Exploring Rocket Propulsion: A Classification Guide
This is, hands down, my favorite aspect of Aerospace technology. Every time I witness a rocket launch, I can’t help but feel the tingles, goosebumps, and a surge of emotions. There’s something really magnificent and beautiful about the moment of ignition. It’s an experience I don’t think I’ll ever get over. Over time, my passion led me to delve deeper into this captivating technology, resulting to hours and hours of videos and books. Let me walk you through my reading notes. This article is based on the classification of propulsion.
What is Propulsion?
Propulsion is the act of changing an object’s motion with respect to a fixed reference frame, and propulsion systems make things move from rest, change their speed, or overcome resistance in a viscous medium.
Classification
So, there is a bring umbrella called Jet Propulsion and under this umbrella we have Rocket Propulsion and Duct Propulsion. Jet Propulsion is a type of motion whereby a vehicle moves forward by pushing out material in the opposite direction. Rocket Propulsion is a type of jet propulsion that creates thrust by expelling matter, known as the propellant or working fluid. Duct Propulsion use the surrounding air or gas as propellant which mixes with the vehicle’s stored fuel to create combustion.
Duct Jet Propulsion
They are often referred to as air-breathing engines, as they draw and energize airflow within a duct. They include turbojets, turbofans, ramjets, and pulsejets. They have higher specific impulse, meaning they can travel long distances. This gives them an advantage over chemical rocket propulsion systems at low altitudes.
However, rocket propulsion systems excel in rarefied air and space environments due to their unique characteristics like high thrust to weight and frontal area, as well as thrust being almost independent of altitude.
Rocket Propulsion
Rocket propulsion can be classified in different ways:
- Energy source type: chemical, nuclear, or solar.
- Basic function: booster stage, sustainer or upper stages, attitude control, orbit station keeping, etc.
- Type of vehicle they propel: aircraft, missile, assisted takeoff, space vehicle, etc.
- Size: Small-scale model rockets used for educational purposes and amateur rocketry projects. Large-scale rockets like the Saturn V used for Apollo moon missions.
- Type of propellant: Hydrogen-oxygen propellant used in the Space Launch System (SLS). Kerosene and liquid oxygen propellant used in SpaceX Falcon 9.
- Type of construction,
- Number of rocket propulsion units used in a given vehicle: Multi-stage rockets like the Saturn V had multiple stages, each with its own rocket propulsion unit. Space Shuttle used solid rocket boosters and liquid rocket engines.
- Method of producing thrust: The thermodynamic expansion of a gas in a supersonic nozzle is utilized in most common rocket propulsion concepts. Nonthermal types of electric propulsion use magnetic and/or electric fields to accelerate electrically charged atoms or molecules at very low gas densities.
Energy sources are central to rocket performance and several kinds, both within and external to the vehicle
Rocket propulsion systems use either heat or electricity as useful energy inputs. The resulting thrust (output) comes from the kinetic energy of the expelled matter and the pressure of the propellant within the chamber and at the nozzle exit. In essence, rocket propulsion systems convert input energies into the kinetic energy of the exhausted gas.
The expelled mass can be in solid, liquid, or gaseous form, and sometimes combinations of two or more phases. At extremely high temperatures, the expelled matter may also exist as a plasma, which is an electrically conducting gas.
Chemical Rocket Propulsion
Chemical propellants, like fuel and an oxidizer, create energy through combustion. This energy heats up gases to very insane high temperatures (around 2500 to 4100 degrees Celsius or 4500 to 7400 degrees Fahrenheit). These hot gases are then pushed out through a supersonic nozzle, making them move really fast (around 1800 to 4300 meters per second or 5900 to 14,100 feet per second).
Types of propellants include:
Liquid propellant rocket engines use propellants stored as liquids that are fed under pressure from tanks into a thrust chamber.
- The bipropellant consists of a liquid oxidizer (e.g., liquid oxygen) and a liquid fuel (e.g., kerosene).
- A mono propellant is a single liquid that decomposes into hot gases when properly catalyzed.
In solid propellant rocket motor, all the ingredients needed for burning are already stored inside a combustion chamber. The solid propellant, called the grain, contains everything needed for burning. When ignited, it burns smoothly at a set rate on its surfaces. As it burns, the internal cavity expands, and hot gases flow through a supersonic nozzle to create thrust. The burning process continues in an organized way until almost all the propellant is used up.
