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.[ citation needed ]
Cold gas thrusters are predominantly used to provide stabilization for smaller space missions which require contaminant-free operation. [1] Specifically, CubeSat propulsion system development has been predominantly focused on cold gas systems because CubeSats have strict regulations against pyrotechnics and hazardous materials. [2]
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 accelerated as it approaches the throat (the narrowest part of the nozzle), where the gas velocity matches the speed of sound.[ citation needed ]
Cold gas thrusters benefit from their simplicity; however, they do fall short in other respects. The advantages and disadvantages of a cold gas system can be summarized as:
Thrust is generated by momentum exchange between the exhaust and the spacecraft, which is given by Newton's second law as where is the mass flow rate, and is the velocity of the exhaust.
For 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.[ citation needed ]
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 gases is given by
where is standard gravity and is the characteristic velocity which is given by
where is the sonic velocity of the propellant.[ citation needed ]
Cold gas systems can use either a solid, liquid or gaseous propellant storage system; but the propellant must exit the nozzle in gaseous form. Storing liquid propellant may pose attitude control issues due to the sloshing of fuel in its tank.
When choosing a propellant, a high specific impulse, and a high specific impulse per unit volume of propellant should be considered. [3]
Overview of the specific impulses of propellants suitable for a cold gas propulsion system:
| Cold Gas | Molecular weight M (u) | Theoretical Isp (sec) | Measured Isp (sec) | Density (g/cm3) |
|---|---|---|---|---|
| H2 | 2.0 | 296 | 272 | 0.02 |
| He | 4.0 | 179 | 165 | 0.04 |
| Ne | 20.2 | 82 | 75 | 0.19 |
| N2 | 28.0 | 80 | 73 | 0.28 |
| O2 | 32.0 | ? | ||
| Ar | 40.0 | 57 | 52 | 0.44 |
| Kr | 83.8 | 39 | 37 | 1.08 |
| Xe | 131.3 | 31 | 28 | 2.74 |
| CCl2F2 (Freon-12) | 120.9 | 46 | 37 | Liquid |
| CF4 | 88.0 | 55 | 45 | 0.96 |
| CH4 | 16.0 | 114 | 105 | 0.19 |
| NH3 | 17.0 | 105 | 96 | Liquid |
| N2O | 44.0 | 67 | 61 | Liquid |
| CO2 | 44.0 | 67 | 61 | Liquid |
Properties at 0°C and 241 bar (3,500 psi).
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 using 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 lbf (6.2 N) of thrust. The two propellant tanks onboard provided a total of 40 lb (18 kg) of gaseous nitrogen at 4,500 psi (31 MPa), which provided sufficient propellant to generate a change in velocity of 110 to 135 ft/s (34 to 41 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]
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]
The main focus of research as of 2014 [update] is miniaturization of cold gas thrusters using microelectromechanical systems. [10]
A rocket is a vehicle that uses jet propulsion to accelerate without using any surrounding air. A rocket engine produces thrust by reaction to exhaust expelled at high speed. Rocket engines work entirely from propellant carried within the vehicle; therefore a rocket can fly in the vacuum of space. Rockets work more efficiently in a vacuum and incur a loss of thrust due to the opposing pressure of the atmosphere.
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric entry.
A resistojet is a method of spacecraft propulsion that provides thrust by heating a typically non-reactive fluid. Heating is usually achieved by sending electricity through a resistor consisting of a hot incandescent filament, with the expanded gas expelled through a conventional nozzle.
A pulsed plasma thruster (PPT), also known as a Pulsed Plasma Rocket (PPR), or as a plasma jet engine (PJE), is a form of electric spacecraft propulsion. PPTs are generally considered the simplest form of electric spacecraft propulsion and were the first form of electric propulsion to be flown in space, having flown on two Soviet probes starting in 1964. PPTs are generally flown on spacecraft with a surplus of electricity from abundantly available solar energy.
An antimatter rocket is a proposed class of rockets that use antimatter as their power source. There are several designs that attempt to accomplish this goal. The advantage to this class of rocket is that a large fraction of the rest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higher energy density and specific impulse than any other proposed class of rocket.
Specific impulse is a measure of how efficiently a reaction mass engine, such as a rocket using propellant or a jet engine using fuel, generates thrust.
Delta-v, symbolized as and pronounced deltah-vee, as used in spacecraft flight dynamics, is a measure of the impulse per unit of spacecraft mass that is needed to perform a maneuver such as launching from or landing on a planet or moon, or an in-space orbital maneuver. It is a scalar that has the units of speed. As used in this context, it is not the same as the physical change in velocity of said spacecraft.
A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordance with Newton's third law. Most rocket engines use the combustion of reactive chemicals to supply the necessary energy, but non-combusting forms such as cold gas thrusters and nuclear thermal rockets also exist. Vehicles propelled by rocket engines are commonly used by ballistic missiles and rockets. Rocket vehicles carry their own oxidiser, unlike most combustion engines, so rocket engines can be used in a vacuum to propel spacecraft and ballistic missiles.
