# An Overview of Cube-Satellite Propulsion Technologies and Trends
## 11 Miscellaneous Propulsion Technologies and Design Methodologies  
### Propulsion Technologies  
A 1U electrolysis propulsion system called HYDROS, Bothell, WA, USA is currently being developed by Tethers Unlimited. HYDROS is a hybrid electrical/chemical propulsion system with water as the propellant. It has four principal components: water tank, a Proton Exchange Membrane (PEM) electrolyzer, gas storage and a bi-propellant thruster system comprising a nozzle and a combustion chamber. In HYDROS, the propellant (water) is electrolyzed into gaseous oxygen and hydrogen that undergo combustion to provide thrust. Note that the electrolyzed gases can also be directly ejected out through the nozzle (without combustion) similar to a CGP system [38]. Apart from resistojets, other types of electrothermal thrusters have been investigated. While in resistojets, the thrusters use resistive heating to heat the propellant, other electrothermal thrusters pass the propellant through a plasma discharge. This plasma can be generated in one of two ways: through high-current discharge in the case of Pulsed Electrothermal Thrusters (PET) and by absorption of microwaves in the case of Microwave Electrothermal Thrusters (MET) [48]. METs have better electrical and thermal efficiency and longer lifetime than other electrothermal thrusters [54]. Arcjets are also a kind of electrothermal device that heat the propellant to temperatures higher than those observed during combustion processes [48]. With increase in propellant temperature and stagnation pressure, higher thrust and specific impulse are achieved in arcjets; however, this poses a limitation on the materials (electrodes) that can be used. To address this problem, a new technology called TIHTUS (Thermal-Inductive Hybrid Thruster of the Universitat Stuttgart) has been proposed, and this technology contains a 2-stage plasma generator wherein the propellant is heated by the arjet thruster during the first stage and is reheated inductively by an afterburner in the second stage [135]. A new experimental ion thruster concept called Ion Cyclotron Resonance Heating (ICRH) was recently proposed. ICRH accelerates the propellant flow resulting in a high specific impulse. In ICRH, a Radio Frequency (RF) wave is directed into the magnetized plasma where it accelerates the ions by increasing their rotational speed around the magnetic field lines [136]. Note that these ion thrusters are different from the gridded ion thrusters (RITs) that were discussed previously. Laser accelerators have been in use for over 25 years in laboratory experiments, but their use on larger spacecraft has been limited mainly due to their high power requirements. Recent advancements have made it possible to develop a compact Tera-watt laser system that can excite plasma waves better than the the conventional RF accelerators [137]. This new laser acceleration technology is utilized in the Laser Ablation Propulsion (LAP) system where an intense laser beam/pulse (with higher energy threshold than the propellant) strikes the propellant (liquid/solid) producing a jet of vapor or plasma [137; 138]. By adjusting the laser intensity and laser-pulse duration, variable I\({}_{sp}\) (3500-5000 s) can be achieved. Thrust of a LAP system, on the other hand, can be varied independent of I\({}_{sp}\) by changing the laser-pulse repetition rate [138]. Note that there is no flight heritage for this propulsion system. Yet another futuristic propulsion concept called Plasmonic Force Space Propulsion (PFSP) is currently being studied at the Missouri University of Science and Technology, Rolla, MI, USA in an effort to maximize performance and minimize the design restrictions (power, mass, volume) posed by propulsion systems [139]. In this technology, solar energy is directly converted into propulsive thrust. PFSP uses plasmonic (plasma oscillations consist of plasmons) antennas and lenses to focus sunlight directly onto deep-sub-wavelength metallic nano-structures.