Propulsion systems are usually the most complicated and expensive components of spacecraft. To maximize its potential, it is crucial that we understand their capabilities.
This survey foregoes direct technology maturity assessments (TMAs) using NASA’s TRL scale, acknowledging its complexity is challenging to perform accurately without sufficient in-depth technical insight into individual devices.
Chemical
Chemical spacecraft propulsion systems typically are derived from heritage technologies that have been modified for use on small spacecraft buses. Sometimes new technologies have also been developed specifically to meet this challenge and offer greater propulsive capability than miniaturizing legacy devices can.
FEEP (field emission electric propulsion), which uses electrical energy to produce ions that accelerate propellant and create thrust, makes use of Iodine as it stores as a solid with high density and a low vapor pressure, offering benefits over high pressure propellant storage in spacecraft integration challenges.
Dawn Aerospace and AAC Hyperion’s iodine-based propulsion system for LituanicaSAT-2 CubeSat achieved 1 mN of thrust to perform formation flying, orbital deorbit, drag makeup with an overall delta-V of 2.25 ms-1 (figure 4.13). This system used SO2 as a self-pressurizing liquid fuel that was electrothermally heated in an insulated reactor to power its thrusters (52). For flight demonstration, this FEEP system used SO2, but now uses helium propellants for increased performance (61). This chapter forgoes direct technology maturity evaluation using NASA’s Technology Readiness Level (TRL) scale. Accurate TRL evaluation requires both in-depth technical understanding of a device as well as knowledge about its intended mission environment.
Electric
Similar to your electric car, a spacecraft with an EP thruster draws power from the sun. Solar arrays convert solar energy into voltage that is sent directly to an EP thruster where an electromagnetic process ionizes propellant fuel (typically Xenon) before speeding it up for use as an exhaust plume that pushes it in its desired direction.
Different kinds of electric thrusters exist, but Hall-type and gridded ion systems have proven particularly popular due to their ability to provide high specific impulses with relatively low secondary energy consumption. Electrothermal systems may provide higher performance but can only heat propellant to certain degrees before it begins burning off prematurely.
Hybrid
Hybrid propulsion systems combine the advantages of solid and liquid rocket engines. Fuel and oxidizer particles are held together with binding agents into a solid block similar to that of solid propellant, but are ignited via heat generated in their combustion chamber, just like liquid rocket engines.
Hybrid rocket upper stages show great promise as viable replacements for both solid and liquid motors, typically with higher Isp than solids and more throttleable performance than liquid motors allowing greater mission flexibility and less explosiveness as they require fewer tanks/valves than liquid motors.
However, several disadvantages still remain. Low regression rates tend to force designers to use multiple ports for a given thrust level, leading to propellant grain slivering and lower inert weight efficiency. IN Space’s hybrid engine technology achieves high regression rates and multiple restarts, helping overcome this issue and deliver performance required by NewSpace applications. Furthermore, IN Space’s rocket engine technology is compatible with in-space resource utilization techniques for orbital refueling refueling operations.
Cold Gas
Cold gas spacecraft propulsion systems use pressurized propellant expansion rather than combustion to generate thrust, with a fuel tank providing propellant, regulating valve controlling thrust production, and a nozzle channeling thrust into flight. Fuel can usually be found in its liquid state but may also exist as solid or gaseous form at a vapor pressure above ambient temperatures.
Numerous cold gas devices have been flight-proven on CubeSats. For instance, VACCO’s MiPS module with R-236FA propellant can generate 75 N-s of thrust for attitude control and trajectory correction maneuvers on CubeSats (figure 4.15).
SSTL’s butane propulsion system for Innosat (figure 4.12) features high specific impulse and small volume capabilities, comprised of butane propellant tanks, resistance jets and solenoid actuators powered by an electrical driver.