• Post category:Rocket Science
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Today, we will be discussing Cold Gas Thrusters.

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 that require a contaminant-free operation.

Schematic of a cold gas propulsion system
Schematic of a cold gas propulsion system

Design

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.

Advantages

  • 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.
  • 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.

Disadvantages

  • 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.
Cold gas thruster in action, during landing of first stage of Falcon 9 rocket.
Cold gas thruster in action, during landing of first stage of Falcon 9 rocket.

Propellants

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.

I hope the above was clear. Feel free to ask questions.

This Post Has 2 Comments

  1. David Field

    This is very interesting. If 4 of the same type of cold gas thrusters as used in the spaceX rockets were attached to a small airship, 40 feet long, with propellant gas tanks inside, could they make enough thrust to move the airship?

    1. Arjit

      It will completely depend on total weight of the airship.

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