrogen. Due to the extremely high temperature, conventional nozzles would be quickly destroyed, necessitating a technology that separates the superheated hydrogen plasma from the actual nozzle. Since the exhaust is already ionized by the combusion heat, it can be manipulated with strong magnetic fields from superconducting electromagnets. This also inherently allows a variable nozzle geometry, combining adjustable expansion ratio for efficiency with a form of thrust vector control. For low-atmospheric operation, the metallic hydrogen has to be further mixed with a secondary propellant to lower the operating temperature for protection of nearby facilities and life. Liquid hydrogen and water are commonly used for this application.Utility
Metastable Metallic Hydrogen as a propellant has significant benefits due to its very high specific impulse. However, even with this high efficiency, on most planets it is not realistic to reach orbit with a single MMMH propulsion stage, requiring a booster stage. Most booster stages are reusable and return to the launch site for refueling and rapid reuse, however some small exploration craft designed for atmospheric use may allow usage of a separately-landed single-use booster stage for return to orbit from planets without appropriate infrastructure.
Large vessels also often use MMMH-based thrusters for maneuvering due to their superior efficiency, however the difficulty of miniaturization limits this use for smaller ships.
Manufacturing
MMMH engines are fairly complex, requiring powerful superconducting magnets and low-latency control systems to contain and control the superheated exhaust. Strict tolerances and calibration is required to avoid the engines damaging themselves while in operation.
Metastable Metallic Hydrogen requires extremely low temperatures and high pressures for manufacturing, however this can also be done at a small scale in-situ, if at a very slow rate. Larger scale manufacturing plants are typically found at orbital launch sites.
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