Some missions requires to change velocities of the satellite after its detaching from the launcher. The satellites propulsion systems give to spacecrafts possibility to make maneuvers 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 utilze this Newtons 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:
Solid fuel propulsion systems
Solid rocket fuel is filled with mixture of relatively hard, rubbery, combustible mixture of fuel, oxidizer and binder. Engine is ignited by pyrotechnic device – igniter. 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 for reach 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. These kind of engines are used for smaller orbital maneuvers such as station keeping.
Bipropellant engine uses separated tanks for fuel and oxidizer. In the combustion chumber fuel and oxidizer are mixed. This kind of engine produce a greater thrust for the same weight of fuel. The examples of fuel-oxidizer combinatos are kerosene-liquid oxygen, liquid hydrogen-oxygene, hydrazine-nitrogen tetraoxide. The engines produce thrust up to \(\)10^6N\(\) and have impulse 300-400s. Bipropellant engines are used to major orbital changes requairing large amount of thrust.
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 maneuvers monopropelant thrusters.
Cold gas propultion systems
Relatively simply is a engine which uses gas at high pressure which fed to a number of small thrusters. Kinetic energy of the nozzle exhaust is solely determined by the driving pressure in the reservoir. Typically gases are nitrogen, argon, freon, propane. Propellant are selected for the simplicity of their storage and its indifference to spacecrafts’ 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 electricity 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, temperature that can be attained in the thruster. The example is an power-augmented hydrazine thruster (PAEHT).
- Arcjet: propellant is heated by electric arc, which it passes through on its way to exhaust nozzle. The engnines use electric power in the range of 1KW to 20KW, and is 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 positivly charged particles in a intense electrical field. The stream of positivly 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 acceleration of charged particles to a heigh velocity.
- Ion thruster with elementary gas as a propellant can producing impulse of the order 3000s at an electical power 1KW. Gas is ionized by electrons emited from an axially mounted thermiodic cathode towards a concentric cylindrical anode.
- Ion thruster with acceleration of charged fluid droplets can generates thrust bigger than engines with ionized gas, but have lower impulse. Inside the engine fluid cone is formed and breaks up into a fine spray of pisitively charged droplets.
Some engines use magnetic field together with electricity to accelerate propellant.
The one of the example is an engine which utilize Hall effect. This kind of engine was developed in 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 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 have big impulse (2500s) and thrust about 500mN. SpaceX Starlink satellites use ‘Hall efect 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 propultion
The Sun emits radiation, which can be reflected by mirror sails, and thus the sail will produce a thrust. Additionaly energy of the solar wind can by utilized. The possibility of of using Suns’ emmision pressure has been seriosly discussed since 1924, when Friedrich Zander publicate 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 usefuly application have appeared with starting age of small satellites. Small satellites with low masses can use solar sail for different manevrous, especialy 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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