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The Rotating detonation rocket engine (RDRE): How it Works and What it Means for Space Travel

A rocket engine is a reaction engine, an engine that expels mass to generate thrust. Thrust is the force that propels a rocket or spacecraft and is measured in pounds, kilograms or Newtons. Physically speaking, it is the result of pressure which is exerted on the wall of the combustion chamber. Rockets carry all of their own working fluid, called propellant, in contrast with air-breathing engines. Furthermore reaction engines are only considered rockets when they use gasdynamic expansion of the working propellant to accelerate it. This contrasts with, for instance, many types of electric propulsion thrusters which use body forces such as the Coulomb electrostatic force to accelerate gasses.


In a normal rocket engine we use fuel and oxidizer in a chemical reaction to create hot combustion products. The combustion process involves the oxidation of constituents in the fuel that are capable of being oxidized, and can therefore be represented by a chemical equation. During a combustion process the mass of each element remains the same. t should be pointed out that in the combustion process there will be a dissociation of molecules among the products. That is, the high heat of combustion causes the separation of molecules into simpler constituents that are then capable of recombining.


The common feature of all of these engines  is a converging / diverging nozzle De Laval nozzle. Such nozzles are the essential feature of rocket engines as this is what enables the engine to gasdynamically accelerate propellant gasses from a stagnant state to high velocity imparting momentum on the engine.  To create high speed exhaust gases, the necessary high temperatures and pressures of combustion are obtained by using a very energetic fuel and by having the molecular weight of the exhaust gases as low as possible. It is also necessary to reduce the pressure of the gas as much as possible inside the nozzle by creating a large section ratio. The section ratio, or expansion ratio, is defined as the area of the exit Ae divided by the area of the throat At.


A good parameter for the effectiveness of a rocket is  called effective exhaust velocity which is the quotient of thrust (what we want) with propellant mass flowrate (what we have to pay). Specific Impulse,  is popularly spoken of as the “gas mileage” for a rocket cycle and it fundamentally indicates how much bang for the buck you get .For a rocket with Isp = 100s a unit mass, m of propellant can generate enough thrust to support its weight in Earth’s gravity for 100 seconds or 100 times its weight for one second.


It takes a lot of fuel to launch something into space. Sending 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. But a new type of engine — called a rotating detonation engine — promises to make rockets not only more fuel-efficient but also more lightweight and less complicated to construct.

Rotating detonation engine

A rotating detonation rocket engine (RDRE) is a type of rocket engine that uses a continuous detonation wave instead of a conventional deflagration flame to burn fuel and oxidizer. In a conventional rocket engine, the fuel and oxidizer mix and then burn in a deflagration, a subsonic combustion process. In contrast, a detonation wave is supersonic and compresses the fuel and oxidizer mixture to very high pressures and temperatures, resulting in a more efficient and powerful combustion process.

The RDRE consists of a combustion chamber that is shaped into a ring, and fuel and oxidizer are injected into the chamber from opposite sides. As the mixture ignites, it creates a rotating detonation wave that travels around the ring, continuously igniting and burning the fuel and oxidizer mixture as it goes. The rotation of the wave helps to distribute the heat and pressure evenly around the combustion chamber, resulting in a more stable and efficient combustion process.

A conventional rocket engine works by burning propellant and then pushing it out of the back of the engine to create thrust. “A takes a different approach to how it combusts propellant,” Koch Researcher at the University of Washington that have developed a mathematical model that describes how these engines work. said. “It’s made of concentric cylinders. Propellant flows in the gap between the cylinders, and, after ignition, the rapid heat release forms a shock wave, a strong pulse of gas with significantly higher pressure and temperature that is moving faster than the speed of sound. This produces high pressure and temperature that drives exhaust out the back of the engine at high speeds, which can generate thrust.”

RDREs have the potential to be more efficient and powerful than conventional rocket engines, which could lead to significant advances in space exploration and travel. However, the technology is still in its early stages of development and faces several challenges, including maintaining stable combustion over long periods and managing the high temperatures and pressures generated by the detonation wave.

In addition to the potential for increased efficiency and power, RDREs offer several other advantages over conventional rocket engines. One key advantage is their ability to use a wider range of fuels and oxidizers, including less expensive and more environmentally friendly options such as methane and oxygen. They also have the potential to be smaller and lighter than conventional engines, making them more suitable for use in small satellites and other space vehicles.

However, as mentioned before, the technology is still in its early stages of development and faces several challenges. One major challenge is achieving stable and continuous combustion over long periods of time. The rotating detonation wave is highly complex and can be difficult to control, and any instability or fluctuations in the wave can cause the engine to fail. Additionally, the high temperatures and pressures generated by the detonation wave can cause materials to degrade quickly, leading to durability issues.

Despite these challenges, RDREs have garnered significant interest from the aerospace industry and several research groups around the world are actively working to improve the technology. In the coming years, it is possible that RDREs could become a key component of future space exploration and travel.


