Cold gas thruster

• A lack of combustion in the nozzle of a cold gas thruster allows its usage in situations where regular liquid rocket engines would be too hot. This eliminates the need to engineer heat management systems.
• The simple design allows the thrusters to be smaller than regular rocket engines, which makes them a suitable choice for missions with limited volume and weight requirements.
• The cold gas system and its fuel are inexpensive compared to regular rocket engines.
• The simple design is less prone to failures than a traditional rocket engine.
• The fuels used in a cold gas system are safe to handle both before and after firing the engine. If inert fuel is used the cold gas system is one of the safest possible rocket engines.[1]
• Cold gas thrusters do not build up a net charge on the spacecraft during operation.
• Cold gas thrusters require very little electrical energy to operate, which is useful, for example, when a spacecraft is in the shadow of the planet it is orbiting.

• A cold gas system cannot produce the high thrust that combustive rocket engines can achieve.
• Cold gas thrusters are less mass efficient than traditional rocket engines.
• The maximum thrust of a cold gas thruster is dependent upon the pressure in the storage tank. As fuel is used up, the pressure decreases and maximum thrust decreases.[3]

Thrust is generated by momentum exchange between the exhaust and the spacecraft, which is given by Newton's second law as ${\displaystyle F={\dot {m}}V_{e}}$where ${\displaystyle {\dot {m}}}$is the mass flow rate, and ${\displaystyle V_{e}}$is the velocity of the exhaust.

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

${\displaystyle F=A_{t}P_{c}\gamma \left[\left({\frac {2}{\gamma -1}}\right)\left({\frac {2}{\gamma +1}}\right)\left(1-{\frac {P_{e}}{P_{c}}}\right)\right]+P_{e}A_{e}}$

Where ${\displaystyle A_{t}}$ is the area of the throat, ${\displaystyle P_{c}}$ is the chamber pressure in the nozzle, ${\displaystyle \gamma }$ is the specific heat ratio, ${\displaystyle P_{e}}$ is the exit pressure of the propellant, and ${\displaystyle A_{e}}$ is the exit area of the nozzle.

The specific impulse (I_sp) 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

${\displaystyle I_{sp}={\frac {C^{*}}{g_{0}}}\gamma {\sqrt {\left({\frac {2}{\gamma -1}}\right)\left({\frac {2}{\gamma +1}}\right)^{\frac {\gamma +1}{\gamma -1}}\left(1-{\frac {P_{e}}{P_{c}}}\right)^{\frac {\gamma -1}{\gamma }}}}}$

where ${\displaystyle g_{0}}$is standard gravity and ${\displaystyle C^{*}}$is the characteristic velocity which is given by

${\displaystyle C^{*}={\frac {a_{0}}{\gamma \left({\frac {2}{\gamma +1}}\right)^{\frac {\gamma +1}{2(\gamma -1)}}}}}$

where ${\displaystyle a_{0}}$is the sonic velocity of the propellant.

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.[4] 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."[5]

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)[6]

Main article: Vernier Engines

Larger cold gas thrusters are employed to help in the attitude control of the first stage of the SpaceX Falcon 9 rocket as it returns to land.[7]

In a tweet in June 2018, Elon Musk proposed the use of air-based cold gas thrusters to improve car performance.[8]

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.[9]