A report proposes a miniature electrothermal spacecraft engine. The engine would include a chamber with an inside length of ≈1.5 cm and inside diameter of ≈0.25 cm, with a dielectric (ideally diamond) sidewall lining, a metal-coated expansion nozzle at one end, and a metal electrode at the other end. A propellant liquid (ammonia) would be vaporized into the cavity. To heat the NH3 vapor and dissociate it to a nitrogen/hydrogen plasma, the cavity would be excited with an electromagnetic field at a cavity resonance frequency of about 25 GHz (also the frequency of a dielectric resonance of NH3). The expansion of the plasma through the nozzle would generate thrust. The electric field would be of such a strength and configuration as to prevent contact between the plasma and the inner surface of the chamber. The plasma skin depth would be great enough that the plasma could absorb a large proportion of the electromagnetic energy. By use of refractory electrode and dielectric materials, pulsed operation, and, preferably, evaporative cooling of the chamber wall by the propellant liquid, it should be possible to achieve high plasma temperature and pressure and, thus, high thrust.
This work was done by Frank T. Hartley of Caltech for NASA's Jet Propulsion Laboratory. To obtain a copy of the report, "Miniature Microthermal Thruster," access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Machinery/Automation category.
NPO-20969
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Miniature Electrothermal Thruster
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Overview
The document presents a technical report on a Miniature Electrothermal Thruster developed by Frank T. Hartley at NASA's Jet Propulsion Laboratory. This innovative spacecraft engine is designed to be compact, with a chamber measuring approximately 1.5 cm in length and 0.25 cm in diameter. The thruster utilizes ammonia (NH3) as a propellant, which is vaporized and heated to create a nitrogen/hydrogen plasma.
Key features of the thruster include a dielectric sidewall lining, ideally made of diamond, and a metal-coated expansion nozzle at one end, with a metal electrode at the other. The chamber is excited with an electromagnetic field at a resonance frequency of about 25 GHz, which corresponds to the dielectric resonance of ammonia. This setup allows for the efficient dissociation of ammonia into its constituent gases, enabling high thrust generation as the plasma expands through the nozzle.
The design addresses several challenges faced by previous electrothermal thrusters, which were typically larger, operated at lower frequencies, and were limited by the propellant temperatures that their chambers could withstand. The miniature thruster's performance is enhanced by its ability to achieve extremely high propellant temperatures and pressures, facilitated by the use of refractory metals and advanced cooling techniques, such as evaporative cooling of the chamber walls.
The thrust produced by the engine is proportional to the chamber pressure, while the specific impulse is related to the square root of the propellant's absolute temperature. This relationship underscores the importance of maximizing both temperature and pressure to optimize the thruster's performance.
The report emphasizes the reliability and longevity of the thruster, which is crucial for space missions. The use of advanced materials and pulsed operation allows for a low-pressure feed system, simplifying the ignition process and enhancing overall efficiency.
In summary, the Miniature Electrothermal Thruster represents a significant advancement in spacecraft propulsion technology, promising high efficiency and performance for future space exploration endeavors. The innovative design and operational principles outlined in the report highlight the potential for this thruster to overcome previous limitations in electrothermal propulsion systems.
