# An Overview of Cube-Satellite Propulsion Technologies and Trends
## 1 Introduction  
Recent years have witnessed a tremendous increase in interest in Cube-Satellites (CubeSats) among the space community including space agencies, industry and academia. Two factors have influenced this spurt of interest: first, low-cost access to space as a secondary payload for the purpose of technology demonstration, science proof-of-concept validation, communication and education; second, utilizing commercial-off-the-shelf (COTS) technologies in the design architecture. These two factors have led to a significant low overall cost of a CubeSat mission [1]. Several commercial companies offer services to launch typical (1U-3U) CubeSats and they charge between $50,000 to $200,000 per CubeSat based on its design size and altitude of deployment [2]. Boeing, Interorbital Systems, Lockheed Martin, SpaceX, and Virgin Galactic have announced to lower this price to anywhere in the range of $10,000 and $85,000 by 2020 [2]. Many space agencies also provide CubeSat launch opportunities to academia through different proposal solicitations, resulting in Universities across the globe launching their own CubeSats and providing valuable space systems engineering education to students. A 1U (unit) CubeSat has dimensions of \(10\times 10\times 10\) cm\({}^{3}\)[3] and a mass of 1 kg [4]. CubeSats have primarily flown in 1U and 3U form factors, but CubeSats beyond 3U are not uncommon; other platforms that have been considered include 6U (12 kg), 12U (24 kg) and 27U (54 kg) [5]. To date, CubeSats have been utilized only for near earth missions, however, a few far Earth and interplanetary missions (INSPIRE and MarCO) have also been proposed [6; 7]. CubeSats have mainly been restricted in their operations due to their small size that limits their on-board capabilities (power,mobility and payload) leading to a limited mission life and range of travel (after being deployed). There are multiple sub-systems on-board a CubeSat, including payload, communication and data handling, mobility (propulsion and attitude control systems) and power systems. This paper gives an overview of the propulsion systems that are suitable for CubeSats. A propulsion system is the primary mobility system of a spacecraft and helps with various maneuvering operations like orbit changing and station keeping. A key parameter that differentiates a propulsion system is its dependence on the on-board power. Accordingly, propulsion systems can be classified into two types: electric and non-electric systems. Electric propulsion systems are mainly classified into resistojet, electrospray, ion, Hall and pulsed plasma systems, and they actively require on-board power for their operation, while the non-electric propulsion systems can be classified into cold gas, liquid and solid rocket systems, and they require on-board power only to regulate (initiate and terminate) the propulsion process. Next, four important performance factors for any propulsion system are outlined: Thrust (\(\tau\)), specific impulse (I\({}_{sp}\)), effective exit velocity (or exit velocity) (\(v_{e}\)) and delta-v (\(\Delta v\)). It is essential to understand these factors to better comprehend the operation of a propulsion system. The thrust (shown in Equation (1)) generated is a combination of momentum thrust and pressure thrust. Momentum thrust depends on the mass flow rate (\(\dot{m}\)) of propellant and the exit (exhaust) velocity (\(v_{e}\)) while, pressure thrust, on the other hand, is a function of the exit area (\(A_{e}\)), exit pressure (\(P_{e}\)) and ambient pressure (\(P_{a}\)) [8]. The ambient pressure for the case of spacecraft propulsion systems is approximated to zero due to the vacuum conditions experienced in space. Nozzles form the expansion zones for the propellants (in cold gas, liquid, solid rocket and resistojet systems) and their geometry plays a significant role in accelerating the propellants and in generating high thrust (higher than typical electric propulsion systems).