Propulsion systems of satellites
Some missions require to change velocities of the satellite after its detaching from the launcher. The satellites propulsion systems give to spacecraft possibility to make manoeuvres for different purpose e.g transfer to target obit, station keeping, collision avoidance, decommissioning.
Propulsion based on Newton third principle
For every action, there is an equal and opposite reaction – classical propulsion systems utilize Newton’s principle. The engines expel mass in one direction to produce thrust in opposite direction.
Thrust force and impulse
Two properties characterize classical propulsion system: thrust force in Newton or pounds and impulse. Impulse is a measure which tells us how fast propellant is consumed to produce thrust. In mathematical terms:
F-thrust, g-acceleration due to gravity, dM-mass of propelent consumed in time dt
Solid fuel propulsion systems
Solid rocket fuel is filled with a mixture of relatively hard, rubbery, combustible mixture of fuel, oxidizer and binder. The engine is ignited by pyrotechnic device – igniter. The engine usually contains two igniters to have redundancy.
The combustible mixture when ignited burns very rapidly, producing a intense thrust, which can reach . The engine produce impulse about 300s. Solid fuel motor is suitable for major orbital manoeuvres such as apogee or perigee kick operations. The motor can be attached to the bottom of the spacecraft and detached after use, or integrated inside the spacecraft body.
The example is a motor of Voyager spacecraft, which weighed 1,123 kg including 1,039 kg of propellant, developed an average 6,805,440 N thrust during its 43-second burn duration. It was used to reach the final Jupiter trajectory velocity. The motor was jettisoned after burnout its propellant.
Liquid fuel propulsion systems
Liquid propellant motors can by classified as monopropellant and bipropellant
Monopropellant solution uses a single combustible propellant like hydrazine, which on contact with catalyst decomposes into its constituents what produce energy and thrust. It gives thrust in range 0.05N to 0,25N and specific impulse 200s. This kind of engines is used for smaller orbital manoeuvres such as station-keeping.
Bipropellant engine uses separated tanks for fuel and oxidizer. In the combustion chamber, fuel and oxidizer are mixed. This kind of engine produces a greater thrust for the same weight of fuel. The examples of fuel-oxidizer combinations are kerosene-liquid oxygen, liquid hydrogen-oxygen, hydrazine-nitrogen tetraoxide. The engines produce thrust up to \(\)10^6N\(\) and have impulse 300-400s. Bipropellant engines are used to major orbital changes requiring a large amount of thrust.
A good example of use liquid fuel by spacecraft is a Cassini – Saturns’ orbiter. For major orbital changes it used bipropellant engine ( exactly it had two main engines, includes spare), and for positioning manoeuvres monopropellant thrusters.
Cold gas propulsion systems
Relatively simply is an engine which uses gas at high pressure which led to a number of small thrusters. The kinetic energy of the nozzle exhaust is solely determined by the driving pressure in the reservoir. Typically gases are nitrogen, argon, freon, propane. A propellant is selected for the simplicity of their storage and its indifference to spacecraft’s materials which can be hit by exhaust plume. The thrust levels are low (~10mN), and the impulse is comparatively small: ~50s. This kind of propulsion system is used to attitude control and station keeping of small satellites like nano-satellites, CubeSats.
Electric propulsion systems
There is a set of propulsion system which use electricity to accelerate expellant. Electric power may be used to heat up propellant, to interacts with propellant ions or to use electromagnetic field to induce a Lorenz force on plasma.
The simplest powered by electric propulsion systems accelerate expellant by heats up the propellant. The typically propellants are hydrogen, nitrogen, ammonia and hydrazine. We can distinguish two kinds of electrothermal engines:
- Resistojet: closely allied to chemical propulsion, ohmic heating caused by an electric current through a heater raises the temperature of the propellant stream, then the hot exhaust gas is accelerated aerodynamically in a nozzle. The performance of this type of limited by the properties of the propellant, a temperature that can be attained in the thruster. The example is a power-augmented hydrazine thruster (PAEHT).
- Arcjet: propellant is heated by an electric arc, which it passes through on its way to exhaust nozzle. The engines use electric power in the range of 1KW to 20KW and are capable to produce specific impulse in the range of 500s to 800s with thrust an order of magnitude smaller than monopropellant hydrazine liquid thruster.
Thrust is produced by accelerating positively charged particles in an intense electrical field. The stream of positively charged particles must be neutralized to avoid a charge, opposite to that carried away from the spacecraft in the beam. The engine carries very little fuel and relies on the acceleration of charged particles to a high velocity.
- Ion thruster with elementary gas as a propellant can produce impulse of the order 3000s at an electrical power 1KW. Gas is ionized by electrons emitted from an axially mounted thermionic cathode towards a concentric cylindrical anode.
- Ion thruster with an acceleration of charged fluid droplets can generate thrust bigger than engines with ionized gas but have a lower impulse. Inside the engine fluid, a cone is formed and breaks up into a fine spray of positively charged droplets.
Some engines use magnetic field together with electricity to accelerate propellant.
One of the examples is an engine which utilizes the Hall effect. This kind of engine was developed in the former Soviet Union. In ‘Hall effect engines’ high voltage accelerates ions of a propellant (gas xenon, krypton, argon, bismuth, iodine, magnesium or zinc). Additionally, radial magnetic field interacts with electrons, holds them for a while inside the engine and creates from them Hall current. A large number of high energetic electrons which stay inside engine effectively ionize the gas, so almost all of its mass is accelerated by the electric field (even 90%-99% of the gas become ionized). The engine has big impulse (the 2500s) and thrust about 500mN. SpaceX Starlink satellites use ‘Hall effect engines’.
In The magnetplasmadynamic arc jet, both Joule heating and electrodynamic forces accelerate neutral plasma. The self-induced magnetic field provides the dominant acceleration mechanism.
Solar sail propulsion
The Sun emits radiation, which can be reflected by mirror sails, and thus the sail will produce a thrust. Additionally, the energy of the solar wind can be utilized. The possibility of using Suns’ emission pressure has been seriously discussed since 1924 when Friedrich Zander public technical paper.
The problem is the size of the sail, which must be very big, for example, to get thrust about 5N near The Earth, the sail will need to have size 800x800m.
The realistic and usefully application has appeared with the starting age of small satellites. Small satellites with low masses can use solar sail for different manoeuvres, especially to speed up deorbitation. The examples of projects are PWSAT and Light Sail. Japanese mission IKAROS successfully demonstrated solar sail technology in interplanetary space.
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