Steering Satellites: The Science of Propulsion
Curious about how satellites effortlessly move through the cosmos? In today’s Space Curiosity edition, you’ll be able to understand how propulsion and its diversity of solutions influence it!
First, let’s start to understand what propulsion is. The propulsion systems are the backbone of the satellite and rocket technology, enabling precise navigation and maneuvering in space. These systems rely on principles of physics and engineering to generate the thrust needed to lift off from Earth, maintain orbits, or explore the far reaches of the cosmos. We’ll go through the fascinating range of propulsion technologies and their applications!
What is Specific Impulse?
The Specific Impulse (Isp ) is a critical parameter in propulsion technology, representing how efficiently a rocket engine uses its propellant. It measures how long 1 kg of propellant can generate 1 N of thrust and acts as a “fuel efficiency” metric for space propulsion systems.
It can be measured through the proportional relationship between specific impulse (Isp ) , thrust (F), and the mass flow rate (m) of the propellant – how much mass is expelled per second.
Therefore, there exist two types of engines:
- High ISP Engines: These systems are highly efficient and consume less propellant to produce thrust, but they typically generate low thrust levels. They’re ideal for precise, long-duration satellite maneuvers.
- Low ISP Engines: These systems produce high thrust but require more propellant, making them well-suited for launch stages where power is paramount.
The Different Types of Propulsion Systems
Each propulsion system is tailored for specific applications, from launching rockets to precisely adjusting satellite orbits.
- Solid Propulsion
Solid propulsion systems use a solid mixture of fuel and oxidizer, which is ignited to release high-speed exhaust gases. This method is:
- Powerful: Solid rockets produce immense thrust, making them ideal for applications like ballistic missiles and booster stages of rockets.
- Stable: The solid propellant is chemically stable at room temperature and ambient pressure, ensuring long-term reliability.
However, solid propulsion systems have a major limitation: once ignited, they cannot be shut down or controlled. This lack of flexibility makes them unsuitable for satellites, which require precise thrust modulation. Additionally, the thrust profile is dictated by the shape of the propellant inside the booster, as the burn rate changes over time.
- Liquid Propulsion
Liquid propulsion is one of the most widely used propulsion systems in the aerospace industry, thanks to its versatility, efficiency, and controllability. It combines an oxidizer, such as liquid oxygen (LOx), and a fuel, like liquid hydrogen (LH2), to create a redox (reduction-oxidation) reaction that generates immense energy. This energy powers rockets, enabling them to lift heavy spacecraft into orbit and beyond. Let’s dive into the fascinating details of how liquid propulsion works and why it’s a cornerstone of modern rocketry!
The Science Behind Liquid Propulsion
When LOx and LH2 mix in a rocket engine, a spark ignites the reaction, releasing energy in the form of heat and thrust.
This reaction is fine-tuned by carefully balancing the ratio of oxidizer to fuel to maximize efficiency. Think of it like following a recipe: if a cake recipe calls for 4 eggs and 200g of flour but you use 2 eggs instead, the result will fall short of expectations. Similarly, an incorrect fuel-to-oxidizer ratio in rocket propulsion will result in suboptimal energy production.
Liquid propulsion systems boast high thrust and a relatively high specific impulse, making them well-suited for in-space propulsion of large spacecraft. The specific impulse measures how effectively a rocket uses its propellant, which is crucial for optimizing performance.
Unique Features of Liquid Propulsion
What sets liquid propulsion apart from solid propulsion, despite the latter’s raw power, is its adaptability:
- Throttle Control: By adjusting the flow rate of the propellants, engineers can modulate thrust, allowing for precise maneuvers.
- Shutdown and Restart: Liquid engines can be turned off and reignited, which is essential for operations like stage separation or orbital adjustments.
For instance, companies like SpaceX and Maia Space utilize liquid propulsion systems in their rockets. SpaceX uses a combination of LOx and methane (CH₄) for its first stages. This setup enables innovative maneuvers like the suicide burn, where the engines reignite seconds before landing to decelerate the rocket and achieve a soft touchdown. Solid boosters, by contrast, cannot be restarted or throttled, making liquid propulsion indispensable for reusable rockets.
Why are the Propellants Liquid?
While oxygen and hydrogen are commonly found as gases, storing them as liquids in rockets is far more efficient. To understand the thermodynamics of these elements, one may use the ideal gas law (P = ρrT), where:
- P = pressure in Pascal (Pa)
- ρ = density in kilograms per cubic meter (kg/m³)
- r = specific gas constant (J/kg/K)
- T = temperature in Kelvin (K)
While this model is only applicable for cold gases (below 3000K or about 2730°C), it greatly helps visualize how the density (how much mass of the element in each volume) of the propellants change with temperature and pressure, and gives an insight into the density of the liquid later on.
By cooling and pressurizing the propellants, their density increases dramatically. For example, liquid oxygen is stored at approximately 90 K (-183°C), and liquid hydrogen is stored at about 20 K (-253°C). These cryogenic conditions allow rockets to pack 800 times more propellant molecules into the same volume compared to their gaseous state, resulting in much higher energy density.
When you watch a rocket pre-launch, you’ll often notice pipes feeding the rocket’s central body. These continuously replenish the tanks to compensate for natural evaporation as the propellants expand and heat up. Safety vents ensure overpressure is avoided.
- Electric Propulsion
Electric propulsion represents the cutting-edge of satellite maneuvering technology. Unlike chemical propulsion, which relies on combustion, electric systems use electricity to ionize propellant (such as metals, water, or gases) and accelerate it to generate thrust. These systems have:
- The highest ISP: Electric propulsion is incredibly efficient, consuming minimal propellant to produce thrust.
- Low thrust levels: These systems aren’t suited for rocket launches but excel in orbit, where small, precise adjustments are needed.
Electric propulsion is especially well-suited for small satellites like Sternula-1. The electricity required to power the thrusters is typically generated by solar arrays. However, for deep-space missions like Voyager-1 and Voyager-2, where sunlight diminishes with distance, onboard nuclear reactors are used to produce electricity.
There are various types of electric thrusters, but they all work on the same principle: using electric or magnetic fields to increase the energy of the propellant, either by heating it or accelerating its velocity. This technology is paving the way for efficient satellite operations throughout the Solar System.
Why Propulsion Systems Matter
Each type of propulsion plays a unique role in space exploration:
- Solid propulsion ensures reliability and power for missions that require brute force, like missile launches or initial rocket stages.
- Liquid propulsion offers unmatched control, making it the backbone of reusable and modular rockets.
- Electric propulsion is revolutionizing satellite technology with its efficiency, allowing satellites to operate for extended durations while conserving propellant.
From enabling reusable rockets like SpaceX’s Falcon 9 to powering small satellites like Sternula-1, propulsion systems are the unsung heroes of space technology. Without these advances, humanity’s ability to explore, understand, and utilize space would not be possible.
Whether it’s reaching Earth’s orbit, exploring the Solar System, or venturing into deep space, propulsion technology fuels our journey to the stars!
Published on 20/12/2024, Written by: Cyriaque Guillot, Marta Marinho Araújo dos Santos
