Fuel Systems
Fuel systems are essential components in engines that generate thrust for aircraft and spacecraft, dating back to the early 20th century. They operate on the principle of Newton's third law of motion, where combustion of fuel creates hot gases that are expelled to propel the vehicle forward. There are three primary types of rocket fuel systems: liquid, solid, and hybrid. Liquid-fuel rockets use separate tanks for fuel and oxidizer, allowing for controlled combustion and speed adjustments. In contrast, solid-fuel rockets combine fuel and oxidizer in a stable form, which simplifies design but limits control once ignited. Hybrid rockets blend the benefits of both types, using solid fuel and a liquid oxidizer for safer and more efficient operation. Additionally, there is ongoing research into nuclear-powered rockets, which could significantly reduce travel time to distant planets, although safety and budget concerns have hindered their development. Each fuel system presents unique advantages and challenges that are critical to the advancement of space exploration and aerospace technology.
Fuel Systems
FIELDS OF STUDY: Aerospace Engineering; Orbital Mechanics; Space Technology; Spacecraft Propulsion
ABSTRACT: A fuel system is the part of a vehicle’s engine that combines fuel and air in amounts that allow for combustion. This process of burning fuel generates the vehicle’s thrust, or motion. Without fuel systems, rockets could never reach space, and the development of new and more efficient types of fuels and fuel systems will further space exploration.
Fuel System Basics
Since the Wright brothers’ first flight in 1903, every engine-propelled vehicle of flight has required a fuel system to generate thrust, the force that propels forward movement. While systems may vary in design and type of fuel burned, thrust is always generated by the application of Isaac Newton’s (1642–1727) third law of motion. This law states that for every action, there is an equal and opposite reaction. In the case of rockets, the hot gases produced by fuel combustion flow backward and out of the rocket. This causes the rocket to move forward with an equal force. The speed of a rocket can be altered by changing the amount of fuel used and the way the resulting gases are concentrated through the nozzle (shaped opening) as they exit the fuel system.
All combustion systems require both a fuel source and an oxidizer to produce the heat that creates thrust. However, space vehicles must compensate for the fact that there is no oxygen in space. Therefore, their systems must be built to contain both the fuel and the oxidizer.
Types of Rocket Fuel Systems
There are three main types of rocket engines: those that use liquid fuel, those that use solid fuel, and those that use a combination of the two. In a liquid-fuel rocket, the fuel source (usually liquid hydrogen) and the oxidizer are stored separately as liquids. Then they are combined in a process known as fuel injection. The fuel injectors combine the two in computer-controlled proportions in the combustion chamber, where the fuel burns and provides the force for thrust. The speed of the rocket can be controlled by regulating the flow of fuel into the system. As a result, liquid-fuel systems are more versatile; however, the additional mechanics needed to control the injection process make them more complicated and heavier than their solid-fuel counterparts.
In a solid-fuel rocket, the fuel source and oxidizer are combined and loaded into a cylinder. In the case of the booster rockets that launch space shuttles into orbit, the solid, dry fuel mix contains ammonium perchlorate for an oxidizer and aluminum for the fuel, plus a binder. Solid-fuel rockets are the simplest of rocket designs and generally the most stable. They will not burn until exposed to an igniter. However, once combustion begins, it continues until all the fuel is used up. The only way it can be stopped is by destroying the cylinder containing the fuel. This makes solid-fuel rockets less versatile, but they are both lighter and less complicated in design.
Hybrid rockets use a combination of liquid and solid propellants, usually a solid fuel and a liquid or gaseous oxidizer. These combine the advantages of both solid-fuel and liquid-fuel rockets: the fuel is stabler and require fewer components for the fuel-injection process, but the use of fuel can still be regulated. These rockets are considered safer, cheaper to operate, and more environmentally friendly than the other types. Their fuel sources are not combustible on their own, and they have fewer working parts to break down and allow accidental explosions. They are cheaper because the oxidizer can be oxygen, hydrogen peroxide, or nitrous oxide—all substances that are readily available and less expensive than custom-made solid fuel mixes. And because the fuels used in hybrids produce fewer hazardous by-products, they are considered to be a greener alternative.
Some vehicles, such as the space shuttle, use several types of fuel systems. Solid-fuel booster rockets are used to launch the vehicle, which is then propelled through space by a cryogenic, or very low temperature, liquid-fuel system. In the late 2010s and early 2020s, the private aerospace company SpaceX developed its Raptor engine, which uses a combination of methane and liquid oxygen fuel. The Raptor is designed as a more efficient, cleaner-burning engine. SpaceX’s Raptor 2 engine was to be used aboard the company’s Starship spacecraft, which was planned for a late 2022 launch.
Nuclear Engines
Solid, liquid, and hybrid fuel systems all share an inherent problem for space exploration: their weight and slow speed make reaching objects farther out in space impractical. The increased efficiency of nuclear power might mean lighter, faster spacecraft that could allow astronauts to reach Mars in as few as four months. Nuclear-powered rockets could trim the flight to Saturn from seven years to as few as three years. Even before the first successful moon landing in 1969, scientists were working on the idea of a nuclear-powered spacecraft.
While no such spacecraft have yet been launched, scientists have created designs for liquid-, solid-, and gas-core fuel systems with variations including closed- and open-core systems. Some solid-core prototypes were created, and NASA established programs for long-duration missions that would use nuclear-powered spacecraft, but they were canceled due to safety concerns and budget cuts.
Principal Terms
- combustion: a chemical process in which a fuel source combined with an oxidizer reacts to create heat.
- fuel injection: a computer-controlled process in a vehicle’s fuel system in which the fuel and oxygen are mixed in the ratio needed to power the vehicle’s engine
Bibliography
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Lockett, Will. "SpaceX’s Latest Rocket Engine Will Dominate Space." Medium, 27 Feb. 2022, medium.com/predict/spacexs-latest-rocket-engine-will-dominate-space-d35213cd3e9e. 16 June 2022.
"Nuclear Thermal Rocket Propulsion." Space Propulsion and Mission Analysis Office. NASA, 9 July 2008. Web. 21 Apr. 2015.
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