Researchers Develop Groundbreaking New Rocket-Propulsion System

A University of Central Florida researcher and his team have developed an advanced new rocket-propulsion system once thought to be impossible. The system, known as a rotating detonation rocket engine, will allow upper stage rockets for space missions to become lighter, travel farther, and burn more cleanly. The results were published this month in the journal Combustion and Flame. “The study presents, for the first time, experimental evidence of a safe and functioning hydrogen and oxygen propellant detonation in a rotating detonation rocket engine,” says Kareem Ahmed, an assistant professor in UCF’s Department of Mechanical and Aerospace Engineering who led the research.


The rotating detonations are continuous, Mach 5 explosions that rotate around the inside of a rocket engine, and the explosions are sustained by feeding hydrogen and oxygen propellant into the system at just the right amounts. This system improves rocket-engine efficiency so that more power is generated while using less fuel than traditional rocket energies, thus lightening the rocket’s load and reducing its costs and emissions.  Mach 5 explosions create bursts of energy that travel 4,500 to 5,600 miles per hour, which is more than five times the speed of sound. They are contained within a durable engine body constructed of copper and brass.The technology has been studied since the 1960s but had not been successful due to the chemical propellants used or the ways they were mixed.


Ahmed’s group made it work by carefully balancing the rate of the propellants, hydrogen and oxygen, released into the engine. “We have to tune the sizes of the jets releasing the propellants to enhance the mixing for a local hydrogen-oxygen mixture,” Ahmed says. “So, when the rotating explosion comes by for this fresh mixture, it’s still sustained. Because if you have your composition mixture slightly off, it will tend to deflagrate, or burn slowly instead of detonating.” Ahmed’s team also had to capture evidence of their finding. They did this by injecting a tracer in the hydrogen fuel flow and quantifying the detonation waves using a high-speed camera.


“You need the tracer to actually see that explosion that is happening inside and track its motion,” he says. “Developing this method to characterize the detonation wave dynamics is another contribution of this article.” William Hargus, lead of the Air Force Research Laboratory’s Rotating Detonation Rocket Engine Program, is a co-author of the study and began working with Ahmed on the project last summer. “As an advanced propulsion spectroscopist, I recognized some of the unique challenges in the observation of hydrogen-detonation structures,” Hargus says. “After consulting with Professor Ahmed, we were able to formulate a slightly modified experimental apparatus that significantly increased the relevant signal strength.”


“These research results already are having repercussions across the international research community,” Hargus says. “Several projects are now re-examining hydrogen detonation combustion within rotating detonation rocket engines because of these results. I am very proud to be associated with this high-quality research.” “These research results already are having repercussions across the international research community.” — William Hargus, study co-author The study was supported with funding from the U.S. Air Force Office of Scientific Research and an Air Force Research Laboratory Contract.


NASA Validates Revolutionary Propulsion Design for Deep Space Missions

As NASA takes its first steps toward establishing a long-term presence on the Moon’s surface, a team of propulsion development engineers at NASA have developed and tested NASA’s first full-scale rotating detonation rocket engine, or RDRE, an advanced rocket engine design that could significantly change how future propulsion systems are built.

The RDRE differs from a traditional rocket engine by generating thrust using a supersonic combustion phenomenon known as a detonation. This design produces more power while using less fuel than today’s propulsion systems and has the potential to power both human landers and interplanetary vehicles to deep space destinations, such as the Moon and Mars.

Engineers at NASA’s Marshall Space Flight Center in Huntsville, Alabama, and primary collaborator IN Space LLC, located in West Lafayette, Indiana, are confirming data from RDRE hot fire tests conducted in 2022 at Marshall’s East Test Area. The engine was fired over a dozen times, totaling nearly 10 minutes in duration.

The RDRE achieved its primary test objective by demonstrating that its hardware – made from novel additive manufacturing, or 3D printing, designs and processes – could operate for long durations while withstanding the extreme heat and pressure environments generated by detonations. While operating at full throttle, the RDRE produced over 4,000 pounds of thrust for nearly a minute at an average chamber pressure of 622 pounds per square inch, the highest pressure rating for this design on record.

The RDRE incorporates the NASA-developed copper-alloy GRCop-42 with the powder bed fusion additive manufacturing process, allowing the engine to operate under extreme conditions for longer durations without overheating.

Additional milestones achieved during the test include the successful performance of both deep throttling and internal ignition. This successful demonstration brings the technology closer to being used with future flight vehicles, enabling NASA and commercial space to move more payload and mass to deep space destinations, an essential component to making space exploration more sustainable. Because of NASA’s recent success with the RDRE, follow-on work is being conducted by NASA engineers to develop a fully reusable 10,000-pound class RDRE to identify performance benefits over traditional liquid rocket engines.


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