A cold gas thruster (or a cold gas propulsion system) is a type of rocket engine which uses the expansion of a (typically inert) pressurized gas to generate thrust. As opposed to traditional rocket engines, a cold gas thruster does not house any combustion and therefore has lower thrust and efficiency compared to conventional monopropellant and bipropellant rocket engines. Cold gas thrusters have been referred to as the "simplest manifestation of a rocket engine" because their design consists only of a fuel tank, a regulating valve, a propelling nozzle, and the little required plumbing. They are the cheapest, simplest, and most reliable propulsion systems available for orbital maintenance, maneuvering and attitude control.
Cold gas thrusters are predominantly used to provide stabilization for smaller space missions which require contaminant-free operation. Specifically, CubeSat propulsion system development has been the predominantly focused on cold gas systems because CubeSats have strict regulations against pyrotechnics and hazardous materials.
The nozzle of a cold gas thruster is generally a convergent-divergent nozzle that provides the required thrust in flight. The nozzle is shaped such that the high-pressure, low-velocity gas that enters the nozzle is expanded as it approaches the throat (the narrowest part of the nozzle), where the gas velocity matches the speed of sound.
Cold gas thrusters benefit from their simplicity; however, they do fall short in other respects. The following list summarizes the advantages and disadvantages of a cold gas system.
In the case of a cold gas thruster in space, where the thrusters are designed for infinite expansion (since the ambient pressure is zero), the thrust is given as
Where is the area of the throat, is the chamber pressure in the nozzle, is the specific heat ratio, is the exit pressure of the propellant, and is the exit area of the nozzle.
The specific impulse (Isp) of a rocket engine is the most important metric of efficiency; a high specific impulse is normally desired. Cold gas thrusters have a significantly lower specific impulse than most other rocket engines because they do not take advantage of chemical energy stored in the propellant. The theoretical specific impulse for cold gasses is given by
where is the sonic velocity of the propellant.
Cold gas systems can use either a solid, liquid or gaseous propellant storage system; but the propellant needs to exit the nozzle in gaseous form. Storing liquid propellant may pose attitude control issues due to the sloshing of fuel in its tank.
When deciding which propellant to use, a high specific impulse and a high specific impulse per unit volume of propellant must be considered.
The following table provides an overview of the specific impulses of the different propellants that can be used in a cold gas propulsion system
Properties at 25°C and 1 atm
Cold gas thrusters are especially well suited for astronaut propulsion units due to the inert and non-toxic nature of their propellants.
Main article: Hand-Held Maneuvering Unit
The Hand-Held Maneuvering Unit (HHMU) used on the Gemini 4 and 10 missions used pressurized oxygen to facilitate the astronauts' extravehicular activities. Although the patent of the HHMU does not categorize the device as a cold gas thruster, the HHMU is described as a "propulsion unit utilizing the thrust developed by a pressurized gas escaping various nozzle means."
Twenty-four cold gas thrusters utilizing pressurized gaseous nitrogen were used on the Manned Maneuvering Unit (MMU). The thrusters provided full 6-degree-of-freedom control to the astronaut wearing the MMU. Each thruster provided 1.4 lbs (6.23 N) of thrust. The two propellant tanks onboard provided a total of 40 lbs (18kg) of gaseous nitrogen at 4500 psi, which provided sufficient propellant to generate a change in velocity of 110 to 135 ft/sec (33.53 to 41.15 m/s). At a nominal mass, the MMU had a translational acceleration of 0.3±0.05 ft/sec2 (9.1±1.5 cm/s2) and a rotational acceleration of 10.0±3.0 deg/sec2 (0.1745±0.052 rad/sec2)
Main article: Vernier Engines
In September 2018, Bosch successfully tested its proof-of-concept safety system for righting a slipping motorcycle using cold gas thrusters. The system senses a sideways wheel slip and uses a lateral cold gas thruster to keep the motorcycle from slipping further.