# An Overview of Cube-Satellite Propulsion Technologies and Trends
## 3 Liquid Propulsion (LP) Systems
### Design Considerations and Technologies  
Liquid propulsion systems, like CGP systems, deal with issues related to storage and operational pressures of the propellant. To this end, the Maximum Expected Operating Pressure (MEOP), which is the highest possible pressure at which the propellant is expected to operate, is an important design parameter. MEOP of a propellant should be desirably high so that thruster performance (thrust, specific impulse) can be maximized [25]. Highly toxic propellants like hydrazine have been successfully used in larger spacecraft for over 60 years. Recently, there has been a global emphasis on the development and use of lower-toxic green propellants to reduce risks incurred due to contamination during laboratory testing and mission phases while in space. Green mono-propellants are less hazardous due to one of two reasons: either due to their benign toxicology even for probable  
Figure 2: Schematic of a Bi-propellant Liquid Propulsion System.  
levels of unintentional ingestion, or their low vapor pressure posing no significant risk of being inhaled. Many of the emergent green propellants (Sulfur Hexafluoride (SF\({}_{6}\)), AF-M315E, Ammonium Dinitramide (ADN)) provide significant additional advantages like better physical characteristics (higher density), better performance for the propulsion system (higher thrust and specific impulse), and reduced thermal conditioning requirements for storage compared to hydrazine. However, they do pose a disadvantage with requiring higher preheat temperatures, higher than the typical 120-150 \({}^{\circ}\)C of hydrazine thrusters [26].  
Recently developed AF-M315E is a high performance Hydroxyl-Ammonium Nitrate (HAN) based green propellant (high mixture stability apart from being low-toxicity hazard) developed by US Air Force Research Laboratory (AFRL), Wright-Patterson Air Force Base, OH, USA. AF-M315E propellant has 50% higher density-specific-impulse (specific impulse/unit volume) than the hydrazine [27] and is still being tested for CubeSat applications; however, note that it is yet to fly on any spacecraft. The minimum storage and operating temperatures of AF-M315E potentially makes it a high interest application in cold environments where continuous thermal conditioning may not be feasible [26]. Additionally, a system using AF-M315E propellant will not likely encounter design issues and failure modes associated with control of mixture ratio of propellant, vapor diffusion and reaction, and oxidizer flow decay [27]. A major disadvantage of AF-M315E is that it is difficult to ignite due to its Ionic Liquid (IL) (high water content) form [26; 28]. Experiments have been conducted with multiple catalyst systems to augment its ignitability, but room temperature ignition does not currently exist and the preheating process can consume large amount of energy (up to 15,000 J). These requirements on the energy consumption impose severe restrictions on spacecraft and even more so on CubeSats [28].  
In order to cater to the needs of different CubeSat missions and to the increase their lifetime, micro-propulsion system developers have come up with form-factor customization based on the amount of on-board propellant that can be carried. MPS-120 CHAMPS, HPGP, BGT-X5 and VACCO/ECAPS are examples of micro-propulsion systems designed in multiple configurations varying from 0.5 U to 2 U. For a given system, the difference in configurations results mostly in the amount of propellant they carry. Note here that none of the above have flown on a CubeSat; however, HPGP micro-propulsion system has flown on two larger spacecraft. Table 2 provides a summary of the surveyed liquid propulsion systems and their performance factors. Note that, though HYDROS (developed by Tethers Unlimited, Bottell, WA, USA) is a hybrid electric/chemical propulsion system, however because the propellant is water it is summarized in the table below.