A propellant is a mass that is expelled or expanded in such a way as to create a thrust or another motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the engine that expels the propellant is called a reaction engine. Although technically a propellant is the reaction mass used to create thrust, the term "propellant" is often used to describe a substance which contains both the reaction mass and the fuel that holds the energy used to accelerate the reaction mass. For example, the term "propellant" is often used in chemical rocket design to describe a combined fuel/propellant, although the propellants should not be confused with the fuel that is used by an engine to produce the energy that expels the propellant. Even though the byproducts of substances used as fuel are also often used as a reaction mass to create the thrust, such as with a chemical rocket engine, propellant and fuel are two distinct concepts.
A de Laval nozzle is a tube which is pinched in the middle, making a carefully balanced, asymmetric hourglass shape. It is used to accelerate a compressible fluid to supersonic speeds in the axial (thrust) direction, by converting the thermal energy of the flow into kinetic energy. De Laval nozzles are widely used in some types of steam turbines and rocket engine nozzles. It also sees use in supersonic jet engines.
The classical rocket equation, or ideal rocket equation is a mathematical equation that describes the motion of vehicles that follow the basic principle of a rocket: a device that can apply acceleration to itself using thrust by expelling part of its mass with high velocity and can thereby move due to the conservation of momentum. It is credited to Konstantin Tsiolkovsky, who independently derived it and published it in 1903, although it had been independently derived and published by William Moore in 1810, and later published in a separate book in 1813. Robert Goddard also developed it independently in 1912, and Hermann Oberth derived it independently about 1920.
Jet propulsion is the propulsion of an object in one direction, produced by ejecting a jet of fluid in the opposite direction. By Newton's third law, the moving body is propelled in the opposite direction to the jet. Reaction engines operating on the principle of jet propulsion include the jet engine used for aircraft propulsion, the pump-jet used for marine propulsion, and the rocket engine and plasma thruster used for spacecraft propulsion. Underwater jet propulsion is also used by several marine animals, including cephalopods and salps, with the flying squid even displaying the only known instance of jet-powered aerial flight in the animal kingdom.
In rocket engine design, regenerative cooling is a configuration in which some or all of the propellant is passed through tubes, channels, or in a jacket around the combustion chamber or nozzle to cool the engine. This is effective because the propellants are often cryogenic. The heated propellant is then fed into a special gas-generator or injected directly into the main combustion chamber.
This is an alphabetical list of articles pertaining specifically to aerospace engineering. For a broad overview of engineering, see List of engineering topics. For biographies, see List of engineers.
A rocket engine nozzle is a propelling nozzle used in a rocket engine to expand and accelerate combustion products to high supersonic velocities.
A reaction engine is an engine or motor that produces thrust by expelling reaction mass, in accordance with Newton's third law of motion. This law of motion is commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force."
Characteristic velocity or , or C-star is a measure of the combustion performance of a rocket engine independent of nozzle performance, and is used to compare different propellants and propulsion systems. c* should not be confused with c, which is the effective exhaust velocity related to the specific impulse by: . Specific impulse and effective exhaust velocity are dependent on the nozzle design unlike the characteristic velocity, explaining why C-star is an important value when comparing different propulsion system efficiencies. c* can be useful when comparing actual combustion performance to theoretical performance in order to determine how completely chemical energy release occurred. This is known as c*-efficiency.
A thermal rocket is a rocket engine that uses a propellant that is externally heated before being passed through a nozzle to produce thrust, as opposed to being internally heated by a redox (combustion) reaction as in a chemical rocket.
A photon rocket is a rocket that uses thrust from the momentum of emitted photons for its propulsion. Photon rockets have been discussed as a propulsion system that could make interstellar flight possible during a human lifetime, which requires the ability to propel spacecraft to speeds at least 10% of the speed of light, v ≈ 0.1c = 30,000 km/s. Photon propulsion has been considered to be one of the best available interstellar propulsion concepts, because it is founded on established physics and technologies. Traditional photon rockets are proposed to be powered by onboard generators, as in the nuclear photonic rocket. The standard textbook case of such a rocket is the ideal case where all of the fuel is converted to photons which are radiated in the same direction. In more realistic treatments, one takes into account that the beam of photons is not perfectly collimated, that not all of the fuel is converted to photons, and so on. A large amount of fuel would be required and the rocket would be a huge vessel.
Microwave electrothermal thruster, also known as MET, is a propulsion device that converts microwave energy into thermal energy. These thrusters are predominantly used in spacecraft propulsion, more specifically to adjust the spacecraft’s position and orbit. A MET sustains and ignites a plasma in a propellant gas. This creates a heated propellant gas which in turn changes into thrust due to the expansion of the gas going through the nozzle. A MET’s heating feature is like one of an arc-jet ; however, due to the free-floating plasma, there are no problems with the erosion of metal electrodes, and therefore the MET is more efficient.
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