Propulsion in Spacecrafts

Introduction and Fundamentals

Rockets play an important role in modern space exploration. As the term “rocket science” is used to describe scientific concepts that are seemingly difficult to grasp, one may imagine that the physics behind launching a rocket involves a great level of complexity. The basic principle of propelling a rocket however, is just an application of Newton’s Third Law: for every action, there must be a reaction. For the case of rockets, vast amounts of particles are released from the *nozzle at a high velocity to generate thrust in the opposite direction of the flow of mass. The greater the momentum a rocket loses, the greater the thrust. A rocket essentially works the same way as when one releases an untied balloon full of air from their hand. From this simple mechanism, rockets have been around humanity for a long time; rockets were originally invented in China back in 1232.

*nozzle: the bottom-most structure of a rocket where the propellent is expelled

Rocket Propulsions

Rockets are made up of 4 main components: the payload system, the guidance system, the structural system, and the propulsion system. The main focus of this paper will be on the propulsion system. The gas that exits the rocket at a high velocity is called the propellent. The propellant includes the fuel and the oxidizer of the rocket.

The thrust of a rocket can be found by using the equations listed on the image below.

Another important concept used in rocket propulsion is called the specific impulse. The specific impulse is defined as the change in momentum per mass of rocket fuel. In other words, specific impulse is the thrust over the amount of propellant it loses in a second. The specific impulse is abbreviated as Isp. The derivation is given on the image below.

Types of Propulsions

1. Liquid Rocket Engines

Liquid rocket engines are the most common type of engines used in rockets. Liquid hydrogen or petroleum is used as fuel and its oxidizer contains liquid oxygen or fluorine. The fuel and oxidizer is pumped to the combustion chamber where combustion occurs to generate thrust. The thermal energy produced in the combustion chamber is converted to kinetic energy as it exists from the rocket through the nozzle. During this process, slow-moving gas with high pressure and temperature gas is converted to fast-moving gas with low pressure and temperature.

2. Solid-Fuel Rocket Engines

Solid-fuel rocket engines contain a mixture of fuel and oxidizer. Once the rocket is ignited, combustion occurs along the surface of the propellant until it is used up completely. Solid-fuel rocket engines are simpler, cheaper and safer than liquid rocket engines however, they have a lower specific impulse, meaning that they are less efficient compared to liquid rocket engines.

3. Ion Propulsion

Ion propulsion is the most efficient propulsion system known to humanity. The fuel efficiency of an ion engine is around 90% whereas the most efficient kind of liquid rocket engines are just 35%! Thrust in an ion engine is produced by the following steps:

A high-energy beam of electrons bombards the target atom (such as Xenon).
Some of the electrons are then stripped off from the *Xenon atom producing Xenon ions = Ionization.
A *plasma of electrons and Xenon ions are created.
The electrons are collected from the positive grid.
The Xenon ions move towards the end of the chamber.
The high electric potential difference between the positive and negative grids causes the Xenon ion to accelerate up to speeds of about 90 km/s.
The high speed Xenon ion is neutralized due to a beam of electrons from the hollow cathode neutralizer.

The amount of thrust each atom produces is very small but can end up creating huge amounts of thrust as there are billions of atoms being emitted at the same time. Although very efficient, ion thrusters only work in a vacuum space; it does not work on Earth as the air particles in the atmosphere interfere with the ions produced.

Xenon is commonly used in ion thrusters for the following reasons.

Xenon has a higher atomic mass so it can generate more thrust per atom.
Xenon is a noble gas and is therefore inert (inactive).
Xenon has a high storage density, meaning that large amounts of atom can be stored per unit length.
Xenon has a relatively low ionization energy – thus requiring minimal amount of energy to eject electrons

*Plasma

Plasma is said to be the fourth state of matter. When enough energy is applied to a substance in a gaseous phase, the electrons will be stripped away from the nucleus to create a pool of electrons and the nucleus. Different from gas, plasma can conduct electricity due to the presence of lone electrons.

References

[1] Benson, Tom. (2014). “Brief History of Rockets”. The National Aeronautics and Space Administration. https://www.grc.nasa.gov/WWW/k-12/TRC/Rockets/history_of_rockets.html Last Accessed: 24 Jan. 2020.

[2] Benson, Tom. (2014). “Rocket Thrust Summary”. The National Aeronautics and Space Administration. https://www.grc.nasa.gov/WWW/K-12/rocket/rktthsum.html Last Accessed: 24 Jan. 2020.

[3] Hall, Nancy. (2015). “Specific Impulse”. The National Aeronautics and Space Administration. https://www.grc.nasa.gov/WWW/K-12/airplane/specimp.html Last Accessed: 24 Jan. 2020.

[4] Hall, Nancy. (2015). “Liquid Rocket Engine”. The National Aeronautics and Space Administration. https://www.grc.nasa.gov/WWW/K-12/airplane/lrockth.html Last Accessed: 24 Jan. 2020.

[5] Hall, Nancy. (2015). “Solid Rocket Engine”. The National Aeronautics and Space Administration. https://www.grc.nasa.gov/WWW/K-12/airplane/srockth.html Last Accessed: 24 Jan. 2020.

[6] Puiu, Tibi. (2013). “Ionic thrusters display huge efficiency, mandating more attention”. ZME Science. https://www.zmescience.com/science/physics/ionic-thrusters-display-huge-efficiency-mandating-more-attention/ Last Accessed: 24 Jan. 2020.

[7] Real Engineering. (2018). “Ion Propulsion – The Plane With No Moving Parts”. https://www.youtube.com/watch?v=IorDYGI1uqc Last Accessed: 24 Jan. 2020.

[8] Kaufman & Robinson, Inc.. (2006). “Ion-Beam Neutralization”. The Ion Beam AuthoritySM. https://ionsources.com/wp-content/uploads/2016/12/TN-02-Ion-Beam-Neutralization.pdf Last Accessed: 24 Jan. 2020.

[9] Dunbar, Brian, et al. (2008). “Ion Propulsion: Farther, Faster, Cheaper”. The National Aeronautics and Space Administration. https://www.nasa.gov/centers/glenn/technology/Ion_Propulsion1.html Last Accessed: 24 Jan. 2020.

[10] (2011). “What’s the difference between liquid and solid-fuel rockets”. Smithsonian National Air and Space Museum. https://howthingsfly.si.edu/ask-an-explainer/what%E2%80%99s-difference-between-liquid-and-solid-fuel-rockets Last Accessed: 24 Jan. 2020.

[11] “Working of Rocket and Jet Plane in terms of Newton’s Third Law”. QS Study. https://www.qsstudy.com/physics/working-of-rocket-and-jet-plane-in-terms-of-newtons-third-law Last Accessed: 24 Jan. 2020.

Thumbnail Image and Source

[12] Cole, Michael. (2016). “Solar electric propulsion: NASA’s engine to Mars and beyond” SpaceFlight Insider. https://www.spaceflightinsider.com/missions/human-spaceflight/solar-electric-propulsion-nasas-engine-mars-beyond/ Last Accessed: 17 Jul. 2020. (Image credits to The National Aeronautics and Space Administration)