Gaseous propellant rocket engines use a stored high-pressure gas, such as air, nitrogen, or helium, as working fluid:
- Cold gas thrusters were used in many early space vehicles for low-thrust maneuvers and for attitude-control systems.
- In a Warm gas propellant rocket propulsion, the gas is heated with electrical energy or by the combustion of certain monopropellants in a chamber to improve their performance.
Hybrid propellant rocket propulsion systems use both liquid and solid propellant storage.
Nuclear Rocket Engines
These are special rocket engines that use liquid propellant, but instead of burning chemicals, they get their power from a single nuclear reactor. Nuclear energy originates in transformation of mass within atomic nuclei and is generated either by fission or fusion.
During the 1960s an experimental rocket engine with a nuclear fission graphite reactor was built and ground tested with liquid hydrogen as the propellant. It delivered an equivalent altitude specific impulse of 848 sec, a thrust of over 40,000 lbf at a nuclear reactor power level of 4100 MW with a hydrogen temperature of 2500 K. No further ground tests of nuclear fission rocket engines have been undertaken.
Rocket engines that use nuclear power have been explored but concerns about accidents spreading radioactive materials on Earth led to the termination of this work. It’s unlikely that nuclear rocket engines will be developed in the next few decades.
Despite the discontinuation of nuclear fission rocket engines, research on nuclear fusion rocket engines continues. Lockheed Martin is a leading company in this exciting field.
Under the contract, issued by NASA and the Defense Advanced Research Projects Agency (DARPA), the Defense Department unit that seeks to develop transformative technologies, Lockheed Martin will work to first fly the engine by 2027. — The Washington Post.
Electric Rocket Propulsion
Electric propulsion offers good thrust with less propellant but is limited by current power supplies. It’s unsuitable for Earth launches due to massive and less efficient power sources, making it better for spacecraft where power can be shared with other systems.
Unlike chemical propulsion, electric propulsion uses energy sources like nuclear power, solar radiation, or batteries that are separate from the propellant being used. It is used in spacecraft with longer mission durations and low thrust levels for tasks like orbit maintenance.
Types:
Electrothermal rocket propulsion is similar to the liquid-propellant chemical rockets we talked about earlier. In electrothermal thrusters, a propellant is heated using electricity (using solid resistors or electric arcs). Then, the hot gas is pushed out through a supersonic nozzle to create thrust.
These thrusters have a thrust range from 0.01 to 0.5 N and can reach exhaust velocities of 1000 to 7800 m/s. They use different materials like ammonium, hydrogen, nitrogen, or hydrazine for the propellant.
Electrostatic or ion propulsion thrusters and the electromagnetic or magneto plasma thrusters: Here, no thermodynamic gas expansion in a nozzle is necessary, and they both work in a vacuum.
In an ion thruster, a special gas like xenon is turned into ions (atoms missing some electrons). Then, using electric fields, these ions are given super high speeds (from 2000 to 60,000 meters per second!). After that, to keep things balanced, the ions are made neutral again by adding back the missing electrons to prevent the buildup of a “space charge” on the vehicle.
In electromagnetic thrusters, a plasma (a special kind of energized gas with ions, electrons, and neutral particles) is accelerated using electric currents and magnetic fields. This plasma is then ejected at really high speeds (from 1000 to 75,000 meters per second!).
Solar Rocket Propulsion
One idea is the solar thermal rocket. It uses big mirrors or lenses to concentrate the sun’s rays onto a special receiver made of high-temperature metal. The metal heats up a working fluid like hydrogen, and then the hot gas is used to produce thrust. This type of rocket can perform better than chemical rockets and has low thrust levels.
The big mirrors must always face the sun, so adjustments are needed if the spacecraft orbits around planets. These rockets work best in space, not in the atmosphere. One such system was tested in 2012, but it’s not widely used yet due to some challenges like lightweight mirror structures and hydrogen storage.
The solar sail is like a big shiny mirror that reflects sunlight to move. It gets its power from the sun, but it can only go away from the sun. Some ideas have been suggested to use lasers or microwaves from Earth to give it energy, but they haven’t been tested yet.
Solar radiation may originate from other sources than the sun like:
- Transmission of energy by ground-based microwaves.
- Laser beams.
Reference: Rocket Propulsion Elements by GEORGE P. SUTTON
