Rajan Wilcox
Rocket fuel weight is a critical factor in space exploration. The amount of fuel required to launch a rocket into space is substantial, with NASA's Space Shuttle, for example, needing over 3.5 million pounds of fuel. The type of fuel and engine used can significantly impact the overall weight and cost of the mission. Traditional chemical rocket engines, for instance, tend to use fuel-rich combustion to reduce exhaust molecular mass, favouring lighter elements like hydrogen. However, kerosene has better handling, density, and thrust-to-weight properties, showcasing the complex considerations in fuel selection. Additionally, the cost of rocket fuel can vary, with some propellants costing hundreds of thousands of dollars per launch. The development of more efficient engines, such as rotating detonation engines, aims to address fuel efficiency and weight concerns, but challenges remain before they can be safely implemented.
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What You'll Learn
Photon rockets need 0.03g of fuel to lift 1kg of payload
Photon rockets are a theoretical type of spacecraft propulsion that could drastically reduce the amount of fuel needed to launch a payload into orbit. While traditional rockets require a large amount of fuel to propel a small payload, photon rockets, in theory, could achieve a much higher payload-to-fuel ratio.
The amount of fuel required to launch a payload into orbit depends on several factors, including the type of rocket, the payload weight, and the desired orbit. For traditional chemical rockets, the fuel efficiency is relatively low. For example, the Falcon 9 rocket burns around $200k-300k of propellant to launch 16000 kg of payload, resulting in a fuel cost of about $20/kg.
In contrast, photon rockets offer a much more efficient way to launch payloads into orbit. A photon rocket is a theoretical spacecraft propulsion system that uses light particles, or photons, to generate thrust. By taking advantage of the momentum transfer when photons are reflected off a surface, a photon rocket could, in principle, achieve extremely high velocities.
According to calculations, a photon rocket would require only 0.03 grams of fuel to lift 1 kg of payload to Low Earth Orbit (LEO). This incredible efficiency is based on the assumption that we can construct a rocket with zero mass and a powerful enough laser to provide the necessary energy input. However, it's important to note that these assumptions are highly theoretical and may not be feasible in practice.
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The Falcon 9 burns $200k-$300k of propellant
The Falcon 9 is a partially reusable, two-stage-to-orbit, medium-lift launch vehicle designed and manufactured in the United States by SpaceX. The rocket has two stages. The first (booster) stage carries the second stage and payload to a predetermined speed and altitude, after which the second stage accelerates the payload to its target orbit. The booster is capable of landing vertically to facilitate reuse. The Falcon 9's launch sequence includes a hold-down feature that allows full engine ignition and systems check before liftoff. After the first-stage engine starts, the launcher is held down and not released for flight until all propulsion and vehicle systems are confirmed to be operating normally.
The Falcon 9 burns somewhere around $200,000 to $300,000 worth of propellant. This figure was stated to be $200,000 in 2015, but the vehicle has grown in size since then. For non-expendable launches, it puts about 16,000 kg into orbit, which equates to about $20/kg. The Full Thrust upgrade (also known as FT, v1.2 or Block 3) made major changes, including the addition of cryogenic propellant cooling to increase density and allow for 17% higher thrust. The stage separation system was improved, the second stage was stretched to hold additional propellant, and the struts holding helium bottles were strengthened.
Both stages of the Falcon 9 are powered by SpaceX Merlin engines, using cryogenic liquid oxygen and rocket-grade kerosene (RP-1) as propellants. The v1.1 first stage offered a total sea-level thrust at liftoff of 5,885 kN (1,323,000 lbf), with the engines burning for a nominal 180 seconds. The stage's thrust rose to 6,672 kN (1,500,000 lbf) as the booster climbed out of the atmosphere. These improvements increased the payload capability from 9,000 kg (20,000 lb) to 13,150 kg (28,990 lb).
In February 2025, a propellant leak caused the upper stage of a Falcon 9 rocket to crash to Earth over Europe. SpaceX confirmed that a small liquid oxygen leak had developed, which ultimately drove higher-than-expected vehicle body rates.
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Traditional ion thrusters use expensive Xenon propellant
The weight of rocket fuel depends on the type of rocket and the payload it is carrying. For example, the Falcon 9 rocket burns around $200,000-300,000 of propellant, putting about 16,000 kg into orbit, which equates to approximately $20/kg. Meanwhile, the Starship rocket burns cheaper methane fuel, with propellant costs estimated at about $500,000 per launch, with a total payload of around 100-150 tonnes, resulting in a cost of around $5/kg.
Now, traditional ion thrusters use Xenon propellant, which costs approximately $850 per kg. Xenon is a noble gas that can be easily ionized to create a cloud of positive ions, which are then accelerated using electricity to create thrust. This process is known as an ion thruster or ion drive and is a form of electric propulsion used for spacecraft propulsion. Ion thrusters can be categorized as either electrostatic or electromagnetic, with electrostatic thruster ions accelerated by the Coulomb force along the electric field direction, and electromagnetic thruster ions accelerated by the Lorentz force.
