Ever wondered what packed inside a rocket gives such massive energy to take a satellite weighing tons beyond earth’s gravitation? What enables a satellite to stay in its orbit or explore the solar system or go beyond its boundaries? Let’s discuss existing and emergent technologies in rocket and satellite propulsion.
Before we start let’s take a look at some basic terms we need to know…
- Propulsion: It is the process of pushing or pulling to drive an object.
- Propellants: It is the fuel used by a rocket engine to create thrust. In other words, it is a mass that is expelled or expanded in such a way as to create a thrust or other motive force following Newton’s third law of motion, and “propel” a vehicle, projectile, or fluid payload.
- Specific impulse (Isp): Specific impulse is a measure of how efficiently a reaction engine (almost all our rocket and jet engines fall under this category) creates thrust. The larger the Specific impulse, the greater the efficiency.
Solid Propulsion
In solid rocket motors, the propellants used are completely solids. Early solid rocket engines were powered with gun powder, which is used in warfare. Usually, a solid rocket engine has a cylinder with a gap in its core and a nozzle at the end. A propellant mixture consisting of various components such as fuel, oxidizer, binder, catalysts, promoters, etc., is thickly lined along the walls of this cylinder leaving some space in the middle. The hole through the cylinder serves as a combustion chamber where the combustion of this mixture takes place. At the top of the gap, there will be an igniter that is used for starting the engine. When the mixture is ignited, combustion takes place on the surface of the propellant resulting in the exhaust of gases. Atomized aluminum powder (fuel), ammonium perchlorate (oxidizer), and HTPB(Hydroxyl-terminated polybutadiene)(binder) are commonly used propellant components.
- They are relatively better at withstanding sudden shock, vibration, and acceleration.
- No propellant pumps are required which makes this rocket engine less complicated.
- Low specific impulse(i.e. less efficient) than liquid rocket engines.
- These engines can’t be throttled or operated in start-stop mode.
- Thrust can’t be regulated.
- Uses an igniter.
Liquid Propulsion
A typical liquid propellant engine uses a liquid fuel-oxidizer mixture that ignites upon contact; to be precise a pressure-regulated hypergolic liquid bipropellant feed. In a liquid rocket engine, there are mainly two storage tanks, one for fuel and the other for oxidizer in addition to that, it consists of a turbine, pumps, preburner (or gas generator), combustion chamber, nozzle, and heat exchange system.
A part of propellants (in a fuel-rich composition), undergoes partial combustion in the gas generator or pre-burner which results in the formation of hot gases. These hot gases are later used for starting the turbine and the pumps connected to our storage ranks. Fuel and oxidizer get pumped into the combustion chamber and the ignition occurs. Exhaust gases escape through the nozzle resulting in the generation of thrust. Usually used liquid fuels are UDMH(Unsymmetrical dimethylhydrazine), MMH(Monomethylhydrazine), etc and oxidizers are N2O4, MON(Mixed oxides of nitrogen). A part of the fuel is also used as a coolant for preventing the meltdown of the nozzle.
- Liquid propellant engines generally have higher specific impulses than solid propellant engines.
- The engine can be throttled or operated in start-stop mode.
- The flow of propellants can be monitored and regulated to precisely control the magnitude of the thrust.
- No need for an igniter.
- The need for pumps, piping, and separate storage for the fuel and oxidant results in extra mass that has to be carried by the launch vehicle.
Cryogenic and Semi-cryogenic propulsion
A cryogenic rocket engine is just another liquid rocket engine in which both fuel and oxidizer are stored at cryogenic temperature (i.e. less than -150°C), specifically liquid hydrogen and liquid oxygen. The very low temperature of propellants makes these engines more complex.
Unlike a cryogenic engine, a semi-cryogenic engine is the one in which either fuel or oxidizer is at cryogenic temperature. Usually, a semi-cryogenic engine uses kerosene and liquid oxygen. It also makes the semi-cryogenic engine more cost-effective than the cryogenic engine.
- Higher specific impulse than liquid rocket engines.
- Cryogenic engines need refrigerants.
Mono propellant engines
Mono propellant engines generate thrust by breaking down propellant molecules (exothermal chemical decomposition) in the presence of a catalyst. The most commonly used propellant is hydrazine (N2H4) along with granular alumina (aluminium oxide) coated with iridium which is a spontaneous catalyst. This Alumina decomposes hydrazine upon contact. Hydrogen peroxide can also be used as a propellant.
- Relatively simple engine.
- Relatively high specific impulse (i.e. high efficiency).
- Quick and easy to control.
Cold gas thruster
As the name suggests, in a cold gas rocket engine, no production of heat occurs. Thrust is generated by the expansion of certain pressurized gas stored as the propellant. Typically inert gases or gases that are inert at lower temperatures are used, such as helium, nitrogen, etc.
- Propellants are cheap compared to regular rocket engines.
- It’s simple design makes it less prone to failures
- Propellants are safer to handle.
- Have a low specific impulse.
Electric propulsion systems
An electrically powered propulsion system makes use of electromagnetic fields to change the velocity of a spacecraft. It’s of 3 types: Electrostatic, Electrothermal, and Electromagnetic propulsion systems.
- Electrothermal– Propellant is heated electrically and expanded to increase its pressure. Later this gas is accelerated to supersonic speeds as it passes through the nozzle.
- Electrostatic– Acceleration is achieved by the interaction of electrostatic fields on charged propellant particles such as atomic ions, droplets, or colloids. Electrostatic ion thrusters use the Coulomb force.
- Electromagnetic – Acceleration is achieved by the interaction of electric and magnetic fields within a plasma. Electromagnetic ion thrusters use the Lorentz force to move the ions.
Ion thruster: An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It can be electrostatic or electromagnetic. Neutral atoms were hit with an electron beam to produce ions and plasma. These ions are then accelerated in an electrostatic or an electromagnetic field to generate thrust.
- The flexible location of the machinery component lets it carry more payload.
- More efficient.
- Lesser pollution.
Solar sail
A Solar sail, also known as a light sail or photon sail, is a method of spacecraft propulsion using radiation pressure exerted by sunlight on large mirrors or specialized films. i.e. using the pressure exerted by the collision of photons in the sail the spacecraft is accelerated like wind sails in ships.
- Can be accelerated without carrying any propellant.
- Can’t be used for orbital missions.
Nuclear propulsion
A nuclear thermal rocket (NTR) is a type of rocket in which heat is produced from a nuclear reaction, often nuclear fission. Usually, liquid hydrogen is heated to a high temperature in a nuclear reactor and then expanded through a rocket nozzle to create thrust.
- Theoretically allows a higher effective exhaust velocity.
Many of these propulsion technologies are already in use while some of them are still a prototype. Depending on the countries and the resources they have, their go-to propulsion technology varies and it has an impact on the kind of missions they could undertake.
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