## 3 Background
### Propulsion System Technologies Overview
#### 3.3.1 Chemical Propulsion Systems  
As chemical compounds store high densities of energy, they are ideal for small craft such as CubeSats as they can provide large amounts of energy in a compact volume. Chemical propulsion systems are desirable as they can satisfy high thrust impulsive maneuvers [23]. Typically, chemical propulsion systems have lower specific impulse outputs compared to electric equivalents but have greater thrust to power ratios, making them suitable for larger craft with higher masses. Table 5 illustrates propulsion systems for small spacecraft. This system generates a chemical reaction to generate a high-pressure gas. This gas is then channeled through a nozzle to produce a reaction force and propel the satellite forwards. The gas exhaust temperature is usually high, and the reactant could potentially be in a liquid or gas state [23]. Previously, a mixture of liquid and gas has been used to produce a chemical reaction. A monopropellant is often used in cold gas thrusters. This process uses a single propellant to produce a reaction when passed over a catalyst. Alternatively, a bipropellant can be used, this is the reaction of an oxidizer and a fuel [3]. Monopropellant systems are commonly used to power CubeSat systems. They provide a high thrust output (Propellant-dependent) while maintaining a low complexity. This system has a low electrical power requirement and a very high reliability. Monopropellant thrusters are often utilized for orbit raising. The alternative bipropellant system offers better performance for this purpose as it produces more thrust giving more directional control; however, the system is much more complex than a mono as it has a complex feed system and occupies a larger volume [23]. With current technology, solid rocket motors are not suitable for nanosatellites. Even the smallest systems would exceed the weight and volume limits set for a CubeSat. Monopropellant systems are often powered by passing hydrazine over a catalyst. They can comfortably meet the specific impulse and volume requirements of a CubeSat. However, this power system requires a much higher power source than the allowed 1 W to actuate the thruster valve [30]. The hydrazine would also require heating to prevent it freezing once in orbit. This thermal regulation cannot be accommodated by the limited CubeSat power supply. Several Hydrazine blends have been developed which have far lower freezing points, meaning constant heating is no longer required. However, the power required to actuate the valve is still an existing issue [31]. For altitude control, thrusters often operate in pulse mode (Figure 11), delivering regular bursts of impulse. The amount of impulse needed depends on the rate of spin around the precession spin axis. It is desirable to be able to control this rate of spin. A cold gas thruster and Hydrazine monopropellant would be suitable for delivering short, accurate pulses [3]. Liquid monopropellant systems can provide significantly larger burst of impulse compared to the gas alternatives. It is extremely viable to state that a mono system could be used for both altitude control and orbit raising. The only limiting factors are volume and power supply for valve actuation [23]. Hydrazine Propellant propulsion systems are typically integrated into large craft, as they represent a reliable power source. The hydrazine system utilizes a double stage flow control valve to regulate propellant supply, which is fed to a catalyst heater surrounded by insulation. An advantage of incorporating Hydrazine thrusters is the ability to perform multiple cold starts, which is useful for power critical short CubeSat missions. Available systems on the market include CHAMPS (CubeSat High-Impulse Adaptable Modular Propulsion System) [32]. See Figures 12 and 13. Below (Table 6) is the manufacturers (Aerojet) published specification for the product [32]. Figure 12: CHAMPS 1U Hydrazine Propulsion System [32]. Figure 13: CHAMPS Functional Schematic [32].