# An Overview of Cube-Satellite Propulsion Technologies and Trends
## 4 Solid Rocket Propulsion (SRP) Systems
### Design Considerations and Technologies  
To overcome the issues with thrust regulation in SRP systems, a unique addition (to the existing SRP system design) was proposed and designed by Aerospace Corporation, El Segundo, CA, USA for the \(I_{sp}\) 30 s Motor SRP system. This addition consists of an external movable mass (pitch/yaw system) with 8 jet paddles. The jet paddles are located just aft of the nozzle, and constitute rectangular moving arms (plates or slabs) with one of their faces exposed to the exhaust flow. Thrust regulation occurs by controlling the orientation of the paddles and imparting desired directionality to the flow [39]. Note that this technology could potentially lead to benefits for other micro-propulsion systems as well, specifically LPS. Figure 3: Schematic of a Solid Propulsion System. A disadvantage of SRP systems is their one-shot use owing to a lack of control over propellant burn rate. To mitigate this disadvantage, a system of hundreds of Solid Propellant Micro-thrusters (SPMs) has been proposed in Ref. [40]; these micro-thrusters can be used by forming a tightly spaced matrix (within the constraints of available external surface area). In SPMs, solid energetic propellant is burnt (during combustion process) and the resultant gases are accelerated through micro-nozzles. The size of the thruster can be modified to suit the thrust requirements and programmable thrust delivery can be achieved via simultaneous or sequential firing of multiple thrusters [40]. A typical SRP micro-thruster makes use of MEMS technology and comprises of several laminated layers containing a combustion chamber, an igniter, a nozzle, and a seal [41]. Combustion chamber stores the solid energetic-propellant and igniter section heats the propellant by means of a resistive heating element. Silicon or nichrome are generally used as materials for the heating element [41]. Nozzles are designed to meet mission-specific thrust requirements and _de-Laval_ (CD) nozzles are commonly chosen for their higher performance. The seal comprises of an epoxy or similar material or mechanisms. Also, silicon wafer is used in these micro-thrusters because it improves the ignition efficiency by minimizing the current leakage [42]. In addition to addressing the issue of a lack of control over propellant burn, SPMs avoid additional external-surface-area requirements owing to the use of a traditional nozzle (instead micro-nozzles are distributed over the surface of the spacecraft). Furthermore, SPMs provide the capability to generate differing torque values depending on thruster distance from the center of mass. An alternative technology that provides improvement of burn rate regulation was proposed by Digital Solid State Propulsion (DSSP), Reno, NV, USA through the invention of a new Electric Solid Propellant (ESP). ESP is a Hydroxyl Ammonium Nitrate (HAN) based Green Mono-propellant (GEM) and offers higher theoretical performance than AF-M315E propellant (discussed earlier in LP systems section). This new technology is inherently safe because ignition is possible only through continuous supply of electrical power, thereby reducing the chances of flames generated due to accidents and it has the potential to be used as propellant in both chemical and electrical propulsion systems [43]. This technology has flown on a larger satellite; SPINSAT, a spherical satellite with 22 inch diameter and a mass of 57 kg, was launched in 2014 and housed 72 DSSP thrusters [44]. Many inexpensive alternate propellants have been researched for SRP systems and recently, aluminum wool as a propellant, along with the mixture of sodium hydroxide and water as an oxidizer, was tested and was found to produce a thrust of 32 mN and specific impulse of 45 s. A major advantage of these propellants is that they are low cost, easy to handle, and can be stored over a long duration without any decomposition [45]. Table 3 provides a summary of the surveyed SRP systems and their performance parameters.