NASA has been at the forefront of developing ion thrusters, with their NSTAR ion engine being used for interplanetary science missions beginning in the late 1990s. The Japanese Aerospace Exploration Agency's Hayabusa space probe, launched in 2003, was powered by four xenon ion engines. Xenon is preferred over other gases like helium because it is cheaper and easier to turn into plasma. Xenon also has a higher "propellant efficiency" (Isp) compared to helium, which means that with the same amount of energy, it can achieve a higher velocity.
However, there are some drawbacks to using xenon propellant. Firstly, it needs to be stored at high pressure (~200 bars) to liquefy or become supercritical, which means that xenon tanks need to be very strong and heavy, making it unsuitable for smaller spacecraft. Additionally, xenon is expensive, with traditional ion thrusters costing around $850/kg to operate. As a result, alternative propellants such as helium, hydrogen, lithium, krypton, argon, bismuth, and iodine have been explored for use in ion thrusters. These propellants offer advantages such as lower cost, higher efficiency, and easier storage.
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Kerosene has better handling, density and thrust-to-weight than hydrogen
The choice of rocket fuel depends on the specific requirements of the rocket engine and the mission. Kerosene (RP-1) and Hydrogen are the two dominant fuels in the industry.
Kerosene has better handling characteristics compared to hydrogen. It can be stored in an unpressurized tank at room temperature, whereas hydrogen must be kept cryogenic and under very high pressure. This makes hydrogen harder to store and handle. Kerosene's higher density means that more fuel can be pumped through the turbo-pump and into the combustion chamber every second, resulting in greater thrust. Kerosene's higher density also leads to lighter tanks, which makes the rocket overall less complex and lighter.
Kerosene's higher thrust-to-weight ratio is advantageous during the first stage of a rocket launch, where high thrust is required to lift the entire rocket. Its higher density also means that kerosene tanks can be smaller, which is beneficial for the lower stages of a rocket that need to escape the planet's gravity.
Hydrogen, on the other hand, offers greater fuel efficiency and provides more energy relative to its weight, allowing rockets to travel longer distances. Hydrogen's lower density means that its tanks tend to be larger and heavier, which can be a disadvantage for certain rocket stages. However, hydrogen's higher efficiency makes it a better choice for the upper stages of a rocket in outer space, where higher efficiency is more important than high thrust.
In summary, kerosene is favoured for its better handling characteristics, higher density, and superior thrust-to-weight ratio, especially during the first stage of a rocket launch. Hydrogen, with its greater fuel efficiency and energy-to-weight ratio, is preferred for the upper stages of a rocket where efficiency and longevity are more critical.
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A rotating detonation engine could make rockets more fuel-efficient
Sending a rocket into space requires a lot of fuel. For instance, launching NASA’s Space Shuttle into orbit required more than 3.5 million pounds of fuel, which is about 15 times heavier than a blue whale. However, a new type of engine, called a rotating detonation engine (RDE), promises to make rockets more fuel-efficient, lightweight, and less complicated to construct.
RDEs are designed to increase efficiency and reduce costs due to their lower complexity, allowing for more mass to be allocated to other subsystems, such as fuel and payloads. The basic concept of an RDE involves a detonation wave travelling around a circular channel (annulus). Fuel and oxidizer are injected into the channel, and after ignition, the rapid heat release forms a shock wave, resulting in a strong pulse of gas with significantly higher pressure and temperature that moves faster than the speed of sound. This combustion-driven shock wave naturally compresses the flow as it travels around the combustion chamber, eliminating the need for additional machinery to direct and control the combustion reaction.
The development of RDEs has garnered interest from organizations such as NASA, the US Navy, and the US Air Force. Researchers at the University of Washington have developed a mathematical model to describe the workings of RDEs, aiding in improving engine stability. However, the RDE field is still in its infancy, and challenges such as unpredictability and instability need to be addressed before the technology can be safely utilized in actual rockets.
Despite the challenges, RDEs show promising results in experiments. Researchers have conducted tests with various fuel mixes, including hydrogen/oxygen, liquid oxygen/gaseous methane, and kerosene. These tests have achieved notable milestones, such as producing thrust levels of up to 22,000 N and successfully testing an RDE in space for the first time. With further advancements in stability and performance, RDEs could revolutionize rocketry by enhancing fuel efficiency and reducing costs.
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Frequently asked questions
The weight of rocket fuel depends on the type of rocket and the payload it is carrying. For example, a photon rocket needs just 0.03 grams of fuel to lift 1kg of payload to LEO, while a conventional rocket engine like the one used to launch NASA's Space Shuttle requires more than 3.5 million pounds of fuel.
The weight of rocket fuel is determined by the type of propellant used, the power source, and the efficiency of the engine. For example, traditional ion thrusters use Xenon propellant, which is heavier and more expensive than other propellants such as helium, hydrogen, or lithium.
Researchers are developing new types of engines, such as rotating detonation engines, that promise to make rockets more fuel-efficient and lightweight. These engines work by creating a shock wave that naturally compresses the flow of propellant, reducing the need for additional machinery to control the combustion reaction.
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